Strange looking craters on Mars could display evidence of an ancient ice-covered tundra.
Scientists believe over 600 "double-layer ejecta" (DLE) Martian craters were once covered by ice. A second layer would have been formed when debris slid down the craters' icy sides, a Brown University press release reported.
The craters are characterized by "a large outer layer with a smaller inner layer sitting on top."
"Recent discoveries by planetary geoscientists at Brown and elsewhere have shown that the climate of Mars has varied in the past," James W. Head, a professor of geological science, said. "During these times, ice from the polar caps is redistributed into the mid-latitudes of Mars as a layer about 50 meters thick, in the same place that we see that the DLEs have formed. This made us think that this ice layer could be part of the explanation for the formation of the unusual DLE second layer."
Head believes the layering was formed by a "landslide' of debris. The debris were made up mostly of rock, and other material "spat out" from below the ice when the object of impact broke through the thick icy layer.
Icy landslides make sense in a number of ways. The craters have radial striations ("grooves radiating out from the crater rim"), these markings have been observed in many Earthly landslides, especially when it takes place on a glacier.
Ice reduces the coefficient friction of a slope, making it more likely debris will slide down it. This is what the researchers believed they were looking at with the craters' inner rim.
"When I did a quick calculation, I realized that the landslide wouldn't be expected to happen [on crater rims] unless the ejecta was landsliding on an ice layer," graduate student David Kutai Weiss, said.
A steep rim would be required to create the calculated landslides, but larger craters have less of a slope.
Weiss calculated the craters must be under 25 kilometers in order to illicit a landslide capable of creating the DLEs. The researcher found nearly all of the 600 DLE craters were below that size.
Another factor pointing towards ancient ice was the absence of a second, surrounding crater. These craters are normally a result of chunks of rock or other debris blasted from the main crater upon impact.
A thick sheet of ice could have protected the planet's surface from the ejected debris, and the secondary crater would only have been ice-deep.
"There are over 600 DLEs on the Martian surface, so reconciling how they formed with our knowledge of the climate of Mars is pretty important," Weiss said. "It could tell us a lot about the history of the martian climate on a global scale."
