SpaceX have a striking video showing Mars spinning faster and faster, transforming from the current red Mars to a planet with a small ocean and with the deserts tinged with green in seven revolutions.

Of course that is poetic exaggeration - it wouldn't terraform in a week. So how long would it take? Science fiction enthusiasts who have read Kim Stanley Robinson's "Mars Trilogy" may remember that in his book, it is terraformed in a couple of centuries. But that's science fiction, not a terraforming blue print.

Is it possible at all, and how long do scientists think it would take? The answer is that if it is possible, it would take thousands of years. Here is how they envision it in the Mars Society - a thousand years to get to trees, water and a landscape where humans can go out of doors without a spacesuit, using scuba gear:

Images from the Big Idea (National Geographic Magazine) See also, How we will terraform Mars - by Jason Shankel and Terraforming Mars by Nicole Willet

Note that after a thousand years, with trees, still you can’t breathe the atmosphere (that's why they are wearing scuba gear in the artist's impression), and there are no animals or birds yet. Also, if you research into their plans, you find out that behind the scenes there is a lot of mega technology to make this possible, if it works. For one thing, they will need to have hundreds of factories constantly producing greenhouse gases, and / or planet sized thin film mirrors in space to reflect extra sunlight onto the planet to keep it warm.

This may perhaps seem a minor achievement to aim for, used as we are to breathing on Earth, but for a prospective Mars colonist, it as a big step forward. At the moment the air on Mars is so thin that the moisture lining your lungs would boil. This is not something you can adapt to. It is simple physics that no warm blooded creature with lungs can breathe there. If someone put you on the surface of Mars with scuba gear, you couldn't take one breath of air because your lungs would stop working immediately in the low atmospheric pressure of 1% of Earth's or less.

Could we do this much?

Our biosphere took hundreds of millions of years to develop on Earth and it might have gone in many different directions. The hope is that the development can be speeded up to a few thousand years, and directed so that you end up with a biosphere like that of Earth at the end, to pretty much the planet you want.

Suppose we had another planet exactly like Earth, but with a thin carbon dioxide atmosphere instead of an oxygen / nitrogen one. Let’s stack all the cards in our favour and make it as easy to terraform as you can for a planet that doesn't have life on it. It’s got continental drift, magnetic field, the works. Now imagine it is in the place of the Moon as close and easy of access as that, I will suggest that we are not anywhere near the level of understanding needed to terraform it with any assurance of success. And it's going to be much harder, with a planet that, though related, is also so different from Earth, further from the sun, much less water, no continental drift, no active volcanism right now (though not quite inactive), lower gravity, different surface chemistry, superoxigenated with perchlorates and hydrogen peroxide, etc etc.

But not to be disheartened if we can't do it. Perhaps terraforming is more like a grand goal for a mature civilization thousands of years old. But if so, there is much we can do as a young civilization, by way of building city domes, lava tube caves and free flying space settlements. We could settle the entire solar system right out to beyond Pluto with space settlements spinning for artificial gravity before the terraforming project has got off its starting blocks.

Video fly through of a Stanford Torus style space habitat by Uzi Bento

Sunlight is usually brought to the habitat via mirrors in these designs e.g. here is how it's done for the Stanford Torus.

This is a modern update of the design, Habitat 2, with  a big 2 km across mirror (aluminised Mylar) - the animation leaves out the cosmic radiation shielding for artistic reasons: 
Video of Habitat 2 and mirror

Sunlight gets reflected around the cosmic radiation shielding into the habitat. UV light can be absorbed on the way. Cosmic radiation, as highly energetic particles, goes right through the mirror.

As the authors of the Stanford University 1975 publication "Space Settlements: a Design Study" wrote,

"At all distances out to the orbit of Pluto and beyond, it is possible to obtain Earth-normal solar intensity with a concentrating mirror whose mass is small compared to that of the habitat.”

Indeed, I wonder a bit if we need to start thinking about galaxy protection, as there isn't much after that to stop us spreading to the entire galaxy - is that such a good idea at our young stage in our civilization?

Also it may be more interesting to find out what happens on Mars if we work with what comes naturally to Mars instead of against it. Perhaps the issues with ideas for terraforming it are partly due to fighting against "Gaia". What if the natural ecosystem for Mars is different from that for Earth? Anyway let's start by looking at conventional terraforming there.

TERRAFORMING ATTEMPTS CAN GO WRONG - BADLY WRONG

There would be so much to go wrong. And I mean badly wrong, mistakes that would make it impossible for anyone to terraform it in the future. Especially if they involved introducing the wrong kind of microbe, there'd be no way you can roll that back.

This can be something that builds up slowly, underground, or a few microbes spreading in the wind and weather. By the time you notice it is going wrong, it could have spread far. Indeed, it may be a while before you notice the microbe at all amongst the hundreds of billions or even a trillion distinct species of microbes, on a partially terraformed world. It may well have spread throughout the planet before you know it is there.

The biologist Cassie Conley gave a simple example. She's the NASA planetary protection officer. And this is just for ordinary expedition to present day Mars, right now not even any attempt to terraform yet. Some Earth microbes, in the anoxic conditions on Mars and in the presence of methane (which may well be present there), could form calcite in underground aquifers - so turning them to cement.

"Conley also warns that water contaminated with Earth microbes could pose serious problems if astronauts ever establish a base on Mars. Most current plans call for expeditions that rely on indigenous resources to sustain astronauts and reduce the supplies they would need to haul from Earth."

"What if, for example, an advance mission carried certain types of bacteria known to create calcite when exposed to water? If such bacteria could survive on Mars, Conley says, future explorers prospecting for liquid water instead might find that underground aquifers have been turned into cement."

Going to Mars Could Mess Up the Hunt for Alien Life

In more detail what she is talking about there is the anaerobic oxidation of methane that leads to formation of calcium carbonate in anoxic conditions . It's done by a consortium of methane oxidising and sulfate reducing bacteria. See summary here in wikipedia: Calcite - formation process - which links to this technical paper which goes into more detail.

Calcite - calcium carbonate. In the anoxic conditions on Mars, in presence of methane, a combination of methane oxidizing and sulfate reducing microbes can cause calcite to form and so, basically, could turn underground aquifers on Mars into cement. Cassie Conley’s example of one way that accidentally introduced microbes could have unpredictable effects on Mars.

When it comes to microbes introduced to an unfamiliar planet that behaves differently from Earth, with many differences in the chemistry, atmosphere, environment - any number of unexpected interactions could happen.

Here are a few more examples

• You want to grow green algae to take the carbon out of the atmosphere and generate oxygen, but haloarchaea take over. These salt loving microbes are likely to feel at home on Mars, and they convert the sunlight directly to energy by a process using bacteriorhodopsin similar to the way our retina works - and produce no oxygen at all. How do you change the balance back to the green algae?
• As the climate warms up and it gets damp, but with no oxygen - those are ideal conditions for microbes that produce bad egg gas (H2S). Hydrogen sulfide smells like a sewer - the whole planet would stink.
  
  Maybe you want that (it’s a greenhouse gas and this is one suggestion for a way to start a process of terraforming). But maybe instead you are trying to get a carbon dioxide / oxygen ecology warmed by those planet sized mirrors, and the hydrogen sulfide producers take over and kill nearly everything.
  
  One theory of many for the Permian extinction, the largest mass extinction in world history, 251 million years ago, is that it may have been caused by an initial upwelling of hydrogen sulfide that was then maintained by purple and green sulfur bacteria that thrived in the anoxic conditions. See description of their research here, and paper here. Whether or not that's what happened on Earth at the end of the Permian period, that it's a possibility for Earth might suggest that something similar could happen while terraforming Mars, which would start off naturally anoxic.
  
  Or, maybe this happens the other way around, you are trying to produce H2S as a greenhouse gas and the cyanobacteria take over.

Then, there's the possibility that there might be native life as well with unexpected capabilities. They could interact with your ecosystem and may not behave in the same way as the microbe whose niche they take over. Or may hybridize with Earth life via gene transfer. This is an ancient mechanism (GTAs) which works between organisms as different as fungi and aphids, and between microbes that split from each other billions of years ago. If Mars has any life that split of from Earth life after the development of DNA, it may well be able to share genes with Earth microbes in the same way - and indeed in suitable environments such as warm salty water, it could do this rapidly, overnight.

Or it could be some microbe from Earth that’s harmless here and not even been noticed finds the different and unusual Mars environment to its liking and spreads everywhere. Spreading through a new ecosystem, within a few years or a decade or two it could become the most important microbe on Mars - and again maybe it doesn’t behave as you’d like it to.

SO MUCH EASIER WITH A SMALLER ECOSYSTEM

With a small free space or lunar habitat of a few cubic kilometers, then we'd be bound to make many mistakes even with an experiment that small. But it is easily reversed (comparatively).

If there’s a build up of some problem gas, you can scrub the atmosphere. If there’s an infestation of some diseases, insects, or mold, say, you can treat it with chemicals or tackle it biologically and eliminate it. In the very worst case, if something comes up which you can’t fix - you can purge the atmosphere, sterilize it if necessary and start again, learning from your mistakes.

You can't sterilize a planet or purge its atmosphere and start again. Nor can you scrub it of problem gases or easily eliminate some problematical organism. Look at how hard it is for us to do anything about carbon dioxide amounting to an extra hundredth of a percent of the Earth's atmosphere, or to keep out invasive species from an island or continent, even for higher animals never mind microbes.

It might be a species introduced deliberately, because you think it will help with the terraforming, and then it causes problems you never expected. The European starling in the US, for instance, introduced by Shakespeare enthusiasts in the 1890s. That's not going to happen on Mars any time soon, as there won't be any birds in an atmosphere with little or no oxygen. But plants, yes, if they were successful. Kudzu might be a good analogy. US citizens were encouraged to plant it for erosion control, as a livestock feed and to make paper. Then it became a problem, smothering large areas, and it grows so quickly it is hard to keep it under check. Imagine a situation where some plant like that is out of control on Mars, what do you do?

Even feral camels are an issue in Australia. Deliberately introduced, but in such a vast continent, it's hard to get rid of them. Rabbits also, famously. Then moving in the other direction to the very small, diseases of microbes might also become a terraforming issue. For instance bacteriophages, viruses of bacteria, significantly reduce the amount of hydrogen sulfide produced by sulfur bacteria. This can be used as a biological control if the problem is too much H2S - but it may be a nuisance on Mars if your aim is to produce as much of the gas as possible to warm up the planet.

