Now that Philae has woken up, we may be on the brink of a major step forward in our understanding of comets, and we expect to find some interesting chemistry. Perhaps 30% of its composition consists of organics, and now we'll be able to look at it close up. Why, though, do most scientists expect to find only pre-biotic chemistry? Is there any chance of finding life? Also, where else can we look for life and prebiotic chemistry in the solar system? 

So far, the only place we have searched is Mars, and our search for life there is very much in the early stages. Recently we have started to plan spacecraft missions to the oceans beneath the surface of the icy moons Europa and Enceladus. What about other ideas about how life could survive in our solar system however? Life in liquid CO₂, in liquid nitrogen, sulfur based life on Io, etc. Even the lava flows of Venus have been suggested as a possible though unlikely habitat. And what happened to Carl Sagan's idea of life in the atmosphere of gas giants? Stephen Hawkings is keen on that idea also. Is there any possibility of this?

So, I thought I'd do an article, first about comets, because of Philae and then covering these less discussed places to search for life.

WHY MOST SCIENTISTS EXPECT ONLY PREBIOTIC CHEMISTRY IN COMETS

Comets seem rather attractive places to look for life at first. After all, they have ice, and organics, and indeed, all the ingredients for life. Also short period comets come close to the sun regularly. For instance Comet 67P/Churyumov–Gerasimenko has an orbital period of 6.45 years.

However other things make it less likely to find water there. The problem is, the comet is of course surrounded by vacuum. And in the vacuum of space, water can't stay stable. The water just boils immediately into water vapour. That is unless the water is trapped. For instance after an impact, a temporary lake could form below the surface of the comet, and the surface could freeze over and trap the water before it freezes solid.

So far though, Comet 67p seems unaltered. It seems to be made up of many 3 meter diameter spherical "goose bumps" piled together, as you see in places where the interior is exposed to view

The roughly spherical bumps on the sides of this pit in the surface of Rosetta are all about 3 meters in diameter, and remarkably consistent in size wherever they are spotted on Rosetta, suggesting that the entire comet interior may well be made up of these - which may be the earliest forming "building blocks" that later came together to make comets. 

This all points to a picture where the comets are undifferentiated primordial objects, and not likely to have liquid water habitats.

However, there is one puzzling observation to explain, which suggests this can't be the whole picture.

They are a result of the stardust experiment which collected comet particles from Comet Wild. Comet fragments don't survive impact into the Earth's atmosphere, are too soft and mainly made of ice - so the only way to find out about them is to send our spacecraft to comets to study in situ or return samples.

Shows impact into the aerogel

Closeup of two of the particles detected in the Stardust aerogel, impact was from right to left.

Samples returned from Comet Wild showed evidence of alteration by liquid water in some of the smaller particles.The original paper in 2011 looks at various nickel, copper and zinc bearing iron sulfides in the comet and found evidence that they must have formed in low temperature hydrothermal environments (below 210 C).

There's an overview of the latest research in this paper under "Aqueous alteration in comets". So I'll summarize what they say there

This finding of materials altered by liquid water is confirmed by several independent studies. However, they didn't find the clay like materials that are common in carbonaceous chrondite meteorites (polysilicates). And all the particles larger than 2 microns were completely dry (anhydrous).

So, it doesn't seem likely that the comet as a whole was altered by water - or at least - the part that was the source of the dust collected in the flyby mission. Because if it was, it would have clays as well as the iron sulfides. 

So, the main question is - did this water form on the comet, or was it brought there from elsewhere in the solar system (incoming meteorites for instance). If it formed on the comet, then it may have been the result of impacts into the comet melting the surface.

Most people think it came from elsewhere, or may possibly have formed as a result of impacts. There's one dissenting voice here, however, Chandra Wickramasinghe.

WICKRASAMINGHE'S THEORY

For a long time he and Fred Hoyle argued for panspermia with the life spread in comets. Recently the idea of transfer of life between planets became mainstream, but through a different mechanism from the one they suggested, through lithopanspermia, life spread in meteorites rather than comets.

