AN EXOBIOLOGICAL STRATEGY FOR MARS EXPLORATION

John F. Kerridge
Department of Chemistry
UCSD
La Jolla, California 92093

In 1994, a writing group was convened by Dr. Michael A. Meyer, Discipline Scientist for Exobiology at NASA HQ, and charged with the task of formulating a strategy for the exobiological exploration of Mars. This group (i) reviewed the state of knowledge about Mars as a planetary environment, (ii) defined the major exobiological goals that could be addressed by exploration of Mars, (iii) showed how achievement of those goals would require a sequence of robotic missions culminating in return of a martian sample to Earth, (iv) summarised the status of planned missions to Mars in this country and elsewhere, (v) discussed the key role of site selection in achieving exobiological goals, and (vi) made a series of recommendations, including several regarding development of technology for future missions (see vu-graph #4). Because of the key role envisaged for sample return in exobiological exploration, and the fundamental scientific importance of resolving the question of whether life ever arose on Mars, it seemed appropriate to include a summary of the group's findings [1] in the report of the Mars Sample Return Science Workshop.

Prior to the Viking missions, exobiological interest in Mars centered on answering the question: Is there life on Mars? However, the emphasis shifted significantly as a result of those missions: Viking revealed a martian surface that today appears highly inhospitable to life, but also dramatically confirmed earlier suggestions that in the past the surface of Mars was heavily affected by liquid water, implying significantly warmer and wetter conditions than pertain today. Furthermore, in the years since Viking, much has been learned about the nature and timing of the earliest life on Earth, and also about the environmental limits within which life can exist on Earth today. The confluence of these findings leads to the following conclusion: Early environments were apparently sufficiently similar on Mars and Earth, and life arose so rapidly on Earth once conditions became clement, that emergence of life on both planets is scarcely less plausible than emergence on only one.

The major exobiological goals for Mars exploration may be defined as:

Although the writing group devised strategies for addressing all three goals, they noted that the Viking results make it likely that any contemporary life on Mars must be well shielded from the hostile surface environment and consequently hard to detect by spacecraft. However, evidence of past life, though potentially vulnerable to destruction at the very surface, could be preserved in relatively accessible locations, such as the interiors of impermeable sedimentary rocks deposited during the epoch of intense aqueous activity on Mars. (The survival of such rocks on Mars is greatly enhanced relative to the Earth because of the subsequently dry climate, low temperatures and lack of tectonic overprinting on the former.) Furthermore, conditions suitable for preservation of evidence for extinct life would also be ideal for preservation of a record of prebiotic chemistry, in the event that life failed to arise. Consequently, a properly planned search for evidence of extinct life on Mars would have a high probability of yielding evidence for possible prebiotic chemical evolution, even if its primary goal were not achieved. For the foreseeable future, therefore, the emphasis of the exobiological exploration of Mars will be on the search for evidence of an ancient biosphere.

It quickly became apparent that the search for evidence of past life on Mars would require a different strategy from that of Viking. Instead of one or two complex spacecraft searching directly for evidence of extant metabolic activity, it would be necessary to mount a campaign, consisting of a series of relatively small missions, each building upon the results of its predecessors, which would focus ever more closely on those samples that might have preserved evidence for ancient life or ancient organic chemistry. The initial task would be site selection. This would involve orbital missions capable of identifying promising lithologies on the martian surface by means of their IR spectra, and determining their global distribution. Typical target lithologies would be aqueously deposited chemical sediments, such as cherts, carbonates, or phosphates, which are known to be effective at preserving biosignatures on Earth. (The term "biosignature" refers to any piece of evidence indicative of the former presence of life. Examples could include biofabrics, microfossils, chemical biomarkers, or isotopic signatures characteristic of bioprocesses.) Because the spatial scale of such deposits on Mars is presently unknown, and because of the difficulty of resolving mineral mixtures using such spectral data, the acquisition of IR data at high spatial resolution (30-100m/pixel) from selected locations is considered n ecessary.

Although the group considered the feasibility of in situ robotic detection of evidence for extinct life, it concluded that acquisition of such evidence on a returned sample would be necessary. The reasons are twofold: First, the vastly greater scope of analytical procedures available in a terrestrial laboratory; and second, the likelihood that the science community would require that such an epochal finding, if made robotically, be confirmed in the laboratory. Consequently, the search for ancient life on Mars would culminate logically with at least one mission dedicated to the return to Earth of a sample selected on the basis of its potential for having preserved a biosignature. Clearly, the analytical protocol employed on returned samples will require particularly tight control in order to minimize the danger of either false positives or false negatives.

Between the orbital missions and sample return, however, it will be necessary to have landed precursor missions capable of confirming site selection(s) made from orbit, and of selecting at the optimum site those samples that should be subsequently returned to Earth. Requirements for such precursor missions would include mobility sufficient to permit exploration and sample acquisition anywhere within the spacecraft's landing ellipse, a capability for remote surveying of individual rocks and possible outcrops with the objective of identifying promising lithologies for biosignature preservation, and a capability for confirming such remote identifications by means of chemical and mineralogical analyses in contact with selected rocks and/or outcrops. In addition, a capability to extract a sample from a few mm depth within a rock will almost certainly be necessary, either to avoid weathering rind or to acquire a sample of suitable size for return to Earth, or both. It may well also be desirable to analyse the interiors of promising rocks in situ for a limited range of organic compounds that might indicate either a prebiotic chemical record or the possible presence of more definitive biosignatures.

Consideration of the issues described above leads to a series of exploratory steps (see vu-graph #7), each of which needs to be taken in turn if a rigorous search for ancient life on Mars is to be conducted. The writing group did not explicitly consider the timeframe on which such a sequence of missions should be carried out. However, it is fair to conclude that an optimum timeframe for such an endeavour would place the sample return mission somewhat later than 2005. Whether launching a sample return mission in 2005 is compatible with the sequence of steps necessary to pursue one of the major exobiological goals on Mars is unclear at this time. However, it seems unlikely that such an accelerated schedule could be carried out within the constraints of the Surveyor program.

It should be noted that there are goals of secondary exobiological interest that could be pursued by a sequence of missions that conform to less stringent criteria than those given above. Such goals might include acquisition of information on volatile inventories on Mars, the timing of aqueous activity on Mars, the nature of climate evolution on Mars, etc. However, it must be pointed out that missions designed to acquire such information are unlikely to yield a reliable answer to the question: Did life ever emerge on Mars? The converse is not necessarily true, however. A properly designed campaign to bring back an optimal sample in which to search for evidence of ancient life would also provide optimal samples with which the secondary goals given above could be addressed. If suitable care is taken during the planning process, there could be a high level of compatibility among the needs of the different components of the "Goldin-Huntress" contract.

Reference:

[1] An Exobiological Strategy for Mars Exploration, N ASA SP-530, Washington DC, 1995.


Vugraphs:

VG 1: Emergence of Life on Mars is not Far-Fetched

VG 2: Science Goals

VG 3: The Exobiological Exploration of Mars

VG 4: Principle Recommendations

VG 5: The Search for Ancient Life on Mars

VG 6: Requirements for Site and Sample Selection

VG 7: The Search for Ancient Life on Mars

VG 8: Return of an Exobiology Sample in 2005?

VG 9: Ancient Life on Mars

VG 10: General Comments

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