Impacts are the most ubiquitous geologic features in our solar system. Roughly (and countless lesser pits) scar the Moon鈥檚 ancient surfaces. On Earth, where wind and water continually wear down the land, the census of stands at just 176.
Mars, a mixed bag of ancient and modern terrains, lies somewhere in between. Over the years spacecraft have glimpsed ever-finer features in the Martian landscape. These days, the HiRISE camera aboard NASA鈥檚 Mars Reconnaissance Orbiter (MRO) can pick out objects only 0.3 metres in size; the High Resolution Stereo Camera on the European Space Agency鈥檚 Mars Express is no slouch either, with a ground resolution of 2 metres.
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So HiRISE researchers were elated, but not particularly surprised, to discover some small, freshly gouged craters in images taken in 2008. Seen at five sites over a latitude range of 43掳 to 56掳 north, the excavations are typically 3 to 6 metres across and a third to two-thirds of a metre deep. One cluster must have appeared sometime between June and August, and a showed up between January and September.
What did astound the team were splashes of white seen in and around a handful of these craterlets. Could it be water ice? Colleagues operating the spacecraft鈥檚 CRISM instrument soon confirmed, for the one case large enough to yield a spectrum, that it was! Apparently fist-sized impactors had punched into a layer of ice hidden by a topping of dust about a third of a metre deep.
Disappearing act
In the months that followed, these snowy splashes gradually faded from view. Water ice isn鈥檛 stable at the craters鈥� latitudes, so most likely it gradually sublimated, or vaporised, into the atmosphere, leaving behind a veneer of any dust that had been mixed with it.
The disappearing act might also be due in part to a coating of dust blown in from the atmosphere. Either way, notes HiRISE investigator Shane Byrne of the University of Arizona, the icy deposits had to be at least a couple of inches (several centimetres) thick, and they couldn鈥檛 have been unearthed from more than a foot or two (0.3-0.6 m) down.
Byrne on Friday at the Lunar and Planetary Science Conference in The Woodlands, Texas. He points out that prior surveys, particularly one done by the neutron spectrometer aboard NASA鈥檚 Mars Odyssey orbiter, show that vast reservoirs of ice lay barely buried across most of the planet鈥檚 polar and mid-latitude regions.
So close
But scientists are only now realising just how near the surface the ice lies 鈥� and how easily it can be reached. When NASA鈥檚 Phoenix lander dropped onto a northern polar plain last May, its braking engine blew off a few inches of loose dirt and revealed slabs of nearly pure ice.
The irony in all this is that the Viking 2 lander, which arrived in September 1976, sits just 800 km southeast of the ice-splashed craterlet shown above, and scientists now realise that a layer of water ice almost certainly lies not far beneath its footpads.
鈥淚t鈥檚 probably just tens of centimetres down,鈥� says HiRISE team leader Alfred McEwen. Had Viking鈥檚 sampling scoop been able to dig a little deeper, he adds, 鈥渨e might have sampled ice on Mars 30 years ago.鈥�
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