There is no mistaking them: the tracks of tobogganers on the frigid slopes of Mars. The Martian toboggans in this case are slabs of dry ice that are now believed to slide down sand dunes and create long, narrow channels that had previously been ascribed to some sort of weird water seeps.
The new explanation for the strangely long, high-sided channels in dune faces has been put forth by a team of researchers who have tested the idea with blocks of dry ice on sand dunes of Earth, as well as theoretical work to see how it would differ in the lower pressure and temperatures of Mars. They conclude that despite it being a process that would happen nowhere on Earth naturally, it's well suited to the Red Planet, where wintertime polar conditions cause vast amounts of carbon dioxide to freeze out of the atmosphere and pile up as a polar ice cap, as well as smaller dry ice deposits at high latitudes.
"What you do is find a place where the ice has been sequestered at the top of a dune after most of the dune has defrosted," said Candice Hansen, a planetary scientists at the Planetary Science Institute a coauthor on the paper that was published in the latest issue of the journal Icarus. "That's what we've seen in images."
Then, as spring progresses, the remaining slabs warm up, de-gas and travel down the dune face. Because of the thin air on Mars, dry ice above its freezing temperature there does the same thing as on Earth: it sublimates, which means it goes directly from solid to gas, without a liquid phase between. This outgassing of the slabs appears to help lubricate them as they slip downhill.
The dry ice explanation of the seasonal channels could also solve the puzzle of why the dune-side channels end abruptly, often with small pits at the end of them. This makes little sense if they were caused by flowing water. However, if there were chunks of dry ice sitting there, subliming away, they make a lot of sense, said Hansen.
The narrow channels, are among many kinds of channels and streambed-like features on Mars that have been imaged by orbiting spacecraft for decades, and have puzzled scientists for just as long. Twelve years ago Australian geologist Nick Hoffman stirred things up with what he called the "White Mars" hypothesis, which ascribed most of the major erosional features of Mars to dry ice lubricating flows of rocks. Today the view of Mars processes seems to be one of a very mixed planet.
"Mars is a very active planet and there are multiple processes that can cause similar looking features," said planetary scientist Timothy Titus of the U.S. Geological Survey. "The dry ice is theory is not mutually exclusive of water."
In fact further from the coldest, polar regions of Mars--where the linear channels on the sand dunes are found--it's even more likely the processes are caused by water rather than frozen carbon dioxide, Titus said.
And while the discovery may not be exciting to those looking for water and life on Mars, it's still pretty neat, said Hansen.
"Dry ice on Mars is a fascinating side story," she said.