The deep furrows carved in the sides of Martian craters were most likely formed by snowmelt in the planet’s recent geological past, according to a new study. The findings indicate that seasonal flows of liquid water may have streamed down the craters’ flanks when Mars was a wetter planet, as recently as a few hundred thousand years ago. Today, the Red Planet is a colder and drier place; although the Mars Phoenix Lander found water ice buried under the dirt near the north pole, no liquid water currently exists on the planet, and any ice exposed to air quickly turns into vapor due to the low atmospheric pressure.
The gullies were first sighted several years ago, but researchers couldn’t immediately determine what had caused them. [S]ome scientists proposed that the features were formed either by dry avalanches or by groundwater pushing up from below the surface and running down the sides of craters [SPACE.com]. But in a new study of crater images taken by the Mars orbiters, researchers found evidence that ice and melting snow were the culprits.
Researchers believe that glaciers formed on Mars when a change to the planet’s orbital axis caused the planet to tilt more drastically than it does now, causing a Martian ice age. The new study, published in the Proceedings of the National Academy of Sciences [subscription required], found evidence of these glaciers in telltale piles of rock and debris that fan out at the base of the crater, in patterns that resemble those found in Antarctica‘s Dry Valleys. As temperatures warmed, due to seasonal changes, the ice sublimated into a gas, leaving behind the depressions [Discovery News].
On Mars, snow could have fallen relatively recently on the crater rim, drifting into the depressions. “You don’t need much snow to create a gully,” Marchant said. “The little bit that falls is transported to favored areas. There it can melt. When it does so, it does so quickly and creates enough meltwater to carve gullies.” The slope and shapes of the debris fans at the base of the gullies indicates that they are geologically quite young, he added…. Thus Mars is by no means a dead planet, he said: “It undergoes climate cycles much like Earth” [National Geographic News].
Image: National Academy of Sciences, PNAS