By Jonathan Amos
Science reporter, BBC News
Water-ice is seen to fade over time in this 12m crater within Arcadia Planitia
Large deposits of nearly pure water-ice may lurk just below the Martian surface, much nearer the equator than previously thought, suggest new images.
The pictures acquired by a Nasa orbiter show white material exposed by fresh meteorite impacts 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 near-surface layers.
To date, exposed water-ice has only been seen at very high latitudes. The US space agency's (Nasa) recent Phoenix 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."
MRO has produced "earlier" and "later" images at five fresh impact sites made in 2008.
These were all halfway between the north pole and the equator on Mars.
The craters were small, just a few metres across, gouged 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 wither over time, something exposed water-ice cannot help but do in the low-pressure Martian atmosphere. It is bound to sublimate - to turn directly from a solid into a vapour.
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 sublimated 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 relatively 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 diffuse through the particles of the soil until it gets to a certain depth where it then freezes.
The locations of the exposed ice fit with the models used to predict where ground ice might be stable, i.e. from mid up to high latitudes.
"The more humid the Martian atmosphere, the more extensive the area of this stable ice," explained Shane Byrne.
"Based on the locations of these craters, we are able to say something about how much water was in the Martian atmosphere recently, and that turns out to be a lot more than is in the atmosphere today - maybe almost double what's in the atmosphere today."
The Viking-2 lander was probably close to a remarkable discovery
CRISM team member Selby Cull, from Washington University in St Louis, said there was a "scientifically heart-breaking" consequence of the MRO discoveries.
This is the realisation that had Nasa's Viking-2 lander, which visited the Red Planet in 1976, dug a little deeper into the Martian soil, it would almost certainly have struck the water-ice MRO is now seeing at the base of impact craters.
"That would have been a major discovery for our understanding of Mars," she said.
"It was literally just inches away from our robotic fingertips. And instead, it has taken more than 30 years to finally make this discovery."