When Curiosity's successor and ExoMars land on Mars around 2021, we will see two different approaches to the search for life on Mars side by side. NASA's mission is the first stage of a two decade sample return program, while the ESAs ExoMars rover is focused on an in situ search for biosignatures, with the capability to drill two meters below the surface. Which is the best approach? 

NASA's decision was based on the last decadal review in 2012, for the decade 2013 to 2022. 

NASA do one high cost "flagship mission" in each decade. For the next decade, they had a choice between a Mars sample return, or the Jupiter Europa Ocean mission. They chose the sample return, but with the funding available, they could only pay for the first half of the mission, sample caching on Mars. They left return of those samples to Earth as a decision for the next decade. So essentially, it's a double decade flagship mission. This makes it one of the most expensive decisions NASA has committed to in the field of planetary sciences in recent years.

Actually NASA are just first off the block. There are many other countries that may do a sample return in the near future including Russia, China, and indeed the ESA themselves who have explored the idea for many years. So these ideas may be relevant to them also.

Is a sample return the best way to use our funds for Mars exploration at present?

The only comparison study I found is a white paper submitted to the decadal review itself by eight exobiologists. And, surprisingly given the outcome of the decadal review, this study comes out firmly in favour of in situ exploration and against a sample return, at least for the purposes of the search for life.

COMPARISON STUDY BY EIGHT EXOBIOLOGISTS

This study is in a white paper submitted to the decadal review by eight exobiologists (from Scripps Institution of Oceanography, NASA Jet Propulsion Laboratory, SETI Institute, University of California Berkeley and NASA Ames Research Center.). It concludes that there is a strong possibility that a sample return would return results as controversial for biology, and as hard to interpret as the Mars meteorites we have already such as ALH 84001.

Now there's no doubt at all that samples from Mars similar to our Martian meteorites would be of great interest to planetary science. We have only 130 meteorites known so far, from Mars, and only a few of those are witnessed falls. Many have been on the Earth for thousands of years and have had opportunity for contamination, though limited due to the Antarctic conditions where many were found.

However, if the conclusions of the white paper is right, that we might get "more of the same", are they interesting enough to justify the price tag of many billions of dollars of a sample return mission?

ALH64001 remains one of the most interesting Mars meteorites for this search for life, partly because it is so ancient, four billion years old geologically (that's its Mars age - its age since formation on Mars). At one point, briefly, researchers thought that they had discovered unambiguous traces of early Mars life in this meteorite. Some of you may remember the news stories about this in the world media.

ALH84001 perhaps the oldest Mars meteorite we have, formed on Mars about 4 billion years ago, sent into space by an impact on Mars, and after millions of years in transit in space landed on Earth about 13,000 years ago.

A chain of magnetite crystals, "like a string of pearls,” within meteorite ALH 84001. Some microbes use magnetite as a compass, and the purity of the magnetite in this meteorite was one of the main lines of evidence for detection of life. Other lines of evidence include the biomorphic features, and detection of large organic molecules, but most of the later discussion focused on the magnetite as one of their strongest points.

The main alternative explanation is that the magnetite could form during shock events. Credit: NASA. See The Continuing Controversy of the Mars Meteorite (NASA Astrobiology Magazine, 2010)

For the latest developments in the storyline for ALH64001, see Tattletale Mars Rock: ALH84001 New Findings.

Another example of a fascinating Mars meteorite is the Tissint meteorite in the news recently. It's a witnessed fall, so one of the least contaminated of all the Mars meteorites, only been sitting around for a few days before it was collected. Again, for various reasons, some scientists see this as good evidence of early life on Mars.

This also is proving as controversial as ALH84001. See Meteorite May Contain Proof of Life on Mars, Researchers Say and Experts Cast Doubt on Meteorite Study's Claims of Martian Life

Here you can see a fragment of this meteorite from London's Natural History Museum, discussed by Caroline Smith, their meteorite expert.

It would be wonderful to have a few more samples like these two meteorites. And especially so, it would be great to have the context, the exact location they come from on Mars.

The thing is though, is it worth the huge expense, to get more samples like this, even ones that are brought straight to Earth from Mars in a spacecraft? How much will this help with the study of exobiology and the possibility of life on Mars? You have to bear in mind the impact this has on other missions, which won't be flown because the funding is used instead for a sample return.

This was the question the exobiologists addressed in their study. And they came down strongly in favour of in situ exploration at this stage of our exploration of Mars.

"Two strategies have been suggested for seeking signs of life on Mars: The aggressive robotic pursuit of biosignatures with increasingly sophisticated instrumentation vs. the return of samples to Earth (MSR). While the former strategy, typified by the Mars Science Laboratory (MSL), has proven to be painfully expensive, the latter is likely to cripple all other activities within the Mars program, adversely impact the entire Planetary Science program, and discourage young researchers from entering the field."

"In this White Paper we argue that it is not yet time to start down the MSR path. We have by no means exhausted our quiver of tools, and we do not yet know enough to intelligently select samples for possible return. In the best possible scenario, advanced instrumentation would identify biomarkers and define for us the nature of potential sample to be returned. In the worst scenario, we would mortgage the exploration program to return an arbitrary sample that proves to be as ambiguous with respect to the search for life as ALH84001."

Instead of a sample return they recommend (to pick out some of the points from their paper):

• Need for increasing mobility, and precision landing, supported by orbital observations, to access the many and varied habitable environments including subsurface, layered sediments, gullies and ice sheets.
• The "follow the water" strategy should now be followed by a "follow the nitrogen" phase combined with a search for biosignatures.
• The biosignature search can use exquisitely sensitive in situ electrophoresis techniques to identify and characterize and find the chirality of amines, nucleobases, polycyclics and other essential organic molecules.
• This search should include drilling to the greatest depth possible for the best chance of success for detecting biosignatures of past life on Mars
• They recommend that we should do a sample return only after we either identify biosignatures on Mars, or have exhausted all other possibilities by in situ research.

(for details, see the conclusions of: Seeking Signs Of Life On Mars: In Situ Investigations As Prerequisites To A Sample Return Mission

As you can see, the main difference in the perspective of the astrobiologists, and the geologists, is in the timing. They recommend a sample return at a later stage in exploration, once we have explored Mars more thoroughly and definitely identified biomarkers on Mars.

Alternatively, if we never find biomarkers on Mars, we could return samples after we have exhausted all the in situ technologies available to explore for the biomarkers on Mars itself directly.

DECADAL SUMMING UP DOESN'T DISCUSS THIS STUDY

The decadal review does list Bada et al's paper amongst the submitted white papers at the end of the report. But it is not cited in the body of the report or discussed. Nor is it mentioned in their final presentation (which is available as a video online).

This is what the decadal review says:

The Mars community, in their inputs to the decadal survey, was emphatic in their view that a sample return mission is the logical next step in Mars exploration. Mars science has reached a level of sophistication that fundamental advances in addressing the important questions above will only come from analysis of returned samples.