His ideas are interesting then, though often controversial.

Chandra Wickramasinghe has argued for many years that there may be liquid habitats inside comets. In a 2009 paper he constructs a model where the liquid water forms beneath a crust of organics. 

His theory has two stages. 

He suggests that the comets originally had radioactively heated liquid cores in the inner solar system, where life could originate. As the radioactive elements decayed, then this source of heating was lost. However life could have evolved in those comet cores, and then later exposed to solar heating. So - he would expect a differentiated core, originally liquid, and with clays forming in it, at some point, as you get towards the center of the comets.

CRUST OF "SUNBURNT ORGANICS"

Then in his model, the comet has a crust of "sun burnt" organics about 1 - 2 cm thick, which can frequently break and reseal. The solar heating heats the ice in a layer below this surface crust of organics. The crust then traps the water vapour, at a saturation vapour pressure of 6 millibars. He predicts that pools at depths of 30 to 50 centimeters below the surface could remain liquid throughout the perihelion passage of the comet. As the water vapour pressure increases then some of the water would escape through the surface layer through the organic layer which would then reseal.

This then makes it possible to have liquid "puddles" of water just a short way below the surface of the comet which microbes could suvive in.

Comet 67p does seem to have a hard crust, the "trampoline" effect, from the way that Philae lander bounced off the surface, 

Of course you don't need to subscribe to his entire "life started in liquid core of ancient comets" theory to be interested in his model here. This hypothesis of liquid water beneath a surface crust of organics is logically independent of the hypothesis of a liquid core in the early solar system sustained by radioactive decay. 

These puddles, if they exist, could also be seeded by life from elsewhere or it could have prebiotic chemistry but in the presence of liquids.

The same idea could also create habitats on asteroids (e.g. Ceres) or even, possibly, the ice at the poles of Mercury.

WERE THE CHEMICALS IN COMET WILD RESULT OF PROCESSES WITHIN THE COMET OR OUTSIDE IT?

So - were the chemicals found in Comet wild the result of processes inside the comet, such as either a temporary impact lake, or these crust covered puddles? Or were they the result of delivery of material from elsewhere in the solar system?

Will Philae shed any light on this question? 

EXCESS OF LEFT OR RIGHT HANDED MOLECULES

Philae has almost the first ever planetary exploration instrument designed with the capability to spot chiral imbalances.

Not the first ever, because Curiosity has a little known chirality experiment. It's part of SAM (Sample Analysis on Mars) . One of the traps for its gas chromoatograph is a chiral compound, so the rate at which compounds pass through it can depend on their chirality. On this page scroll down to GC4; trap; ChirasilDex (chiral compound separation).

But Philae's instrument COSAC, the Cometary Sampling and Composition experiment is a dedicated "chirality module". It uses gas chromatography, so gases percolate through a liquid column which separates out different molecules according to the speed with which they move - which then are analysed as separate molecules by a mass spectrometer. 

The COSAC instrument used for chirality testing on Philae. The two large spheres here hold the carrier gas Helium. It has three chiral columns,  Chirasil Dex CB,  Chirasil L Val and Cyclodextrin G-TA. These have only one version of a chiral compound (only one enantiomer) instead of equal numbers of both - and as a result, separate out molecules in the sample according to their chirality (Techy details of how it works).

An even more capable version of this technology will fly to Mars with ExoMars in 2020.

Meteorites also have chiral imbalances sometimes. That is to say, the molecules can come in two forms, "left" and "right" one the mirror image of another. All Earth life uses amino acids with the same handedness, conventionally, left handed.

Some meteorites, and comets, of course, are rich in organics. A famous example is the Murchison meteorite,

Fragment of the Murchison meteorite, and particles extracted from it in the testube. The meteorite was a witnessed fall, collected soon after it landed, and has many organics in it. It includes rare amino acids such as Isovaline: 

Isovaline, a rare amino acid found in the Murchison meteorite. This helps confirm that the organics in it are of extraterrestrial origin as this amino acid is not involved in Earth life. Incidentally, it may be of value for treatment of acute and chronic pain.

