Mars and the search for traces of life- interviews

(ESA) – The European Mars Science & Exploration Conference: Mars Express and ExoMars has just concluded. We present interviews with selected experts on some hot topics.

Jean-Pierre Bibring
Mars Express OMEGA Principal Investigator and ExoMars Microscope co-Team Coordinator, Institut d’Astrophysique Spatiale, France

Subjects: Mars mineralogy and surface water evolution

As the person responsible for the mineralogical mapping spectrometer on board Mars Express, you have been actively involved in Martian science for many years. Which one of your team’s discoveries do you consider most important?

OMEGA has profoundly revised the history of Mars, on the basis of minerals identified at its surface, which record the evolution of its environment.

My answer to your question about what I consider most important would be the discovery, identification and mapping of hydrated phyllosilicates (clays) a specific family of minerals formed by aqueous alteration. By finding and studying them, OMEGA has opened a window in ancient Mars History during which Mars might have harboured a key ingredient for habitability: stable liquid water.

Does this relate to the search for traces of life on Mars, and how?

Yes it does. It indicates where one should search for potential bio-relics.

If life ever emerged on Mars, it likely required liquid water to be stable for long durations. The discovery of hydrated phyllosilicates in the most ancient terrain indicates that such conditions might have occurred. In the search for sites and samples that have preserved this past record: phyllosilicate-rich areas are favoured targets for future in-situ missions. Potential bio-relics would be associated with such minerals.

As team coordinator of the MicrOmega instrument on board ExoMars, what are your expectations for the future?

Supposing ExoMars lands in a phyllosilicate-rich area, that will be extremely exciting. MicrOmega would be able to analyse, at microscopic scales, the structure and composition of samples of key relevance to astrobiology. It would study the association of phyllosilicates and other phases such as carbonates, on which carbon-rich molecules might have grown.

If life ever started on Mars, MicrOmega would be in position to identify bio-relics and their mineralogical environment, either or both through their structure and molecular composition.

Agustin Chicarro
Mars Express Project Scientist, ESA-ESTEC, Netherlands

Subjects: Mars Express mission and science overview, Mars geology and planetary evolution

As a geologist and an expert in rocky planets, what in your opinion makes Mars so interesting?

When it comes to landscape, geology, climate and the possibility of life having appeared and evolved, Mars is the planet most similar to Earth.

It is also the only planet that humans can even dream of exploring on foot. The profound knowledge on Mars, Venus and Earth is essential to determine the planetary evolution and habitability of terrestrial planets; also the origin of life.

What do we know today about Mars that we did not know before Mars Express was launched? What do you consider as the most unexpected and exciting results so far?

Mars Express has shown that volcanic and glacial activity on Mars has probably continued until the present.

It has helped establish a sequence of events with respect to the amount of liquid water on the surface, which was abundant in the early times of martian history and became more scarce as the planet turned colder.

Mars Express has discovered mid-latitude aurorae on Mars and has also confirmed the presence of methane. It has shown the existence of the solar wind scavenging mechanism, which penetrates its atmosphere and contributes to atmospheric escape.

What are the outstanding questions you would like to see addressed by future missions to the Red Planet?

Even after so many years of exploring Mars, key scientific questions remain unsolved, in particular the determination of the deep internal structure of Mars, the precise rotational parameters of the planet, the detailed meteorology, the absolute ages of rocks and soils on the surface, the link between atmospheric escape and paleomagnetic anomalies in martian crust.

To this end, ESA has started studying the Mars-NEXT mission under the Aurora Programme, which includes an orbiter and at least three landers on the surface that will constitute a network allowing a variety of simultaneous measurements.

Roberto Orosei
Roberto OROSEI, Ma_Miss infra-red spectrometer (ExoMars drill) Team, Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF), Italy

Subjects: Exomars drill operations & subsurface mineralogy

As an expert in planetary mineralogy, why is it important to drill the surface of Mars and look underneath?

Drilling into the Mars’ subsurface means both, accessing a different environment with respect to the surface physical, chemical and thermal conditions, and going back in time to the age when water may have flown on the Martian surface. That’s because many geological processes result in the deposition of more recently modified or new material (such as wind-blown dust, lava flows or impact debris) getting heaper on top of older material that had formed through different physical processes, under different climatic conditions.

Is it also important in search for traces of life?

