Mercury falling: the polar deposits imaged by Earth-based radar are shown in yellow (Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory)

You might have thought there was a snowball’s chance in hell of finding ice on Mercury, the closest planet to the sun. Now measurements by NASA’s Messenger spacecraft suggest there is about 100 cubic kilometres of frozen water at the planet’s poles – roughly enough to fill the Dead Sea.

Mercury experiences the widest extremes of temperature anywhere in the solar system, from around -170 °C by night to a scorching 400 °C by day. Its axial tilt is nearly zero, though, raising the possibility that craters at the poles might never see sunlight and so could harbour ice.

Hints that they did came in 1992, when radio telescopes on Earth discovered bright spots at Mercury’s poles. These were consistent with patches of ice, because ice provides a smooth surface that reflects the telescopes’ radar signals well. However, uncertainty remained as deposits of substances such as sulphur could also have been responsible for the bright spots.


Now instruments on board Messenger have confirmed the presence of hydrogen at these locations, a clear indication that water is present. The spacecraft also took measurements of the planet’s surface with a laser, which again suggested patches of ice – although some of the bright regions in the radar images showed up dark in the laser’s readings.

“The new Messenger data has enabled fairly conclusive proof that the radar bright features are due to the presence of water ice,” says David Paige at the University of California, Los Angeles, the lead author of one paper on the findings published today. His team has shown that the brightest regions detected by the radio telescopes match up with the coldest areas on Mercury, where ice can exist on the surface. The areas that appeared bright in the radio images but dark in the laser observations correspond to warmer patches on Mercury, where ice would be stable only if buried under a 10-centimetre-thick layer of other material.

“However, it’s pretty hard to just order up a 10-cm layer of Mercury soil when you need it,” says Paige. If the dark regions in the laser images do show where ice is buried beneath other material, then that other material may be in the form of organic compounds that were once mixed with the ice. Such organic-rich ice may have arrived on Mercury through comet or asteroid impacts.

“Shirt-sleeve” zone

The findings suggest a small sliver of Mercury is more hospitable than you might guess. “Between the burning hot equator and freezing cold poles, there is a subsurface zone that has completely habitable temperatures, a true shirt-sleeve environment if you like,” says Paige.

Earth’s subsurface is now known to be teeming with life to depths of several kilometres, and many think the best hopes for finding life on Mars will come from subsurface investigations.

Life is extremely unlikely to exist in these subsurface regions of Mercury, however, as pockets of liquid water would be very small, says Gregory Neumann of the NASA Goddard Space Flight Center in Greenbelt, Maryland, a lead author on a second paper. They could potentially support a human base on Mercury, although such a mission will not happen any time soon. “I would test out robotic landers before volunteering to go and mine rock-hard water and hydrocarbons in a complete vacuum,” says Neumann.

“It looks pretty compelling to me,” says David Rothery of the Open University in Milton Keynes, UK, and a researcher on BepiColombo, a joint European and Japanese mission to Mercury set to launch in 2015. But taking a sample directly is the only way to rule out any doubt over the presence of water, he says. To do so would probably require a probe capable of surviving a hard landing on Mercury, one that would also break up the surface enough for it to sample any water beneath.

Journal references: Science, DOI: 10.1126/science.1231106 and 10.1126/science.1229764