The site will be selected on the basis of compelling evidence in the orbital data for aqueous processes and a geologic context for the environment (e.g., fluvial, lacustrine, or hydrothermal). The sample collection rover must have the necessary mobility and in situ capability to collect a diverse suite of samples based on stratigraphy, mineralogy, composition, and texture. Some biosignature detection, such as a first-order identification of carbon compounds, should be included, but it does not need to be highly sophisticated, because the samples will be studied in detail on Earth.

Vision and Voyages for Planetary Science in the Decade 2013-2022

As we've seen, Bada et al's paper is hardly "emphatic in their view that a sample return mission is the logical next step in Mars exploration.". Indeed, their conclusion could be somewhat more accurately summarized as "a sample return mission is not the logical next step".

Here I pick out some of the things they say in the summing up, and compare them with the statements in Bada et al's paper.

• Summing up: "fundamental advances in addressing the important questions above will only come from analysis of returned samples"
• Bada paper: "We have by no means exhausted our quiver of tools, and we do not yet know enough to intelligently select samples for return

And

• Summing up: "Some biosignature detection, such as a first-order identification of carbon compounds, should be included, but it does not need to be highly sophisticated, because the samples will be studied in detail on Earth."
  
• Bada paper: "We argue here that when in situ methods have definitively identified biomarkers, or when all reasonable in situ technologies have been exhausted, it will be time for MSR. We are not yet at that crossroad."

If the decadal review did discuss his paper - it would seem that none of that has got into the final review.

Do any of you reading this know more about the background to this? Did they discuss it? If so, did they make a decision not to mention it, and if so why? Or was it just ignored?

THE DECADAL REVIEW HAVE MADE THEIR DECISION - WHAT CAN BE DONE TO MAKE IT A SUCCESS?

I'll look into a possible reason why this might have happened later on. The decadal review have made their decision, and it doesn't seem likely they will change track. Rather, the idea is that this discussion may help us to better understand what role this NASA decision can play in the future, for astrobiology.

That is if it happens at all. There have been many earlier plans for a sample return that never came to anything.

The idea of a Mars Sample Receiving laboratory was first studied in 1978. The idea then was for an orbiting quarantine facility called Anteus to receive the samples. Antaeus Orbiting Quarantine Facility (1978) 7th July, 2012

Other proposals were explored in the 1980s, including direct entry of sample container to Earth's atmosphere, recovery by the space shuttle, recovery to space station, recovery to a dedicated Antaeus space station, and several intermediate proposals. Mars Sample Recovery&Quarantine (1985)

Perhaps this time it will happen. If so, what can we do to help make it a success, and a valuable part of our space program? Can we do anything to reduce the huge cost? And can it be done safely, given the issues for back contamination of the Earth? At reasonable cost?

First, let's look into the background to Bada et al's reasoning, some more.

You'd think the exobiologists would be the ones most in favour of returning a sample from Mars. Surely they, of all the planetary scientists, would be the ones who would most long to get hold of a sample of exobiology from Mars to study in their laboratories rather than remotely. So why didn't these eight exobiologists support a sample return?

INFLUX OF ORGANICS TO MARS FROM SPACE

The basic problem is that Mars gets a constant influx of organics from micrometeorites every year, also from comets and meteorites. The organics found by Curiosity need not have any connection with life as such.

You might not realize this from the enthusiastic news stories when Curiosity finally discovered organics, but the biggest surprise was that it didn't find them sooner. All those organics "raining in" from space have to go somewhere. For instance in the Yellowknife bay organics found by Curiosity, the findings were consistent with presence of 300 to 1200 parts per million (ppm) organic C from meteorites. These organics may well be mainly of meteoritic origin. A 1990 paper predicted that between 2 and 27% of the Martian soil would be contributed by meteorites.

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, which

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.

The organics from meteorites may even have a chiral excess also. In the case of the Murchison meteorite this imbalance is subtle and controversial,

But in other meteorites, much larger excesses have been detected. 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.

Meteorites like this surely fall on Mars just as they do on the Earth. So what happens to all the organics they bring to Mars?

Since it took so long for Curiosity to find the organics, there clearly has to be some process, probably chemical, actively destroying organics on Mars.

SEARCHING FOR LIFE AMONGST THESE ORGANICS

So that's the picture, Mars has a continual influx of organics from space. It probably also has organics created by indigenous non biological processes (e.g. serpentization deep below the surface). These organics are then actively destroyed by surface processes. And mixed amongst all those processes involving non biological organics, it may also have some life based organics. But the organics from life may be rare and hard to distinguish.

That's the same for both past and present life, though for different reasons.

• Present day life on Mars is likely to be in low concentrations. Our rovers are currently targeting regions with low chance of present day life. But even the most hospitable spots, even if they have life, are likely to be as inhospitable as the heart of the Atacama desert and the McMurdo dry valleys. These have life, but have such low concentrations of organics that a Curiosity type mission would be unlikely to be able to detect them.
• Past life, if close to the surface, gets degraded rapidly by ionizing radiation from cosmic radiation and solar storms, about a thousand fold every 650 million years. Cosmic radiation has little effect over time periods of years, decades, centuries or millennia. But over time periods of hundreds of millions of years the effects are huge.

  It's an exponential process. Every 650 million years the remaining organics are reduced in the same proportion. After 1.3 billion years, a thousand tons of amino acids gets reduced to a milligram, with the rest converted mainly to gases like carbon dioxide, water vapour, methane and ammonia. After 2.6 billion years it's down to a nanogram (billionth of a gram) and after 3.9 billion years you are down to less than a picogram (10^-12 grams) of your original thousand tons deposit. And of course this radiation leads to deterioration of the sample making it hard to identify it as life.

  This makes it a major challenge to find any life at all preserved from billions of years ago. Even if there were large quantities originally, there may not be so much to find now, especially when you combine this degradation with the continual influx of new organics from space.

• It is hard to predict where you will find life based on geology. In Antarctica for instance, or the Atacama desert, some rocks have life and others, apparently identical, don't. This may be true for both present day and past life on Mars. In both cases, apparently habitable regions may have no life and apparently less habitable regions may have life.

  For instance,what if life on Mars never developed photosynthesis. Then perhaps the only place we can find life on Mars, past or present, is in hydrothermal vents or hydrothermal features?

  Or what if it flourished briefly in some favoured habitat (whatever is your preferred location for origins of life) and then became extinct?

  For present day Mars - some of the habitats may be uninhabited just because the life hasn't got to them yet. It might have life in one dune field or or region of warm seasonal flows, and not in another.

IMPOSSIBILITY OF SAMPLING EVERYTHING

This region of the Mawrth Vallis area of Mars gives some idea of the complexity of the situation on Mars.

Imagine trying to study this region by returning samples to Earth for analysis? And now, imagine that you also have to drill below the surface to find samples less affected by ionizing radiation?

Close up image of a region of stratified clays in the Mawrth Vallis region of Mars

With current ideas, the sample would only return small quantities, probably less than a kilogram in total. So there is no way we can do a complete survey of any moderately complex region of Mars and return samples from all the interesting points in the region.

