Science Priorities for Mars Sample Return
|
|  Yüklə 1.81 Mb.
|
V.C.Low temperature altered rock suite.Low temperature alteration processes occur at near ambient conditions on the Martian surface (typically less than about 20°C) and include, among other things, aqueous weathering (including, certain forms of palagonitization) and a variety of oxidation processes. Spectral observations made by Viking and Pathfinder first inspired the notion that rock surfaces on Mars are coated with thin veneers of altered material. Crude depth profiling provided by the RAT experiment on the MER rovers revealed thin (mm-scale) alteration rinds on most rock surfaces studied. The exact nature of the alteration processes remains under discussion, but most investigators agree that low-temperature, relatively acidic aqueous conditions were involved (e.g., Haskins et al., 2005; Hurowitz et al., 2006; Ming et al., 2006). Low temperature processes also influence the regolith during and after its deposition. The sulfur-rich composition of regolith has long been attributed to low temperature aqueous processes that yielded sulfate and other secondary minerals. This was confirmed when the MER rovers identified reactive magnesium and ferric sulfate minerals in the soils (Yen et al., 2007). The Viking gas exchange and labelled release experiments also demonstrated that a reactive and oxidizing compound in the regolith was capable of breaking down many organic species. The nature and origin of this compound remains controversial, but various models call for low temperature processes, such as photochemical alteration, impact crushing, or oxidizing acid interactions (Yen et al., 2000; Hurowitz et al., 2007). Understanding the conditions under which low temperature alteration processes proceed would provide important insight into the near-surface hydrological cycle, including fluid/rock ratios, fluid compositions (chemical and isotopic, as well as redox conditions), and the mass fluxes of volatile compounds (see also MacPherson et al., 2001, 2002). It would be particularly important to analyze complete alteration profiles, whether on rock surfaces or within regolith columns, because they would also constrain the kinetics of these alteration reactions. Representative, intact (or at least reconstructed) profiles on rock surfaces would be required in order to understand these alteration reactions. Recent experimental work has shown that parent rock compositions (mineralogy) are an important variable in understanding these processes (Tosca et al., 2004; Golden et al., 2005). Consequently, a diverse compositional suite would be highly desirable and would require sample site characterization during sample selection. Alteration profiles on rock surfaces would most readily be acquired by coring. The scales of alteration profiles range from less than one mm to perhaps as much as one cm, and so sample sizes of at least 2 cm would be needed. Because alteration profiles are likely to contain small amounts of sulfate and perhaps other reactive minerals, these samples would be susceptible to degradation during sampling and transport to Earth by processes such as dehydration and chemical reaction, which in turn could also affect their physical integrity. Accordingly, sample encapsulation is deemed critical. V.D.Igneous rock suite.The igneous rocks on Mars are expected to be composed primarily of lavas and shallow intrusive rocks of basaltic composition (McSween et al., 2003; Christensen et al., 2005), along with volcanic ash deposits (e.g. Wilson and Head, 2007). Although more and less evolved silicic and ultramafic magmatic rocks may potentially be present and would be of great interest, they have not yet been unambiguously identified on the surface. Igneous rocks would be central to investigations that reveal the geologic evolution of the Martian surface and interior because their geochemical and isotopic compositions constrain both the composition of mantle source regions as well as the processes that affected magmas during their generation, ascent, and emplacement (see also MacPherson et al., 2001; 2002). Although spacecraft instrumentation could measure many major elements, Earth-based analyses of returned samples would be necessary to determine most trace element and isotopic abundances of rocks. Melting and crystallization experiments in terrestrial laboratories would be based on the compositions of igneous rocks. Trace siderophile element abundances and isotopic compositions in igneous rocks could constrain the nature of the core and possibly its interaction with the mantle. Because magmas carried dissolved volatiles to the surface, these rocks would also be critical to understanding the inventories of degassed volatiles and the cycling of water and carbon. Only igneous rocks could be reliably dated using absolute radiometric dating techniques, therefore they would be critical for calibrating the Martian stratigraphic timescale. Quantifying cratering rates would allow absolute ages of Martian surfaces to be derived from crater densities (Hartmann and Neukum, 2001). Unaltered igneous rocks that are geographically linked to extensive terranes with known crater densities would be required. This linkage would likely be accomplished by comparing their geochemical/mineralogical characteristics with local bedrock and by characterizing regional units using orbital remote sensing. A large proportion of rocks on the Martian surface are likely to have experienced at least some low-temperature alteration (Wyatt et al., 2004). However significantly weathered samples would not satisfy the needs of these investigations and instead would be better suited to investigations involving rock/water interactions. Consequently, the low-temperature alteration products associated with the weathering of the igneous rock suite are discussed separately. To accommodate these investigations, a suite of igneous samples with as much chemical and textural diversity as possible would be required. Although some basaltic rocks may appear similar in terms of major element abundances and mineralogy, a suite collected over some geographic area would be likely to exhibit differences in trace element and isotopic compositions that would be highly informative. If different types of igneous rocks are present, (e.g. ultramafic or silicic rocks), additional samples of these rocks should be collected, as these could constrain fractionation processes on Mars. It is important to note that many different scientific objectives could be met with the same samples. For example, radiometric dating of a lava flow that overlies a sedimentary sequence might constrain the cratering rate, the mechanisms and timing of planetary differentiation and evolution, and the period when sedimentation occurred. The igneous rock suite is relatively robust, therefore most geologic objectives could be met with minimal temperature control and encapsulation procedures. However, interactions with fluids derived from dehydration of other samples, physical mixing, and the abrasion of rock chips during transport could all be detrimental to these investigations. FINDING. MSR would have its greatest value if the rock samples were organized into suites of samples that represent the diversity of the products of various planetary processes. Similarities and differences between samples in a suite can be as important as the absolute characterization of a single sample. Four primary suites of rock samples are called for:
|