1. THE SEARCH FOR ORGANIC MATTER ON MARS

Mars is Earth’s nearest planetary neighbour and it is unsurprising therefore that effort to find life outside the Earth focuses on the Red Planet. Yet Mars has environmental conditions that are distinct from the Earth. Mars is cold and dry with a thin atmosphere. Liquid water, the solvent of life, is not stable at the planet’s surface. Under Martian conditions water passes directly from ice to gas, a process termed sublimation. Moreover, the lack of a magnetic field ensures that the Martian surface is subjected to levels of radiation not generally seen on the Earth.

The search for life on Mars therefore presents particular challenges. The harsh conditions on Mars will ensure that any extant life will have adaptations similar to those found in extremophilic organisms on Earth. Extant life, if present, is also likely to exploit the subsurface environment which provides more equable conditions than those at the surface. The subsurface is also important because evidence of Mars life may be present in fossil form, entombed in sedimentary rocks. Conditions on Mars were more conducive to life in the early history of the Red Planet. Liquid water was once widespread on the Martian surface and these conditions coincided with the probable timing of the origin of life on Earth.

2. PRESERVATION OF ORGANIC MATTER ON MARS

Organic geochemical principles can be used to inform life search protocols. Organic geochemistry deals with the influence of life on rocks, including the production of organic matter, its preservation and modification in the subsurface, and the interpretation of the consequent organic records in sedimentary sequences. The preservation of life’s remains occurs through a complex series of processes involving degradation, selective preservation and adsorption on mineral surfaces, all of which depend on the physical and chemical environment of deposition.

The cold and dry climate of Mars has an effect on the preservation of organic matter. Amino acids for example suffer decarboxylation (removal of the acid functional group) rapidly under terrestrial conditions at 25 °C. Yet at colder temperatures destruction becomes exponentially slower. For instance, at 0 °C the half life for this process is between 0.8 and 1.1 Ga and at -20 °C the half life increases further to between 2.3 and 220 Ga (Aubrey et al. 2006). Notably this duration exceeds the age of Mars itself suggesting that in some cases remnants of life may survive better on Mars than Earth.

In chemical terms, however, life usually differs from fossil life quite dramatically and the search for fossil life on Mars must be cognisant of this fact. Extant life is dominated by macromolecules but not all of these entities or their sub-units resist degradation at the same rate. On Earth, within days to weeks, bacterial degradation removes delicate macromolecules such as nucleic acids and proteins. Simple organic molecules that are important energy providers such as amino acids, sugars and short-chained carboxylic acids are also lost on this timescale. Within months to years unresistant macromolecules such as polysaccharides are removed, leaving only the lipids to persist for periods of thousands to millions and even billions of years. It must be noted however, that the degradation rates mentioned reflect the presence of protein catalysts called enzymes which can accelerate the process by a factor of 10¹⁷ (Wolfenden et al. 1998). In the absence of enzymes, days become hundreds of years and years become millions of years. In either case, it is the lipids that represent the most likely organic matter to be encountered on Mars (Table 1).

3. THE LIPIDS OF MARS

With lipids representing the most stable molecules in sedimentary rocks it is appropriate to consider the information they could provide if encountered on Mars. Biological markers or “biomarkers” are lipids that represent a rich source of information on source organisms, environmental conditions and subsurface conditions following deposition. Even the simplest interpretation of lipid biomarkers would provide important information on Mars (e.g. the domain of life to which the source organism belonged).

All known living organisms have lipid membranes. Table 2 list the characteristic membrane molecules found in organisms of the three domains. These molecules have additional units attached at their polar ends (e.g. phosphates or sugars). Isoprenoids, hopanoids and steroids have similar biosynthetic pathways and represent progressive evolutionary adaptation. The Archaea have isoprenoidal structures, bacterial membranes have fatty acid layers with hopanoids that help modify membrane rigidity. Eukaryotic membranes contain steroids in the place of hopanoids. Importantly, the biosynthesis of steroids requires aerobic conditions (Summons et al. 2006). Such conditions never existed on Mars so it would appear unlikely that steroids will be part of any inventory of biomarkers in Martian rocks.

Table 2. Lipid biomarkers of the three domains of life.

4. THE SIZE FRACTIONS TO SAMPLE

Preparation of rock or soil samples for analysis on Mars may require some form of drilling, crushing, grinding or sieving (e.g. Beaty et al. 2005). These important first steps must be performed carefully so as to not introduce any sample bias or to render the samples unsuitable for organic detection. Terrestrial procedures provide a valuable insight into the effects of preparation steps but fundamental differences are likely to exist between organic matter on Earth and any counterpart on Mars.

