Other reservoirs II Shergottite Ar

Just as the nakhlites contain noble gases that do almost, but not quite, match the atmosphere, so do the shergottites. In this case, however, the evidence is in the ratio of the gas trapped within them. 

On Earth, after decades of searching, it is not clear that anyone has ever found a sample of paleoatmosphere with noble gas isotopic ratios distinct from the modern ratios. For Mars, with only 20 samples to work with, a strong case can be made that paleoatmosphere has been found. 

But does it have paleoatmosphere The identification has largely been made based on expectations of what the Martian atmosphere might have been like 4 Ga ago. A definitive identification would require some demonstration that the identified component was implanted 4 Ga ago, which would be difficult. 

The highest Xe/ Xe in ALH84001 is about 2.16, 10% lower than measured in the glasses in the shergottites. In addition, the Xe/ Xe/ Xe systematics are not consistent with a combination of current Martian atmosphere, a Martian interior component like that identified in Chassigny, and terrestrial contamination. On this basis, the Xe has been argued to be an ancient component, rather than modem atmosphere (Gilmour et al. 1998 Mathew and Marti 2001 Murty and Mohapatra 1997). 

Identifying Martian paleoatmosphere is fascinating in its own right, but also could put tight constraints on Martian atmospheric evolution. For example, based on Xe isotope systematics in ALH84001, Mathew and Marti have argued that not only had the Martian atmosphere not acquired its full complement of Xe by 4 Ga ago, the isotopic fractionation that is required (see Fig. 3 and discussion above) had not yet occurred then either (Mathew and Marti 2001). Such a late fractionation would be very difficult to explain by atmospheric loss through hydrodynamic escape (Hunten et al. 1987 Pepin 1994 Pepin and Porcelli 2002, this volume). 

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