V.G.Atmospheric gas - Science Priorities for Mars Sample Return


^ V.G.Atmospheric gas
Scientific objectives related to investigating the geochemistry of Martian atmospheric volatiles, would include determining the following:

As discussed in Appendix II, the systematics of Martian volatiles should be studied in two V.G.Atmospheric gas - Science Priorities for Mars Sample Return complementary ways: analysis of atmospheric gas and analysis of gas released by the thermal decomposition of rocks of various types and ages. Collected gas samples could be utilized to determine V.G.Atmospheric gas - Science Priorities for Mars Sample Return the stable isotopic abundances, including noble gases, and trace gas composition of the present-day bulk atmosphere (Appendix II, IIB-1). Likewise, the thermal decrepitation studies of solid samples could help to determine V.G.Atmospheric gas - Science Priorities for Mars Sample Return the history of the composition of the atmosphere (Appendix II, IIB-3). However, ND-SAG has concluded that determining the production/loss, reaction rates, and global 3-dimensional distributions of key photochemically reactive species V.G.Atmospheric gas - Science Priorities for Mars Sample Return would not easily be possible using sample return, because the species of interest are present in trace quantities, and the species degrade relatively rapidly. The gas placed in the container on V.G.Atmospheric gas - Science Priorities for Mars Sample Return Mars would not be the same as the gas received in the lab on Earth. Characterizing organic gases to interpret possible biologic implications, although important to Goal I (Appendix II; e.g. IA-4, IB-1, IB V.G.Atmospheric gas - Science Priorities for Mars Sample Return-3), may also encounter similar difficulties in sample preservation. Thus, for the remainder of this section, the scientific objectives are considered in the context of the major inorganic gases, including V.G.Atmospheric gas - Science Priorities for Mars Sample Return the noble gases.

Our present knowledge of the Martian volatile system comes from previous measurements by the 1976 Viking landers, and from analysis of gases trapped in Martian meteorites. Those results show that that V.G.Atmospheric gas - Science Priorities for Mars Sample Return some atmospheric species (e.g., N, H, Ar, Xe) have been isotopically fractionated by atmospheric loss into space. Models of both continuous loss and early episodic loss have been advanced (e V.G.Atmospheric gas - Science Priorities for Mars Sample Return.g., Pepin, 1991), but the details of volatile loss remain largely unanswered. Atmospheric loss also has occurred on other terrestrial planets, such as Earth. To understand the specific atmospheric loss mechanisms, it is important to V.G.Atmospheric gas - Science Priorities for Mars Sample Return know the initial isotopic compositions of these gas species. Such knowledge may also indicate to what degree these volatiles were acquired during the accretion of Mars and later degassed from the interior, versus V.G.Atmospheric gas - Science Priorities for Mars Sample Return to what degree volatiles were added after accretion by, for example, comet impacts.

Knowledge about initial isotopic compositions mainly would derive from analyses of volatiles trapped in solid samples, either ancient V.G.Atmospheric gas - Science Priorities for Mars Sample Return rocks containing volatiles accreted with Mars, or in condensed phases such as carbonates and hydrates, which represent major inventories of these volatiles on Mars. Earlier atmospheric gases also could be V.G.Atmospheric gas - Science Priorities for Mars Sample Return trapped in impact melts. The potential exists to use gaseous isotopes formed over time through radioactive decay (e.g., 40Ar, 129Xe) and measured in samples of different ages to characterize early Martian differentiation V.G.Atmospheric gas - Science Priorities for Mars Sample Return and evolution of the atmospheric inventory. Ancient volatiles trapped in Martian meteorites give hints of initial volatile compositions, but some initial isotopic compositions (e.g., D/H, 13C/12C, light noble gases) are largely V.G.Atmospheric gas - Science Priorities for Mars Sample Return unknown, as are details of variations in isotopic compositions through Martian history, generated by volcanic degassing, loss to space, climatic cycles, etc. The 13C/12C, 18O/16O, and D V.G.Atmospheric gas - Science Priorities for Mars Sample Return/H isotopic ratios of atmospheric gases are important parameters in understanding chemical equilibria among atmospheric and condensed volatile phases, but these ratios are poorly known. Further, measurements of various volatiles in solid samples (igneous V.G.Atmospheric gas - Science Priorities for Mars Sample Return and sedimentary rock, chemical precipitates, impact glass, etc) could elucidate the important volatile-containing phases within Mars and possibly variations in these phases across Martian history and climatic cycles. An understanding V.G.Atmospheric gas - Science Priorities for Mars Sample Return of the C, O, and S isotopic compositions in condensed Martian phases could be important in determining formation temperature and distinguishing biotic from abiotic chemical reactions that produced such phases (e V.G.Atmospheric gas - Science Priorities for Mars Sample Return.g., Farquhar et al., 2000; Valley et al., 1997).

