has been accepted, the only viable sample is one for which there already is compelling evidence for life or at least a high probability of there having been life. This would seem to require obtaining a sample from a place at which detailed in situ astrobiological analysis has already been done.
That approach also presupposes that the only astrobiological value in returning a sample is determining whether life is or has been present. As already pointed out in this report, the astrobiology science goals for Mars are much broader than this, and researchers can make major progress in understanding the history of Mars, its volatiles and climate, and its geological and geophysical history by returning samples.
Related to the question of the right sample is the number of sample-return missions likely required to achieve the identified scientific goals. While some groups have attempted to estimate appropriate values for this number,25 programmatic and fiscal realities are likely to dominate purely scientific considerations. Returning samples from Mars will cost many billions of dollars, and the history of NASA’s space science activities clearly demonstrates that each major scientific community gets only one multibillion-dollar mission per decade. Thus, the only realistic number of sample-return missions that can be contemplated within the predictable time horizon is one.
Finally, reasoned and thoughtful determinations can be made today concerning where to obtain samples that would have good chances of providing valuable information on whether life is or has been present. These include likely sites of longstanding liquid water and of the geochemical potential for life. While additional data always will provide better information, sufficient data already exist to choose important sites.
The combination of the high astrobiology science value in returning samples, the seemingly impossible task of defining a single “right sample,” the likely number of sample-return missions, and the ability to choose sites today that have a high potential for providing details about life suggests that the appropriate strategy is to return samples at the earliest possibility rather than continuing to wait for more data.
The selection of promising landing sites using orbital data is not infallible, and in all likelihood surprises will be encountered at every landing site. The MERs have demonstrated the ability to traverse significant distances and to analyze the diverse materials encountered on the surface and in the subsurface (exposed in craters). That demonstrated ability suggests a plausible strategy for collecting samples to be returned to Earth: Utilize all the highly mobile, well-instrumented rovers (and possibly stationary landers) in the NASA Mars exploration strategy to cache collections of interesting samples for possible future sample-return missions. A subsequent sample-return mission could then land near (possibly guided by a beacon on the cache) and retrieve a sample cache, perhaps using a tethered rover (minimizing mobility, communication, and navigation requirements) and eliminating the need for analytical instruments to assess the nature of the samples. The storage of samples should involve very simple mechanisms. Such a strategy would increase the likelihood that astrobiologically interesting samples could be obtained, and it obviously would decrease the cost of a sample-return mission (or multiple missions) significantly.
Planning for the 2009 Mars Science Laboratory is too far advanced to allow adding even a simple sample-caching capability, but this strategy should be considered for any subsequent rover (or lander, if it is to acquire subsurface samples). It may also be desirable to coordinate with the European Space Agency (ESA), to see if this strategy could be adopted for other (non-NASA) Mars rovers and landers.
While sample caching can help facilitate a Mars sample-return mission, this strategy may, according to some observers, raise issues relating to the implementation of current planetary protection policies. These concerns can be divided into those associated with forward contamination and those associated with back contamination. The former concerns arise in particular because of the potential for contaminating the martian materials collected with organisms that hitchhiked from Earth in the sample-acquisition and sample-handling systems or the caching container.26 The latter concerns are associated with ensuring that the caching container, the samples within it, and any other spacecraft subsystems exposed to the martian environment and scheduled for return are completely sealed within the Earth-return canister prior to ascent from the surface of Mars.27
None of these concerns is unique to a sample-caching strategy. Indeed, all Mars sample-return scenarios must contend with all of the above planetary protection issues. Caching causes complications because some of the procedures that would only have to be implemented on a sample-return mission must now be implemented on the preceding missions that are caching samples for later return to Earth. Of particular concern is the need to ensure that the sample acquisition and handling systems undergo appropriate bioload-reduction prior to launch. If this is not done then martian materials might be contaminated with organisms from Earth as they are collected