With a Stanford Torus or O'Neil habitat, or a lava tube cave or a city dome, none of these are an insoluble issue. You are not going to have a problem with feral camels, and rabbits also wouldn't be hard to deal with. With invasive plants like kudzu, at worst you have to sift the soil to remove the roots. Even sulfur bacteria, cyanobacteria, or microbes that could turn your water supply to cement - none of these are an insoluble issue. Even with bacteriophages, if you can't control them in any other way, just press the "reset button" of sterilizing the habitat, analyse what went wrong, and start again.

VAST TIMESCALES OF MILLENNIA TO COMPLETE THE PROJECT

With the plans to terraform Mars, as proposed by the Mars Society, they are counting on everything going right for a thousand years to get to the point where you have an atmosphere suitable for trees.

The humans still can't live there even with an oxygen supply. It turns out that carbon dioxide is a poison to us at concentrations above 10% in the atmosphere (not many know that, and I am not mixing up carbon monoxide with carbon dioxide). If the Apollo 13 hadn't found their duct tape MacGyver type solution to the problems with their carbon dioxide scrubbers, this risked killing them all, dying in a carbon dioxide rich atmosphere, with plenty of oxygen still available.

You can't live in a mixed CO2 / O2 atmosphere. They would need to use closed system air breathers like aqualungs rather than an oxygen mask.

Then, there is not enough CO2 in the ice caps to do more than double the atmospheric pressure to around 12 millibars, under 2%, far too little for humans to take off their full body spacesuits. So to go any further then they have to assume lots of CO2 in the form of dry ice mixed in with the soil to considerable depths - but there's no evidence either way about this yet.

They usually suggest using greenhouse gases to warm it up to the point that the dry ice sublimates into the atmosphere. That's a big megatechnology project, 500 half gigawatt power stations according to one estimate, and mining cubic kilometers of fluorite ore on Mars per century to make the gases. That’s the ‘easy solution’. The harder solution is to use planet sized thin film mirrors in space to reflect extra sunlight on Mars.

Chris McKay et al estimate that you could get Mars warm enough to trigger a runaway greenhouse effect (if it has the dry ice available for it) by producing greenhouse gases 24/7 for a century using 245 power stations each half a gigawatt running 24/7 producing gases. The amount of fluorite you'd need to mine on Mars would be equivalent to the contents of eleven cubic kilometers of pure fluorite ore. That's in his 2004 paper with other authors. At forty billion metric tons it is just not practical to transport them from Earth.

In a 2005 paper with Margarita Marinova as the principle author, they do detailed modeling of various greenhouse gases, and come up with a mixture of gases that can tip it into a runaway greenhouse at 0.2 pA. Since one pA is about a thousandth of the current Mars atmosphere (100,000 Pascals to a bar), that's 0.025 teratons, or 25 billion tons of the gases, a somewhat smaller figure.

If this succeed, then a thousand years later, you end up with an atmosphere that only trees and plants can tolerate which is poisonous to humans. If everything breaks in your favour. At that point you have to continue production of the greenhouse gases, however, if you want Mars to stay warm, because it requires a far warmer greenhouse to keep it warm at that distance with half the sunlight levels of Earth. You might be surprised at the difference, but it's because the absolute temperature depends on the balance between the input sunlight and the heat radiated. The absolute temperature of Earth is very high and it doesn't take much of a reduction to make the average temperatures uninhabitable for humans and most vegetation.

A recent study, looking at all the evidence, comes to the conclusion that even this much is probably not possible. It finds that

• The adsorbed CO2 in the regolith, if it exists can't be released quickly
• Even if the atmosphere could be increased to 100 mbar it would raise the temperature by only 10 K and would still not lead to stable liquid water
• If the polar CO2 was sublimed, e.g. by covering with soot, doubling to 12 mbar, then it would not be an equilibrium situation and would be likely to soon recondense back again on the surface.

Their conclusion is:

The ability to release enough CO2 into the Mars atmosphere to provide any significant greenhouse warming is extremely limited. This is the case even if most of the CO2 present on early Mars still remained on the planet, locked up in adsorbed gas and carbonates. Greenhouse warming is further limited in the likely event that the bulk of the early CO2 has been lost to space, as suggested by recent measurements.

While greenhouse warming is still conceivable by the mechanism described by McKay et al., large scale manufacturing of chlorofluorocarbons, that approach is very far into the future at best.

It is not feasible today, using existing technology or concepts, to carry out any activities that significantly increase the atmospheric CO2 pressure and/or provide any significant warming of the planet. Terraforming in the near term is not feasible.

Perhaps you get the carbon dioxide from comets from the outer solar system? At present that's not very feasible but maybe in the future we can move comets around the solar system with ease. So, let's stick with that and explore a bit further.

GENERATING AN OXYGEN RICH ATMOSPHERE - IS IT 900 YEARS (ZUBRIN) OR 100,000 YEARS (MCKAY)?

There are no natural sources of oxygen outside of Earth - so you won't find an oxygen rich comet. The usual suggestion is to produce the oxygen from carbon dioxide so you start with a carbon dioxide atmosphere and then use plants to extract the carbon as organics. If you use photosynthesis it's around 100,000 years to pull all the carbon out of the atmosphere according to Chris McKay. Perhaps that can be speeded up, but somehow or other, you have to create a layer of meters thickness of organics averaged over the entire planet to get rid of the carbon.

To get a rough idea of how much carbon you need to pull out of the atmosphere - if you achieve an Earth pressure atmosphere, it's 2.63 times as much mass as on Earth because of the low gravity. So about 26 tons per square meter of atmosphere. Of that, if it's a pure carbon dioxide atmosphere, then the molecular mass of CO2 is 22, atomic mass of carbon is 6, so the mass of carbon in the atmosphere is 6*26/22 = 7 tons. So you need to extract about 7 tons per square meter of carbon from the atmosphere. Since organic matter has other elements as well, hydrogen and nitrogen, and is typically lighter than water even when dried, then you are talking about more than 7 meters of thickness of organics. For instance the element content of wood is approximately 50% carbon, 42% oxygen, 6% hydrogen, 1% nitrogen, and 1% other elements. Its density is typically up to 0.7.. So that would make it around 20 meters thickness of solid wood that you'd need to accumulate, at a mass of 14 tons per square meter. If the atmosphere is only a tenth of Earth's atmospheric pressure, the thinnest that humans can breathe, it's only 2 meters but then you have the issue that at those concentrations it has to be pure oxygen which is a fire hazard. I think you can understand why Chris McKay worked out that this would take 100,000 years using natural processes.

If you use photosynthesis to extract carbon, this will bury nitrogen and water as well, so taking it out of the atmosphere. If concentrations are similar to those in wood, then getting on for half of it is water, so you would need a lot of water available, before you can extract that much carbon from it. Enough to cover the surface of Mars to a depth of around 10 meters. That is, unless you find a way to burn the biomass to form biochar, all over the planet regularly. That would deal with the water and nitrogen problem but it would be hard to arrange for it to happen.

Zubrin is much more optimistic and he estimates, it could be accomplished as quickly as 900 years, with mega-engineering.

His proposal is to first release one millibar of oxygen from the perchlorates which he estimates requires 2200 terrawatt years of power (that’s the equivalent of 20,000 half gigawatt power stations operating for 220 years). It’s a lot of power but he would use space mirrors, and assumes a 3,125 km radius space mirror focusing the power of the sun as the source of energy.

After that he envisions higher plants genetically engineered with an efficiency of 1% spread over the planet. He doesn’t explain, but for this to work, for all that photosynthesis to be dedicated to increasing the oxygen content of the atmosphere - the plants have to be harvested and buried and more grown on top. For his objective, it’s no good just having them in a cycle returning the material to the atmosphere when they die, as that is just a seasonal oscillating cycle of more and then less oxygen.

With this background he writes:

“… they would represent an equivalent oxygen producing power source of about 200 TW. By combining the efforts of such biological systems with perhaps 90 TW of space based reflectors and 10 TW of installed power on the surface (terrestrial civilization today uses about 12 TW) the required 120 mb of oxygen needed to support humans and other advanced animals in the open could be produced in about 900 years. If more powerful artificial energy sources or still more efficient plants were engineered, then this schedule could be accelerated accordingly….”

As he goes on to say, if we had easy access to fusion power we’d have far higher levels of power available which could change many things. But as it is now it’s a huge megatechnology project.

Photosynthesis cools down the planet by removing carbon dioxide (a greenhouse gas of course). So now you have to step up your greenhouse gas production even more. You are committed to greenhouse gases or large thin film mirrors for as long as Mars remains habitable.

An Earth atmosphere transferred to Mars would not be anything like Earth without artificial means - the water would all freeze and it would be too cold for trees even at the equator. That's simple physics, it is too far from the Sun for Earth's atmosphere to work there.

Also plants on Mars will have to work three times as hard as on Earth to maintain the same level of oxygen, because in the low gravity you need three times the mass for the same atmospheric pressure. But they also have to do that with half the level of sunlight. This is why Chris McKay says it would take so long, 100,000 years, to generate an oxygen atmosphere there. On Earth in similar conditions it could happen in a sixth of the time.

MAGNETIC SHIELD DOESN’T HELP

The idea of a giant magnetic shield got some publicity, but is just useful to protect the Mars atmosphere long term. It is not going to thicken it up in the near term. Even if you had Earth’s volcanic production of CO2 on Mars it would take around a million years to reach 6 millibars, which is the point at which they think a runaway effect could happen.

The reason it is so inactive is probably because of its lack of continental drift. And perhaps it may have thick deposits of carbonates that if they were subducted beneath another continental plate would then lead to many volcanic eruptions and carbon dioxide building up the atmosphere. But even on Earth - you are talking about a very long cycle there. There are some figures for Earth here.

"On average, 10^13 to 10^14 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year"

So, with the upper estimate, that's up to 10^11 kg from Earth volcanoes per year. The Mars atmosphere is 2.3*10^16 tons.

So, you are talking about 100,000 years of Earth level of volcanism to duplicate the current atmosphere at about 0.6 % of Earth normal, so about a million years to reach 6% which is the point at which a runaway greenhouse might be possible (if there is enough dry ice there to be liberated, which is unknown)..