Organic matter in terrestrial rocks is distributed across a number of size fractions. The relative contributions of particular organisms to these size fractions is well known in palynology where samples are screened to isolate the larger, most information rich and morphologically recognizable organic entities. Inevitably these are Eukaryotic organisms and Bacteria and Archaea rarely produce palynomorphs (Traverse 2007).

Microscopic Eukaryotes, Bacteria and Archaea contribute to what is called “amorphous” organic matter which resides in the smallest size fractions in rocks and is often removed in palynological preparation steps. Organic geochemical analysis of palynological size fractions has revealed the chemical consequences of screening by size with Eukaryotic remains, such as land plant debris, dominating the larger size fractions (Sephton et al. 2005).

Bacterial and Archaeal signals are the most likely responses from organic investigations on Mars. These organisms have diameters between 500 nm and 2 μm. Their small size ensures that their biochemistry is simple with cellular processes that are not compartmentalized into organelles. This lack of complex parts provides benefits in extreme environments because there is little to fail under environmental stress. More complex organisms are most likely precluded by the extreme conditions on Mars. Martian organic signals therefore will be derived from the finest, i.e. in practice <10 μm, size fractions of Martian rocks.

5. THE MINERALS OF MARS

The OMEGA/Mars Express imaging spectrometer has recognized mineralogically and temporally-distinct areas on Mars (Bibring et al. 2006). Some of the most ancient rocks on Mars contain phyllosilicates produced during early wet conditions on the Red Planet. Less ancient rocks contain sulphates generated by aqueous conditions that were markedly acidic. The least ancient deposits are characterized by ferric oxides in a liquid water free environment. The three types of deposit have been used to define three sequential eras on Mars, namely the "phyllosian", the "theiikian" and the "siderikian" eras (Bibring et al. 2006). Each era reflects distinct environmental conditions that would have had a major control on the probability of life. The earliest liquid-water rich era would represent the most habitable conditions and the latest anhydrous era the least habitable.

The different rock types from the three eras will also affect how any organic matter may be preserved. Terrestrial organic-inorganic relationships provide hints as to how such associations may occur on Mars. Ninety percent of Earth’s organic matter accumulates in coastal margins and is intimately associated with mineral surfaces. Studies suggest that organic contents are directly related to mineral surface area and imply monolayer coatings that provide mean values of 0.86 mg C_org m^-2 (Meyer 1994). Once adsorbed organic matter is relatively sheltered from degradation and this process is so efficient that 80% of organic matter on minerals can be irreversibly adsorbed (Hedges and Keil 1995). If monolayer adsorption is in operation on Mars then the three types of deposit will be characterized by differing preserved organic contents.

Surface areas of Martian mineral analogues were calculated by Pommerol et al. (2009) and the data can be used to predict monolayer organic contents (Table 3). It is clear that phyllosilicates are particularly important minerals for the entombment and preservation of organic matter. Phyllosilicate formation requires water and therefore conditions that are conducive to life. Moreover, phyllosilicates and organic matter are hydrodynamic equivalents indicating that they settle from water at the same locations. Once juxtaposed, the high surface areas of phyllosilicates provide ample sites for organic adsorption. Hence, phyllosilicates provide a combination of relatively promising opportunities for life and a high probability for the preservation of its remains. Such deposits therefore are important targets for life search missions.

Table 3. Minerals

6. MARS METHANE

In 1965 James Lovelock suggested that life produces atmospheric gases that are incompatible on a long term basis (Lovelock, 1965). The apparent absence of a non-equilibrium situation in the Martian atmosphere was a setback for life detection studies. Yet recently just such a non-equilibrium system has been recognized and methane has been detected in the atmosphere of Mars. Earth based telescope observations reveal an average concentration of 10±3 ppbv (Krasnopolsky et al., 2004), while the Planetary Fourier Spectrometer on the orbiting Mars Express spacecraft provides concordant data at 10±5 ppbv (Formisano et al., 2004) and 14±5 ppbv (Geminale et al., 2008).

Calculations for the lifetime of methane in the Martian atmosphere suggest durations of approximately 300–600 yr (e.g. Atreya et al., 2007). The annual resupply of methane needed to maintain the Martian atmospheric abundance is approximately 126 tonnes if estimated methane loss near the surface is considered (Atreya et al., 2007; Formisano et al., 2004), and 270 tonnes if the estimate is widened to accommodate methane loss throughout the entire atmospheric column (Krasnopolsky et al., 2004).

