Last year there was a flurry of stories of water ice detected in subsurface layers over quite a bit of the surface. The Mars Reconnaissance Orbiter spotted glaciers in the southern hemisphere at latitudes of 30-60 degrees. Later detections of ice in fresh craters led to the suspicion that a similar layer extended beneath a widespread area. The Phoenix lander also saw a few possible drops condense on its leg, adding to suspicions of atmospheric vapour.
Now the mapping of the ice is complete, showing a layer extending halfway between the poles and the equator. So far that had NASA’s Viking 2 probe dug just ten centimetres deeper than it did into the soil of Mars (launched in September 1975), it would’ve encountered the ice. The layer is around a metre thick, up to ten metres and in some places down to a centimetre. It is believed to be a relic of wetter times on the red planet.
An astonishing find from observations of fresh craters was the purity of the ice thrown up by the impacts. It was believed the layers represented 50:50 mixes of ice and dust, however the stuff thrown out was 99 percent ice, 1 percent dust.
Once the ice was thrown up, it sublimated (turned directly to steam) in a couple of hundred days. MRO was able to use the time to point a spectrometer at it and confirm what it was. NASA’s press release on the subject is here and images are here.