Black glass holds first Mars soil sample on Earth

Veins of black glass in a meteorite that recently crashed in Morocco contain the first chemical traces of Martian soil brought to Earth. The find represents a rare chance to look closely at ancient surface conditions on Mars.

Robots sent to Mars, such as NASA's Curiosity rover, only have limited capabilities to analyse the soil samples that they take. Until the launch of a sample-return mission, the most thorough way to study Martian rock is via meteorites that originated on Mars, says Hasnaa Chennaoui Aoudjehane of Hassan II University in Casablanca.

In July 2011 people saw a fireball streak across the sky and smash into the Moroccan desert. The 7-kilogram meteorite, dubbed Tissint, broke apart as it fell, and both scientists and private collectors quickly retrieved the fragments.

Initial analysis showed that the rock's chemical composition matches that of a type of Martian basalt. The meteorite was most likely thrown up from the planet about 700,000 years ago as the result of an asteroid impact.

Pristine sample

Tissint is only the fifth Martian meteorite collected promptly after falling to Earth. Most of the 90 or so known Mars rocks that have been found on Earth had been lying around for years. By contrast, the Tissint fragments should provide an unadulterated look at Mars's geology. "It's so fresh, such pristine material," says Aoudjehane.

Her team found that the meteorite is laced with a large amount of bubbly black glass. It contains carbon and nitrogen isotopes that are characteristic of those found in Mars's atmosphere, something that has been seen in other Martian meteorites.

More surprisingly, the glass contains relatively high amounts of light rare-earth elements not found in the rest of the meteorite, including an unusual ratio of cerium isotopes that indicates some of the cerium got oxidised. Conditions that would oxidise cerium are most likely to exist close to Mars's surface.

The team says weakly acidic water may have leached rare-earth elements from Martian soil and deposited them in fractures in surface rocks. Heat from the asteroid impact that ejected Tissint melted the material in the fractures, which crystallised as it cooled to form the black glass.

Further analysis of Tissint should reveal more details of such geochemical processes on Mars, rounding out our picture of the planet's past.

"The history of Mars is interesting for us because it's related to the history of the Earth, and it's important to know how Mars was in the past and how it evolved with the times," says Aoudjehane.

Journal reference: Science, DOI: 10.1126/science.1224514

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