Just as the case for martian water (discussed earlier), other atmospheric species (e.g., carbon dioxide and Ar) were probably much more abundant early in martian history. What was the origin of these volatile elements (mantle degassing, late accretion of carbonaceous chondrites and comets?) and where have they gone? The isotopic compositions of several volatile elements measured in martian meteorites indicate that they have experienced mass fractionation, probably as a consequence of major atmospheric losses to space. What was the nature of these loss mechanisms (hydrodynamic escape, solar wind sputtering, solar UV ionization, giant impacts?) and when did they occur? Can the isotopic compositions of carbon and oxygen in martian surface materials define the nature of chemical and physical processes that have affected these elements over time?

Some elements in the martian atmosphere display large isotopic fractionations that suggest early loss of a major portion of the atmosphere (Figure 6). These data are: A D/H ratio (measured by earth-based spectra and in martian meteorite water) that is five times that of the Earth; a ¹⁵N/¹⁴N ratio that is 60% enriched over Earth's (measured by Viking and also found in martian meteorites); A ³⁸Ar/³⁶Ar ratio that is 30% enriched over Earth's, and a ¹³⁶Xe/¹³⁰Xe ratio that is 16-25% enriched over solar Xe and that found in carbonaceous meteorites (both Ar and Xe measurements are from shock-implanted gases in EETA79001). In contrast, Kr isotopes in EETA79001 closely resemble solar Kr, as do Xe isotopes in Chassigny. A minor component of N in EETA79001 and Zagami has a ¹⁵N/¹⁴N ratio similar to Earth's and quite different from Mars' atmospheric N. These findings indicate that at least two reservoirs of these gases exist on Mars, one a mass-fractionated atmospheric component and one likely an unfractionated mantle-derived component (Figure 6). How can such different reservoirs be used to define in detail the volatile evolution of Mars?

Isotopes of O and C in water and carbonates of martian meteorites show much less fractionation than expected due to atmospheric loss. This is probably because losses of these elements are buffered by condensed phases such as crustal water and carbonate. Even so, variations in ¹³C/¹²C among analyses are greater than one expects from equilibrium reactions. Has the martian ¹³C/¹²C changed with time, and if so can this be used to determine martian volatile evolution? Values of ¹⁸O/¹⁶O in ALH84001 carbonate are also about 1% enriched over oxygen in silicate, but ¹⁸O/¹⁶O in martian water is less enriched (Figure 7). Therefore, a potential exists for using these differences to deduce the temperatures and CO₂/H₂O ratios for equilibrium reactions involving these two species with silicates. Future analysis of returned martian surface materials containing a wider variety and abundance of C, O, and other volatile species (e.g., S, halogens) offers great potential for deciphering the long-term volatile evolution of Mars.