In this 2006 analysis the EET92042 and GRA95229 meteorites had chiral excesses ranging from 31.6 to 50.5%.

GRA95229 - another chrondite, collected in Antarctica, had chiral excesses of +31.6‰ for a-AIB to +50.5‰ for the (non terrestrial) amino acid isovaline, while the EET92042 meteorite ranged from +31.8‰ for glycine to +49.9‰ for L-alanine. It's thought that these excesses are extraterrestrial and not due to contamination by Earth life.

These are not thought to be signs of life in these meteorites, but results of prebiotic processs. The idea is that Earth may have been seeded by organics with a chiral imbalance already.

In the case of meteorites, then these chiral imbalances need to be investigated further with in situ study of meteorites and a sample return mission to a meteorite.

We don't have any samples from comets though. They formed much further out in the solar system, and any that hit the Earth's atmosphere will just disperse completely. This will be our first chance to look close up at the ingredients of a comet.

Could comets explain why there’s life on Earth? A lesson in chalk

So it will be interesting to see if the organics in 67p have a chiral excess, and if they have it in the same direction as Earth life. Most scientists don't expect to find evidence of life there, but they wouldn't be surprised to find a chiral imbalance, such as we have already for some of the carbonaceous chrondites.

LIFE IN LIQUID ("SUPERCRITICAL") CO₂

We are used to CO₂ as "dry ice" which turns instantly into a gas when it is heated. But under high pressures CO₂ is liquid. Indeed, not many know this, but there are places on the Earth where you can find liquid CO₂. Well not quite on the surface, we are talking about the ocean depths here.

At the high pressures of the ocean depths, anywhere below around 0.8 kilometers depth, CO₂ is a liquid.

This video is taken at a depth of 1.6 kilometers at a white smoker vent system on one side of the small undersea volcano Eifuku off Japan

The "bubbles" in this video are in fact bubbles of liquid CO₂. And there's life there, not actually living in the bubbles but apparently not bothered by it at all. See "Life in liquid carbon dioxide".

The first discovery of natural liquid CO₂ in the oceans goes back to 1990.

Interestingly, as you raise its temperature, liquid CO₂ at around 31.1 C and 73.8 atmospheres in pressure becomes supercritical. That's a phase where the distinction between a gas and a liquid disappears and the properties often change.

Table from the paper Supercritical Carbon Dioxide and Its Potential as a Life-Sustaining Solvent in a Planetary Environment by  Ned Budisa and Dirk Sculze Makuch

CO₂ at these temperatures and pressures is actually a solvent like water. And indeed researchers Ned Budisa and Dirk Sculze Makuch found that it has advantages over water. Enzymes are more stable in liquid CO₂ than they are in water, and it makes enzymes more specific about the molecules they bind to.  

Liquid CO₂ is often used for sterilizing. However, some microbes and their enzymes can tolerate living in liquid CO₂. This was a surprising recent discovery reported in February of thsi year. They found six strains of microbes, isolated from three sites targeted for geological carbon dioxide sequestration - that have the astonishing ability to grow on the interface between water and supercritical CO₂. See Microbial growth under supercritical CO₂.

There are fairly large reservoirs of liquid CO₂ beneath the Earth's oceans. For instance a CO₂ lake found off the coast of Taiwan at a depth of 1.4 kilometers.

At that depth, liquid CO₂ is lighter than water, so it must be kept in place for instance by an overlying layer of clathrates (which they observed). They found life there, even in low numbers within the liquid CO₂ layer itself. (Though this is below the temperature at which CO₂ is supercritical as used for sterilization).

It is heavier than water below a depth of around 3,000 km, so there may be deeper reservoirs.

You get supercritical CO₂ on other planets too. 