But Earth has dozens of active volcanoes. Mars, despite many searches using thermal imaging from orbit, has turned up no volcanoes to date. Not even a geothermal hotspot or a fumarole. It could conceivably have some ice fumaroles with the heat signatures hidden by towers of ice but that's a long shot.

There is good evidence that Olympus Mons erupted 25 million years ago. AFAIK that is the most recent eruption there is any evidence for.With the current level of volcanism on Mars you must be talking about getting on for billions of years. And indeed estimates of current outgassing rates confirm that.

In this paper studying outgassing from Mars volcanism, updated with the latest Curiosity results, here Ga means billions of years ago. They estimate 70 mbar outgassed in the last 3 billion years with their most extreme outgassing scenario. The more likely scenario is essentially 0 mbar in the last 3 billion years. In both scenarios most of the outgassing happens over 2 billion years ago.

For completeness, we test the evolution scenarios by both increasing the total outgassing rate and prolonging the outgassing activity. Specifically, we assume the mantle plume scenario of Grott for outgassing, at an oxygen fugacity of IW+1 and a degassing efficiency of 0.4. The total outgassing amount since 3.8 Ga would then be 420 mbar, in which 350 mbar would be outgassed between 3.8 and 3.0 Ga and 70 mbar would be outgassed between 3.0 Ga and present. This is compared with the standard models in which 48?mbar would be outgassed between 3.8 and 3.0 Ga, and essentially 0 would be outgassed after 3.0 Ga.

If you look at their figure 3, you see that even with the highest figure (red solid line in the diagram) most of the outgassing happened over 2 billion years ago so most of that 70 mbar happened before then. Almost none in the last 2 billion years.

Over a time long enough for humans to evolve from shrew like mammals scurrying at the feet of dinosaurs it still wouldn't have thickened up noticeably. Indeed, even over the half billion years it took for humans to evolve all the way from our first microscopic multicellular ancestors, you still wouldn't notice a significant thickening of the atmosphere. This idea is a non starter. It could be useful if Mars already had a thick atmosphere and you needed to retain it long term for millions of years.

NITROGEN AND WATER

Finally, an oxygen only atmosphere is a fire hazard so you have to add nitrogen as a buffer gas (it absorbs some of the heat from a fire and so acts as a fire retardant). Mars seems to be rather deficient in nitrogen. Perhaps its original atmospheric nitrogen is partly buried underground in nitrates beneath its northern sea? Nobody yet has a clear idea where to find it except to import it by hitting Mars with ammonia rich comets.

There's enough ice in the polar caps for a few meters of water over the surface of Mars but that's assuming you have a Mars with no ice caps - how likely is that?

Its ice caps are much smaller than Earth's and an Earth-like Mars would have larger ice caps, not smaller ones.

Mars north pole ice cap - Composite of 1000 Viking Orbiter red- and violet-filter images (NASA / JPL / USGS) - got it from: When Humans Begin Colonizing Other Planets, Who Should Be in Charge?

A terraformed Mars would still have ice caps, larger if anything. Terraformers hope that there are large supplies of ice in the southern uplands beneath the surface. Some of this at least has been found so there may be some reason for optimism, but how much is there?

Also the equatorial regions are dry to considerable depth and any water would have to fill the desert sands to levels of hundreds of meters to kilometers before you have any on the surface.

If you are optimistic you hypothesize enough ice from the earlier oceans trapped in the southern uplands to pour out onto the northern ocean area, and maybe even the ocean that used to cover the entire northern hemisphere is still there somewhere, trapped as ice underground.

If you are pessimistic then nearly all that ice got lost to space as water vapour split by solar storms with the hydrogen escaping and the oxygen reacting with the surface of Mars and rusting it - or it ended up in the hydrosphere kilometers below the surface. After terraforming, unless you import ice from comets - at most you have a few flowing streams of water and a lake or two, in a still largely dry planet covered mostly in desert.

NOT AN AUTOMATIC OUTCOME

As well as that, supposing you do find all the water you need, it took hundreds of millions of years for Earth to develop a microbial biosphere supporting an oxygen rich atmosphere.

But as well as that we don't know that Earth's present biosphere is the automatic outcome. There are probably thousands of possible ways that our planet could have gone, with a variety of ecosystems, and perhaps only a few of those habitable to humans. Some perhaps totally uninhabitable as a result of some runaway effect that tipped it out of habitability.

And we are hoping to speed up those hundreds of millions of years into centuries. If it can be terraformed that quickly it can probably unterraform just as quickly.

PAUL BIRCH'S TERRAFORMING MARS QUICKLY

The article is here: Terraforming Mars quickly.

This is a bit like Robert Zubrin's 3,125 km radius space mirror but far more ambitious, with the aim to terraform Mars in 120 years. He uses his mirror to boil the rock in the regolith at over 3000 C. The largest component is an annular mirror 25,000 km in diameter and 300 km in width of the ring, focuses light via a secondary mirror Total mass of that plus the soletta 50 million tons.

It also has a very low density 920 km diameter "lens" floating at a height of 400 km in the Mars upper atmosphere which is too flimsy to be built at ground level and so must be built in orbit and somehow "lowered" into the atmosphere to rest there floating partly through buoyancy in the upper atmosphere and mainly through the hot air generated beneath it. The aim is to focus all the sunlight into an 80 km diameter spot on the Mars surface which will be so hot it volatilizes the regolith into vapour.

Once an atmosphere forms he has to ge rid of the carbon dioxide. He proposes to use photosynthesis. and inhibit uptake of oxygen by giving the plants sunlight 24/7 using mirrors to illuminate the night side of the planet.

He doesn't really explain how he generates a growing media for the plants - just says there are the elements needed. But he has to generate meters thickness of organics, so the plants have to grow in the remains of previous plants - so how do they do that except by decomposition? But if there is decomposition, then there are microbes returning carbon dioxide back to the atmosphere and using oxygen.

He assumes that we can control which microbes are on Mars so that there is an

"absence of animals, pests, heterotrophic organisms or other competitors".

But he also talks about humans being on the surface already saying they can live on the surface during the terraforming.

This is the most implausible part of it for me. How can you have humans on the surface without heterotrophs (microbes that eat other microbes)? But that is essential to his plan as if you have unexpected microbes there then there is no way to be in control of what the plants do. There's also the possibility of native life and who knows what that would be able to do if it is there.

On nitrogen he proposes a much lower level than on Earth and claims that the low gravity will reduce convection and so reduce the risk of fire. But a pure oxygen or near to pure oxygen atmosphere is generally considered a fire risk in space habitats - that's why they have an Earth normal atmosphere in the ISS rather than an oxygen atmosphere. It would make things much easier to have an oxygen atmosphere at the same pressure as spacesuits when it comes to EVAs.

I don't know of any study saying that an oxygen atmosphere without nitrogen is only a fire hazard in Earth gravity. He doesn't give any cite there to support it. If it is a fire hazard in zero g - then surely it is a fire hazard in Martian g too, so I'm not convinced by that

When it comes to water, he is not sure if there is enough of it. He talks about releasing the water from the polar ice caps. This has to mean the final terraformed world has no ice caps - is it warmed so much, at least at the poles, that it no longer has ice caps? He talks about finding it in the regolith and possibly importing it by transporting Saturn's Hyperion moon to Mars (he has already earmarked Enceladus for Venus).

It's a fun paper and gives n idea of what lengths some future megatechnology civilization would need to go to to terraform Mars quickly. It is major megaengineering and assumes a great deal of confidence and command in ones ability to predict or somehow control what microbes introduced to a planet will do there.

I do recommend reading his papers. They are a fun read for a space geek and though I don't think we will be "terraforming Mars quickly" any time soon, they could spark imaginative science fiction books and maybe lead to ideas of what other millions of years old extra terrestrial civilizations may be able to do. His page with his papers all available for download is here.

KEEPING MARS WARM

In Chris McKay's analysis he assumes continued production of those very potent fluoride based greenhouse gases once you get to an oxygen rich atmosphere. Once the CO₂ is gone, the natural equilibrium average temperature of a  Mars even with a thick Earth like atmosphere is very low. That's because nitrogen and oxygen do almost nothing to help retain heat and it gets half the sunlight that Earth does, and the temperature dependence on distance is for the temperature measured from absolute zero.

Chris McKay et al in Making Mars Habitable estimate that Mars in thermal equilibrium with an Earth atmosphere would average -55°C, and that includes a greenhouse effect due to CO₂ set to 1%, so much more than for Earth, as that also means three times the column mass for the same pressure in the lower gravity.

Earth average temperatures are 16°C which includes a greenhouse effect of about 33°C, much of that due to water vapour. But on Mars at below -50 °C the amount of the greenhouse effect due to water vapour would be small.

For trees you need a temperature of around 3 °C (page 105 of Chris McKay's paper). With a terraformed Mars without substantial continuous warming by greenhouse gases or orbital mirrors, it isn't possible to have trees growing even in equatorial regions.in summer. Even to have trees growing at low levels in equatorial regions would require considerable warming.

To reach average temperature levels similar to Earth, your mirrors or greenhouse gases, assisted by the warming effect of water vapour, have to raise the temperature by about 88 °C, though only around 55 C increase in temperature if you take account of the warming effect of water vapour (if it has as much water vapour in the atmosphere as Earth). And they have to do this in perpetuity for as long as the planet remains habitable.

WASTEFUL OF RESOURCES - PLANETARY CHAUVINISM

It is just so incredibly wasteful of resources to try to terraform a planet. All that water, all that carbon dioxide, all the resources on Mars for habitability together with gathering thousands of comets too, probably, for both water and carbon dioxide too (probably), and smashing them into Mars in order to create 2-3 meters thickness of breathable air (we don't care about the rest) and all that water poured into the dry sands in order to have a tiny amount on the surface for habitability.

Then all those greenhouse gases or large planet sized mirrors to keep it warm enough to be habitable. And that's not just for the start, but you have to keep the mirrors maintained or keep producing the greenhouse gases right through into the foreseeable future, for as long as you want the planet to remain habitable.

This is why in the 1970s, many space settlement advocates turned their attention away from Mars and looked into space habitats. They calculated that you can do settlement far faster, more efficiently, more easily and with fewer resources if you built space habitats. And if you are doing that, you don't need to build them on a planetary surface.

