That planetary scientists are eager for chemical information about the rocks of planet Mars is probably unnecessary information, a vast amount of money having been spend to get three spindly vehicles equipped with miniaturized petrographic instruments onto the Martian surface. Meteoriticists might say, ‘Well, we already have some Mars rock in our lab, and we can collect some more from deserts or ablated blue ice in Antarctica’. Four classes of meteorites are alleged to have been flung from Mars by impacts: the allegation is supported by the materials having oxygen isotope proportions that are different from those in rocks from the Earth or Moon.
Another class of meteorite has joined the Martian family, and it it’s a doozy. Found in the northwestern Sahara Desert the rock is a breccia containing a variety of rocks in the form of clasts (Humayun, M. and 10 others 2013. Origin and age of the earliest Martian crust from meteorite NWA7533. Nature online doi:10.1038/nature12764). In fact four other meteorites looking much the same were found near NWA7533. The bulk of the material is impact melt rock, now devitrified. Some of the clasts are also melt fragments and spherules, while others are fine-grained basalts, broken crystals and, most exciting, coarser igneous rocks rich in alkali and plagioclase feldspar. Their rare-earth element contents, like those of the Earth’s average continental crust, show evidence of fractional crystallization, particularly the removal of plagioclase to produce a marked depletion in the element europium. Slowly cooled and evolved monzonites of this kind are candidates for Martian crustal material. Overall, the texture of the breccia meteorites closely resembles the material that coats the lunar surface – regolith – but it has been lithified rather than remaining a dust.
Highly evolved igneous rocks, broadly speaking those of granitic composition, are the most likely to contain the mineral zircon, and the monzonite clasts yielded five that the US-Australian-French team subjected to U-Pb dating. The results are astonishing. These zircons formed around 4425 Ma ago, in the first hundred million years of the planet’s evolution, at the same time – within statistical error – as did the earliest materials from Earth and the Moon. Other putative Martian meteorites have yielded evidence from their neodymium isotopes that the earliest event there was the formation of a magma ocean, much as postulated for the Earth-Moon system. The latter is widely regarded as having resulted from a mega impact of the proto-Earth with an object roughly the size of Mars. The Martian monzonites may well be products of fractionation from that magma, subsequently excavated and shattered by a series of later, lesser impacts. If it did come from Mars, NWA7533 probably represents part of the early, heavily cratered highlands of the southern hemisphere of that planet.
It will be a long time before rocks can be lifted from the actual surface of Mars and transported back to Earth, and meteorites with a Martian provenance are so rare, that one can foresee a lot of very frustrated planetary petrogeneticists in the near term and a great deal of field work on desert and ice-cap surfaces looking for similar lumps of far-flung regolith.
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