The Venus atmosphere actually is supercritical at the base of its atmospheric level. However - it's too hot for organics to be stable, but as well as that it's more of a gas than a liquid at those temperatures. That's because it is below the "Frenkel line" - the line above which a supercritical fluid has liquid like properties.

Diagram from Structural Evolution of Supercritical CO₂ across the Frenkel Line
However it may have a layer of supercrtical liquid CO2 a short way below its surface, and in the past, when it's atmospheric pressure was probably several times higher, and temperatures cooler, it might have had supercritical liquid CO₂ oceans, which might have flowed like a fluid and carved out some of the features found on the surface.

You don't need a thick atmosphere or oceans for pressure though. Surface ice and rock layers would also lead to layers of liquid CO₂ on Mars. 

Liquid CO₂ would be stable on Mars beneath about 100 meters of rock in the cold conditions there. At one point this was the favoured explanation of the Mars gullys as many of them start at a depth of about 100 meters below the top of the cliff.

Combine those conditions with high enough temperatures, say from hydrothermal heating, and again you would have supercritical CO₂ on Mars. Which might be a habitat for an exotic form of life.

LIFE IN ATMOSPHERES OF GAS GIANTS

This is an idea suggested by Carl Sagan in his Cosmos series.

For Carl Sagan's scientific paper on this, see Particles, Environments and Possible Ecologies in the Jovian Atmosphere.

And more recently by Stephen Hawking in his "Into the Universe"

In Cosmic Biology: How Life Could Evolve on Other Worlds, Louis Irwin and Dirk Schulze-Makuch look at this.

First they say about the clouds, that they appear to be too cold for liquid water. And lower down, where the temperatures are warm enough for liquid water, then there is little water. See page 166.

The main problem though, is how the life could evolve there in the first place. There are trace amounts of hydrocarbons, nitrogen compounds, and sulfur complexes. But there's a lack of oxygen, and there's no mechanism for concentrating the organics into one place. Any region where life could form is liable to be torn apart by the strong winds, and turbulence, and affected by strong radiation.

However, they thought there might be possibility of complex organics even forming structures with large scale chemical networks of precursors and products, but without a hard and fast physical boundary or exact replication, then it wouldn't fit the criteria to count as life.

They thought it unlikely that Earth life could survive there, not being pre-adapted to the conditions. And indeed, Galileo at the end of its mission was sent to crash into Jupiter, for planetary protection reasons. For the same reason Cassini will be sent to crash into Saturn. The idea is that Earth life can't survive in either of the gas giant atmospheres, and by doing this we avoid the possibility of contaminating the interesting moons of these gas giants with Earth life.

Another idea is that the gas giant atmospheres could be seeded by life from the planetary moons, such as Io for Jupiter, and Titan for Saturn, which may be similar enough in chemistry to the planets to survive there. In case of Io because it is pre-adapted to radiation and temperature extremes and to a sulfur based chemistry that could be found in the Jupiter atmosphere. And in case of Titan because it is adapted to dense, cold, organic atmosphere, again conditions that could be found in pockets in the Saturn atmosphere.

For details see page 168 of their book, most of which is available online.

SUPERCRITICAL LIQUID HYDROGEN LAYERS IN GAS GIANT ATMOSPHERES

Another possibility is life at a much lower layer in the atmosphere where hydrogen becomes supercrtcial under high pressure and, like liquid CO₂, it becomes a solvent for organics. On Jupiter this zone is very narrow but on Saturn then it is quite wide. This is suggested as a possiblity in The Limits of Organic Life in Planetary Systems which was produced by the Space Studies Board.

LIFE IN LIQUID NITROGEN - PLUTO OR TRITON

This photograph of Neptune's largest moon Triton shows dark streaks thought to be nitrogen geysers.