We may be planetary chauvinists because of our familiarity with living on a planet. Isaac Asimov explains here that he got the term "Planetary Chauvinism" from Carl Sagan. He can't say for sure that Carl Sagan invented it, but that was the first he heard of it. He talks about this 35 minutes into this video:

Isaac Asimov Interview with Bill Boggs

Settlements in space provide much more living area than planets can, for much less investment of effort and much less use of resources. As he says in that interview, our future, for most of humanity, is likely to lie in space habitats rather than on the surface of planets. From his interview in that video:

"I'm convinced that we will build space settlements in space, we will live inside small worlds, and we will eventually recognize that as the natural way to live. It is economical. You have just a relatively small amount of mass, and it is all used. In the case of the Earth, you've got an enormous mass, and almost all of it is not used. It's down deep where we can't get at it, and the only purpose is to supply enough mass to produce enough gravitational intensity to hold stuff onto the outside. And that's a waste! With the same mass you can build a trillion space stations carrying incredible numbers of people inside. And this is what we will eventually come to. I'm sure we will use the asteroid belt to build any number, thousands upon thousands, hundreds of thousands of space stations, which will eventually flee the solar system altogether."
Isaac Asimov

Anyway that's the idea. That planets are not where it is at for the future of space settlement. One calculation they did back then is that there are enough resources in the asteroid belt to build habitats for a thousand times the surface area of Mars (its surface area is also about the same as the land area of Earth).

For any here who are unfamiliar with it, the idea of mining the asteroid belt is to turn the materials there into habitats that spin to create artificial gravity. This is an idea developed by many engineers and scientists in the 1970s including O'Neil and some scientists at Stanford university who drew up detailed engineering plans for how to do it. This has been rather forgotten recently with all the fanfare about Mars.

It's not a case of living on Ceres. Nor is it a case of hollowing out an asteroid and spinning it and living inside it. It's a case of making a habitat out of asteroid materials and then spinning the habitat. And you can generate any level of gravity. Typically the design is to have a habitat under full Earth gravity because the designers assume that's best for human health. If a lower level of gravity is better they can just spin it more slowly. You could have a thousand times the land area of Mars as space habitats with Mars gravity if that was preferred. And you can have any level of sunlight you like and any length of day or night, using thin film mirrors to reflect the sunlight into the habitat, and then shades to simulate night and day.

I discuss it in my Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths

If you do it that way you end up with a thousand terraformed planets in terms of living area, for much less by way of megatechnology than would be needed to attempt to 'terraform' a single planet Mars. You can do it faster too, far faster, with the first habitats completed in decades.

And what's more, they can be customized to whatever gravity level you like. The atmosphere, temperature, ecology, all easily regulated and within your control.

If this is not feasible, there is no way that terraforming Mars is feasible.

I think myself that it is only after we have thoroughly mastered the art of making space settlements like that of a few cubic kilometers that we should even contemplate terraforming.

These are my articles about it on my Science 2.0 blog:

• To Terraform Mars with Present Technology - Far into Realms of Magical Thinking - Opinion Piece
• Trouble With Terraforming Mars
• Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?

MARSFORMING MARS - AND MARTIAN GAIA OR ANTI-GAIA

Perhaps the problem with terraforming Mars is that we are fighting against Gaia? Perhaps that is why we would need all that technology?

Gaia is the idea of James Lovelock that a planetary biosphere can be self maintaining and as conditions change it responds to stay in balance much as a microbe does - but on a vaster scale.

This leads in to an idea for Mars by Chris McKay - if we find interesting Martian life on Mars, perhaps independently evolved, that we try to “turn back the clock” to create conditions hospitable to Mars life.

Well if we do this, perhaps the answer is to set up an ecosystem with hydrogen sulfide as a major component of the atmosphere as this is a strong greenhouse gas. Or methane, or both.

Perhaps early Mars had powerful greenhouse gases to keep it warm. Or perhaps it was habitable only briefly when its orbit is much more eccentric than it is now and with its axis optimally tilted to reduce the polar caps to a minimum. Whether or not that is how it did it originally - maybe we could set up an ecosystem like that today?

So what if we make this the objective anyway, whether or not there is life there already? If we work with Mars rather than against it, find where the Mars climate “wants to go” rather than try to make it into a copy of Earth then it may be easier to achieve. We need to find what kind of ‘Gaia’ comes naturally to Mars. Then - rather than a pale shadow of Earth, Mars becomes whatever it is best suited to become. We help it to realize its own potential - which then may well also benefit us too.

An Earth like ecosystem, without megatechnology, on Mars might even be a kind of “anti Gaia” (my own suggestion here, do let me know if you have seen someone else suggest it).

Photosynthetic life would cool down the planet just as it does on Earth, by pulling carbon dioxide out of the atmosphere. But that is the opposite of what you want on Mars to maintain a Gaia like balance. It’s going to keep pushing the planet to colder conditions again as soon as it gets warm enough for life.

That’s why we need all the artificial greenhouse gases and planetary sized mirrors in all the terraforming plans and why none of them can do it all using biology alone. It is too cold, not too hot, so a thermostat that ‘Gaia’ can use to cool it down when life flourishes, based on photosynthesis can’t work there.

But perhaps some thermostat based on hydrogen sulfide producing bacteria can? Maybe biologists can think of a way this could work? That the cooler it gets the more hydrogen sulfide it produces? Anyone got any thoughts about how that could work? Or any other way to help it become self regulating?

If we can't make it self regulating, it might need constant attention and megatechnology to keep it in balance, and as soon as we stop, then it reverts to its current state. So, to get a sustainable world warmed by greenhouse gases, we would need to establish something more, some form of 'Gaia' in the "weak Gaia" sense. We’d also need the magnetic shield on a very long timescale, otherwise any attempt at "Marsforming" it is going to make it even drier once the atmosphere is stripped by solar storms over long time periods.

To be clear, I'm not suggesting we do this right now. But we can certainly think about it, just as we do for the terraforming. We may learn a fair bit just by thinking it over.

There'd be no hurry to do that on Mars - we can try out such things in the small habitats, and there it's a real experiment where you can change things and try again.

When it comes to Mars, well, it's the only other terrestrial planets with habitats on it in our solar system. Even if it is such a common type of planet that nearly every star has a Mars analogue - still, it's the only example we have at hand to study, here on Earth. So for us it is unique. We probably want to study it "as is" before changing it to something else we think is "better" - who knows what discoveries we can make. Even if there is no native Mars life, still, it's our only chance to see what happens to a terrestrial planet, left to billions of years, if it doesn't develop life.

PARATERRAFORMING THE MOON AS THE NATURAL STARTING POINT

We can do “paraterraforming” which is covering a body with habitats. The idea is to eventually cover the entire planet with a transparent "roof" to hold in the atmosphere. To start with you can just build city domes, and greenhouses, covering larger and larger areas with greenhouses and habitats.

The main disadvantage compared with free space settlements is that you have to use the local gravity and also work with the local levels of light.

• Mars has half the levels of sunlight you get on Earth, so photosynthesis has to work twice as hard or be supplemented by artificial light,
• It is occasionally blocked out for weeks on end during the dust storms, which go global every decade or so, sometimes more often.

Compare this with the Moon:

• The Moon has plenty of sunlight and no dust storms.
• But a cycle of a night of 14 Earth days and a similarly long day
• Though it has 24/7 sunlight close to year round in some favoured areas at the two poles.

I suggest that the natural starting point is a habitat at the lunar poles, perhaps eventually even a city dome there. This is where ESA plan to build their "Lunar village" in collaboration with other space agencies, worldwide, and also many private companies. It's much more like a village than like the ISS with separate habitats sharing some common facilities. I think myself that it is perhaps the most exciting idea we've had for ages in the field of humans in space.

After that, the best starting point is to paraterraform a lunar cave. These are known to exist and in the low lunar gravity they may be vast, kilometers in diameter and over 100 km long.

The 14 Earth days long night is not such a drawback for the lunar caves as you might think. Modern LED lighting and especially the purple lights optimized to produce only the light needed for photosynthesis reduce the lighting you need a lot, to only a hundred watts per square meter.

I am going there by this High Efficiency Full Spectrum LED Grow Light -  uses 20 watts of power to illuminate 0.2 square meters. So that's 100 watts of supplied power needed per square meter. It is recommended for crops that require bright sunlight such as lettuces in this roundup in 2015: Top 10 Best LED Grow Lights

Remember that on Mars you have to supplement the low levels of light anyway if you want to grow crops that need a lot of sunlight (tropical plants) and you also have to be able to supply sunlight through weeks long global dust storms, unless you just stop all agriculture whenever that happens. It’s not much of an engineering difference if you do it for an unpredictable length of time of at least a few weeks every few years or you do it every 14 days. You still need some way to supply the power for 2 weeks, and in the case of Mars probably longer.

You can grow just about all the food for one person and also provide all their oxygen from 30 square meters of crops, actually tested in the BIOS-3 experiments in Russia. Also, there are many crops that can withstand 14 days of darkness so long as you cool the roots during the lunar night, and continue to crop as normal with a double length growing season.

So, the lunar caves are far more manageable than they seem at first. Also, the Moon has a major advantage that it is within easy access of Earth), has tourist potential, and it may have commercial potential (enthusiasts struggle to find commercial potential for Mars), and in one comparison after another it comes up better than Mars.

I see lunar caves agriculture as involving LED lights mixed with some areas of crops that just go dark at night, and with light piped from the surface in the lunar day.

There are many ways to store enough energy for the night. But longer term, then they are bound to have solar panels that are constantly in sunlight, as solar panels are easy to construct on the Moon. At any time one of those arrays will be in sunlight and the electricity transferred to settlements throughout the Moon using microwaves or else long distance power transmission using high voltage direct current which should be easy to do with cables laid over the lunar surface. They might well run alongside the lunar railways that take you around the Moon.

This is a fair bit of engineering - large solar arrays, long distance HVDC cables, eventually city domes, lava tube cave settlements, and lunar railways, but it is nothing compared to what is needed to terraform a planet.

Of course you can paraterraform Mars as well, live in Martian lava tube caves much like the lunar caves - or build city domes. We know that Mars does have those caves, though we don't know how large they are, can only see the entrances from orbit (similar to the Moon's situation).

However Mars is by far the most vulnerable from Earth microbes of any place in the solar system, if you are interested in searching for extraterrestrial life.

WHAT IS THE BUSINESS MODEL FOR MARS? OR THE MOON?

