processes. Laboratory investigations of the sulfur oxidative process and the concomitant isotopic partitioning59 suggest that the predominant oxidants were hydrogen peroxide and/or ozone. This indicates that the primary oxidants had sufficient electro-negativity to impart their isotopic composition to any secondary minerals formed. If so, then subsurface water circulation would have been restricted, with water/rock ratios remaining low.
Although the significance of these observations for the origin and persistence of putative subsurface martian life forms is still unclear, the isotopic record of secondary aqueous minerals (carbonates and sulfates) in the SNC meteorites provides a direct record of hydrological processes of great importance for assessing the long-term habitability of the martian subsurface.
In summary, the application of new high-resolution isotopic methods to the study of martian meteorites suggests the following:
Liquid water has existed in the martian subsurface over prolonged periods of geological time;
Active exchanges between surface and subsurface water reservoirs maintained by groundwater circulation provided a means for the active transport of oxidants needed to maintain subsurface redox gradients; and
The abundance of water was sufficient for authigenic mineral precipitation, but relative to the host rock, water volumes have remained low.
The assessment of martian habitability made in the NRC’s 1997 report Mars Sample Return: Issues and Recommendations led to its recommendation that: “Samples returned from Mars by spacecraft should be contained and treated as though potentially hazardous until proven otherwise. No uncontained martian materials, including spacecraft surfaces that have been exposed to the martian environment, should be returned to Earth unless sterilized” (p. 3). The present committee found that the knowledge gained from both orbital and landed missions conducted over the past decade, combined with findings from studies of martian meteorites, has enhanced the prospect that habitable environments were once widespread over the surface of Mars. In addition, the potential for modern habitable environments, both as transient surface environments and as stable habitats in the deep subsurface, is much better understood. This understanding has, in turn, enhanced the possibility that living entities could be present in samples returned from Mars. Therefore, the committee concurs with and expands on the 1997 recommendation that no uncontained martian materials should be returned to Earth unless sterilized.
Recommendation: Based on current knowledge of past and present habitability of Mars, NASA should continue to maintain a strong and conservative program of planetary protection for Mars sample return. That is, samples returned from Mars by spacecraft should be contained and treated as though potentially hazardous until proven otherwise. No uncontained martian materials, including spacecraft surfaces that have been exposed to the martian environment, should be returned to Earth unless sterilized.
The committee found that uncertainties in the current assessment of martian habitability and the potential for the inclusion of living entities in samples returned from Mars might be reduced by continuing research in the following areas:
A vigorous program of remote-sensing and in situ exploration of Mars with the goal of answering questions relating to martian habitability, including those concerned with the presence of water in surface and subsurface environments through time, the distribution of biogenic elements, and the availability of redox-based energy sources (e.g., those based on the oxidation of ferrous iron and reduced sulfur compounds); and
Continued studies of martian meteorites to help refine understanding of the history of interactions of Mars’s rock-water-atmosphere system throughout the planet’s history.