Comparisons of isotopic compositions of volatile elements like C, O, H, and S in various chemical forms and in different phases of returned samples could give V.G.Atmospheric gas - Science Priorities for Mars Sample Return a potential wealth of geochemical information about atmospheric and volatile interactions and evolution. The isotopic compositions of such species are subtly changed when they undergo chemical reactions or phase changes, and these isotopic V.G.Atmospheric gas - Science Priorities for Mars Sample Return differences may elucidate these phases and processes and the temperatures involved. For example, the isotopic composition of carbon differs in predictable ways in carbonates precipitated from carbonate-bearing groundwater and in equilibrium V.G.Atmospheric gas - Science Priorities for Mars Sample Return with atmospheric CO2. Further, such data could provide information about genetic relationships among sulfur- and oxygen-bearing phases, the oxidation pathways for compounds in the regolith that involve atmospheric species V.G.Atmospheric gas - Science Priorities for Mars Sample Return with anomalous oxygen isotope compositions (which could be affected by oxygen sinks), and the sources and mixing of Martian sulfate. Although the isotopic compositions of these elements in the present and ancient Martian atmosphere are V.G.Atmospheric gas - Science Priorities for Mars Sample Return important for such considerations, their atmospheric compositions are poorly known. Solid samples also are certain to contain noble gases produced by cosmic ray bombardment of the Martian surface, and these V.G.Atmospheric gas - Science Priorities for Mars Sample Return have likely altered the atmospheric noble gas composition over time.

In addition to what we know from Viking and study of the martian meteorites, MSL will carry an instrument (SAM) that is capable of V.G.Atmospheric gas - Science Priorities for Mars Sample Return measuring many components of the Martian atmosphere, including the isotopic ratios of Ar, N2, CO2 (both C and O), Kr, Ne, Xe, the concentration of methane and sulfur gases, and the V.G.Atmospheric gas - Science Priorities for Mars Sample Return D/H ration in H2O. The precisions and detection limits of SAM’s capability in these areas is summarized in Table 2 (Appendix V; data from Mahaffy, writ. comm., 2008).

^ Table 2 Planning aspects related V.G.Atmospheric gas - Science Priorities for Mars Sample Return to a returned gas sample.

Notes on Table 2: 1). More abundant Xe and Kr isotopes are known more accurately, but the low abundance isotopes, with the least accurate precision, are important V.G.Atmospheric gas - Science Priorities for Mars Sample Return in order to decipher the starting compositions. 2). CAVEAT on the use of meteorites to interpret gas chemistry:  Assumes gas trapped in martian meteorites is the same as the current martian atmosphere.


ND V.G.Atmospheric gas - Science Priorities for Mars Sample Return-SAG concludes that analysis of a returned martian atmospheric sample for Ne, Kr, CO2 and CH4 and C2H6 would confer major scientific benefit (Table 2). Characterizing the initial Kr component in the primitive atmosphere V.G.Atmospheric gas - Science Priorities for Mars Sample Return would require analytic precision beyond MSL’s capabilities. Understanding processes of exchange between CO2, CH4 and exchangeable crustal reservoirs of carbon, oxygen and hydrogen requires highly precise stable isotopic V.G.Atmospheric gas - Science Priorities for Mars Sample Return measurements. A returned Xe sample would provide an improved estimate of the initial atmospheric Xe component and of minor components that have been added. However, returned samples of Ar, N2, S gases and H V.G.Atmospheric gas - Science Priorities for Mars Sample Return2O would confer minimal benefits relative to what we expect to learn from MSL. The abundance and isotopic measurements of Ar and N2 achievable by MSL in situ would be sufficient to address V.G.Atmospheric gas - Science Priorities for Mars Sample Return the key open scientific questions in those areas. The S gases and H2O have such low abundances and high reactivity that they would not be expected to survive the return to V.G.Atmospheric gas - Science Priorities for Mars Sample Return Earth in unmodified form. Appendix V explains the rationale for these findings in greater detail.



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