Noble Gases in Martian Meteorites

Noble gases have been instrumental in establishing a link between the SNC(O) meteorites and planet Mars. A superior (compared to the previous Viking measurements) analysis of the Martian atmosphere (MA) by the SAM instrument aboard the Mars Science Laboratory (MSL) has recently become available, which only strengthens this link. Nevertheless the results obtained on Martian meteorites still provide the most complete picture of noble gases on Mars, not just atmospheric Martian noble gases but also gases that can be confidently described as being of “interior” origin. Overall three major components of trapped noble gases have been clearly identified in Martian meteorites: (1) ~recent MA; (2) fractionated and possibly ancient MA; (3) interior (mantle?) noble gases. An additional, perhaps minor, component is only observed during crushing of shergottites. While the fractionated atmospheric component may tell us about processes like weathering, the difference between the atmosphere and the interior reservoir on Mars is unlike the difference observed on Earth and points to significant differences in the volatile history of the two planets. The nonradiogenic gases in the atmospheres of the two planets—apart from the ~2 orders of magnitude difference in absolute abundance—however, share similarities in their element abundance patterns, which sets them apart from solar wind or primordial noble gases in primitive meteorites and may indicate similar acquisition/loss processes. In addition, the atmospheric Xe isotopic composition for these two planetary bodies is quite unique (and remarkably similar), which points to a similar fractionation process, or set of processes, leading to the observed composition. A crucial point to be settled is whether the isotopic composition of the (assumed) atmospheric Xe in the unique, >4 Ga old, ALH 84001 differs from that of modern Martian atmospheric Xe. The indication that it may have a solar wind–like composition instead poses a problem for currently popular models for the early evolution of the MA (as well as that of the terrestrial atmosphere).