Triton's south polar terrain photographed by the Voyager 2 spacecraft. The dark lines are the trails of plumes from volcanoes, thought to be caused by eruptions of liquid nitrogen from below the surface. Some of the plumes were actually observed erupting in the Voyager images - by anaglyph projection, where the plumes were obvious as they were closer to the spacecraft than the other features.

Triton - and perhaps Pluto also - could have thin layers of liquid nitrogen, between a surface of solid nitrogen ice and subsurface of water ice. So could there be life in these layers?

Liquid would be a non polar solvent, so not good for organics. But it might be just the thing for polysilanes, a complex molecule that uses silicon in the place of carbon. Maybe Triton and Pluto could have silicon based lifeforms?

Turns out that silanols- a kind of silicon version of alcohol - can dissolve in liquid nitrogen. And - in these conditions, silicon has as diverse a chemistry as carbon, as William Bains has argued in his hypothesis paper Many Chemistries Could Be Used To Build Living Systems.

They have weaker bonds than carbon based organics - but this is just the thing you need in very cold conditions. 

It's "silicon based life" but not in the sense that rocks are made of silicon - any more than humans are diamond or graphite or charcoal based life. The silicon of course is combined in long chains with many other atoms. See also Peter Ward's chapter on this idea.

SULFUR BASED LIFE ON IO

There might be life on Io, sulfur based, in underground pools of liquid SO₂, with the life chemistry probably also using H₂S, because, though less abundant, it forms hydrogen bonds easily, and is more suitable for a solvent within the cells. So life might seek out the liquid H₂S and use this as their intercellular solvent, while living in pools of liquid SO₂.

Volcanoes erupting on Jupiter's inner most moon Io, photographed by the Galileo spacecraft. The surface is rich in sulfur, and one suggestion is that Io could be host to an exotic sulfur based biochemistry.

Louis Irwin and Dirk Schulze-Makuch explore this in some detail in Cosmic Biology: How Life Could Evolve on Other Worlds,

The life molecules would have backbones of sulfur nitrogen, phosphorus and other elements and would take advantage of the complex chemistry of sulfur compounds. Sulfur has a wide variety of oxidation states, even fractional oxidation states such as -0.4 or -2/3. It also forms a variety of ring compounds, and polymers.

The temperature range over which H₂S and use this as their intercellular solvent, while living in pools of liquid SO₂ are both liquid is quite narrow, from -75 C to -60 C. And because these temperatures are quite low, the reaction rate is likely to be slower.

A plume ejected from same general region as one of the regions imaged by Voyager, called Masubi, it erupts 100 km into space from Io. The eruption comes from different places in this region and leaves plumes of SO2 on the surface.

"It would be all underground and able to thrive only when temperatures reached an appropriate narrow range. But that would happen, for at least a brief period, in local pools or over short stretches of ground, every time a hot spot erupted or a sheet of lava advanced."

WHAT ABOUT LIFE IN LAVA FLOWS ON VENUS?

One of the most challenging forms of life would be silicon based life living in liquid lava. 

If this was possible, we might find it on Venus - which has entire rivers of liquid lava flowing in lava tubes - for huge distances across its surface.

These could use Silicones - organosilicon polymers with a silicon-oxygen backbone. These are stable at temperatures so high they would destroy any organics. But - would they remain stable at the temperatures of even cooling magma pools on Venus?

If life is possible in magma pools - then why hasn't it evolved on the Earth also? Why have we never found silicon based life fossils in lava flows on the Earth? 

So - silicon life seems unlikely to us. But is it just that it is a low probability life form? After all - for all we know organics based life in oceans might also be low probability.

Maybe just as we have no silicon based life in our lava, on other planets, there's no water based life in the water either.

Maybe on other planets there are silicon based life forms  reasoning in the same way that carbon based life is impossible.

Perhaps those silicon based life forms, if they exist, living in molten magma have lifeless seas of water, just as we have lifeless flows of lava (as far as we know anyway). To them, water might well seem too cold to sustain life which they think can only occur at the temperatures of molten lava :).

