Fresh Mars Impact Craters Blast Ice Onto Surface

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Fresh impact craters on Mars have revealed more evidence of stable ice that’s been hiding just beneath the surface all along, say scientists working on images sent back by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter.

The craters appeared sometime between January and September of last year, in areas that had been previously imaged without them prior to January 2008. The impacts served as “natural probes” to excavate evidence that gets to a long-standing question about ice on Mars: where is it stable, and where is it residual, in the process of sublimating away?

Study leader Shane Byrne, of the University of Arizona’s Lunar and Planetary Laboratory, presented the new set of images last week, at the Lunar and Planetary Science Conference in Texas. 

Computer models agree that Mars contains stable high-latitude ice, but researchers have encountered difficulty deciding the quantity and geographical boundary of the ice, partly because they can’t see it beneath the surface and partly because pinning down an indirect measure of sub-surface ice — a long-term, global average water vapor concentration in the atmosphere — has proved challenging. 

The new craters are a significant clue, because they hit not in the high latitudes where planetary scientists are fairly certain about stable ice, but in the mid-latitudes where the ice’s reach is unclear.

“Here we report on natural probes of the Martian subsurface which have ‘detected’ ice in this critical mid-latitude zone,” wrote Byrne and his co-authors.

“In five such cases (with latitudes spanning 43.3° to 55.6° N), these impacts have excavated bright material which in High Resolution Imaging Science Experiment (HiRISE) data have a brightness and color consistent with water ice.”

Each of the five new craters is a few meters in diameter, several decimeters deep and with associated bright material a few meters across, the authors report. Four of them showed no spectral evidence of water ice. But one proved a jackpot. 

“Spectra from this site show clear water ice absorption features at 1.25, 1.5 and 2 ?m,” the team reported. Exposed surface ice is not expected to be stable at the latitudes, and the team has already noticed shrinkage and fading.

Based on atmospheric water vapor data, even stable underground ice isn’t expected to be widespread at the mid-latitudes where the reservoir was found: “Thus the ground ice exposed here is probably in the process of retreat from a previously larger extent,” the authors wrote.

Source: LPSC proceedings