Pitted deposits in Mars craters point to subsurface ice

Studies of pitted deposits in crater floors appear to indicate that subsurface ice has been more widespread on Mars than previously thought. That’s the conclusion of a team of reseachers led by Livio Tornabene (University of Western Ontario), who reported (PDF) on the finding at the 43rd Lunar and Planetary Science Conference in The Woodlands, Texas.

CRATERS WITH PITTED deposits lie in many areas of Mars, according to a new study, but only where water or ice likely existed formerly. (Image taken from Figure 2 in the paper.)

The team did a global survey using HiRISE and CTX cameras on NASA’s Mars Reconnaissance Orbiter of fresh craters and found more than 200 between 60° north and south latitudes that show pits on their floor materials. A great many Martian craters have deposits on their floors that are most likely pools and sheets of melted rock, liquified by the heat of impact, but only the 200 or so show pit features.

The craters with pits on their floors range in diameter from 1 to 150 kilometers. About 75% of the pitted craters lie between 10° and 30° north and south of the equator, latitude bands where subsurface water and ice were left by changes in the Martian climate.

Impact melt pools also form in lunar craters, says the Tornabene team. But when one examines Moon craters at high-resolution, pits like the Martian ones are not found anywhere. “The lack of observable pits in meter-scale images of fresh lunar craters suggests that the Martian pits could be due to volatile interactions with impactites generated during the impact process.”

As a possible parallel with the Moon, they point to a fresh crater (dubbed Pangboche) which they found near the summit of Olympus Mons. It shows a pool of impact melt but no pitting because environmental conditions at the volcano’s summit have long been Moon-like, with extremely low air pressure and a short lifetime for subsurface ices.

The researchers explain, “We suggest that the pits form from the interaction between hot impact-melt bearing breccias and entrained water derived from the target materials. Volatilization of water within the deposit leads to rapid, and perhaps explosive, degassing of the deposit, with pits corresponding to locations of degassing pipes.”

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