- © The Mineralogical Society Of America
STUDYING MARS AS ANOTHER PLANET
Noble gas studies have been done on well-documented samples from two very different solar system bodies, the Earth and Moon. There is one other planet, Mars, from which we have samples, but those samples are nearly 20 (at the start of 2002) Martian meteorites, rocks from unknown locales on Mars. Even from those, though, we have learned enough to realize that Mars is a fascinating compromise between the Earth and Moon.
Like the Earth, Mars has a true atmosphere and interior volatile reservoirs, and it has been generating volcanic rocks throughout most of its history. Like the Moon, Mars has a surface that is exposed to enough galactic cosmic rays to generate abundant spallation reactions in surface rocks, and impact is likely one of the most common sources of heating for metamorphism.
In reviewing noble gas studies of Mars, we will touch on topics that are covered for the Earth, Moon or more common meteorites in several other chapters. In most cases, Mars provides another place to apply the techniques developed for the Earth and Moon, and gives intriguingly different results.
DISCOVERY OF MARTIAN METEORITES
The first measurement of noble gas on Mars came not from a Martian meteorite, but from a spacecraft. The Viking landers carried mass spectrometers that were able to measure the composition of noble gases in the Martian atmosphere well enough to determine that the 129Xe/132Xe and 40Ar/36Ar ratios are considerably higher than in the terrestrial atmosphere (Owen et al. 1977). On the other hand, the relative elemental abundances of the non-radiogenic noble gases are very similar to the terrestrial atmosphere, although the absolute abundances are all roughly a factor of 100 lower than in the Earth’s atmosphere. The 15N/14N ratio is also higher than that in the terrestrial atmosphere. In …