Many organic molecules are unstable because of the weathering conditions on Mars – the intense UV radiation, together with large temperature swings and arid conditions. Because of this, the Martian surface is a very unfavourable place for current biological activity, and is also too harsh to have preserved traces of past life.

That’s why, in my opinion, the question whether or not life ever arose on Mars cannot be answered without access to the subsurface.

What are your expectations from the ExoMars mission?

As subsurface sounding radars such as MARSIS and SHARAD added a new dimension to the study of martian geology, so I believe that drilling will open a new world to the study of the biological environment on Mars in-situ.

As an example, let’s take the most important question of the origin of gases such as methane and formaldehyde, potentially produced by organic activity. These gases have been detected in the Martian atmosphere and they require some unseen source to compensate for destruction by UV radiation. For the answer, we must look into the subsurface.

John Parnell
ExoMars Life Marker Chip Science Team and Associate Scientist for ExoMars Raman-LIBS Team, University of Aberdeen, UK

Subjects: Exobiology and biomarkers

Could you explain to our readers what ‘biomarkers’ are?

Living organisms use a wide range of molecules for biological functions, such as sterols (solid alcohols including cholesterol, related to steroids). When organisms die and are buried in sediment, and eventually lithified into rock, these molecules are transformed into derivative molecules that may have long term stability that enables us to detect them during exploration of rocks. These are described as biomarkers (the sterols transform into stable biomarkers called steranes), as we can use them to infer an origin from life.

Can we expect that biomarkers left by living organisms on Earth are similar to those we may expect to find on Mars?

Inevitably, we are influenced by the biomarkers that characterize terrestrial life. However we believe that this is a reasonable approach. For life to exist, it would have to harness a supply of energy, and the way that energy is used then influences all the chemical/biochemical reactions that occur subsequently. The ways in which this might happen are limited, so some similarity with terrestrial organic geochemistry is likely. The non-terrestrial, non-biological molecules that we encounter in carbonaceous chondrite meteorites are of types that are familiar to us, even though some have detailed chemistry unknown on Earth. So it is likely that we would detect the molecules remaining from non-terrestrial life. We have an idea of how molecules formed by an inorganic mechanism should be distributed quantitatively. If we find an assemblage of molecules that are not in the distribution that we expect (i.e. they appear out of equilibrium), then we have something to explain, and the explanation could be biological activity.

With ExoMars, what exactly will you be looking for?

In the first instance we are looking for any traces of organic compounds. We know that some non-biological organic compounds should be introduced through the meteoritic material that lands on the martian surface (we see them in meteorites landing on Earth), but our failure to find them so far suggests that they are altered and/or destroyed at the surface. By looking in protected sites (at depth by drilling, or where recently excavated by natural processes, or sheltered somehow), we hope to find this component. That would be a major step forward. If we don’t find it, that is still a useful measurement that we need to explain. Once we find some organic molecules, we then need to examine their distribution in detail to decide if they show evidence of deviation from what is expected from the meteoritic component, which could reflect biological activity.

Jeffrey Plaut
Mars Express MARSIS co-Principal Investigator, Jet Propulsion Laboratory, USA

Subjects: Martian subsurface sounding, underground water inventory

The MARSIS radar on board Mars Express has marked history by being the first instrument to look beneath the surface of the Red Planet. As a co-responsible for it you must have thrilling expectations. Which were they when the mission was launched? What are they today, after MARSIS has started probing underground?

MARSIS is an unusual experiment in that no similar instrument has ever been sent to another planet. For this reason, we did not quite know what to expect. However, we had confidence that our instrument was capable and robust, and all that we needed for success was a ‘cooperative’ Mars. After two years of observations, we are thrilled with the results we have obtained.

Mars has been especially cooperative in the polar regions, where we have easily mapped the 3D distribution of the kilometres-thick ice caps. MARSIS signals also reveal subsurface materials elsewhere on the planet, and we expect to focus on additional regions as the experiment proceeds.

From what you have seen so far, is there any chance that liquid water is still present below the surface?

Most Mars scientists believe that liquid water is present below the surface. The questions are: how deep, where, and how much? So far MARSIS seems to be telling us that large aquifers are not common in shallow layers of the crust. This could mean that there is not much water (liquid or ice) in the crust, or that the crust is too cold to maintain liquid water near the surface. Alternatively, there may be shallow water that somehow has eluded detection by MARSIS. We are continuing to pursue this part of the investigation.