An in situ search on the surface is not restricted in any way. We can continue studying new samples indefinitely. We can also home in on regions of interest.

If the in situ study finds that a particular band of rocks, or type of rock, for instance, is of especial interest - say with a chiral signature or detection of amino acids or other biologically interesting molecules - then the in situ rover can then focus the search on other rocks of that type. Or it can drill into the layer to get deeper samples and so on. It can make decisions about where to go next based on the analyses already done. But if you have to return the rocks to Earth to search for biosignatures, this is impossible.

CHALLENGE OF FINDING WELL PRESERVED SAMPLES

Mars is a great place for preservation of organics in some ways. Eventually, we can hope to find samples of organics on Mars that are relatively unchanged since the first few hundred million years of the early solar system.

• Cold conditions keep organic molecules stable for billions of years against the effects of deracemization (which removes the chirality signal of organics in warm conditions).
• With no continental drift, much of the surface of Mars is billions of years old, hardly changed since soon after the formation of Mars itself.

But other conditions on Mars make preservation difficult

• Later episodes of flooding
  
  Artist's impression of gale crater as it might have looked during one of its flooding episodes (by Kevin Gill). Curiosity Rover Data Indicates Gale Crater Mountain Used to be a Lake

  Of course, floods like this may make it briefly habitable, but they can also wash out earlier deposits. Especially as the later floods on Mars were often rapid flash floods.

• High levels of cosmic radiation - either originally when the deposit is formed, if it is not buried rapidly enough - or later when it is unearthed again on or near the surface
• Chemical degradation of near surface materials. We know that there must be processes actively removing organics that come to Mars from meteorites and comets.
• Influx of organics from meteorites and comets can get mixed with the organics from life which we are looking for.
• Mars was most habitable billions of years ago. This is a long timescale for preservation of organics, during which Mars lost most of its water, had many floods, and changed its inclination, orbital eccentricity, atmospheric density, and climate many times..
• Present day and geologically recent life is likely to be rare.
• Life is most likely in places that had water in the past. These are the very places where warmth, flooding, consumption of the organics by other lifeforms, and other forms of degradation can happen.

So, for a clear signal, for past life, we have to look for life in the right place (e.g. hydrothermal vents, or salt lake deposits or the warm seasonal flows or whatever turns out to be best). And then your sample needs to be:

1. Preserved quickly (dried out, caught in clays or salt, or the microbes rapidly entombed in fast forming rocks like chert)
2. Plunged rapidly into freezing conditions.
3. Buried quickly, ideally within a few tens of millions of years, to a depth of several meters (or it would degrade beyond recognition through cosmic radiation)
4. The life wasn't washed out with later floods, or chemically altered or decayed or mixed with other sources of organics, or returned to the surface temporarily for more than brief time periods.
5. Returned to the surface rapidly (perhaps as a result of a meteorite strike), and did this in the very recent geological past. Or else, your rover needs to be able to drill deep, or search in caves protected from the surface cosmic radiation.

On the plus side, Mars is a huge and varied planet, with surface area the same as the land area of the Earth. There are plenty of opportunities to look for this life on Mars. Surely somewhere on the surface of Mars we will find the ideal conditions leading to preservation of past life, and optimal conditions for present day life.

The downside of this vast search ares is that we don't know where to look. On Earth one key to discoveries of early life was the realization that gunflint chert is a "magic mineral" that preserves traces of early life.

Galaxiopsis, one of the fossil microbes found in gunflint chert, which has turned out to be a "magic mineral" for search for evidence of early biology on Earth.

What are the "magic minerals" for the search for life on Mars, in the very different conditions that prevail there? Where are the best places to look? We don't know yet.

We are making a great start with Curiosity. We will find out more with future missions like Curiosity's successor and Exomrs. But there are many more steps still to go through. See Habitability, Taphonomy, and Curiosity's Hunt for Organic Carbon

LIFE FROM MARS LIKELY TO BE MICROSCOPIC, AND MAY WELL BE NANOSCALE

Though it's possible that Mars had multi-cellular life, or stromatolites, or biofilms, the traces you are looking for are perhaps more likely to be remains of microscopic creatures.

For present day life, that's because the most Mars like landscapes on Earth, cold dry deserts, are often inhabited only by single cell life. It may have lichens, but all the other lifeforms currently suggested as Mars analogues are single cell organisms.

Past life may not have evolved as far as multicellularity (multicellularity, for the most part, is only 500 million years old on the Earth).

The earliest lifeforms, if we can find them, would be even more challenging to idenetify, as they are sure to be sub optical resolution in size, of the orders of tens of nanometers rather than the hundreds of nanometers of modern cells. It's impossible that the modern cell in all its complexity arose directly from inorganic chemicals.

Indeed, if life on Mars hasn't evolved as much as it did on the Earth, or perhaps evolved in a different direction, maybe a more efficient metabolism and methods of replication - present day life there might still be nanoscale. It could be of the order of tens of nanometers in size - an order of magnitude smaller than the smallest known cells on Earth and well beyond optical resolution.

Then, we don't know what we are looking for, yet, it may be unknown biology. It could be based on XNA (like DNA but with a different backbone) or it could be something else not DNA at all.

This is likely to make it harder to find past or present day life on Mars, or to identify it. In short:

• Likely to be single cell micro-organisms
• We don't know what it looks like
• We don't know what chemical signatures to look for
• It may only form nanoscale fossils, which are notoriously hard to identify as life or non life.

POSSIBILITY OF THICK DEPOSITS SUCH AS OIL SHALES

There might be thick deposits of life on Mars, shells like our chalk beds, or oil rich shales. If so, that would make the task easy. Seems not impossible, as life may have survived on Mars for hundreds of millions of years in relatively stable conditions. That's plenty of time to build a thick deposit of oil shale.

If we found something like this, even without the multicellular life fossils, just the remains of single cell life but in deep meters thick beds of organics, it would be easy to identify it as life:

Fossils in Ordovician oil shale (kukersite), northern Estonia

If deposits like this exist on Mars, they haven't been found yet. Maybe conditions on Mars were never favourable for creating thick deposits like this. Maybe the life was always sparse.

Or, it could also be that they were washed out by the later floods, and what's left, destroyed by the cosmic radiation on the surface, which could get rid of entire layers of organics. Maybe deposits like this are still there tens of meters below the surface?

At any rate if those deposits exist, we don't know where to look for them yet. There is no sign of them from orbital observations.

EVEN MULTICELLULAR FOSSILS WOULD BE HARD TO FIND

What if Mars had or has multicellular life such as lichens, or large structures formed by microbes such as stromatolites?

Or indeed, what if it developed multicellular animals? They would face a big issue in present day Mars. But in the earlier system, Mars could have had a relatively oxygen rich atmosphere, perhaps created by ionizing radiation dissociation of the water. It would have had to have had a very rapid start to have true multi-cellular life back then. On Earth it developed only half a billion years or so ago. But on the other hand, multi-cellular life on Earth boomed after a "snowball Earth" episode. Mars has surely frozen over many times, could that have lead to evolution of multi-cellularity far sooner?

