Abundance and Diversity of Martian Meteorites from Northern Africa

Anthony Irving
University of Washington
March 31, 2005

Out of the 35 known unpaired Martian meteorites recognized through 2004, 15 of them (or 43%) were found in Northern Africa (dominantly Morocco and Algeria, with one each from Libya and Egypt). When the first Antarctic Martian meteorite was found in 1977, only six other Martian meteorites were known (and four of those were the witnessed falls of Chassigny, Shergotty, Nakhla and Zagami). Since 1977, a further 10 Martian meteorites have been found in Antarctica, as well as 3 from Oman and one from the United States. The prevalence of finds from Northern African sites cannot be because more specimens fall there; instead, it must be because local nomads and a few professional meteorite hunters in these areas are more diligent. If these same efforts were directed to other areas of the world, we should expect to find many more (and possibly different) pieces of Mars rocks. Although the averaged global frequency of recovery is about one specimen per year, the number of Martian meteorites found in all but one of the years since 1998 has exceeded this (see chart).

Column chart showing annual frequency of 29 unpaired Martian meteorite finds from 1977-2004.

This region (especially Northwest Africa) also boasts the greatest variety of Martian rock types recovered, from olivine-orthopyroxene-phyric shergottites (DaG 476, NWA 1195, NWA 2046 and NWA 2626) to olivine-phyric shergottites (NWA 1068) to basaltic shergottites (NWA 480, NWA 856, NWA 1669 and NWA 3171) to lherzolitic shergottites (NWA 1950 and NWA 2646) to nakhlites (Nakhla, NWA 817 and NWA 998) to chassignites (NWA 2737). Even so, few of these specimens resemble any of the rocks found at landing sites on Mars. Although the Mossbauer spectrum of NWA 3171 is fairly similar to that of Bounce rock at Meridiani Planum, the spectrum of NWA 1195 shows inverse relative amounts of olivine and pyroxene in comparison to the olivine-rich basaltic rocks at Gusev Crater (see Seda and Irving (2005) at http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2204.pdf).

Rather than being a disappointment, this mismatch between meteorites and Mars surface rocks (from only five landing sites) probably means that the meteoritic samples found so far were ejected from just a few locations (as is supported by cosmic ray exposure ages), and so we should expect to find other, very different types of Martian meteorites on Earth. Diverse igneous rocks, sedimentary rocks and even impact breccias (which so far have not been observed at Mars landing sites) may be waiting to be recognized and picked up by observant people in remote places.

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