Mars has also got many other disadvantages. For instance, when asked how his Mars colony is going to support itself commercially, Elon Musk says that prospective colonists with half million dollar homes will sell their homes to go to Mars and he says this is how the colony will sustain itself to start with through the sale of the homes of its prospective colonists. Here is his interview where he says so (on the SpaceX channel):

What is the Business Model for Mars?

But even a spacesuits currently cost two million dollars each and are good for a couple of dozen or so space walks from the ISS (much less wear and tear than you get for a Mars suit) and need constant servicing.

It's no surprise they cost so much as they have many tasks to perform. They have to give thermal protection, protect against micrometeorites (usually with multiple layers), cool the astronaut as you get too hot easily in a spacesuit, usually done at present with a a Liquid Cooling and Ventilation Garment, hold in the pressure, supply the oxygen, scrub carbon dioxide, it needs a power supply, a fan to circulate the air, they need joints that can remain flexible in a vacuum - or if it is one of those form fitting ones proposed by Dava Newman it will need to be designed to fit the astronaut snugly, basically tailor made. It needs strong transparent material for the face plate that can hold in the high outwards pressure of the oxygen and is also able to withstand micrometeorite impacts, somehow it needs to protect the hands from damage when working inside very stiff gloves (unless they are also tailored biosuit style). See What is a Spacesuit? (NASA).

Surely they will be reduced in cost with mass production - but by how much? Let's guess a ten-fold reduction in cost to $200,000 but that may be optimistic for the near future.

Even if they are reduced in cost ten fold. they will need to be repaired - and will need to be replaced from time to time. Maybe they can be made more durable, but surely not likely to become so durable they last for years any time soon. Then what about the cost of your habitat on Mars? It will probably cost hundreds of millions of dollars. And it will have a finite life - if it is like the ISS it will last a few decades before most of it has to be replaced.

You have got to Mars, but you are planning to live there for the rest of your life and raise a family there too, presumably, or it is not really a colony.

Your half million dollars won’t last long. Some is gone on the ticket out there - Elon Musk says the ticket price is $200,000. So after buying your spacesuit as well, now you have a $100,000 deposit on your (much smaller) habitat on Mars and that’s your half million dollars already gone before you have done anything on Mars.

What next?

He says in that interview that a colony can't support itself by exports from Mars.

The only other source of income he suggests for the colony is intellectual property rights - the income from their inventions and other IP that they export from Mars. Both he and Robert Zubrin think that by living in such harsh conditions the Martian colonists will become extraordinarily inventive and make many extraordinary inventions that will then earn them big bucks back on Earth.

I don’t “get that” myself. Here in the UK we pride ourselves as a nation of inventors and unlike the US we don’t credit it to a “frontier spirit” but just think we are naturally inventive. The French also think of themselves as great inventors and so do many other countries.

But - let’s give him this for now and see where it takes him. So how is that supposed to work?

Presumably the idea is that some of the billionaires there employ others, from their earnings from sales of their inventions on Earth, and so keep the colony going. But what's to stop a new billionaire who has invented something on Mars from emigrating back to Earth which is where his or her new invention will be marketed anyway? It will seem a paradise to them too after Mars And then why would they pay billions of dollars of their income to Mars?

Sorry, I just don't "get" their business model.

Anyway, whether it would work or not, In one way after another, the Moon actually wins over Mars - I was surprised. There's a lot of work to be done, but at least on paper it seems promising.

There are several enthusiastic space engineers, geologists and scientists who have written books with detailed working out of what seem to be practical economics for the Moon on paper, involving exports of platinum, or exports of ice from the lunar poles to LEO (if those are easily mined) or by setting up tourist hotels on the Moon and many other ideas.

See for instance:

• The Value of the Moon by Paul Spudis
• Moonrush by Dennis Wingo
• The Moon: Resources, Future Development and Settlement by Madhu Thangavelu, David Schrunk, and many other contributing scientists

Much of the material in these books is devoted to a detailed business case for the Moon.

You just don't get this for Mars. The nearest I've seen is David Kuck's idea of the Deimos water company - and that could be feasible if Deimos does indeed have ice in it (as it might) - though if there is ice at the lunar poles, it's hard to compete (except for export to Mars itself, which can't work unless Mars has a business case).

For exports from the Mars surface, just not much. Robert Zubrin talks mainly about the same idea of export of intellectual property. His deuterium export idea from his section on Interplanetary Commerce in “Case for Mars” page 239 chapter simply doesn't work, as it's only saving one step in a process that on Earth is done in bulk in 27,000 tons deuterium factory, the size of a skyscraper, and powered by a large hydro-electric scheme with an output of 128 MW. Hard to compete with that on Mars. He has other speculative ideas there, but none of the details of the lunar ideas.

FAR EASIER TO EXPORT FROM THE MOON

It's far easier to export from the Moon because of the low delta v to get back to Earth. Then you also have the fixed distance, the short travel time there and back (especially for tourism), and that you can go there any day of the week, any month of any year. There's even the Hoyt cislunar tether system, which acts a bit like a siphon feeding a waterwheel - it takes materials from the surface of the Moon, and through two rotating orbital tethers, one in low lunar orbit and one in LEO, it transfers it to LEO, lower in the gravitational gradient of the Earth, and if you time things carefully with a net flow in the Moon - Earth direction it actually generates power instead of using power and fuel.

It is hard to see how Mars could compete. And if there is this premium on inventions from living in a hard place in space - well what about inventions from lunar colonists? You have all the other exports from the Moon and then if Zubrin and Musk are right, you have the inventions too.

I go into this question of whether there is any commercial case for Mars and compare it with the Moon in the sections in Case for Moon First:

• Commercial value for Mars
• Would a space colony survive with only exports of intellectual property to pay for imports?

I based my answer on those sections of the book, which go into a lot more detail with quotes and cites. I searched the online forums, and included all the methods of possibly paying for a Mars colony by Mars enthusiasts I could find along with the few suggestions in the short chapter on the topic in Robert Zubrin’s Case for Mars.

It originated as a Quora answer

• Could colonizing Mars be a lucrative or potentially profitable venture?

And was run by Forbes magazine as

• Is There A Fortune To Be Made On Mars?

PLANETARY PROTECTION AND CASE FOR MOON

I think also there are serious planetary protection issues involved in sending humans to the Mars surface. Not us, but the microbes that come with us, may make native Mars life extinct, maybe before we even know it is there. That would be tragic. There is so much we could learn from it. And we simply haven’t looked yet. See my

• Is This Why We Haven't Found Life On Mars Yet? Value Of Actually Looking

For these and many other reasons, I suggest that the Moon is the best place to start with human settlement. I cover it in Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart

IDEA THAT MARS WILL HAVE ITS OWN INTERNAL ECONOMY LIKE EARTH - WHICH DOESN’T NEED EXPORTS

This is a nice idea in principle, to set up its economy - like another planet. Earth doesn’t need exports so why would Mars?

However the difference is that conditions are so hostile on Mars. Remember if you could do anything there, you could do it far easier in a desert on Earth. We have not yet got to the point where someone could do that.

Even if you had e.g. a spacesuit making factory in the middle of the Gobi desert then it would need imports, and the people in the factory would need to be fed, and their clothes imported and generally it would have loads of imports which it would pay for by selling its spacesuits to the rest of the world.

But you can't do that on Mars. Not until you have all that stuff already there.

I don't know if eventually you could, if you had big city domes and inside can do Earth industry with an ultra low maintenance outer skin to the dome - or if you paraterraformed it and covered it with a low maintenance reliable meteorite proof transparent roof over the Valles Marineres or something. Maybe then you have areas where humans can work on Mars as easily as on Earth and produce things at low cost locally. It might then have an economy that can work on its own without imports or exports to other planets.

But if so, there is a long way from here to there. Right now, there's no way that it would work unless you have exports to Earth from Mars to pay for your spacesuits and other high tech imports - or you have people on Earth with big pockets willing to pay out trillions to get a Mars colony underway.

TERRAFORMING A SMALL AREA OF A PLANET WITH A "GREAT WALL OF MARS"

This is an idea described by Isaac Arthur towards the end of his video. I haven't come across it before, so I thought I'd share it here

So, the idea is to have a "Great wall of Mars" enclosing a large area, say the summit of Olympus Mons, which you then terraform normally. So you have the complete depth of atmosphere but it is only above the point that your colonists live. This could fit in with ideas to have an orbital mirror that instead of warming the entire planet just warms one spot, in geostationary (or in this case aerostationary) orbit above it.

My main concern for a project like this would be the same as for any Mars colonization - the issue of bringing Earth microbes to another planet that may have its own indigenous interesting native life, as well as the impact of introduced Earth microbes on the planet anyway - such as Cassie Conley's accidental conversion of subsurface aquifers to cement.

But it's an interesting idea, if we can work through such issues somehow.

He leaves out one final option, although it would not fit with the story line of the other approaches and couldn't be done simultaneously with them.

If we find native Mars life that is of outstanding interest or we decide that Mars is of great interest 'as is' as the only terrestrial planet untransformed by life available to study - or just want to delay modifying it until we know what we are doing, then we might want to stay in orbit. We could then operate rovers on the surface and humanoid robots, but live in orbital colonies. You work on Mars by getting into a VR suite in orbit, and can walk around Mars experiencing it far more directly than in a spacesuit. The robots on the surface would be a bit like the inhabitants of civilizations in the game of "Civilization" where you leave them doing things automatically, and then step in from time to time when they need direction.

You could still grow plants on the surface, if they use sterilized seeds and sterile aeroponics. Then you can export to orbit using sterilized spacecraft. It is possible to have 100% sterile robots, in principle. Just heat them up to 500 C and so long as you have electronics capable of withstanding such temperatures, leave them at that temperature for a while and they will be sterile. Or assemble them in sterile conditions in a vacuum on Phobos or Deimos, or on the Moon.

WHAT ABOUT THE GRAVITY LEVELS?

As for gravity levels - first nobody knows what is needed for health. We may even be healthier in lunar gravity. And - though there is no way to know for sure - at least it seems likely that after months or a year at lunar gravity it would be much easier to adapt to the Earth on returning home than if you spend the same time in zero g. Not likely that you are unable to stand at all and have to be taken off the spaceship in a stretcher until you adapt back to Earth gravity as happens for many astronauts who spend a long time in zero g.

