Researchers have been debating for years whether the water that once flowed on the red planet's surface formed by either water bubbling up from the ground or precipitation. (Also see "Mysterious Martian Grooves Carved by Dry Ice Chunks?")
Now powerful new computer models of wind circulation and precipitation patterns in the Martian atmosphere around four tall Martian mountains and crater rims show the strongest evidence yet for ancient snow or rainfall down their steep slopes.
The team's simulations indicate significant orographic precipitation—that is, where snow or rain falls due to moist winds blowing up mountainsides—were particularly heavy at the heads of valley networks. (Read "Mars Snow Falls Like Dry Ice Fog.")
"We found that crater rims and other topographic highs strongly enhanced precipitation on ancient Mars," said study leader Kat Scanlon, a graduate student at Brown University.
"The fact that the most valleys are located in the areas where this effect should have been particularly strong—on the upwind side of craters, as determined by the [global climate models'] wind directions—further supports the idea that the water that flowed through these valleys originated as precipitation."
In addition to the models, Scanlon found the very same unique precipitation patterns here on Earth in Hawaii.
On some islands in Hawaii, moisture-laden tropical winds blowing in from the east run up an island's mountainsides, releasing rain on the eastern slopes that leads to the growth of tropical jungles. In contrast, the western edge of the mountains experience near-desert conditions, since the rain never makes it over the peak.
The research team hopes that if their meltwater theory holds up, it may offer new clues to the early climate and atmosphere of ancient Mars, which is thought to have been much wetter and more Earth-like in the past. (See more Mars pictures.)
What About Rain?
Scanlon and her team now plan on testing the alternate rain theory, which says that rains caused the valleys. However, models of the composition of the planet's atmosphere in ancient times suggest it never got warm enough for rain.
"What we were able to find is that snowfall does put the precipitation in the right place," she added.
The case for rain falling instead of snow, however, is far from closed, said planetary scientist Caleb Fassett, who is not associated with the study.
"Scanlon's team are influenced by recent results of [global] climate modeling ... that have a good deal of trouble raising the surface temperature warm enough on early Mars for it to rain," explained Fassett, of Mount Holyoke College in Massachusetts.
"But since we don't have a clear mechanism for producing melting, rain still is a definite possibility. This has been a very persistent problem in trying to understand early Mars—people have been going back and forth on this for decades. "
But Fassett does acknowledge the new evidence for snowmelt is quite compelling and may be a significant source of groundwater. Current calculations show as much as an inch or two (a few centimeters) of valley-carving runoff may have flowed every day for many millions of years.
The next step for Scanlon's team will include modeling how the snow would melt on ancient Mars under various climate models.
"We plan on modeling snowmelt rates under a variety of climate scenarios in order to determine which processes—like rainfall, snowmelt from impacts, or seasonal snowmelts—are most consistent with the calculated runoff amounts," she added.