Evidence of past life on Mars in ALH84001

Blue Flower

Details: Written by Stephen Parker; Category: Space Exploration; Published: 13 June 2016; Hits: 4149

In a seminal paper by David McKay and coworkers in 1996[1], it was proposed that a Martian meteorite – ALH84001[2] - contained evidence of biological activity on Mars. The ramifications of this proposal were enormous – the first possible evidence of life outside Earth, and accordingly McKay’s findings (and the meteorite itself) have been subject to great scrutiny and debate. This paper will attempt to outline the evidence behind this conclusion, and the key supporting arguments, as well as counter-arguments against ALH84001 showing evidence of Martian life.

ALH84001

ALH84001 is a 1.93 kg meteorite, formed about 4.5 billion years ago, that is believed to have been ejected from Mars roughly 16 million years ago via a collision event[3], and collided with Earth 13,000 years ago, landing in Antarctica[4]. It Martian origins are not disputed –gas mixtures captured in pockets within the meteorite match those of Mars, and are very different from Earth and other solar system sources. Evidence exists for a shock event (most likely a meteor or comet impact nearby) 4 billion years ago, which created cracks in the rock, and crucially, it is believed that the rock encountered a liquid flow, possibly 3.6 billion years ago. This interaction was marked by the deposit of rounded carbonate globules within the cracks[6]. These globules offer the potential evidence of biological activity[7]. ahl_image1 Figure 1. The famous rock itself.[5]

Evidence for biological activity

Before examining the arguments made by McKay and others, it is important to stress that at no stage has any direct claim of the discovery of Martian life been made. Rather, the arguments focus on evidence consistent with the operation of biological processes. To this end, four non-specific requirements exist before any case may be made.

The meteorite is from Mars. Gas mixture samples provide overwhelming evidence to support this[8].
The meteorite encountered liquid water on Mars. Liquid water is a necessary pre-requisite for biological processes, and if it cannot be shown that the meteorite had encounter Martian water, then any arguments for biological activity would be seriously undermined. The absence of water-bearing minerals[9] raises some questions here, but does not preclude the presence of water. Studies of Martian landform are highly suggestive of significant quantities of surface water in the past[10].
The carbon globules were formed in a temperature range conducive to life. If the globules were produced in a high-temperature environment (say > 200° C), this would essentially preclude biological activity. Several cases have been presented arguing that the globules were formed in a high-temperature environment[11], but most studies argue for a temperature compatible with biological processes[12].
Contamination, both at the impact site (and over subsequent millennia prior to discovery), and in the laboratory (through handing and preparation) must be ruled out as the source of observed phenomenon put forward as evidence of life. Potential results created by contamination will be discussed where pertinent.

Evidence showing the carbon globules were deposited early in Mars’ development are also positive, as it is believed that a much warmer climate, more conducive to the development of life, existed at that time. However, this is not a critical requirement. Beyond these base requirements, the McKay work focused on three main pieces of evidence supporting biological process:

1. Microscopic features resembling terrestrial bacteria

Electron microscope images of the carbon globules in ALH84001 showed an “irregular, grainy texture”[13], with oval, elongated, and cylindrical structures in fractures in the carbonate mineral globules. McKay compares them to known nano-bacteria on Earth, in appearance and dimensions. Although significant work has been done to rule out contamination by preparation materials (such as the gold and palladium plating used in electron microscopic analysis), this is the least compelling of the arguments, as the lower range (20 nm) is far below any known terrestrial bacteria size (and several magnitudes smaller than ancient fossilised bacteria), and is unlikely to contain all cell componentry identified in earthly bacteria[14]. Although this is a rather earth-centric view of life, various components (to allow energy production and reproduction) would be needed to qualify for out definition of life, and a likely lower limit on the size of a single celled organism must exist. Figure 2. Possible bacterial fossils?[15] Lacking evidence of cell walls (differentiating bacteria from their surroundings), reproductive processes (bacteria shapes dividing or budding), growth (differing shapes and sizes, with the largest showing signs of reproduction), and cells colonies are all serious short-falling of this observation.[16] Finally, several inorganic process can also produce these structures, reducing their value as pointer to biological activity. However, contamination by terrestrial bacteria seems unlikely, as testing of a number of other Antarctic meteorites failed to find similar bacteria in them (although contamination by terrestrial microbes in situ is a problem[17]), supporting the conclusion that the bacteria-shaped structures are not recently introduced.