But if we do need more gravity then we may well need it for only a short time per day, e.g. while exercising, or eating or sleeping which we could do spinning for artificial gravity.

Spinning is much better tolerated in zero g than on Earth with astronauts able to tumble end over end without any feeling of nausea or dizziness and this might be the case for the Moon too.

HIGH TOLERANCE OF ASTRONAUTS IN ZERO G TO SPINNING RAPIDLY - WITHOUT FEELING SICK

Rats don't get nauseous when they spin so it is to do with human physiology. On page 95 of Packing for Mars, by Mary Roach she mentions that NASA Ames researcher Bill Toscano has a defective vestibular system. He only realised this when they put him on the spinning chair and he experienced no nauseous effects at all from the spinning.

So, some people won't get nauseous or dizzy anyway. But most will, and the habitat has to be built for everyone of course. However there's some intriguing evidence that perhaps everyone might be nausea free like Bill Toscano in zero g. Or nearly so anyway, at least tolerant of much higher spin rates than on Earth.

Tim Peake tried tumbling end over end in the ISS and he could tolerate fast spin rates he couldn't tolerate before or after on Earth.

Of course astronauts are trained to be able to tolerate tumbling motions better than most. But he says that he couldn't have handled this on the ground. Spinning so fast is a challenge even for an astronaut - they are not trained to withstand such rapid spins to the same degree as, say, an ice dancer.

He is not first to notice this, indeed it's well known by astronauts - and it actually goes back to Skylab experiments (Chapter 11, Experiment M131. Human Vestibular Function from: Biomedical results from Skylab.).

The crew as they ran inside Skylab found they could tolerate spins very well - and there were a series of experiments in a spinning chair - the "litter chair" designed to help them get a better understanding of space sickness. In the process they also found out about astronaut tolerances to spins in zero g. All the astronauts tested were better able to tolerate rapid spins in zero g than they were either before or afterwards on the ground. They tested them all up to 30 rpm in space, the highest spin rate available, and they were all symptom free, while they couldn't tolerate such high spin rates before or after.

Interestingly the lack of susceptibility to nausea actually persisted for a day or two after the flight. There was only one exception in all their experiments. The commander of skylab 3 did experience a "vague malaise" in one experiment at 30 rpm, on mission day 52 which persisted for around 30 minutes, but it was not typical of acute motion sickness and so might have had other causes.

His explanation is probably wrong however. It's not the vestibular system that shuts down. It's the otolith that senses linear motion, according to the Skylab experimenters. The vestibular system is still active because he can sense his orientation in space very well.

During the Skylab experiments with the rotating litter chair experiments,the experimenters found evidence that the otoliths are not stimulated due to the lack of any linear acceleration along the axis of the spin. They hypothesized that the reason the astronauts don't feel nauseous when they spin in space is because there is no conflict between the spin which is a circular motion and the gravity on the axis which is a linear acceleration.

Whether that's the reason or not, it does seem that astronauts in zero g can tolerate spins that they couldn't before or after.

WHAT ABOUT PEOPLE WHO ARE VERY SUSCEPTIBLE TO LOW SPIN RATES OF ONLY A FEW RPM?

Some people are very susceptible to spinning . And some are the opposite - I'm hardly susceptible to it at all myself. I tried just spinning on the spot at 30 rpm for an hour like a whirling dervish to test my susceptibility and felt only mild dizziness on stopping and no nausea at all :). That may be why I find it easier to accept that spinning for AG would work in zero g.

But in the Skylab zero g experiments mentioned in the last section, note that NONE OF THE ASTRONAUTS experienced any nausea or symptoms at all at 30 rpm although most of them did before or after. (Apart from one instance which seems a coincidence as it didn't match typical spin nausea symptoms).

Probably someone susceptible to 4 rpm wouldn't pass astronaut training so I know it's just a selection. But if it is true that the otoliths that sense linear acceleration are de-activated in zero g and if this applies to everyone, and if it is true that nausea from spinning is triggered by this conflict - I know it's a lot of if's but they are each individually quite plausible - it's possible that NOBODY SUFFERS FROM SPIN INDUCED NAUSEA IN SETTLEMENTS SPINNED FOR AG AT LEAST UP TO 30 RPM.

Of course this will also depend on the otoliths remaining shut down after long term residence in a spinning habitat.

30 rpm is enough for full g with a radius of only one meter so that would mean that a spinning habitat can be as small as you like effectively. It's just impossible to know the answer to that without more experiments.

The MIT researchers recommended it in this paper.

"In order to truly address the operational aspects of short-radius AG, a centrifuge must be made available on orbit. It's time to start truly answering the questions of "how long", "how strong", "how often", and "under what limitations" artificial gravity can be provided by a short radius device."

EASY TO DO ARTIFICIAL GRAVITY TESTS IN LEO - IDEA FIRST PROPOSED IN THE 1960S FOR A VOSKHOD IN RUSSIA

Artificial gravity was a priority for the ISS up until the loss of Columbia in 2003, first in NASA Ames, then later on the project was passed on to the Japanese space agency, then called NASDA, now called JAXA, who built a Centrifuge Accommodations Module which however never flew because the Space Shuttle was needed to get it into orbit. See page 55 of this paper. Then in 2010 there were proposals to send a centrifuge to the ISS, but it never happened.

Joe Carroll's idea is to develop an artificial gravity research gravity in LEO to let us test multiple levels of gravity at multiple spin rates can start off as simple as just one space module with a counterweight. And indeed the first experiments are even simpler than that. He has been advocating it for years. He's an expert on space tethers and several of his tethers have flown in space.

The idea actually dates back to the 1960s. We now know that Sergey Korolev had a plan to tether a Voskhod with its spent final stage which he put forward in 1965-6. It was going to be a 20 day flight to upstage the Americans. It would have included a pilot, and a physician and the artificial gravity experiments would have lasted for 3-4 days during the flight. He died unexpectedly in January 1966 and the mission was postponed to February 1966 then cancelled outright. So we came very close to doing this experiment way back in 1966. (See page 17 of this thesis).

Joe Caroll's idea similarly is to start with a tether spin experiment with a module tethered to its final stage, as this goes into orbit anyway. He keeps it attached to the spaceship by a tether. Then he spins it up with a series of thrusts during the perigee, when closest to Earth to spin it up (spinning in the plane of the orbit). Those thrusts boosts its apogee while at the same time setting the combination spinning for AG. After staying in that elliptical orbit for a while to test AG (which has much less drag than a final stage normally has), then he cuts the tether at apogee, so circularizing the orbit of the spaceship at a higher altitude and meanwhile sending the final stage down to a targeted re-entry in the southern Pacific (at present its re-entry is uncontrolled). It's a really neat idea!

The nice thing is that all the delta v put into spinning up the assembly gets released at the end of the experiment. It uses no extra fuel unless the tether is severed by space debris, which from his experience in improving tether design is now a very low probability event. The Soyuz always carries extra fuel in event that more is needed than expected during the launch. So he would use this, and only use it if it hasn't been used up during the launch (usual situation|). As a result, the Soyuz would still get to the ISS even in that worst case scenario where the tether is cut at the worst possible time by space debris. Similarly, they could also cut the tether at any time in an emergency and simply continue with the normal approach to the ISS. It would also be done in such a way that even if the tether is cut by orbital debris, there is no chance of it going near the ISS without an extra burn. So there are no safety issues.

It is worked out in detail and could be done right away, as quickly as the Gemini tether mission was put together, for a near future crew mission to the ISS. They'd use the longer phasing approach of several days, so you could test several days of artificial gravity. It could be done with the Soyuz TMA or any other crewed mission to the ISS. The cost wouldn't be much as human spaceflight experiments go, just to add a tether to a Soyuz TMA mission that is going to the ISS anyway.

Though this would be a short experiment, there are many things you can test in a short mission. It would of course test things such as tether dynamics and tether spin up. Also radio communications during tether spins, and orientation of the panels to achieve adequate solar power throughout the orbit.

Also, in particular, it would give us the first real data on spin tolerances of humans in artificial gravity long term. It's a different experiment from the short arm centrifuges in the ISS, because these can be much longer tethers, far too long to fit inside the ISS, so with slower spin rates

MIGHT TOURISTS BE THE FIRST TO TEST ARTIFICIAL GRAVITY IN SPACE?

There is so little interest in testing AG amongst the space agencies at present, that I wonder if the first to test it may be tourists, in space hotels for the private sector.

In the near future I think we will definitely get spinning toilets in zero g if it is true that everyone can tolerate 30 rpm. A toilet constructed with a 2 meter radius in say a Bigelow habitat would need only 22 rpm. I see that as likely to be one of the first applications because zero g toilets are difficult and unpleasant to use - and so once we have tourist hotels in space they are likely to go for an AG toilet as one of the first luxuries to develop. And you only need to spend a few minutes in it, and can set the spin rate you like, only a fraction of a g would make a huge difference.

It is the same for sleeping, and for eating, eating is far easier if you have at least some g. Also exercise ,easier to do exercise to keep you fit with some gravity to work against. And - even if you can only tolerate minutes at 22 rpm (say), it may make a big difference to health.

So, perhaps our first AG experiments in space will be done "accidentally" by the space tourism industry?

If you want to have a go at a "design your own" spinning habitat, and try different spin rates and sizes to check the artificial gravity levels, here is a fun web browser app by Tom Lechner together with many presets. You can even try the effect of throwing a ball in AG :).

Press and hold to "throw" a constant stream of dots and you can vary the strength of the throw by repeatedly pressing "throw harder" or "Throw softer". You can design your own or choose from lots of presets such as the Kaplana One, Stanford Torus and other space stations of the engineers or fiction.

• Space Station Numbers (if the numbers don't seem to "take" - enter the number as text and then use the up / down arrows - it seems to respond when you use those but not immediately when you type in the numbers by hand).

. Also there's a good online calculator here, works out the level of AG from the spins.

• Spincalc

COULD A "STANFORD TORUS" TYPE HABITAT BE REALLY SMALL, JUST A FEW METERS ACROSS

If the data from Skylab is right, and stands up for longer duration spins, if astronauts can spin for hours at 30 rpm symptom free, then the Stanford Torus maybe doesn't need to be that large at all. Maybe it can just be 10 meters or even 4 meters, who knows. We simply have never done the experiments needed to find out the answer to this either way.

