New Scientist would like to offer its readers the chance to win a rare and fabulous prize: a genuine scrap of Mars.
The rock is a fragment of a larger piece that was blasted off the surface of the red planet by an impact long ago.
But how do we know it is genuine?
We bought this piece of Mars rock from an established collector, Luc Labenne. He in turn obtained it from Michael Farmer, who undertakes the risky business of hunting for specimens in north Africa, and bought it in Erfoud, Morocco, in 2005.
It is thought that it is part of a meteorite that crashed into the desert in Algeria, designated NWA – “north-west Africa” – 2975. In all, there may be over 100 pieces, estimated to total 1.6 kilograms.
The rarity of NWA 2975 can be gauged by its value – one 102-gram sample is on sale for $100,000. Our prize piece of rock, however, weighs around 1.7 grams; its dimensions are around 17×10×10 millimetres.
The original scientific description of NWA 2975 is based on the work of Ted Bunch of Northern Arizona University in Flagstaff and Tony Irving of the University of Washington in Seattle.
To confirm that New Scientist‘s prize is part of NWA 2975, we went to Colin Pillinger’s team at the Open University in Milton Keynes, UK, which has been studying Martian meteorites since the early 1980s. There, Andy Tindle obtained a slide of the NWA 2975 specimen from Ted Bunch for further analysis.
Our sample is still encrusted with desert sand and its reddish colour is probably also terrestrial, having resulted from weathering on Earth. You can take a (very) close look at it here through the Open University’s Virtual Microscope.
The microscope reveals that the meteorite consists of pale green prismatic pyroxene (augite and pigeonite) intergrown with a white or colourless material that started out as the mineral plagioclase feldspar. However, the shock of being launched into space by an impact on Mars converted this mineral into a glassy material called maskelynite.
Minor mineral constituents include the phosphate merrillite and three opaque species (ilmenite, pyrrhotite and ulvöspinel).
The ratio of iron oxide to manganese oxide in the pyroxene can reveal whether a specimen comes from Mars. In ours, the ratio is 28 to 38, typical of Martian rocks and distinct from volcanic rocks on Earth, which have much higher ratios.
But to be really sure that the New Scientist meteorite is Martian in origin, Richard Greenwood and Jenny Gibson removed around 0.01 gram for further analysis. Using a mass spectrometer, they studied the proportions of three isotopes of oxygen – oxygen-16, oxygen-17 and oxygen-18 – in the meteorite’s silicate minerals. Because the relative abundance of these isotopes varies throughout the solar system, they can indicate whether a meteorite comes from the moon, an asteroid or Mars (see data).
Greenwood and Gibson used a laser to melt the silicate minerals in the presence of chemicals that liberate oxygen from the sample. They then made isotope measurements on this oxygen with the mass spectrometer.
Meteorites from Mars have a slight excess of oxygen-17 and oxygen-18 compared with terrestrial rocks. This is how we were able to authenticate the New Scientist prize meteorite with total certainty: the excess was greater than it could possibly be for an Earthly rock. “The sample is probably slightly weathered, but the test is 100 per cent conclusive,” said Greenwood. “A terrestrial rock could not have this value.”
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