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Trapped gas proves a meteorite came from Mars

Gas sealed in shock glass inside a 7.9 kg Antarctic stone matched the air Viking had measured in 1976. Terrestrial laboratories suddenly held free samples of Mars.

The flat, freshly sawn face of a pale grey stone, streaked with fine vertical saw marks and scattered with dark rounded patches, resting on a black stand beside a centimetre scale.
The cut face of EETA79001, photographed at Johnson Space Center after the first saw cut; the dark rounded patches are pods of shock-melted glass, and one of them held the trapped gas. · Image: NASA

On 12 August 1983 Science ran "Martian Gases in an Antarctic Meteorite?", four pages by two geochemists at NASA's Johnson Space Center. Bogard and Johnson heated one of the twenty-odd pods of black shock-melted glass in the sawn faces of EETA79001, a 7,942-gram stone collected at Elephant Moraine, Antarctica, in 1979. Pod BRAVO gave an argon-40 to argon-36 ratio above 2,000, xenon enriched in xenon-129, and krypton and xenon near 15 — against about 300 on that argon ratio for Earth's air, and the roughly 3,000 Viking had reported for martian air in 1976.

Shergotty fell in Bihar in 1865, Nakhla near Alexandria in 1911, Chassigny in Haute-Marne in 1815. By the 1970s dating put the nakhlites near 1.3 billion years and the shergottites closer to 180 million, against 4.5 billion for almost every other meteorite. A basalt that young needs a parent body still molten inside: a planet, and Mars was the only candidate. The objection was mechanical — escape velocity is 5.03 km/s, and an impact that violent should vaporise rock, not throw it clear.

Shock melt traps the gas around it at the instant of impact, and this gas matched no meteorite, only Mars. Becker and Pepin added nitrogen isotopes in 1984, Ott and Begemann confirmed in 1985, and by 1987 the literature had dropped the question mark. Melosh supplied the mechanism: spallation flings near-surface rock off the edge of the shock front, fast and barely damaged. EETA79001 crystallised 169 ± 23 million years ago, then spent about 600,000 years in space on the cosmic-ray clock and perhaps twelve thousand in Antarctic ice.

NASA's compendium listed 62 named martian meteorites in 2012 and 104 by 2017; the wider catalogue, counting desert fragments separately, has passed two hundred. They carry much of what we claim about the interior: the hafnium-tungsten clock on the core, the chemistry behind seismic models of the mantle. Ten years on, mineral chemistry and oxygen isotopes made ALH84001, a misfiled stone in a Houston cabinet, martian.

They are also the standing argument for Mars Sample Return. One arrives with no crater, no lava flow, not even a hemisphere, and delivery favours igneous rock: coherent basalt survives launch and entry, the mudstones Curiosity drills in Gale crater would not. EETA79001 grew terrestrial weathering along its glass veins from the moment it landed. A sealed tube in Jezero has neither problem.