We have so many wonderful fossils in our museums. But when you go for a walk in mountainous rocky regions - how often do you see fossils in the rocks? Some rocks have them, but they are hard to spot. They are only easy to find in special types of rock, for instance chalk, rocks that are entirely made up of fossil lifeforms.

Even if Mars had birds, and fish, in the early solar system, the chances are that we wouldn't have found any signs of it yet.

This picture shows Archaeopteryx.

Archaeopteryx was hard to find. They had to search through tons of quarry material to find the few thin flakes with Archaeopteryx preserved.

You could send a rover to Earth and set it to explore rock formations in our desert regions for decades, and it might never spot a single fossil, depending where you send it.

The same is likely to be true on Mars, we may need to search many tons of sediments to find the few still recognizable remaining traces of past life on Mars, even if it developed multicellularity at an early stage.

HOW WOULD WE RECOGNIZE FOSSILS ON MARS?

We don't know what to look for on Mars. If we found a fossil archaeopterix it would be obvious. But what if we find these?

These are now known to be early stromatolites. But it took a lot of work and evidence before they were accepted as such.

There are many formations on Earth that look for all the world as if they were some fossil lifeform, such as this.

Baryte Rose from Cleveland County, Oklahoma, photograph by Rob Lavinsky

If Curiosity found this on Mars, I'm sure many people would be convinced it was a fossil. But no. It's a "Desert rose" - a crystal like structure that can form in desert conditions.

However if it found this:

While of course if we are lucky enough to spot something like this (fossil fish from the green river oil shale deposit) there would be no question about what we have found.

We will be very lucky indeed if we find a lifeform on Mars that we can conclusively identify as living just by its physical shape.

Even if it turns out that the planet had stromatolites, or even multicellular fish and birds, in the past.

So we don't hope to find life based on the morphology, not in the near future. Not too likely that Curiosity's successor will find a fossil fish or a clear unambiguous stromatolite. So we are left with the search for traces of organics as the only likely way ahead.

As for present day life, Mars could have lichens, some suggest, but we are not currently looking in the most likely place to find it.

RHYTHMS FROM MARTIAN SANDS - WHAT IF VIKING DETECTED LIFE?

There is one scenario here though, that could mean that we return life from Mars at an early stage. What if there is life there already in the sand dunes, and Viking detected it? Gilbert Levin has been saying this for decades, and recently some other scientists have found new evidence that may support him. See Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know

If that is correct - then since Viking didn't land anywhere special on Mars, it probably means that life is present in low concentrations almost everywhere on Mars.

In this scenario - then ExoMars will probably detect biosignatures of life at an early stage, since it's instruments are sensitive enough to detect life even in the Atacama desert (where Curiosity and Viking could not detect the organics). In this scenario, Curiosity's successor would be expected to return life in the sample.

But of course it doesn't go the other way. If the sample return doesn't contain life, it just shows that Gilbert Levin's interpretation of the Viking data is probably incorrect (or that both Viking landers were very lucky). Doesn't mean that there is no life on Mars, or even in the equatorial regions!

MARS SAMPLE RETURN AS A GEOLOGY MISSION AND A TECHNOLOGY DEMO

The decadal review summing up motivates the Mars sample return using examples of previous sample returns of comet and interplanetary dust, and moon samples.

Tracks of particles from comets collected in the stardust aerogel, first sample return of a comet to Earth

These undoubtedly were hugely valuable in advancing our understanding. But those are all astrogeological missions, and their value was geological.

Geological specimens don't deteriorate in the same way as organics. There is no problem of racemization, or of lifeforms eating them, or them being washed out by flooding.

They are also easier to find. They are also relatively easy to identify. The sample return from Mars may well be of great value for geology. Nobody controverts that.

ASTROBIOLOGICAL MOTIVATION

The big difference is that a Mars sample return is motivated mainly by its value for exobiology. Geologists would love to get hold of a sample from Mars. However, it's hard to motivate a multi-billion dollar mission to return a kilogram or so of samples from Mars for the geology only. Especially since we are making great strides in understanding the geology of Mars robotically and have many meteorites from Mars already.

If the motivation is geological, you listen to the planetary geologists. But if the motivation is astrobiological - surely you need to listen to the astrobiologists? Methods that work for astrogeology may not work so well for astrobiology.

WHY DID THE DECADAL REVIEW CHOOSE A SAMPLE RETURN?

So why did the decadal review choose a sample return mission over in situ exploration? Why didn't they listen to the astrobiologists?

I don't know the answer to that, since they don't discuss the Bada paper at all as far as I can see. They don't give any reasons as far as I can see (do correct me if you know of anything they say on this subject, anyone). But I can offer a few thoughts that might be relevant.

NASA, of all the space agencies, is the one most focused on a human landing on Mars. There are two main reasons for this I think. First, they want to repeat the USA's success in the 1960s on the Moon, with human boots on Mars. Also, the Mars Society colonization advocacy group is especially strong in the US, and space colonization advocates such as Robert Zubrin are influential there, and they are not quite so influential with the ESA and other space agencies outside the USA.

"SAFE ON MARS" - COULD A SAMPLE RETURN TELL US IF MARS IS SAFE FOR ASTRONAUTS?

This is just a guess, but I wondered if it is possible that they were motivated partly by the Safe on Mars report in 2001. This recommended a sample return from Mars to check to see if there are any biological hazards for humans on the surface. The decadal review does cite Safe on Mars.

If so, it's interesting to note that Safe on Mars is not really that supportive of a sample return if you read it carefully. It actually came to a similar conclusion to Bada et all.

The sample return is presented as a second best solution. They recommended it only because at the time they wrote the report, there were no instruments sensitive enough to do a good search in situ, in their view. They say:

As stated above, there are currently no measurement techniques or capabilities available for such in situ testing. If such capabilities were to become available, one advantage is that the experiment would not be limited by the small amount of material that a Mars sample return mission would provide. What is more, with the use of rovers, an in situ experiment could be conducted over a wide range of locations.
(Page 41 of Safe on Mars)

Well, now, these capabilities are available. Many instruments that were huge laboratory filling machines even as recently as this report in 2002, with no chance at all of sending them to Mars - they have now been miniaturized and a fair number also tested in space simulation conditions, and could easily be sent to Mars.

These include DNA sequencers, electron microscopes, ultra sensitive biosignature detectors able to detect a single amino acid in a sample, and updated versions of the Viking Labelled release using chirality to eliminate false positives, including the exquisitely sensitive electrophoresis "lab on a chip" methods mentioned by Bada et al. Another new idea is the Solid3 approach of using monoclonal antibodies - which can detect, not just the organics you find in animal bodies, but a wide range of organics, again with exquisite sensitivity.

Also the geology of Mars is much more varied than realized in 2002 when that report was written. We have ideas now for potential habitats for life even in equatorial regions such as the advancing sand dunes bioreactor and the warm seasonal flows. Also there are ideas for ways that life could survive (perhaps just below the surface) using just the night time humidity with no water at all.