Note, that even on Earth many people can tolerate very high spin rates. You can learn to do so. Ice dancers are noted for this - also skiers etc. There were lots of people doing multiple tumbles and spins in the winter olympics. That takes them years of training of course, but many ordinary folk also can come to tolerate high spin rates up to 30 rpm with just three training sessions according to an experiment done by some MIT researchers. Others who can tolerate high spin rates are the whirling dervishes, who are used to twirling for long periods of time too.

So, could we all tolerate high spin rates of 30 rpm or even more in zero g, for hours on end, symptom free? We have simply never done the experiments needed to find out. For more on this see my:

• Need for adventurous experiments in life support and artificial gravity in LEO first
• Astronauts don't get nauseous when spun rapidly in zero g - so could a device as simple as a spinning hammock be all that is needed to keep us healthy in space?
• Spaceship tethered to final stage and then spun up - experiment first proposed for a Voskhod in 1965-6 but never flown - could this solve all the zero g health problems?

in my online book Touch Mars?

However we don't have to have massive Stanford Torus type habitats even for slow spin rates. If you use tethers between habitats to spin up, then they can be as far from the center of gravity of the system as you like.

So, When you want a habitat spinning for artificial gravity but just want it small, for instance to travel on an interplanetary journey - just set up a tether between your habitat and another one. Or for instance, when traveling to Mars, every time a spaceship goes to Mars, nearly always its final stage is on the same trajectory. Well - tether it to its final stage and spin them slowly around their common center of gravity. With a tether system like that,you can also change course while still spinning .

It's not like a wheel spinning under gravity - there isn't anything rigid there and so it won't resist a change of direction by wobbling like a bicycle wheel. It's more like running and changing direction while whirling a stone around on a rope in one hand. Indeed you can even change course with only minor interruption in the passenger's sense of artificial gravity if you boost carefully.

LOWEST MAINTENANCE HABITATS AND HABITATS WITH LEAST OUTLAY

Any space habitat requires some level of constant maintenance, if just the airlocks and the spacesuits. The inhabitants probably do have to be able to get out of it occasionally, however maintenance free it is inside - or send autonomous robots or semiautonomous telerobots to do the necessary maintenance tasks.

However, remember, so does a terraformed Mars with constant production of the greenhouse gases or maintenance of the planet sized mirrors to reflect more sunlight onto it. If you do succeed in terraforming it, then on the surface it may seem an easy place to live but you are dependent on a lot of technology working “behind the scenes” to keep it going long term.

There are also many biogeochemical cycles you need to complete, carbon cycle, nitrogen cycle, oxygen cycle, phosphorous cycle etc and those won’t necessarily work automatically as they do on Earth. It might be a constant on-going challenge to keep its ecology on track and stop it “unterraforming” - if you succeed in terraforming in the first place.

It is also a very long term commitment. In the Middle Ages there were some projects to complete cathedrals on a timescale of centuries. But this far exceeds any of those projects. It’s like the inhabitants of the Lascaux caves starting a project that would take so long that we’d still be at the early stages of it 17,000 years later.

Any civilization that can contemplate such immensely long timescales has to be very mature. I think that a civilization that takes on a terraforming project with confidence of success, and of seeing it through to completion would probably be at least thousands of years old, and more likely millions of years old. It’s probably completed many centuries and millennia long projects before it tries this one.

Even houses on Earth of course take time to build, and need maintenance. However, Earth is the only place where humans can survive without any technology at all, like the gorillas do, in at least some places. Then, with minimal pre-industrial technology, we can survive anywhere from the Kalahari desert to the Arctic (San people, to Inuit).

There are some places outside of Earth where we can live with fairly low levels of technology, though nowhere we can live without any at all, not anywhere that we know of.

Saturn's largest moon Titan with its dense methane atmosphere actually has an atmospheric pressure greater than Earth’s. You need thermal insulation, and you need an Earth atmosphere inside your habitat, but you do not need to hold in the internal pressure. Habitats on Titan could be any shape and be lightweight flimsy things like houses on Earth, indeed flimsier and easier to construct in the lower gravity (apart from any artificial gravity requirements if you need spinning for AG for health).

The Venus cloud colonies are similarly lightweight and also arguably low tech. These float just above the clouds at the level where the temperature and pressure is similar to that on Earth. It needs the technology of an airship + sulfuric acid resistance. But the acid protection is only for the outer skin of a large habitat. Arguably acid resistance is easier to engineer for than holding in Earth’s atmospheric pressure against a vacuum - and is far less mass at least, just a thin layer of teflon or similar.

When it comes to paraterraforming or the large spinning habitats in space, then as for the Venus cloud colonies what matters is how easy it is to maintain the outer skin.

If you think about it that way, then perhaps the lava tube caves are strong contenders too. Most of the mass is already there - in the form of the lava tube itself. Perhaps you just need to fill in cracks and make it impervious to air. If so, the launch mass from Earth could be very low, lower even than a cloud city or Titan dwelling.

If you can make any of the big structures nearly maintenance free and within it you have an Earth normal atmosphere, it might well end up being lower maintenance than e.g. living on Titan without a city dome . Once it is built, that is. If it is exceptionally low maintenance it could be easier than living on a house on Earth.

However nowhere in space can be lower maintenance than living in a tropical jungle on Earth, unless you find a way to make the maintenance totally automated with robotic machinery (as is the case in many science fiction stories).

Note - large spinning habitats do not need any form of propulsion to keep spinning. Maintenance, and the level of technology needed to live in such a habitat would be similar to a city dome, or a lava tube cave.

That’s just the external structure. The internal ecology is likely to require constant monitoring and “gardening”. But that again is the same for a planet.

Only a very mature civilization would have confidence that the ecology of their terraformed planet could continue long term without constant vigilance and monitoring, and then correction of issues as they arise, I think. And it would probably gain that confidence at least in part through working with larger and large enclosed habitats, starting with much smaller scale closed cycle ecosystems of up to a few cubic kilometers, and gradually gaining confidence through those experiences and also through study of exoplanets.

That is, unless, of course, we make contact somehow with a mature ETI that has solved these problems already, long ago. Even then, their solutions may need adaptation to terrestrial biology.

WHAT ABOUT ELON MUSK’S “NUKES TO TERRAFORM MARS”

This was just an off the cuff joking remark he made. He talks about it here at 2 minutes in to this video, where he called it a “fixer upper of a planet”. He says is 

“There’s a fast way and a slow way… The fast way is to drop thermonuclear weapons on the poles":

He gave no details yet it was taken up in many news stories. presented half seriously as a way to terraform Mars. This was not based on any research, just an off the cuff joking remark by a CEO of a space company. It was soon followed by other news stories by the more techy and geeky journalists saying it was impossible to do it that way.

Could his remark he based it on that idea that if you liberate enough dry ice, you could kick start the runaway greenhouse? But there isn’t enough dry ice at the Martian poles anyway to reach the magic 6 mbar to start a runaway greenhouse, at most you could double the current 0.6 mbar. Then, the number of nuclear bombs you’d need if there would involve a vast megaproject, hardly an “easy” solution. You are talking here about hundreds of thousands of hydrogen bombs as powerful as the 50 megaton Tsar Bomba - the largest nuclear bomb ever tested.

This was my article about his idea, in response to those many journalist stories:

• Why Nukes Can't Terraform Mars - Pack Less Punch Than A Comet Collision

When asked for clarification he later explained he meant constantly exploding nuclear fusion bombs to form two “mini suns” above both the lunar poles. Rather a science fiction scenario. See Elon Musk Clarifies His Plan to "Nuke Mars".

Probably many of you saw this as the screen saver as you wait for a SpaceX video to start - it doesn't give any timescale however. 

As far as I know they are focused on space engineering and are not actively researching into terraforming. They leave that to the likes of the Mars society and keen scientists who are researching into it anyway.

WHAT ABOUT KIM STANLEY ROBINSON’S ‘THE MARS TRILOGY’

This is a series of three books, Red, Green and Blue Mars that came out in the mid 1990s together with a book of short stories with the history as backdrop called “The Martians”. In this he envisions Mars terraformed and developing a planet spanning civilization in a couple of centuries. The main focus is on social issues but he has a backdrop of terraforming with plausible sounding science, and this has influenced many people to think that terraforming Mars would be easy and accomplished as soon as a couple of centuries. See Mars trilogy - Wikipedia

The Martians cover

Kim Stanley Robinson himself says that it would take far longer than his trilogy suggests, which is based on 1980s ideas. In a podcast he gives as his main points

• Mars seems to have lost its nitrogen. We need nitrogen
• There could be life in the basement regolith a hundred meters or a kilometer underground and that’s going to be very hard to disprove. So we may be intruding on alien life.
• Its surface is covered by perchlorates, poisonous to humans in the parts per billion. They could be changed into something more benign to humans, by introducing something to eat them, but that would take time.
• The best analogy is Antarctica - beautiful, scientifically interesting, and for Mars, especially of interest for comparative planetology - going to Mars is a way to study Earth
• Mars trilogy is a kind of allegory of people on Earth. We have over 7 billion people and may end up with 9 or 10 billion. There is no way we can use Mars as an escape valve in less than thousands of years.
• He was following Carl Sagan - and Martin Fox who suggested thousands of thermonuclear bombs so deep they heat up the planet. Then you introduce genomes from Earth.
• He thinks terraforming is for later - once we have a sustainable civilization on Earth and proved we will not wreck this one, we can then consider the next great project.
• He does not think that Mars is in the same relation to Earth as the New World is to the Old World. The New World wasn't really a pioneering colonization anyway as the first people were there already. Also, Mars is not habitable without terraforming, with thousands of years needed to terraform. All of this make the analogy not applicable in his view. It’s not going to be a solution in decades. He says he has a profound disagreement with Robert Zubrin on the New World analogy, and says that this is not what Mars is about.

He says that we can’t use Mars as a ‘backup planet’. We have to fix our problems on Earth to have any hope of surviving on the timescales of the book. See the podcast here and summary on Io9 here here

There are many other issues with his book though, if you consider it as science rather than science fiction:

• He assumes that the humans can be genetically engineered to tolerate carbon dioxide instead of nitrogen in the atmosphere - supposedly by using crocodilian hemoglobin then humans become able to tolerate high levels of CO2 as well as becoming nearly immortal
• He greatly accelerates the timescales, e. g. the effects of photosynthesis, a hundred fold or a thousand fold.
• He doesn’t explain how Mars is kept warm with a CO2 / O2 atmosphere
• His Mohole wouldn’t work - he fudges the numbers and there is no serious scientific paper suggesting use of moholes to terraform Mars. He proposes to warm the atmosphere of an entire planet using a number of large radiators at a temperature of 50 C or so.
  
