Large deposits of nearly pure water-ice may lurk¹ just below the Martian surface, much nearer the equator than previously² thought, suggest new images.

新图象显示，火星表面可能埋藏大量纯净冰水，比预想地点更靠近赤道。

crater³ within Arcadia Planitia" width="466" border="0" src="/upimg/090926/4_032734_1.jpg" />
Water-ice is seen to fade over time in this 12m crater within Arcadia Planitia

The pictures acquired by a Nasa orbiter show white material exposed by fresh meteorite⁴ impacts（陨石撞击） fading over time - behaviour expected of ice on Mars.

An onboard instrument also detected the tell-tale chemical signature of water.

Researchers tell Science magazine that the observations suggest vast sheets of ice may reside in（存在于，属于） near-surface layers.

To date, exposed water-ice has only been seen at very high latitudes⁵. The US space agency's (Nasa) recent Phoenix⁶ probe famously dug into water-ice at its "high Arctic" landing site.

The implication, even with the small set of examples scientists now have, is that broad deposits of ice sit just below the red top-soil of Mars.

"There's a consistent picture starting to emerge now that these broad sheets may be common on Mars," observed Shane Byrne of the University of Arizona, a member of the team running the HiRISE camera on Nasa's Mars Reconnaissance Orbiter (MRO).

"The volume of water is probably comparable to the volume that we would have in the Greenland ice sheet on the Earth - in the buried ice deposits that stretch from each pole to mid-latitudes."

Clean ice

MRO has produced "earlier" and "later" images at five fresh impact sites made in 2008.

These were all halfway⁷ between the north pole and the equator on Mars.

The craters⁸ were small, just a few metres across, gouged⁹（凿，挖） out by incoming space rocks that may have been no more than 10cm in size.

The bright-white deposits uncovered by the impacts were seen to wither¹⁰ over time, something exposed water-ice cannot help but do in the low-pressure Martian atmosphere. It is bound to sublimate¹¹（升华物） - to turn directly from a solid into a vapour（蒸汽）.

However, the length of time it took to fade was a good indication that the ice was very pure. Had it contained a lot of dirt mixed in with it, the ice would have sublimated¹² much quicker, scientists said.

The identity and purity of the water-ice was further assessed by the team making observations with MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM).

The discoveries made by MRO are said to indicate that Mars had a more humid climate in the relatively¹³ recent past , within the last 10,000 years.

Scientists suspect much of this ice came out of the atmosphere. Water vapour in the atmosphere will diffuse¹⁴ through the particles of the soil until it gets to a certain depth where it then freezes.