Analysis of a meteorite found in northwest Africa, prosaically named NWA 7533, indicates that it is the first sample of the regolith, or 'soil', of Mars, and is derived from the earliest Martian igneous crust yet identified. See Letter p.513
NASA's decadal survey for planetary science1 concludes that returning samples of the ancient crust of Mars to Earth ranks among its highest priorities for exploring the Solar System. In this issue, Humayun et al.2 (page 513) describe a Martian meteorite sample already on Earth, albeit without the geological context that samples collected on Mars would have. Nonetheless, it is a revealing discovery.
The meteorite, which is called NWA 7533 (Fig. 1; NWA is an acronym for northwest Africa, where it was found), was part of a celestial rock that broke up during its passage through the atmosphere, producing at least five recovered stones. Another member of this group of stones, NWA 7034, was described previously3 as a volcanic breccia, which means that it is composed of fragmentary material produced from basaltic lava. Humayun et al. have interpreted NWA 7533 — and, by extension, NWA 7034 — as being a regolith breccia. Regolith is the planetary surface layer that is pulverized by meteor impacts (planetary scientists often use the terms 'regolith' and 'soil' interchangeably, which drives soil scientists mad). Regolith breccias are soils compacted and cemented into rocks by impact-derived melts. Many lunar samples returned by the astronauts of the Apollo missions are regolith breccias.
NWA 7533 contains clasts (fragments) that texturally resemble impact-derived melts in lunar regolith breccias, but with chemical compositions unique to Mars. The compositions of the clasts are nearly identical to those of basaltic rocks and soils analysed by the Spirit rover during its trek through the Gusev Crater on Mars. The high abundance of normally rare elements in the clasts, such as nickel, osmium and iridium, supports the idea that NWA 7533 is a regolith breccia. Lunar soils are also rich in these elements, because they have been bombarded by chondritic meteors and, over time, become contaminated with their debris. The composition of chondritic meteors is thought to reflect the primordial composition of the terrestrial (rocky) planets before these elements were sequestered into the planets' cores. Contamination by chondritic material also accounts for the high levels of iridium found in strata on Earth from the Cretaceous–Tertiary geological boundary, famously cited as evidence that a meteor impact was responsible for the extinction of the dinosaurs.
The real surprise is the ancient age reported for NWA 7533: 4.4 billion years, demonstrating that this breccia is a sample of the earliest Martian crust. The age was determined by analysing the radioactive-decay products of uranium in zircon crystals, which concentrate this element. Zircon crystals typically form during magma crystallization, and these impervious crystals probably survived pulverization and melting of their host rocks by impacts. The age differs from that previously reported3 for NWA 7034 (2.1 billion years), an age that was obtained using the decay of radioactive rubidium. The younger age determination, based on analysis of the bulk rock, may represent a mixture of the ages of formation of different components that make up the breccia, or may record some isotopic disturbance that occurred long after the igneous crystallization of the original basaltic rocks. The new, older age implies that a thick Martian crust formed within the first 100 million years or so of the planet's history, coeval with the formation of the Moon's crust.
These new Martian meteorite breccias are fiendishly complex rocks, and forthcoming investigations will surely reveal more surprises and conundrums. Detailed studies of the various types of breccia clast, including age dating and analysis of their geochemistry and petrology, could help to unravel the geological record of early Mars.
It has become apparent that Martian meteorites have different chemical compositions from rocks analysed on the planet's surface4. Various explanations have been proffered to explain this difference5,6. But with the discovery of these latest meteorite breccias, we have a handful of paired meteorites that have the composition of Mars surface rocks, as well as one rock from the Martian surface with a composition like that of the meteorites7.
Increasingly, the world's meteorite collections are being augmented by finds in hot (northwest Africa) and cold (Antarctica) deserts. Both sources have revealed previously unknown meteorite types, but it is unfortunate that these unique Martian meteorites fell in Morocco rather than on Antarctic ice. The acquisition of meteorites from hot desert countries for research typically depends on the ability to buy them, as opposed to the case with Antarctic meteorites, which are collected, curated and subsampled under nearly pristine conditions and allocated widely and free of charge on the basis of the scientific quality of proposals to study them. But we will gladly accept more samples of Mars from wherever we can get them.
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Meteoritics & Planetary Science (2017)