Researchers believe a mysterious type of crater was created as a result of Mars having a thick sheet of ice covering its surface. Ejected material from the impact would later return to the surface, slide downward on the ice and create a secondary later at the bottom of the impact site.

According to geologists from Brown University, double-layered ejecta (DLE) craters were produced as a result of a sheet of ice covering the surface of Mars. DLE craters have an impact site, a first layer of ejected debris and a smaller second layer of debris. The second layer may have been formed as ejected debris at the top of the crater rim slid off a sheet of ice as thick as 50 meters.

The DLE craters were first observed from data collected by NASA’s Viking missions in the 1970s. While there was no explanation for these types of craters, the geologists believe, based on recent research indicating a period of Mars’ history when ice was present, an object crashing into a layer of ice would have produced results similar to a DLE crater.

The new study of DLE craters was led by James W. Head, professor of geological science, and David Kutai Weiss, a graduate student at the university, and will be published in the journal Geophysical Research Letters. According to Head, in Mars’ past the climate was cool enough to have ice present at the same location where DLE craters are located. “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,” said Head in a statement.

As the geologists explain, the hypothesis of a sheet of ice causing a landslide of ejected material would explain several features of DLE craters. The inner layer of these craters have ridges flowing outward from the crater rim, called radial striations, that are commonly found in landslides that occur on glaciers on Earth.

Other DLE crater data supported the landslide on ice theory. According to Head, a steep crater rim would have been needed, and larger craters, 25 kilometers (approximately 15 miles) or more in diameter would not have a steep enough rim. Of 600 known DLE craters, all of them were less than 25 kilometers in diameter. DLE craters did not have secondary impact areas, a trait common in other craters, and the researchers believe a sheet of ice could have protected Mars’ surface from large chunks of ejected debris.

Understanding how DLE craters formed creates a clearer picture of Mars’ history. “It could tell us a lot about the history of the Martian climate on a global scale,” said Weiss.