What would the implications of such a break-through be?

Confirmation of the presence of liquid water would of course provide us with a prime target for the search for life on Mars. Every watery environment on the Earth contains life. It would be fascinating to determine if this is the case on Mars too.

Jorge Vago
ExoMars Project Scientist, ESA-ESTEC, Netherlands

Subjects: ExoMars mission and science overview, Martian space environment

What makes ExoMars so special with respect to other past, current and even future missions to Mars?

ExoMars will carry the most complete suite of instruments dedicated to organic and mineralogical studies ever planned for a Mars mission. With its powerful rover and drill, ExoMars will also be the first mission combining mobility and access to subsurface locations where organic molecules may be well-preserved; thus allowing, for the first time, to investigate Mars’s third dimension: depth. This alone is a guarantee that ExoMars will break new scientific ground.

What are the major technological challenges the ExoMars mission has to overcome to achieve its objectives?

The first and most important challenge is to land safely on Mars. To achieve this, ESA is developing a novel landing technology based on a double parachute system, liquid throttled engines and a new type of airbag design.

Other key milestones will be the successful deployment of the rover onto the martian surface, and the utilisation of the subsurface drill and the sample preparation and distribution system (SPDS) in combination with the instruments.

What major criteria do you need to follow to select the final landing site?

The major scientific requirement for ExoMars candidate landing sites is that they show evidence of a past water-rich environment. This evidence must be both morphological (e.g. orbital images of deltas, lakes, or channel systems) and mineralogical (e.g. spectral signatures of minerals that form in water environment and that can preserve organic matter well, like clays, salts, etc.). The site must not contain a lot of dust, as dust is very poor for the preservation of biomarkers.

The other major requirement is that candidate sites must be safe to land on. There is a large number of engineering requirements that must be satisfied. They include altitude, latitude, terrain slopes, rock size and distribution, etc.

Finding a safe place to land that has a very high science interest will entail much work over a period spanning about four years.

How is Mars Express helping in the preparation of ExoMars?

Mars Express has helped immensely because it is a mission that has changed our understanding of many key processes on Mars. In particular, it has confirmed that Mars was a much more hospitable planet during the first of its 4.6 thousand million-year history. At that time, as life started on Earth, conditions on the red planet allowed water to exist on the surface.

Mars Express will also help to identify suitable candidate landing sites. Its instruments, particularly the HRSC camera and the OMEGA spectrometer, will be used to study sites in detail. Besides Mars Express, NASA’s MRO will also contribute to the detailed search for top landing locations.

Frances Westall
ExoMars Microscope co-Team Coordinator, CNRS, France

Subjects: Exobiology, origin of life during planetary history, extreme environments

Looking for signs of life outside the Earth is one the most exciting ventures mankind is undertaking. However, most planets in the Solar System seem pretty inhospitable. What make you think that Mars is – or was – hosting life?

Over the last few decades we have learnt an enormous amount about Mars and its habitability from orbital and in-situ studies, as well as the analyses of martian meteorites. It is clear that, early in its history, Mars had a lot more liquid water on its surface than at present, and that perhaps there existed a small ocean in the northern hemisphere of the planet. Liquid water is, however, only one of the ingredients necessary for maintaining life, the other two being: first a source of essential elements, for example carbon, hydrogen, oxygen, nitrogen, phosphorous and sulphur, and second energy.

Carbon would have been as abundant on early Mars as it was on Earth, coming both from internal mantle sources as well as external sources (carbonaceous meteorites, micrometeorites, comets). Although today, the lighter, more volatile carbon molecules are destroyed by oxidation and ionisation at the surface of the planet. Early on, when the planet still had a considerable carbondioxide atmosphere and a protective gravity field, this was not the case. The other elements were also abundant on the surface of the planet. Thus, early Mars was habitable.

But, just being habitable does not mean that a planet was inhabited. On the other hand, as far we understand at present, the environmental conditions on early Mars and early Earth were similar enough to hypothesise that life arose independently on Mars.

Could Mars still host life?

Conditions are not favourable for the long-term (or even short-term) existence of life at the surface of Mars right now. The lack of liquid water, plus the radiation and oxidising conditions are not conducive to life.

But, it is clear that at certain periods in the history of Mars, water did exist for slightly longer periods of time at the surface without completely and instantaneously subliming.

