A Mars sample return mission in 2005 will be a challenging task, one so ambitious that realistic goals must be made clear at the outset of planning. To help define what is realistic, I will focus on several "nuts-and-bolts" questions.
What kind of geologic sample(s) should (can) we collect, and why?
Both Viking lander sites had concentrations of rocks, possibly ejecta from local craters plus outcrops of layered rock. Small rock fragments were inferred to be scarce, but the rocks themselves showed considerable petrologic variability. The surface sampler was unable to chip or scratch any rock surfaces, implying no weak weathering rinds. Thermal inertia and albedo measurements for Mars are inversely correlated, suggesting mixtures of two materials: bright, low-inertia dust and dark, high-inertia rocks. Modeling of thermal inertia data indicates that surface rock cover averages 6%, with abundances ranging up to 35%. Both Viking lander sites have above average rock abundances (10% and 20%). Although periodically dusted with weathered material, the dark regions consistently reappear after dust activity, so exposures of bedrock are possible.
The mineralogy of the most easily accessible rocks is likely to resemble basaltic shergottites, based on spectral similarity of the dark, rocky regions (obtained by telescope and orbiting spacecraft) with these meteorites. The mineralogy of soils is unlikely to be represented by the alteration phases in SNC meteorites.
Advantages of collecting a soil sample are that it is ubiquitous and relatively easy to sample and it provides information on interaction with the atmosphere and hydrosphere. If such a sample proves to be relatively unweathered, it may provide a great deal of petrologic diversity, analagous to lunar soil. However, it is likely that weathering would obscure such information. Rock samples allow the full arsenal of mineralogy/petrology/geochemistry/isotope techniques to be applied. The evidence in rocks is of discrete events rather than time-integrated events, as in soils. Both rocks and soil access the geologic past better than an atmospheric sample, and linkage with possible life will be more direct than with an atmospheric sample.
What information and tools are necessary for proper geologic sampling?
Instruments already scheduled to be flown on precursor missions are, for the most part, adequate to define a suitable site. On the sample collection mission itself, the following will be needed: descent imaging for geologic context, improved landing accuracy (probably more important for subsequent sample return missions), and mobility in the form of a reasonably capable rover. Mobility is especially critical because of the requirement (see below) to collect small rock samples, as well as to sample the petrologic diversity that is likely to have been provided by meteor impacts. It is not necessary to obtain a core sample on this first mission. Considerable attention must be given to the possible need to sample large rocks. It is unlikely that rocks can be broken, but they might be drilled. However, mass, power, and cost limitations will probably require that the 2005 mission collect small rocks rather than sample larger ones.
A suggested sample payload is 5 small rocks (on the order of 10 g each), one loose soil sample, one duracrust sample, and possibly one atmospheric sample. This total sample size is roughly an order of magnitude smaller than that advocated at the last Mars sample return workshop (LPI Technical Report 88-07). Sample storage can be very simple, e.g. soil can be used as packing for rocks, and atmosphere can be trapped as pore space or head space volume.
Sample storage requirements during launch, cruise, and re-entry have already been specified by NASA Technical Memorandum 4184. This document includes recommendations on contamination, temperature, head-space pressure, radiation shielding, magnetism, and acceleration. As desirable as these target conditions are for maximizing the scientific worth of the samples, they may have to be relaxed for this mission to be flown under the cost constraints.
How can a returned sample help us find and utilize resources, or alternatively, what resources might be used to effect a sample return mission?
For the purposes of the 2005 mission, the only relevant Martian resources are water and atmospheric carbon dioxide. An important goal of the Mars Surveyor Program is to understand the global inventory, long- and short-term repositories, and hydrologic cycle of water. This resource will certainly be critical for human exploration. However, water as a resource will probably not be directly addressed on this mission. Hydrous alteration phases in soil and rock may lead to a better understanding of one repository, but the energy required to extract water from hydrated minerals is considerably greater than from ice. Subsequent missions must address the question of where accessible water can be found. Although water is an economic propellant source, electrolysis to make H2 and O2 requires storing a hard cryogen.
The most accessible Martian resource is atmospheric CO2, which can be obtained by simple compression with no mining or beneficiation required. When used for propellant production on Mars, it can significantly lower launch mass by eliminating fuel for the return trip plus the fuel required to boost the return fuel to Mars. Consideration should be given to in situ propellant production as a means of increasing the mass of the returned sample. The addition of in situ resource utilization would also make the mission more technologically exciting.
References:
Workshop on Mars Sample Return Science (1988) LPI Technical Report 88-07. Scientific Guidelines for Preservation of Samples Collected from Mars (1990) NASA Technical Memorandum 4184.
VG 1: Distribution of Rock and Soil
VG 2: Advantages of Rocks versus Soils
VG 3: Advantages of either over Atmospheric Sample
VG 4: Measurements Required to Select Site and Samples
VG 5: Getting to the Right Location
VG 7: Sample Storage Requirements During Launch, Cruise and Re-entry
VG 9: Atmospheric Carbon Dioxide
