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Input: Billions of years ago  Mars was likely much wetter than it is today. The ice has a series of layers  says  Colin Dundas   a planetary geologist at the U.S. Geological Survey and the paper's lead author. Those newly detected layers suggest that the ice was laid down over a period of time  through a process that happened repeatedly.
"Something caused it to be deposited and then deposited again " says  Dundas . That thing was likely snowfall   Dundas  says  which was eventually compacted into these deposits. The snowfall could have happened as recently as tens of millions of years ago  he says.
The scientists think the exposed ice is not stable at the relatively warm surface temperatures.  Dundas  says ice at the surface is transforming from solid into vapor  causing the slopes to collapse and become further exposed.
There have also been significant questions about how pure Mars ice is. But  Dundas  says that "at these locations  its  quite a thick ice sheet of rather clean ice."

Thick 'Rather Clean' Ice Sheets Are Spotted On Mars : The Two