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Water ice seen in fresh craters on Mars
Posted: September 24, 2009

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NASA’s Mars Reconnaissance Orbiter, MRO, has revealed frozen water hidden just below the surface of the red planet in fresh impact craters located about half-way between the north pole and the equator.

HiRISE took these images of a fresh, six metre wide crater on Mars on 18 Oct 2008, (left) and on 14 Jan 2009 (right). Water ice is the bright material visible in this pair of images, which sublimated away during the Martian northern-hemisphere summer, leaving behind dust that had been intermixed with the ice. The thickening layer of dust on top obscured the remaining ice. Image: NASA/JPL-Caltech/University of Arizona.

Bright icy deposits have been identified at five different locations within craters that range in depth from approximately 0.5 to 2.5 metres, and are about three to four metres wide, MRO scientists revealed in a dedicated press conference this evening. The craters did not exist in earlier images of the same sites, making them ideal candidates for studying how freshly exposed materials respond to the Martian environment. Bright patches darkened in the weeks following initial observations as freshly exposed ice vaporised into the thin Martian atmosphere.

“We think that all five of these impacts occurred during the first half of 2008, and we watched as the ice faded away during the second half of 2008,” says Ken Edgett of Malin Space Science Systems. “This is the first time we have seen places on Mars where new impact craters have exposed water-ice that was below the ground.”

This series of images spanning a period of 15 weeks shows a pair of fresh, middle-latitude craters on Mars in which some bright, bluish material apparent in the earliest images disappears by the later ones. The two craters are each about four meters in diameter and half a meter deep. From left to right, top then bottom row, the images were taken in 2008 on 12 Sept, 28 Sept, 9 and 14 Oct, 22 Nov and 25 Dec. Image: NASA/JPL-Caltech/University of Arizona.

Scientists already knew there was ice below the surface at high latitudes of Mars, but the new findings show that it extends far closer to the equator than originally thought, based on Mars’ climate today. “The other surprising discovery is that ice exposed at the bottom of these meteorite impact craters is so pure,” says Shane Byrne, a member of the High Resolution Imaging Science Experiment (HiRISE) on MRO. “The thinking before was that ice accumulates below the surface between soil grains, so there would be a 50-50 mix of dirt and ice. We were able to figure out, given how long it took that ice to fade from view, that the mixture is about one percent dirt and 99 percent ice.”

The confirmation of the material as ice came from spectral measurements by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). “It was crystal clear, no doubt about it – water ice,” says Selby Cull, a member of the CRISM team. “By comparing the CRISM spectra with model spectra [from the lab] of water-ice that has been mixed with various amounts of dirt, we can estimate how dirty the ice is. The ice is very pure, a solid layer of ice.”

The bright material conspicuous in this image was excavated from below the surface and deposited nearby by a 2008 impact that dug a crater about eight metres in diameter. Image: NASA/JPL-Caltech/University of Arizona.

The Phoenix Mars Lander detected fresh ice at its landing site in the north polar regions but how far the ice sheet extends towards the equator was unknown. “The scientifically heartbreaking aspect of this work is that these craters are located very close [350 miles] to the Viking 2 lander site, which landed on Mars in 1976, and dug a trench about 4-6 inches deep,” says Cull. “What this new study is telling us is that if Viking 2 had been able to dig down a few more inches, it would have hit ice. That would have been a major discovery for our understanding of Mars, and it was literally inches away from our robotic fingertips.”

How far water ice extends toward the equator depends largely on how much water has been available in the Martian atmosphere in the recent past. The way that the ice forms on Mars is that water vapour from the atmosphere infiltrates the soil and when it gets down to a certain depth it becomes cold enough to freeze out as ice. The more humid the Martian atmosphere, the more extensive the area where stable ice can form. Based on the locations of the craters, Bryne speculates that in the past 10,000 years there was double the amount of water vapour in the atmosphere compared with that present today. “The ice is a relic of a more humid climate not very long ago, perhaps just several thousand years ago, but it still wouldn't have been enough to support liquid water on the surface,” he says. “On Mars it would be of great interest if we could discover a process that involved liquid water in today’s climate, and not just in some of the warmest areas of the planet but in some of the coldest areas of the planet in the high latitude regions.”

This map shows five locations where fresh impact cratering has excavated water ice from just beneath the surface of Mars (sites 1 through 5) and the Viking Lander 2 landing site (VL2). The colour coding indicates depths to the top of a water-ice-containing layer, ranging from 1 centimetre in dark-blue coded locations to 10 metres in red-coded locations. Image: NASA/JPL/University of Arizona.

Finding new candidate impact sites is becoming routine when searching through images obtained by Mars orbiting spacecraft such as MRO, Mars Odyssey, Mars Global Surveyor (MGS) and Mars Express. About three-and-a-half years ago 19 new impact sites were found using cameras on MGS, the first time evidence for impact crater formation had been found on any planet. When MRO continued the efforts started by MGS, the inventory soon rose to some 100 impact sites, with the five at the centre of this research offering up evidence of ice. “This tells us that in places where ice may be just a few feet below the ground, we can wait for new impacts to occur and use them to test our various hypotheses about where we think there is ice below the surface,” says Edgett.

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