LIFE ON MARS

For this, I've written a lot about it already. In summary - back in 2008 few people thought that life was possible in present day Mars, except possibly deep below the surface. Now though, there are many suggestions for ways that life there could survive.

• Habitability Of Mars - Salty Seeps, Liquid Layers In Polar Ice, Ice Fumaroles, Sand Dune Bioreactors. ...
• Where To Search On Mars For Droplets,&Shallow Flows Of Liquid Water - Where Microbial Life May Flourish
• Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know

LIVE IN THE OCEANS OF ENCELADUS AND EUROPA

Again I've written a bit about this here:

• "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus

LIFE IN CLOUDS OF VENUS

The idea here is that Venus started off very Earth like in the early solar system. But at some point it dried up, lost its ocean due to a runaway greenhouse effect, and because it had no continental drift (which on Earth continually buries the carbonates).

This is a major challenge - in many cases the most habitable location in our solar system - almost Earth like in temperature, pressure, and atmospheric composition (without the oxygen of course) - but with droplets of concentrated sulfuric acid.

However we do have acidophiles on Earth that survive in conditions not far off the acidity of Venus clouds - in sulfuric acid outflows from mines on Earth.

So - it's possible that there is sulfuric acid tolerant life in the Venus clouds. The planetary protection workshop on Venus came to the conclusion that this was unlikely - but their conclusions were questioned later by some exobiologists. So it's possible a new workshop held today might come to different conclusions.

The other main problem with the high Venus clouds is that there are no solid surfaces of course. So could the life find some way to stay aloft? The residence time of particles is months rather than days - so - that makes it easier - but it's still quite a challenge.

HARD TO JUDGE IF OTHER FORMS OF LIFE ARE LIKELY

You get people arguing that some of these forms of life are impossible because organic chemistry is so much more varied than any of these other forms of chemistry. But how much of that is because organic chemistry is much easier for us to study?

We know a huge amount about how Carbon, Hydrogen, Oxygen and other atoms combine together at Earth pressures and temperatures - but is only rarely that we try e.g. experimental conditions resembling those of Titan, or Triton or Io.

And is no way we could invent organic based life processes ourselves - "inventing" something like a living cell from scratch is still way beyond us, only copying the way life does it and changing it slightly.

So how much are we biased by our knowledge of how our own type of life works? And our natural tendencies to do experiments in conditions we  find comfortable and using substances that are common where we live?

LOOKING FOR YOUR KEYS BENEATH A STREET LIGHT

It's a case, basically - of looking for your keys beneath the street lights first - because you haven't got a torch - but is at least a chance that you might have dropped them there.

“Did You Lose the Keys Here?” “No, But the Light Is Much Better Here”

If life is easy to find, and at least some of it is organic based - then perhaps we will find organic based life and maybe it won't take too long to find. 

If it is rare, or most is not organic based, - then who knows how long it will take to find it or if we will recognize it easily when we find it.

At the moment we can assume almost anything about life in the solar system.

We haven't sent life detection equipment to any of these places (except Viking to Mars in the 1970s). None of this life would be easy to spot from orbit, so there is no way to know at present.

It might be that life in one form or another is present almost everywhere in our solar system, even in the most exotic habitats. Or it could be present nowhere except on Earth.

If there is no life in these places, there may well be prebiotic or complex chemistry in some or many of them. These might hint at the range of possibilities for life elsewhere in the universe and also give insights into our understanding of the very early stages of life evolution.

FIND OUT MORE

I've only touched on a few highlights of a vast subject here.

For some more ideas, see

• The Limits of Organic Life in Planetary Systems (National Research Council) 
• Hypothetical types of biochemistry (wikipedia)
• Cosmic Biology: How Life Could Evolve on Other Worlds, Louis Irwin and Dirk Schulze-Makuch
• Many Chemistries Could Be Used To Build Living Systems, William Bains
• Life as We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life - Peter Ward
• Weird Life: The Search for Life That Is Very, Very Different from Our Own - David Tooney