  They are typically one kilometer in diameter, let's call the area one square kilometer. There are ten of them, so call that ten square kilometers. With this he proposes to warm up a planet with a surface area of 144.8 million km². The atmosphere is in thermal equilibrium with the surface - so he has to warm up at least the top few millimeters of the regolith, not just the atmosphere. I'm not sure how to do the detailed calculations to see what temperature difference there would be, but it's going to be minute.
  
• His windmills idea is plain silly for a physicist - I know it's meant as a deception to illegally spread algae - but how could it deceive the other scientists in his plot line?
  
  The wind is going to be slowed down anyway. Slowing it down prematurely using windmills is just a way of concentrating the energy dissipated by slowing it down into a single place on the surface. So there would be no net heat input into the atmosphere as a result of the windmills. It would only make a difference if the atmosphere was moving as an ideal fluid without friction.
  
  Fun scientific quibble for geeks,: this is a simplification. Actually, as Lee Weinstein (mechanical engineer and energy researcher at MIT) wrote in his blog post "Windmills on Mars", there would be a very minute, but temporary, warming effect. By slowing down the winds with the windmills, this reduces the kinetic energy of the Mars atmosphere due to wind, and by conservation of energy, this has to mean a slight increase in the temperature of Mars. However this is just a temporary effect while they reduce the speed of the wind. Once the average speed of the wind has reached a new, lower, equilibrium, then Mars returns to thermal equilibrium with the rest of the universe. So there would be a really tiny increase in temperature for a short while after the windmills are deployed, after which the temperature returns to normal. When you stop the windmills, the opposite happens, it cools down then returns to normal.
  
  You could use the same argument for the moholes. The temperature of Mars can only increase temporarily as a result, because no heat is being created. The heat from the interior is just being lost more quickly at that point. The rest of the crust of Mars must be getting slightly less heat radiated through it, so eventually this is going to cause Mars to cool down slightly elsewhere, by tiny immeasurable amounts but probably only on long timescales.

The physicist Raymond Pierrehumbert, specialist in climates of planets in our solar system and exoplanet atmospheres puts it like this in his article "Science Fiction Atmospheres":

"Robinson has certainly set up the puzzle correctly, but the physics behind many of the solutions his characters propose is silly. Silliest of all are the windmills, which are supposed to heat the planet by using wind-generated electricity to drive heating coils. (I won’t insult the reader’s intelligence by spelling out why this wouldn’t work.) One could argue that the windmills were really just a ruse for illegally dispersing Mars-adapted algae, but it’s more than a little implausible that all the high-powered physicists among the Mars colonists would be taken in. There is other silliness. Polar caps are dissipated by albedo-reducing algae, and water vapor is added to the atmosphere by cometary impacts and ”moholes” without regard to the constraints imposed by Clausius-Clapeyron.

"On the other hand, there are some interesting and workable ideas in Robinson’s book. There is a space mirror to catch sunlight and turn Martian night into day, but to bring Martian insolation up to Earth levels would require a mirror with a cross sectional area equal to Mars itself; still, a more modest mirror with 10% of the Martian cross-section could make a useful contribution. The question of the microclimate of low-lying areas like the Hellas Basin, where surface pressure will be greatest, bears thinking about, as does the circulation one would get around the rim of the basin. It would be rather like Death Valley (how jolly!) or a drained Mediterranean, only more so. If it were up to me, I’d make some use of algae bio-engineered to release HFC’s, and perhaps also synthetic cloud particles optimized to reflect infrared while letting through a lot of sunlight"

(there Clausius-Clapeyron is an equation used to estimate vapour pressures of liquids such as water and how it depends on temperature).

All of this is acceptable in a science fiction book written for entertainment, especially given that he says his main aim was to reflect on conditions on Earth. But it is not a scientific blueprint for terraforming Mars, and was surely never intended as such.

On the issue of intruding on alien life on Mars, then there are many suggestions now for habitats not just at the base of the regolith, but on the surface or in the top cm or so of the soil as well. There is an almost bewildering variety of possible habitats for surface, near surface and subsurface life. None confirmed but many to be investigated. Here are some of them - the links take you to the section of my online Touch Mars? book.

• Nilton Renno's droplets that form where salt touches ice - why did he call a droplet of salty water on Mars "a swimming pool for a bacteria"?
• Recurring Slope Lineae
• Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
• Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over - that one was reported as uninhabitable but Nilton Renno was not so sure, biofilms might make it habitable
• Transgressing sand dunes bioreactor
• Desert varnish
• Sun warmed dust grains embedded in ice
• Southern hemisphere flow-like features - these may involve fresh water!
• Methane plumes on Mars and the possibility of water deep below the surface in its hydrosphere
• Porous basalt
• Two ways Curiosity's methane spikes could be generated in the shallow subsurface (centimeters deep at most)
• Ice covered lakes habitable for thousands of years after large impacts
• Ice covered lakes from volcanic activity
• Possibility of geological hot spots in present day Mars
• Life in ice towers hiding volcanic vents

Also see Modern Mars Habitability in Wikipedia (one of my contributions to the encyclopedia)

NATIVE MARS LIFE IS NOT NECESSARILY SAFE FOR US OUR ANIMALS OR CROPS

The terraforming plans assume that there is nothing on Mars that can harm us already. If there is - or some ancient microbe is activated as a result of the terraforming - there is nothing to say it has to be safe for us or our animals or crops. They do not have to be adapted to us to harm us. Indeed microbes normally become less harmful when they adapt to humans, and may eventually become symbionts.

For instance legionnaire’s disease infects amoebas and biofilms. It uses the same mechanism to attack the lungs of humans. So a disease of Martian biofilms could easily attack human lungs too.

Microbes can also harm us indirectly through producing a toxin. Examples include botulism, ergot disease, tetanus, and aspergilliosis (a fungus that can cause allergic reactions such as asthma and can be fatal to people with damaged immune system). None of these are adapted to us.

For another example that is plausible for some alien biochemistry, Alzheimer's disease may perhaps be caused by cyanobacteria which produce a mimic BMAA for an amino acid L-serine that is not exactly the same and gets misincorporated in the proteins of our body. This is ongoing research - but it highlights something that could easily happen in response to an alien biology with similar but not identical chemicals to those used by Earth biology.

Similarly some algae blooms that form in Lake Eyrie in the States to kill cows. There is no evolutionary advantage - cows are not their natural “prey”. It’s just a coincidence. The same could happen with Mars life and ourselves. For more on all this see this study, lead by David Warmflash of the NASA Johnson Space Center: Assessing the biohazard potential of putative martian organisms for exploration class human space missions and see the section Many microbes harmful to humans are not "keyed to their hosts" in my online book.

LONG TERM SPACE SETTLEMENT IN FREE SPACE HABITATS

I think one way or another we are likely to find a way to be able to live in large habitats on the Moon. But if not, we have the large habitats in free space, such as the Stanford Torus. They can be positioned anywhere in the solar system and have whatever gravity levels you want and with thin film mirrors, as much sunlight as you want whenever you want it. I think they are the natural end point for space settlement myself.

You could make a habitat like this using materials mined from a small 300 meter diameter NEO such as 4660 Nereus

4660 Nereus, 300 meters diameter, NEO, easier to get to than the Moon, has more than enough material for the cosmic radiation shielding (main part of the mass) for an entire Stanford Torus with 10,000 inhabitants.

Long before you have the capability for terraforming, or can have got even a fraction of the way towards terraforming your first planet, if you ever do succeed at it, you have the capability for these free space habitats.

FREE SPACE HABITATS CAN FILL THE ENTIRE SOLAR SYSTEM TO BEYOND PLUTO - AND GALAXY PROTECTION

Once you have them, just by using larger and larger thin film mirrors to concentrate the sunlight, you can live anywhere in the solar system right out to well beyond Pluto. The mass for larger thin film mirrors will be only a small fraction of the mass of the habitat.

It's a case of do it once, colonize the entire solar system. Indeed it would become so easy to colonize that I am concerned about the effect it could have on the galaxy and wonder how we will achieve galaxy protection, a long way into the future that is at present. See my draft article: Galaxy Protection Solutions to Fermi's Paradox - No Need to be Scared of 'Great Filter' - I'm working on that one at present and plan to post it in my blog here in the near future. I have a long section on Galaxy Planetary Protection in my Touch Mars? book and it's based on that. See

• Travel to other stars
• Planetary protection for other stars and exoplanets
• Galaxy protection - what about colonizing other star systems?

I think we will find a way through it though and if so, well then we could end up with a solar system with trillions upon trillions of humans living sustainably if we so wish.

And such a civilization would be immune to anything. When the sun goes red giant - just move the habitats further out. Or reduce the size of the thin film mirrors reflecting sunlight into the habitats for the already distant settlements.

I cover the lunar gardening in detail in my

• An Astronaut Gardener On The Moon - Summits Of Sunlight And Vast Lunar Caves In Low Gravity

I cover the asteroid habitats in my

• Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths

I cover the idea of space habitats right out to Pluto and beyond in this section of my Touch Mars? book.

• Space habitats made from asteroid and comet materials get plenty of sunlight - right out to Pluto (using thin film mirrors to concentrate the light) (above)

My original motivation for exploring alternatives to Mars was for reasons of planetary protection. For someone keen on humans in space but also keen on science and interested in the possibility of finding life based on a different biology, it would be tragic to make life on Mars extinct in our eagerness to send humans there.

But I then realized that the Moon is greatly favoured anyway and that Mars is not the natural next place for humans that it seems to be at first.

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DO COMMENT ON THIS POST

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Also if you spot any mistakes however minor, be sure to say. Thanks!

TOUCH MARS? BOOK

You can read my Touch Mars? book free online here:

Touch Mars? Europa? Enceladus? Or a tale of Missteps? (equivalent to 1938 printed pages in a single web page, takes a while to load) also available on Amazon kindle

The other ones are

• Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart - and on kindle
• MOON FIRST Why Humans on Mars Right Now Are Bad for Science - and on kindle.
  This includes my An astronaut gardener on the Moon