It doesn't seem likely that a few samples returned from the surface even of a large plain of sand dunes, for instance, would be able to confirm or deny the advancing sand dunes bioreactor hypothesis. There might be only a few sand dunes with the right mixtures of salts to give conditions for life in the entire plain.

In conclusion, given what we now know about the variety of conditions on Mars and the varied possibilities for habitats there - it doesn't seem likely that a sample return from Mars at this stage would settle anything about safety of surface conditions for astronauts.

NASA'S IN SITU UREY SUITE OF INSTRUMENTS

NASA originally planned to explore Mars in situ, along with the ESA. It developed a suite of instruments, the UREY suite to fly on ExoMars. This was able to detect life signatures with exquisite precision.

NASA was going to collaborate with ESA - and launch ExoMars to Mars for the ESA.

However, it pulled out of this collaboration in favour of its own sample return program, leaving ESA develop its own instruments suite and to partner with Russia instead. Then because Russia doesn't yet have the proven capability the USA has to land heavy rovers to orbit, UREY instrument suite was too heavy for this new mission and was descoped.

HOW CAN WE PROTECT EARTH DURING A SAMPLE RETURN MISSION?

A sample return is not only an expensive mission, it also raises unprecedented issues of planetary protection for the Earth, and especially so if the sample is returned at such an early stage, when the planners of the mission can have no idea what is in the sample of biological interest.

If NASA does go ahead with the second half of their proposal, and they do a sample return, this is something that Carl Sagan and others have argued is something we should only do with great caution.

I have two new suggestions here that could, just possibly, help resolve this.

First, though, for those of you who are new to this, let's just summarize the need for back contamination protection. Let's look at a couple of the arguments you may have come across, which may seem to suggest there is no need for planetary protection at all.

SURELY MARS LIFE WOULD BE HARMLESS BECAUSE IT GETS HERE ON METEORITES?

This is one of the things that you hear over and over. Robert Zubrin and others popularized this argument. They argued that any life from Mars must be harmless because any life from Mars must have got to Earth on meteorites already.

It's true that we get many tons of meteorites from Mars every century. But what is not so often realized is that the material we receive today is thoroughly sterilized by several hundred thousand years, minimum, of exposure to cosmic radiation in space.

• Material even from Mars, the closest, has spent hundreds of thousands of years in transit Zunil crater on Mars, diameter 10 km, a young candidate crater source for Mars meteorites on Earth. A crater needs to be around this size or larger to send material to Earth. Most of the material arrives at Earth between 16,000 and 20 million years after impact, and most of it is thoroughly sterilized by cosmic radiation and solar flares in transit.
• Capsules can return microbes that could never get here on a meteorites. For instance many of the habitats on Mars such as the layers in the ice sheets, salts mixed in the sand dunes, or salt pillars, or deliquescing salts would probably never get into space after an asteroid impact. And if they did, any life within them might well not be able to survive the journey of at least a century in the vacuum conditions, also subjected to UV, the deep cold of space conditions, and solar flares.
• Life on Mars may be localized to particular spots which the meteorite would need to hit (E.g. the warm seasonal flows).
• How do we know that meteorites don’t cause extinctions from time to time, over a timescale of many millions, even hundreds of millions of years?

The NRC looked into this last possibility, that life from Mars could have caused negative effects on Earth in the past. They concluded:

"Certainly in the modern era, there is no evidence for large-scale or other negative effects that are attributable to the frequent deliveries to Earth of essentially unaltered martian rocks. However, the possibility that such effects occurred in the distant past cannot be discounted."

WHY DO WE NEED TO CONCERN OURSELVES WITH THIS - IT SOUNDS LIKE SCI FI?

This is another common reaction. Many of you are used to science fiction stories, and these scenarios are often implausible and anyone with a good science background can see they wouldn’t work. E.g. a virus from space able to infect humans. After exposure to such improbable ideas in movies, it may be hard to take the idea of back contamination seriously.

However, there are realistic non sci fi threats that we do need to concern ourselves with. For some reason these are rarely or ever covered in movies.

Here XNA refers to any of the many alternatives to DNA now known. I use this as an example of a lifeform not related to Earth life, different evolutionary history. Of course life from Mars needn't involve anything resemble DNA at all, this is just a "for example".

• XNA based life that Earth life can’t recognize as a hazard, able to take up residence in our ecosystems, our animals, soil, the sea, or even ourselves
• Life that is related to Earth life, adapted to infect Martian micro-organisms (like legionnaires disease in our lungs, uses same mechanism it uses to infect amoeba to infect human lungs).
• XNA life could create molecules that closely resemble life molecules but are not identical and taken by mistake. For instance, this molecule:
  .
  L-serine, resembles
  
  BMAA which is created by green algae. It’s been suggested that BMAA can be misincorporated to cause tangle diseases like Alzheimers.Perhaps in a similar way (my own suggestion here) an XNA based lifeform could generate organic molecules that resemble amino acids used by Earth life and be misincorporated to cause protein misfolding and tangle diseases. It doesn’t need to be adapted to Earth life to do this.
• XNA life that is better than Earth life, more efficient metabolism say. Even better in all respects than DNA, smaller cell, more efficient systems of replication and metabolism. It could take over from other micro-organisms in Earth ecosystems. And may function differently from them.
• GTAs that tranfer new capabilities to Earth life.

On the first possibility, and particularly, the idea of whether it could be a hazard for humans, Joshua Lederberg who first characterized the Archaea said:

“On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse”

Not saying at all that it is going to happen, or is inevitable. As Carl Sagan said once for Mars,it could be that you can ingest kilograms of its life without any ill effects. But if you don’t know what is in it, you have to be prepared for anything

SAMPLE RETURN WITH UNSTERILIZED SAMPLE

If the sample is unsterilized, we have to take precautions to protect the Earth. That is the conclusion of all the studies into the back contamination risks of a Mars sample return to date.

You’d think this was easy, but it turns out to be surprisingly complex and difficult. Back in the 1990s the general idea was that we can just return samples to a glove box facility in a biohazard 4 laboratory.

After a series of studies, though, the precautions needed got more and more complex. The most recent studies require a facility costing perhaps half a billion dollars or more, with capabilities never tested before.

One problem is that it is easy to contain a known pathogen, say smallpox, or anthrax, or the Ebola virus etc. But what can you do when you don’t know what is in the sample, what its capabilities are, what size it is, or even what biochemistry it has? Quite possibly doesn’t use DNA?

The smallest size for early cells if they don't contain all the machinery of modern DNA, is generally estimated as about 40 nm.

Successive studies by the National Research Council (NRC), USA (two studies) then the European Science Foundation (ESF) (one study) gradually lead to more and more stringent requirements. First came the reduction to 200 nm by the NRC after discovery of the ultramicrobacteria Then reduced to 10 nm by the ESF. as a result of discovery of how readily archaea can share their DNA through the tiny Gene Transfer Agents (GTAs)

Credit: Zina Deretsky, National Science Foundation

The red colouration of this pea aphid comes from a unique ability to generate carotenoids itself. It got this ability through horizontal gene transfer from a fungi.

