Mysterious debris fields found far from the poles on Mars were made by glaciers, possibly formed just like glaciers are on Earth — by the buildup of snow, researchers said Friday.
The glaciers would have resembled those found on Earth in places such as Mount Kilimanjaro in Africa or the Andean peaks in South America, the researchers report in Friday's issue of the journal Science.
They probably formed when Mars was tilted on its side five million years ago, Brown University planetary geologist James Head and colleagues said.
Researchers were intrigued when spacecraft data showed curious rock-strewn deposits at the foot of some Martian volcanoes and mountains close to the equator.
These scraped-up piles of rock and sand in certain valleys and along the western flanks of the three giant volcanoes look like what is left behind by a moving glacier on Earth.
It is now very cold and very dry on the Mars surface, and water cannot stand in any one place on the planet for long, even as ice.
Researchers have speculated whether the tropical glaciers came from snow or perhaps oozed up from underground.
The team of U.S. and French researchers ran climate simulations that suggest glaciers could have formed when the Sun heated up the poles, sending snow there into the atmosphere and allowing it to fall elsewhere on the planet.
The models predict locations for these glaciers that match many of the glacier remnants seen today.
"What we found was that the glaciers were formed from snow brought from the polar regions," Head said in a statement.
They note that Mars often has changed its tilt so that the poles face the sun and the equator gets only oblique sunlight.
"Actually, it last occurred only 5 1/2 million years ago," said Francois Forget of the University of Paris, who led the team.
The sun's rays could vaporize the snow in a process known as sublimation, also seen here on Earth. Winds would carry the water vapor south, up and over the soaring slopes of the Tharsis Montes volcanoes and the giant Olympus Mons volcano.
The vapor would cool and condense into snow, which would eventually harden into an icy glacier.
"The findings are important because they tell us that Mars has experienced big climate changes in the past, the kinds of climate change that led to the Great Ice Age here on Earth," Head said.
"The findings are also interesting because this precipitation pattern may have left pockets of ice scattered across Mars. This is good information for NASA as officials plan future space missions, particularly with astronauts."