2. Mineral grains similar to biological markers of terrestrial bacteria

McKay et al identified three minerals (magnetite, pyrrhotite, and greigite) as sign-posts to biological activity, based on similar experiences on Earth. The reason for these mineral production varies (or is unknown), but include use of magnetic particles (magnetite in this instance) for orientation, or waste products Visually, the mineral grains that McKay identified in ALH 84001 conform to the appearance of the mineral by-products of bacteria on earth. Figure 3. Comparisons between terrestiral and ALH84001 (right)[18] Thomas-Keprta et al propose that the truncated hexa-octahedral magnetite crystals are most likely formed by biogenic processes, based on their strong resemblance to “intracellular magnetite crystals produced by magnetotactic bacterium strain MV1[19] Given that no inorganic processes have been identified that produce this mineral in the configuration (such as size and crystal alignment) found in ALH84001, this would provide a key piece of evidence. A key argument in favour of this evidence is the differing nature of these minerals, and their inorganic formation sites, implying a low likelihood of them existing in the context of ALH84001 without the involvement of living organisms. As an example, evidence for partial dissolution of the carbon globules implies an acidic water source, but magnetite and greigite are formed non-biologically in very alkaline water. However, the partial dissolution may be related to another mineral external to these observations, particularly given our limited knowledge of conditions and general geology. Set against this is the differing shapes and sizes of the magnetite crystals, which would require a variety of biological specimens to produce, further raising the bar on the complexity of the problem[20]. Aggregation of these minerals by bacteria must be viewed in context – these are not standard cellular function, but rather environmental adaptations. Two examples cited[21] are:

some greigite production is related to bacterial interaction with plants, an unlikely scenario on Mars
magnetite secretion to allow orientation respective to the earths magnetic field would be less relevant in the much weaker Martian magnetic fields.[22]

Countering the second argument (and many non-biological ones) is our limited knowledge of Martian conditions billions of years ago when these physical features were laid down.

3. Organic chemical compounds similar to terrestrial bacteria decay products - Polycyclic Aromatic Hydrocarbons

Polycyclic Aromatic Hydrocarbons (PAHs – multiple rings of hydrogen/carbon molecules) are by-products of a number of chemical processes, both organic and inorganic, and were found in ALH84001. McKay proposed that their presence could point to bacterial decomposition, particularly as the distribution peaks matched the sites of the carbon globules. However, a number of requirements of the PAHs must be met to support this premise:

not terrestrial - didn’t enter the meteorite while it was in Antarctica,
different from PAHs in other meteorites (such as carbonaceous chondrite meteorites, which also contain PAHs)
structure is consistent with decomposition of simple bacteria.
not introduced during analysis

Distribution analysis with depth shows increased numbers of PAHs within the meteorite’s interior, as compared with the surface, supporting a Martian source. The nature of the PAHs also differs from the dominant background PAHs on earth – as an example, atmospheric PAHs are dominated by sulfur, but such PAHs have not been detected in ALH 84001. Further, PAH densities in ALH 84001 are greater than historical core samples from pre-industrial age high-latitude regions indicate. Significant differences exist between PAH molecules in ALH 84001 and those found in a broad range of meteorites. There is a set of meteorites that have similar PAH profiles, but these are sufficiently different to allow differing explanations of their origin[23]. Evidence for the third point – identification of the PAHs as a product of bacterial decomposition – has not been provided, and represents a short-falling in this argument. There has also been no evidence to show that the PAHs are the exclusive result of organic processes. And studies of another Martian meteorite believed to be formed much more recently, after the disappearance of Martian surface water (and hence an environment conducive to life) shows similar patterns of PAHs, reducing their value as a biological activity marker[24]. In addition to the PAHs, some amino acids have been detected, but appear to be earth-based ice water contaminants[25].

Conclusion

So we can see, that although McKay et al have pulled together a number of disparate observations to argue that their combined evidence may suggest biological activity in the early Martian history, the limited detail we have (particularly with only one meteorite showing these hallmarks), its ambiguous nature, and by necessity, the highly speculative view that must be taken, the study of ALH 84001 has been a much greater success for defining the framework for analyzing life on another planet, than providing strong evidence of Martian biology. With present and future Martian missions constantly expanding our knowledge of the planet, the analytical work triggered by this proposal will add enormously to the analysis of newly emerging data, and further our understanding of the likelihood of life on Mars.

References

McKay, D and others, “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001”, “Science”, Aug 16, 1996 Becker, L., Popp, B., Rust, T., Bada, L. “The origin of organic matter in the Martian meteorite ALH84001”, Earth and Planetary Science letter 67 (1999) Thomas-Keprta, K. and others, “Truncated hexa-octahedral magnetite crystal in ALH84001: Presumptive biosignatures”, PNAS, vol. 98, no. 5 Knoll, A., “A Martian chronicle”, “The Sciences”, July/August 1998 Gibson, E. and others, “Life on Mars: evaluation of the evidence with Martian meteorites ALH84001, Nakhla, and Shergotty”, Precambrian research 106 (2001)