It is possible that, if life had evolved at an earlier epoch, if could be preserved in protected subsurface environments where water is in a liquid phase. Not all life forms need sunlight, but in fact, the most primitive life forms can exist on inorganic sources of carbon, obtaining their energy from chemical reactions taking place at the surfaces of minerals in the presence of liquid water.

Could this subterranean life come to the surface when water or ice is expelled intermittently from the subsurface reservoirs?

In such a scenario, it is possible that cells could survive for long enough to divide and metabolise at the planet’s surface.

Are the chances of finding present life comparable to those of finding signs of past life?

Given the inhospitable conditions at the surface of Mars today, it is unlikely that we will find traces of living cells on the surface, except, in the scenario I’ve just described, when subsurface cells are brought to the surface and they can survive for a brief period of time.

Again, life could still exist in protected subsurface aquifers, but such niches will be very heterogeneous. Moreover, the cells will have to metabolise and divide with sufficient rapidity to overcome the long term effects of mantle radiation. If the organisms are in a very small habitat over long periods of geological time (hundreds to thousands of millions of years), they run the risk of exhausting their sources of nutrients and/or energy and becoming extinct. Since the potential microhabitats of living cells will occur heterogeneously in the subsurface, looking for them will require good analysis of the subsurface and the areas where liquid water may exist. The prospective of finding present life is low, but we cannot exclude this possibility.

Certainly, if life did appear on Mars in its youth, we are more likely to find evidence of past life than present, simply because the environmental conditions conducive to life were far better before about 4.0 thousand million years ago. The process of fossilisation of microorganisms starts within 24 hours (although, not all microorganisms fossilise) and during the process, if there is no oxidation, the organic material making up the microorganism can become trapped in a mineral matrix. It is thus possible to find morphological and geochemical traces of life in rocks formed in environments that could have hosted life.

What kind of life form may we expect to find?

First, we have to assume that primitive life forms on Mars will have been carbon and water-based, as on Earth, given that the starting ingredients and environmental conditions were similar to those on Earth. Although we cannot exclude the possibility of other life forms based on different constituents, with the same starting ingredients, only the cellular, carbon and water-based life form became dominant in the competitive, Darwinian-style evolution on Earth. Presumably the same thing happened on Mars.

We therefore expect small, cellular life forms on Mars that used complex carbon molecules (the equivalent of proteins, sugars and lipids that characterise terrestrial life). The chirality (handedness) of these molecules will also be characteristic, although they may not be of the same chirality as the terrestrial organic molecules.

One of the characteristics of terrestrial microorganisms is that they are never isolated – they occur in colonies that may be loose or dense. Terrestrial microorganisms can live in sophisticated mixed-species communities that can be very large, sometimes forming mats that, at least on modern Earth, can cover some square kilometres on the floors of large lakes, or shallow lagoons and small basins. However, these complex communities are complex. They existed on early Earth, at least 3.5 thousand million years ago, but environmental conditions on early Earth at this stage were far better than those on Mars.

3.5 thousand million years ago, the surface of Mars was probably completely inhospitable to life. The question is: how much time did life on Mars take to evolve from the very first, most primitive cells? Did it have enough time to evolve into mat-forming communities (primarily based on photosynthetic organisms which are the primary producers for the community living in the mats)? If life ever did appear on Mars, studies of its evolutionary steps will greatly help us understand the early evolution of life on Earth (since terrestrial plate tectonic activity has eliminated the first billion years of usable rock record).

At the very minimum, we can expect very small, simple cells occurring in colonies in a mineral environment and having chemolithotrophic metabolisms (using inorganic sources of carbon and energy sources). Perhaps early life forms on Mars managed to evolve into mat-forming communities, in which case we may expect to find the lithified remains of these communities in the form of biolaminations in rocks.

Are there or were there other planets in the solar system that could (have) hosted life?

Certainly, Venus in its early history was very similar to the early earth and early Mars. It is therefore likely that, if life appeared on Earth and on Mars, it might have appeared on Venus.

Another possible contender is Europa, especially early in its history when its rocky mantle was warm and volcanically active. Now, heating of the planet comes through the tidal frictional forces induced by the gravitational field of Jupiter. If life did appear on the planet, early in its history, it could still possibly survive in the cold slush of melted ice beneath the ice crust of the planet.

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