Archaea can also transfer genes between phyla that are as different from each other as fungi are different from aphids. It is an ancient mechanism and so may also be able to transfer genes from life that had last common ancestor with us in the early solar system.

In one experiment 47% of the microbes (in many phyla) in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day

So if the life is at all related to Earth life, you have the possibility of this exchange of DNA bringing new capabilities to Earth microbes from space. Even if the microbes themselves don’t survive.

Another thing that makes the design more complex is the need not just to contain the sample (which is usually done by a positive air pressure from outside) but also to protect it from outside organics (which needs a positive air pressure from inside). You end up with some kind of a double walled facility and they cite this as one of the reasons why you have to have a new design of building, never tested before.

This is one of the designs they came up with in 2008, with telerobotics.

The LAS sample receiving facility uses a fully robotic workforce, including robotic arms that manipulate samples within interconnected biosafety cabinets. Carrier robots would transport the samples around the facility. Credit: NASA/LAS

This is for just a less than 1 kg of samples returned most likely, yet you have to build something like this. And even then, it might not be sufficient.

Every Mars sample return study to date says at the end that their conclusions have to be reviewed continually, based on new research.

For the next study, whenever it is - well there is much active research at present into into a semi synthetic minimal living cell or an artificial minimal cell. Does the 40 nm size limit still apply based on the recent research? In the Programmable Artificial Cell Evolution project, the smallest artificial minimal cells were as small as 103 atoms, based on PNA instead of DNA, making it possible to simulate the whole cell as a quantum mechanical system in a computer. These “cells” were just a few nanometers across.

Also there's much more work been done on possible XNA based life, and I would expect that to feature more in a new study than in previous ones.

And none of the studies to date address issues of human error, accidents, terrorism, a plane crashing into the facility etc. The studies done to date mention these issues, but only to say that these issues were not part of their remit.

And then after all this work - we might find that the sample receiving facility wasn't even needed. The samples returned might be completely harmless.

It seems a back to front way of proceeding to me. Wouldn't it be better to first characterize the sample before we return it? Then design the facility around the samples once we know what they are?

For more about this see my Need For Caution For An Early Mars Sample Return - Opinion Piece

LEGAL COMPLEXITIES

Margaret Race covered these in an excellent paper, there’s far more to it than you’d think.

Back in days of Apollo, then the quarantine rules for the Apollo 11 return were only published on the day that they launched to the Moon, giving no opportunity at all for comment or peer review. That would simply not be permitted today. Also the Apollo regulations have lapsed BTW.

Also, there are many domestic and international regulations to be negotiated and new laws to be passed. She considered the whole process likely to take ten years or more, and it can also potentially involve the domestic laws of nations that are not receiving the sample, because of the potential effect of the worst case scenario could impact on all nations.

See Planetary Protection, Legal Ambiguity, and the Decision Making Process for Mars Sample Return

IDEA OF A PRECURSOR GEOLOGICAL TECHNOLOGY DEMO SAMPLE RETURN

If it is really, as the exobiologists suggested in that white paper, most likely to be a technology demo and a geology mission - perhaps they would be better off just presenting it as such, similarly to the Indian MOM mission? Perhaps this idea can be used to simplify the situation?

Suppose that a technology demo could be designed in such a way that it is safe for the Earth, no biological precautions needed, and yet is able to provide material for geologists, and also still has the opportunity to test for biosignatures on the remote chance that they do return samples of interest for exobiologists?

If you think about it that way, it is hard to justify such a huge expense for the USA on its own in isolation. But it can be justified if it can work in parallel with the ExoMars and its successors.

While the ESA continues with the in situ search for life on Mars, NASA can continue developing technology that will be used to return samples from Mars once we know where to look. Though the first samples returned would be likely to be of geological interest only, still, this would develop the technology necessary later on for astrobiologists.

Then some decades into the future, if or when interesting signs of life are found in situ, the technology will be available and tested. Meanwhile it will also answer many questions of interest to geologists

This though still leaves the problem of the potential for back contamination of the Earth. Even treating it as a technology demo, and the samples as of mainly geological interest, how can we return them in a way that keeps Earth safe from potentially unknown biology in the sample.

FIRST IDEA - SAMPLE RETURN WITH STERILIZED SAMPLE

You can remove the risk of planetary contamination of the Earth almost entirely if you sterilize the sample first before return to Earth. That's a controversial suggestion of course, I'll go into its merits and possible disadvantages in detail in a moment. But it would be a huge cost saving for them if you did that, save hundreds of millions of dollars, which I think could be better spent in other areas at this stage.

There's been a lot of research into signatures that would be preserved even after the equivalent of hundreds of millions of years of Mars surface ionizing radiation. The geology, and any past life would be likely to be hardly affected at all, as it has probably had huge doses cosmic radiation already.

If you are so lucky that you find life in the very first samples returned, then the signatures will still be present, and you can then do a follow up mission to find out more. It would probably be easy to find the funding for it in that situation.

The question here would be, how much sterilization to do.

The most radioresistant microbe currently known is Thermococcus gammatolerans.

Thermococcus gammatolerans - an obligate aerobe from hydrothermal vents, the most radioresistant organism known, able to withstand 30,000 Grays of gamma radiation, and still reproduce. That's about 400 thousand years (30,000/0.076) worth of surface radiation on Mars at the radiation levels detected by Curiosity during the current solar maximum. Lifeforms as resistant as this can probably survive at least that long on Mars in dormant form

That's astonishing, it's DNA must be cut up into a myriad small segments. Yet somehow it is able to re-assemble it and continue living, and then reproduce. But the thing is - these microbes are not even adapted to be resistant to ionizing radiation. It seems to be a side effect of their resistance to other adverse conditions.

Mars life, adapted to conditions of ionizing radiation may be even more radiation resistant. How much radiation would be enough? A million Grays?

Of course this idea would need to be examined carefully. First of all to make sure that the ionizing radiation would be sufficient to sterilize the sample. And also, to make sure that biosignatures would be preserved so that the life could still be detected.

SAMPLE RETURN USING FALCON HEAVY

The actual sample return itself could, perhaps, be hugely reduced in cost using the SpaceX spacecraft. This was a recent suggestion in the news. This study was done by NASA Ames researchers, not SpaceX.

Project 'Red Dragon': Mars Sample-Return Mission Could Launch in 2022 with SpaceX Capsule

This gives an overview of how they would do it:

SECOND IDEA - UNSTERILIZED SAMPLE RETURN TO THE EARTH MOON SYSTEM, BUT SAMPLE REMAINS IN ORBIT

The other approach is to return it to the Earth Moon system, but keep the sample in orbit. And - this is the new idea - to study it by telepresence only. More on that in a minute, but meanwhile some background.

Most ideas of this nature return it to a manned facility in LEO like the early Antares idea - with the idea that you get some level of quarantine by handling it in orbit, rather than on the surface.

