17 - Martian meteorites

Summary

Introduction

This class of meteorites is a group of magmatic rocks whose relatively young crystallization ages implies derivation from a planetary-sized body rather than an asteroid [17.1], leading several authors to consider that the specimens may come from Mars [17.2–17.4]. The suggestion of a potential Martian origin was strengthened by the discovery of argon with a ⁴⁰Ar/³⁶Ar ratio compatible with that of Mars’ atmosphere (which is very different from the terrestrial atmosphere). The gas was trapped within impact-melt glass in shergottite EETA 79001 [17.5]; subsequent analysis of other noble gases, nitrogen and CO₂ [17.6–17.8] confirmed a strong link with the composition of the atmosphere of Mars (Figure 17.1).

For many years, Martian meteorites were referred to as the SNCs (for Shergotty, Nakhla and Chassigny, type specimens of the three original groups within the class). This was before the Martian origin of the meteorites was completely accepted. Now, however, evidence that the meteorites are from Mars is almost indisputable, and the number of groups has increased, so it is prefereable to drop the acronym and simply refer to the samples as Martian meteorites. The Martian meteorites are all igneous rocks and provide critical constraints on the mineralogical, geochemical and geophysical properties of the Martian crust, mantle and core. There have been many reviews of Martian meteorites; the most complete are by McSween [17.1, 17.11], plus the ISSI special issue [17.12, 17.13]. An extremely valuable resource that maintains an updated bibliography of Martian meteorites is the Mars Meteorite Compendium [17.14]. Much of the information in this chapter has been derived from these sources.

Collection of meteorites from Antarctica and hot deserts has dramatically increased the inventory of Martian meteorites. As of June 2014, and not accounting for pairing, there were 132 Martian meteorites listed in the Meteoritical Bulletin [17.15], only five of which are observed falls.

Shergottites (after Shergotty) are the largest group of Martian meteorites, 107 specimens in total at June 2014 (again, not accounting for pairing). Originally, just a single group of basalts [17.1], shergottites are now divided into one of three subgroups: (i) basaltic shergottites (meteorites with a volcanic origin derived from a fractioned magma); (ii) lherzolitic shergottites (meteorites with a plutonic origin and a cumulate texture) [17.16]; (iii) olivine–phyric or picritic shergottites [17.17, 17.18] (meteorites with olivine–porphyritic textures).