NASA’s Dawn spacecraft found evidence of surface ice in the bottom of permanently shadowed craters near the asteroid Ceres’ north pole.
Water on the surface of Ceres is rare, because even in its far-flung orbit, nearly three times farther from the sun than the Earth’s, sunlight is intense enough to cause ice to slowly sublimate into space.
Until recently, the only place ice was seen on Ceres was on the rim of a crater, where a buried ice layer appears exposed by a landslide.
But now, scientists also found ice on the floor of steep-walled craters near the north pole.
There, the endless night allows temperatures to plummet to -163 °C or below – so low some become cold traps in which water can persist indefinitely.
There are more than 600 such craters, Norbert Schorghofer of the University of Hawaii, Honolulu, reported today at a meeting of the American Geophysical Union in San Francisco, California and in a paper in Nature Astronomy.
Finding water in these cold traps is difficult because the same shadows that create them make it hard to peer inside. But Dawn’s cameras are sensitive enough for images to be “stretched” to look for bright spots, dimly illuminated by sunlight reflecting off the crater rim.
Most of the craters proved empty. But a handful, Schorghofer says, did have bright spots. And in one lucky case, a deposit edge poked into summer sunlight, letting scientists determine that it was water.
How thick the ice might be, he adds, is impossible to determine: “Anywhere from 100 microns to 100 metres.”
Similar cold traps exist on Mercury and the moon. But on the moon, there’s not a lot of exposed ice in any traps, Schorghofer says. On Mercury, all cold traps collected icy deposits.
Ceres is somewhere in between: some traps contain ice but many don’t. Why is currently unknown, but it may have to do with changes in Ceres’ axial tilt that exposed many of the now-shadowed crater floors to sunlight at various times, Schorghofer says.
But, while it is clear that ice deposits on the surface are extremely rare, this does not mean that Ceres is parched. To the contrary, there appears to be a lot of water not far beneath the surface, another team of scientists reported at the same meeting. (This work was also published today in Science.)
Thomas Prettyman of the Planetary Science Institute, Tucson, Arizona, and colleagues mapped the hydrogen content of the uppermost metre of Ceres’ surface, based on cosmic rays interactions.
Assuming that all hydrogen was contained in water molecules, they found water is actually plentiful on Ceres. Water content ranged from 16% by weight at the equator to 29% near the poles.
Some of this, especially near the poles, is probably buried ice crystals, Prettyman says. The rest likely takes the form of water-containing minerals known as hydrates.
That much water in the rock, he says, means liquid water must once have been present inside Ceres, raising the possibility that, like Jupiter’s moon Europa and Saturn’s moon Enceladus, it might once have had a sub-surface ocean. Whether that ocean had the ingredients for life, he adds, isn’t clear.
Meanwhile, the Dawn spacecraft remains healthy enough to continue operations for quite a while, says Carol Raymond, the project’s deputy principal investigator.
That’s an improved prognosis from a year ago when scientists worried that two of its four reaction wheels had malfunctioned. Mission controllers used more of the spacecraft’s precious supply of hydrazine fuel during mapping manoeuvres than intended.
But the two remaining reaction wheels have continued to perform well, Raymond says. “As long as they survive, the prospects for continuing to operate are pretty good throughout the next year.”