Sen— Impacts onto a thick layer of ice on ancient Mars could explain the double-layered pattern found on hundreds of craters, according to a new study.
The craters, known as double-layered ejecta (DLE) craters, were first spotted by the Viking probe in the 1970s. Scientists have been puzzled as to how the layered patterns were created.
Craters are surrounded by material excavated by the impact of a space rock smashing into the surface, but what makes DLEs unusual is that the debris around the crater has two distinct layers. The double layer is made up of a larger outer layer with a smaller second layer on top.
The new study by planetary geologists at Brown University suggests the DLEs are the result of impacts onto a layer of glacial ice tens of metres thick. The impact would have blasted through the ice scattering debris onto the surrounding ice. Ejecta around the newly formed crater rim would then form a landslide down the ice from the structurally uplifted rim, creating a second layer on top of the outer ejecta.
In the ancient past it is believed that ice, maybe up to 50 metres thick, migrated from the polar ice caps to the mid-latitudes of both hemispheres where the DLEs formed.
David Weiss, a graduate at Brown University and one of the paper's authors, said: “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.”
The "glacial substrate" model proposed is supported by a number of points. Firstly, grooves radiating out from the rim of a DLE crater -- known as radial striations -- are common in landslides on Earth and especially landslides on glaciers. “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," explained Weiss.
The researchers also noted that landslide would require a relatively steep slope around the outside of the crater rim to have been formed by the impact. The researchers calculated that craters with a diameter of more than about 25 km would not have steep enough slopes to cause a landslide. The 600 DLEs studied were all found to be between 1 km and 25 km in diameter, suggesting they would have steep enough slopes away from the crater rim to create the top layer landslide.
Other evidence supporting the glacial substrate model was the general absence of secondary craters around the double-layer ejecta craters. Secondary craters are caused by large chunks of ejecta smashing into the surface around the main impact site. If such ejecta landed on ice they would be less likely to leave their mark when the ice disappeared.
The study provides further insight into Mars' past. Weiss noted: “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. It could tell us a lot about the history of the martian climate on a global scale.”
The research is published in the journal Geophysical Research Letters.