In The Field

AGU: Martian ice, in a new light

The power of the HiRise camera on NASA’s Mars Reconnaissance Orbiter continues to amaze. Its latest trick, PI Alfred McEwen just told a session here on MRO results, is to look at buried ice in Mars’s mid latitudes.

Seeing ice that’s half a metre or so beneath the surface of flat plains might seem hard even for eyes as sharp as HiRise’s. The trick is to spot small impact craters just after they have formed. Small craters get made on Mars at a reasonably high rate, because little impactors that would just be pretty lights in the sky on earth make it all the way through Mars’s thin atmosphere. So HiRise is already spotting craters that have been made since it got to Mars a few years ago. In one of these, which they know was made in the summer of 2008, HiRise saw a couple of little white dots.

Because the craters were less than 5 metres across, the McEwen and his colleagues reckoned that this put the white dots about 50 centimetres down. They’ve now found four other more or less contemporary craters on the Arcadia plain which have similar white patches, and the spectrometer on MRO, CRISM has confirmed that the largest white patch is water. Over time the patches fade as the ice turns to water vapour, or gets dusty, or both.

There’s nothing new about finding water on Mars, and I haven’t yet had a chance to find out whether ice in these latitudes (about 45 degrees north) is a surprise to people who think about such things. But as McEwen pointed out, it makes it all the more likely that Viking 2, which landed at a similar latitude not that far away, might have been sitting on ice, if only it had been able to scrape down a little farther. It also means anyone with future interest in ice would not necessarily need to go as far north as Phoenix did.

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