There are currently 31 known martian meteorites (see Martian Meteorite Summary table). They have been found on every continent except Australia. Four of them (Shergotty, Zagami, Nahkla and Chassigny) were observed to fall between 1815 and 1962. The 27 meteorites found long after they fell include 11 Antarctic and 16 non-Antarctic samples. The samples vary widely in size, ranging from 12 g to 18 kg, with the smallest samples being several Antarctic samples. The non-Antarctic samples are curated at natural history museums around the world and are provided for research, traded with other museums, or bought and sold by mineral dealers. Even though four samples are falls, they were recovered long before their significance was understood. Special care in collection and curation was unknown at the time. The Antarctic samples are prohibited by international treaty from being used for profit. They are owned by government organizations and provided for research to investigators. This document focuses most attention on the five US Antarctic meteorites collected by NSF and curated by NASA and the Smithsonian Institution (SI) as part of a joint US Antarctic Meteorite Program. These samples are carefully collected and curated in special clean labs.

Classification: S = shergottite, N = nakhlite, C = chassignite, ALH84001 is none of these.

Year is recovery date for non-Antarctic meteorites and date of martian classification for Antarctic meteorites. Year of recovery for Antarctic meteorites is indicated by the first 2 numbers in the name.

Curation Locations: Antarctic meteorites include MWG (JSC) and SI for US program and NIPR (Tokyo) for Japanese program. Non-Antarctic meteorites are curated in natural history museums around the world. The main US collections are at the Smithsonian (SI), Field Museum (Chicago) and American Museum (New York).

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Classifications

All of the martian meteorites are igneous rocks formed by crystallization of cooling magma in Mars' crust. As such all are achondrites formed by differentiation in their parent body rather than primitive chondrites formed by accretion of chondrules in the early solar nebula. Martian meteorites have many similarities to HED achondrites which are basaltic eucrites, cumulate diogenites and howardites formed by mixing the two types. Martian meteorites were grouped with HED meteorites until their young crystallization ages and distinct oxygen isotope trend defined them as a distinct SNC group.

The SNC group was named after the type samples of the first three rock types: basalt Shergotty, clinopyroxene cumulate Nakhla, and olivine cumulate Chassigny. As more samples of the group were identified, some of them fit well into the existing types; Zagami and EETA79001 as shergottite basalts and Lafayette and Governador Valadares as nakhlite cumulates. ALHA77005, however, was found to be related to shergottites, but be a cumulate rather than a basalt. It was nonetheless classified as a shergottite. By 1985 the discovery of martian atmosphere trapped inside EETA79001 had convinced many meteoriticists that the SNC meteorites were from Mars.

ALH84001 was misclassified as a diogenite (orthopyroxene cumulate) because it fit well into that mineralogical classification and because the SNC meteorites were presumed to be limited in their lithologies. In 1993 it was found to contain oxidized iron in its accessory oxide phases, in contrast with the reduced iron in the oxides of HED meteorites. This implied that ALH84001 came from a volatile-rich Earth-like planet rather than a volatile-poor asteroid. Oxygen isotope analyses proved that it was a part of the SNC family even though it is not a member of any of the three types.

There are now five distinct igneous rock types in the martian meteorite family. Bulk compositions are generally consistent with mineralogy. Shergottites are divided into pyroxene-plagioclase basalts (S-B) and pyroxene-olivine lherzolites (S-L). Nakhlites (N) are clinopyroxene cumulates. Chassigny (C) is an olivine cumulate or dunite and ALH84001 is an orthopyroxene cumulate (O). The first three types are populated by multiple samples, ALH84001 and Chassigny are singular members of their classifications. Because Mars is a complex planet one can expect that as more martian meteorites are identified, new rock types will continue to be found.

This review attempts to summarize salient features of martian meteorites for scientists planning to evaluate the evidence for fossil life on Mars. It is not a review of what the petrology and geochemistry of martian meteorites tell us about the planet. An excellent review paper on that topic was recently published by McSween (1994). The abstract and table of contents of that paper are included at this site. Meyer (2003) has completed a catalog of martian meteorites. That work was very useful in preparing this summary and the references from it are included at this site.

References:

• What We Have Learned about Mars from SNC Meteorites
• Pigeonholing Planetary Meteorites: The Lessons of Misclassification of EET87521 and ALH84001
• Mars Rock Compendium - Compiled References (Word document)