Potential sources of Martian methane include life (Krasnopolsky et al., 2004), serpentinization of ultramafic crust (Atreya et al., 2007; Oze and Sharma, 2005), recent volcanism (Neukum et al., 2004) and ablating meteorites/comets (Court and Sephton 2009). Volcanism appears unlikely to be a major contributor because of the absence of hot spots recognised by the thermal emission imaging system (THEMIS) on the Mars Odyssey orbiter (Christensen, 2003) and the paucity of the gaseous consort of volcanic methane, sulfur dioxide, as implied by spectroscopic investigations (Krasnopolsky, 2005). Methane has been measured in carbonaceous chondrites at only 0.14 ppm (Butterworth et al., 2004) but much more could be generated by degradation of the gas prone macromolecular organic fraction that these objects contain at percentage levels. Recent experimental studies, however, have demonstrated that meteorite or cometary addition and ablation is also an implausible source of Martian methane producing only 10 kg of the required 270 tonnes each year (Court and Sephton 2009).

7. SUBSURFACE METHANE ON MARS

With the direct injection of methane into the Martian atmosphere, either by meteorite/comet ablation or volcanic activity ruled out, it becomes seemingly unavoidable that the methane is sourced from the subsurface. If the source is biological and represents a form of methanogenic microbe then these subsurface sites are important scientific targets.

If the source of methane is serpentinisation then opportunities for other forms of life exist in near surface sites and their scientific importance remains high. Serpentinisation occurs at temperatures of 40–90 °C which are believed to occur at a depth of 2 km on Mars, with liquid water being stable at depths of 2–20 km (Oze and Sharma, 2005; Atreya et al., 2007). Any abiotic methane derived from serpentinisation must rise through at least 2 km of Martian material to reach the atmosphere and en route to the surface the methane would encounter oxidants including sulphates and ferric iron.

This redox situation presents metabolic opportunities for life similar to those observed around methane seeps on Earth where anaerobic methanotrophic Archaea gain energy exclusively from the anaerobic oxidation of methane with sulfate as the final electron acceptor (Hinrichs et al. 1999). Microbial utilization of methane is not inconsistent with its passage to the atmosphere and in terrestrial seep analogues not all methane is removed by the microbial community and up to 80% of methane can escape to the atmosphere (Wegener et al. 2008).

Upwardly percolating subsurface methane would also have other effects on Mars. Organic matter is hydrogen-rich and as such is preserved best in oxygen free environments. On Earth, aerobic respiration occurs rapidly in oxygen-rich surface environments. Aqueous environments are more conducive to anoxia and are often the sites of enhanced organic preservation when decomposing organic matter exhausts the supply of oxygen. Methane sourced in the Martian subsurface may provide similar opportunities for enhanced organic preservation. The upward percolation of methane through the Martian subsurface would promote more reduced conditions in which organic matter is protected from degradation.

8. RADIATION EFFECTS ON MARS

Solar energetic particles and Galactic cosmic rays impinge on the Martian surface owing to the lack of a magnetic field which would shield the planet from radiation. The harsh conditions on the Martian surface ensure that the search for life on Mars is beginning to focus on the subsurface. Organic matter is transformed by radiation but at depths below two meters the radiation flux is substantially attenuated and preserved material may be encountered (Kminek & Bada 2005). Yet transformation does not always mean destruction and radiation can immobilize organic compounds providing records of past organic fluxes.

New insights into the effects of radiation chemistry in organic matter-containing rocks have been provided by studies of terrestrial organic residues that appear to have formed from the trapping of methane around radiation-releasing mineral grains that contain small amounts of uranium and thorium (Court et al. 2006). Radiation energy cleaves molecules, the products of which are highly reactive and readily combine to generate new, more complex compounds. For passing methane, progressive radiolytic alteration produces an organic solid comprising of larger organic units. Unsurprisingly it is the highly stable polycyclic aromatic hydrocarbons that dominate the resulting intractable organic material. Such organic deposits may be present in the very near subsurface above methane producing regions on Mars and their extent could indicate the duration or magnitude of methane generated through time.

In addition to polymerizing methane in the Martian subsurface, radiation could also produce de novo abiotic organic matter. The work of Calvin and co-workers (Garrison et al. 1951) revealed that high energy particle radiation reflecting cosmic rays or radioactive radiation can produce low yields of organic compounds from carbon dioxide and water. Existing knowledge of the Red Planet and data from the Neutron Spectrometer instrument of the Mars Odyssey spacecraft’s Gamma Ray Spectrometer instrument suite imply that all three of these components are present at high latitudes on Mars (Mellon et al. 2004) and could provide an additional, small but continuous, source of non-biological organic matter in the near subsurface of Mars. Radiation, it appears, may be an important generator of relatively accessible non-biological organic matter on Mars.

9. TESTING THE HYPOTHESES

The European Space Agency (ESA) is planning the ExoMars mission for robotic in situ analyses of Martian soil for the presence of life’s chemicals. Current proposals are for a drill to collect samples of Martian soil at certain depths. Access to the near subsurface of Mars initiates a number of newly testable hypotheses as described above. Results from the near subsurface provided by ExoMars may reveal an organic record containing evidence of past or present life, mineral-organic interactions, radiation-induced polymerization, or combinations of all three.