August 6, 2013
Researchers Discover How Mysterious Mars Craters Were Formed
redOrbit Staff & Wire Reports - Your Universe Online
The over 600 double-layer ejecta (DLE) craters on Mars resulted because the planet's surface was covered by a thick sheet of ice during the time of impact, geologists from Brown University have discovered.
However, this type of crater is different because the debris forms two distinct layers - a large outer one with a smaller, inner one resting on top. DLE craters were first observed during the Viking missions to Mars back in the 1970s, and ever since that time researchers have attempted to determine what caused the formation of the double-layer pattern.
Now, in research set for publication in an upcoming edition of the journal Geophysical Research Letters, Weiss and Head report they have discovered the answer to the long-standing mystery. The craters were the result of impacts onto surfaces that were coated with a layer of glacial ice tens of meters thick.
"Recent discoveries by planetary geoscientists at Brown and elsewhere have shown that the climate of Mars has varied in the past. 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," explained Head.
He and his colleague Weiss devised a scenario to explain the phenomenon. In their hypothesis, the impactor crashes through the ice layer, causing rock and other ejecta to spew forth onto the frozen water surrounding the crash site. However, since the ejected material lands on a slippery surface, not all of it stays where it originally lands.
"Weiss and Head believe the layering occurs when material near the top of an upraised crater rim slides down the slippery ice and overtops material on the lower slopes," the university explained in a statement. "That landslide, enabled by steep slopes and a slick ice layer, creates the DLEs' telltale two-layered appearance."
"I think for the first time since DLEs were discovered in the 1970s we have a model for their formation that appears to be consistent with a very wide range of known data," said Weiss. Learning more about how these types of craters formed could help researchers recreate the environmental conditions at the time of the impacts, the author noted.
The research, which was funded by a NASA grant, led to discoveries that could help explain several features unique to these types of craters, including the radial striations common on the inner ejecta later of DLEs. Those striations, which are grooves that radiate out from the crater's rim, are common in landslides here on Earth, Weiss explained.
The ice-based model also helps to account for the lack of secondary craters surrounding DLEs. "Secondary craters are the result of big chunks of ejecta blasted out of the main crater, leaving gouges in the surrounding surface when they land," the university said. "But if that surrounding surface were covered by ice, evidence of shallow secondary craters would disappear when the ice disappeared."