WHY HUMAN QUARANTINE IS A REALLY BAD IDEA

I'd argue strongly against quarantine in a human occupied facility. I think that this is a really really bad idea. Let me explain.

• First humans aren't the only creatures we need to be concerned about. We can’t take all the organisms in Earth ecosystems into space for testing to see what the XNA life does to them.
• What Carl Sagan caused the “Vexing question of the latency period”. Leprosy has a latency period of up to several decades. Any effects of XNA based life on either humans or other organisms might not manifest for decades
• The life might be dormant and need special conditions to wake up before it does anything, and these conditions may not be present in the facility.
• Microbes evolve quickly. It may start off not well adapted to Earth life, just able to survive, and then at some point it adapts and then becomes a problem, possibly years later, through changes in gene expression, or through exchange of GTAs with Earth life (indeed it might even be an Earth lifeform that becomes the problem, using capabilities transferred to it from Martian life).

As a simple example, suppose for instance that Mars has life that has a more efficient metabolism than Earth life. Just a few percent more efficient.

Or suppose that the life uses photosynthesis, but has a third method of photosynthesis not yet explored by Earth life?

Salt ponds in San Francisco bay, pink and red with Haloarchaea (salt loving bacteria, the same ones that turn the Red Sea red). These photosynthesize using bacteriarhodopsin, which is what gives them their pink coloration. The light sensitive cells in our eyes use rhodopsin in a similar process. This method of photosynthesis doesn't generate oxygen or fixate carbon but converts light directly into an electrical potential which the lifeform uses for energy.

We have only two essentially different methods of photosynthesis on Earth. What if Mars life has evolved a third form of photosynthesis? What would it to do the Earth to introduce a new lifeform which uses a different method of photosynthesis? What if it is slightly better at photosynthesis than any Earth lifeform?

Mars life might be better than Earth life at metabolism, or at photosynthesis. But this advantage might not manifest right away. Perhaps for decades it is a rare, inoffensive microbe that just manages to get by, in the soil maybe a bit like radiodurans. It's not adapted to Earth and so to start with it finds conditions here too warm, or it can't face the competition - all the reasons people suggest that Mars life is not likely to be a problem.

But then - something flips. Some gene changes expression. Or it exchanges a GTA fragment with Earth life. Or it just evolves. Or even, that the life doesn't change at all, it just "flips" into a new state due to external conditions.

On Earth, harmless seeming short horned grasshoppers in the Acrididae family, for many years, causes no problems to anyone:

But then for reasons little understood, changing conditions, they turn into this

You could observe locusts for decades in an orbital laboratory and never once suspect that it is capable of forming devastating hordes of locusts like this.

Of course no chance of locusts on Mars, they couldn't survive there. But it may serve as an analogy for those lifeforms that might at some point "flip", and turn out to be better at photosynthesis, or metabolism, or in some other way better than Earth life.

Or maybe nothing flips. Maybe it is just a very slow exponential growth. Like the green curve in this diagram.

Suppose those figures in the x axis are decades, the y axis shows population, and this is the exponential growth curve for a photsynthetic XNA based lifeform in our oceans. You may notice nothing for three or four decades, it's an insignificant microbe in low populations. But, even if it is just a fraction of a percent better than Earth life, then eventually, the bulk of the photosynthetic life in the ocean may be XNA based. And, since it's not DNA based life, it could be inedible to Earth based life, or indeed produce chemicals that are toxic to Earth life.

In all this - not at all saying that any of this is likely. Just it's a possibility and we have to look at all the possibilities for something unprecedented such as potential return of life to Earth from another planet.

For that matter, what would you do anyway if the astronauts handling the samples became seriously ill? That was a major flaw in the Apollo quarantine plan I think.

Of course you'd return them to Earth right away. There would be an outcry if they were left to die in orbit, just on the off chance that they might be ill because of the influence of the Mars samples they are handling. It's also probably legally and ethically impossible to do something like this, even if the astronauts agreed to it voluntarily.

What do you think NASA would have done if the lunar astronauts became ill in the quarantine facilities, especially if the illness seemed life threatening? Would they have just left them to die? They could never have done that.

So - I don't think quarantine is the way ahead, not in a human occupied spacecraft. It's little more than a token measure, unless we know what it is that we are returning first, and tailor the quarantine measures to a known risk.

But what if humans, and other lifeforms, never go near the facility?

RETURN TO EARTH MOON SYSTEM - BUT STUDY BY TELEPRESENCE

The idea here is, you return a small piece of Mars to a safe orbit close to Earth - so that your spacecraft can get there in days or even hours, rather than months or years. Once there in a stable orbit, we may enclose it in a larger “holding” spacecraft which is the nucleus of our later telepresence laboratory.

Then anyone can send their own robotic missions up there to study it. It could be an international endeavour, like the ISS, but telerobotic rather than human occupied.

By then probably we will be using telerobotics routinely for repairing satellites. Dextre is gradually step by step working towards satellite repair capabilities, as is Darpa, involved in an initiative for repair of civilian spacecraft in GEO And by then with heavy lift we’ll be able to send hundreds of tons into HEO. And telerobotics surely much advanced. We already have telerobotic surgery as routine.

FIGURE 1: The Da Vinci Telerobotic Surgical System permits the surgeon to perform an operation on a patient from a remote site. Currently, the FDA requires the surgeon to sit physically in the same room as the patient on whom he is operating. The first ever trans-continental telerobotic surgery was done in 2001, when a patient in France was operated on by a surgeon in the US in order to test the feasibility of intercontinental operation – a successful operation to remove a gall bladder. (Surgeons were at hand in France to step in and halt proceedings if needed, but they were not required)

That should completely safe for planetary protection so long as material is only transferred one way. And if we make a good choice of orbit, there is no chance of impact with Earth. So that then would simplify everything. No need to convince general public and ourselves that we have everything covered and that nothing can possibly escape containment.

If this idea is accepted, where is the best place to return a sample in the Earth - Moon system?

WHAT ABOUT L1 AND L2?

The usual suggestion here is to return it to L1 or L2. I thought the same at first.

But with L1 or L2 it could potentially spiral out and then after flybys of Earth and Moon leave the Earth and enter independent orbit around the sun and eventually return and impact Earth.

To understand this discussion we need to know about the two types of Lagrange point in vicinity of the Earth.

In this diagram (not to scale) EL1 and EL2 are the two Earth lagrange points which balance the gravity of the Earth and the sun. For animations to see how this works, see What are the Lagrange points (ESA). A spacecraft can stay in these places with very little use of fuel - but they are unstable equilibria, so it has to do "station keeping". Without continual small thrusts, it will eventually depart from them, perhaps into an independent orbit around the sun, over a timescale of months.

LL1 and LL2 are the lunar Lagrange points. A spacecraft can hover there, similarly, by balancing the forces of the Moon and the Earth. Again, it will drift away on the scale of months unless it does continual small thrusts for station keeping.

