Mars science and telecommunications orbiter


Atmospheric composition: implications for extant processes



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Atmospheric composition: implications for extant processes


It has long been understood that the presence of life on a planet would modify the atmosphere in such a fashion that this "disequilibrium" condition could be detected by remote sensing. Moreover, active abiogenic geological processes also will modify the environment in which these processes exist.

Again, the earliest discussions about detection of life through remote sensing identified methane as one example of a biosignature, say, in the Mars atmosphere, where methane would not otherwise be present simply due to standard atmospheric photochemistry. So the presence of methane in the current atmosphere would be highly suggestive of extant active processes--geological or even biological, and most probably in the subsurface. One line of reasoning that leads to these conclusions is that the atmospheric lifetime is short, ~400 years, so there is no chance that the existing atmospheric methane is an atmospheric remnant from billions of years ago.

In fact, if active subsurface processes do exist in the current epoch, there may be other chemical constituents in the atmosphere, albeit with very small abundances, that may arise from processes that produce methane or from other active geological or biological processes. Thus a sensitive survey of the chemical composition of the Mars atmosphere will provide important insights into the extent of active processes that exist in the Martian subsurface and the character of these processes. These observations should include the isotopic variants of methane and possible species that would be co-generated in potential methanogenic processes to seek an identification of the particular process forming methane.

A broad understanding of the atmospheric chemistry of Mars is important for distinguishing true "disequilibrium" chemical species. In addition, the chemistry of the atmosphere controls the lifetime of potential species that can be introduced into the atmosphere by subsurface active processes. While the recent observations, at the limits of their sensitivity, suggest that spatial and temporal variability in methane has been detected, the current understanding of the methane lifetime and the magnitude of the dynamical atmospheric circulation suggests that methane should be well-mixed. If methane were found to be not well-mixed, there would be significant implications for atmospheric chemical processes that are not known at present (this is discussed further in the following section).

In addition to trying to understand the character of subsurface active processes forming the disequilibrium chemical species that may be detected in the atmosphere, it is equally important to identify specific locations on the surface where these species enter the atmosphere. Such locations would be important sites for further exploration, and may offer the best locales for future sample return. Depending on whether methane is well-mixed in the atmosphere or not, methane itself may not be the best tracer to identify these "oases" of subsurface activity. Other disequilibrium species with shorter atmospheric lifetimes may form distinct emitted plumes and therefore be easier to trace back to the source locale. Knowledge of the chemical lifetimes of these tracer species and the atmospheric circulation are key elements in localizing potential source regions.

Major Questions


The MSTO SAG identified several major science questions related to the escape of the atmosphere over time, the detailed composition of the lower atmosphere, the spatial variation of the composition, and what this reveals in terms of its interaction with the near-surface environment.

(Q1) Could the present atmospheric escape processes currently occurring at Mars account for climatologically significant amounts of loss of atmosphere, particularly CO2 and H2O, in the early history of the planet - i.e., after the era of hydrodynamic escape and after impact erosion ceased to be important?

(Q2) Could liquid water have been present on the surface of Mars at that time?

(Q3) Do present day values for the nitrogen and argon isotopes in the upper atmosphere, when compared with their surface values, suggest a loss of atmosphere consistent with the CO2 and H2O escape? Are the mechanisms responsible for atmospheric erosion consistently manifested through all of the volatile species, C,H,O,N and Ar?

(Q4) What is the present day inventory of molecular species in the lower atmosphere?

(Q5) Are the relative abundances of these species consistent with contemporary models of Mars atmospheric chemistry, including the effects of dust and aerosols?

(Q6) What are the dynamic processes (winds) that distribute and mix the constituents on short time scales?

(Q7) Is there a clear correlation between the spatial variability of trace species and identifiable surface or subsurface features?

(Q8) Are CH4 and its close derivatives present in the boundary layer atmosphere?

(Q9) If so, do the abundance patterns of these species, and their isotopic compositions, provide unambiguous discrimination between geological and biological sources?



Science Objectives


From the science questions detailed above, the MSTO SAG identified the following prioritized science objectives for further analysis:

(O-1) Characterize the present day escape of the atmosphere in sufficient detail, and over a period sufficiently long to account for changes induced by seasonal and solar cycle variability, to allow realistic backwards extrapolation of the atmospheric composition.

(O-2) Determine the relative abundances of the major, minor and trace constituents of the lower atmosphere and their isotopic components to provide a definitive inventory of species, in order to better understand the current interaction between the atmosphere and the near surface environment and, in particular, to determine whether spatially non-uniform species are in chemical equilibrium.

(O-3) Characterize the winds, both zonal and meridional, with good altitude resolution, and their seasonal variations.

(O-4) Map the crustal magnetic field with approximately 100 km resolution. These data are required in part to aid in elucidating the mechanisms contributing to atmospheric escape; they are also of importance in advancing our understanding of the surface mineralogy and evolution of the planetary interior.

Table 1 lists the relationship between the science goals and the recommendations of MEPAG. The science objectives developed in this report are also in alignment with the Decadal Study recommendations related to investigations of the evolution of the Mars atmosphere.





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