Now, just by a coincidence, the Earth and the Lunar lagrange points are at almost exactly the same gravitational potential. You can get from one to the other with almost zero thrust.

This means that a spacecraft at LL1 could end up diverting to EL1 and then escape the Earth Moon system altogether. For more about this see Lagrange and the Interplanetary Superhighway

Lagrange, the mathematician who discovered the "Lagrange points"

Using this method spacecraft can do ultra low energy trajectories to anywhere in the solar system, so long as they can get as far as LL1. This is an artist's impression of the trajectory used by the ISEE3 satellite (launched in 1978) to visit the Earth's (not the Moon's) L1 and L2 points, and eventually also two comets (again not to scale).

So the lunar L1 and L2 points require constant station keeping. Leave the satellite unattended for some months and it may end up anywhere. Including going into independent orbit around the sun and returning to impact Earth.

On the other hand it does mean that a spaceship from Mars could use this low energy trajectory to get down to the lunar LL1 from EL1 without ever needing to do a flyby of Earth.

The LL1 or LL2 points seem a great place to return the sample to first. But they may not be the best place to leave it permanently.

LUNAR RETROGRADE ORBIT

NASA has rather similar requirements for its asteroid redirect mission. In that case, one plan is to return to a lunar retrograde orbit.

As the name suggests, this orbits the Moon clockwise (as seen from the North) - in the opposite direction to the rotation of the Moon around Earth. Though it continues to orbit the Earth in the normal prograde, or anti-clockwise sense. It can orbit above or below the lunar L1 and L2 positions. This shows an example lunar retrograde orbit, along with other orbits for comparision.

As seen from the Earth - this is just an elongated elliptical orbit around the Earth in the same direction as the Moon, and with the same period. Sometimes it is closer to Earth and orbits faster than the Moon. At other times it is further from Earth than the Moon and orbits more slowly, so lags behind. The combined effect of all these motions is that, when seen from the Moon, it orbits in a retrograde direction.

This is a much more stable position than the L1 or L2 positions. Once the sample is in a lunar retrograde orbit, you can relax, it is safe there for at least a century or so. But it may not be long term stable. At this stage we have to treat it as potentially extremely hazardous, even if the chance that it is hazardous is tiny. So, better to have it in a complely stable orbit.

FROZEN LUNAR ORBITS AND LUNAR POLAR ORBITS

One possibility is to put it into a so called "frozen" lunar orbit.

Because of the mass concentrations, low orbits around the Moon are unstable. Satellites in most of these orbits will impact the Moon, often within a month or so. That's because the tugs of the mass concentrations move the satellites into increasingly elliptical orbits until the lowest points fall within the Moon itself.

However, if you get the inclination just right, then the satellite will orbit indefinitely.

These are the orbits used for lunar mapping. However - they are still maybe not totally stable, and it's also inconvenient to get to from Earth. So just mentioning this for completeness.

LEO

This is the most convenient of course. But you can't leave anything here long term. Satellites will fall to the Earth over timescales of years or decades.

We have to contain potentially nanoscale particles. And many meteorites survive almost intact to the surface. So we can't rely on a fireball entry to burn up our sample and sterilize it. LEO is surely ruled out for safety reasons.

GEOSTATIONARY ORBIT

This is the most stable orbit of all. There is almost no atmosphere. In any case any atmosphere would be co-orbiting with the satellite. The main things you'd need to consider are solar wind, light pressure and so forth. But the LL1 point requires a delta v of 1.38 km per second. As for an impact on the Earth, forget about it, it is 4.33 km / second to get down to LEO where you would then spiral down into the atmosphere through atmospheric drag.

The only problem with GEO is that it is full of satellites. Our spacecraft wouldn't be welcome there and there would be a risk of other satellites hitting it also.

GRAVEYARD ORBIT

The graveyard orbit is the place where satellites from GEO go at the end of their lifetime (if possible). GEO is far too high for satellites to de-orbit to Earth easily.

So instead, they are moved upwards a little further, into the graveyard orbit. They are required to be at least 235 km above GEO, plus some additional tolerance to allow for radiation pressure effects. There 200 km is to allow for maneuvers in GEO and 35 km for gravitational perturbations of the Moon and sun.

However - this doesn't seem the best place for our satellite, because - potentially - it might impact with other satellites in the graveyard orbit.

So instead - what about say a 40,000 or 30,000 km orbit - 5,000 km either above or below GEO. Either of those would be as stable as the graveyard orbit - but well out of the way of any other satellites, in an orbit that's not likely to be needed by anything else.

Once there, it is safe for millennia. Even if for some reason humanity abandons spaceflight, we need have no concerns about it at all. It can't impact Earth or escape the Earth / Moon system because the required delta v is too large.

At least - seems so to me. Of course any decision of this magnitude would need to have a large scale review. This is just a suggestion for discussion.

Anyone see any problems with this? Or if not, what do you think?

IN SITU SEARCH FOR LIFE

Meanwhile though, I expect that the main advances in astrobiology for Mars would come from in situ search rather than sample return - at least unless Curiosity's successor, or Curiosity itself is amazingly lucky and finds life already in the first samples returned.

If this is right, perhaps ExoMars will be first to find evidence of life on Mars. Or if not it, one of its successors.

There are many instruments developed for Mars.

• Urey, originally for ExoMars, or astrobionibbler, with liquid extraction using supercritical water. Able to study the organics relatively unmodified
• Planetary In-situ Capillary Electrophoresis System “lab on a chip” – separate the organics by ionic mobility, by electrokinetic methods in sub millimeter capilliaries with the fluid manipulations done within the chip itself.
• LDChip and Solid3, using monoclonal antibodies to detect organics. These detect a wide range of organics not specific to DNA based life.
• Microbial fuel cells, where you check for redox reactions directly by measuring the electrons and protons they liberate. This is sensitive to small numbers of microbes and has the advantage it could detect life even if not based on carbon or any form of conventional chemistry we know of.
• Levin’s idea of chiral labelled release, where he has refined it so you feed the medium with a chiral solution with only one isomer of each amino acid, and if the CO2 is given off only with one isomer and not the other, that would be a reasonably strong indication of life there.This has the advantage that the life just needs to be organics metabolizing and to produce a waste gas that contains carbon.
• Miniaturized scanning electron microscope. Can’t detect whether it is life or not, but useful along with the others.
• Miniaturized DNA sequencer could work if we had a common ancestor that was introduced to both Enceladus and Earth in the very early solar system, is in a reasonably advanced state they say could be made space ready by 2018.
• Raman spectroscopy and gas chromatography of course and using chiral media for those (the Philae lander, if it wakes up again, will be first spacecraft to search for chirality by this method with the COSAC gas chromatography experiment - though of course it is looking for life precursors. ExoMars will also use this method).

There are many instruments like this we could send, and several of them already space qualified but never flown.

Working together in this way, with samples in HEO - perhaps mainly of geological interest to start with - and simultaneously in situ missions to Mars - eventually we may find life on Mars, study it in situ, and then return it to facilities in the Earth Moon system where we can study it via telepresence as well.