A new model of the ancient Martian climate has revealed that the glacial deposits of the planet’s tropics were laid down by snow carried to equatorial regions by monsoon winds. The findings, published today in Science, resolve the mystery about the source of the rocks and debris at the foot of Mars’s tropical mountains and volcanoes spotted by Nasa’s Viking mission in 1976.

A team led by James Head, of Brown University in Providence, Rhode Island, has now established that the deposits are the remains of large glaciers that formed a few million years ago. “What we found was that the glaciers were formed from snow brought from the polar regions,” Dr Head said.

At that time, Mars’s axis was tilted so that its ice caps were pointed more towards the Sun than they are today. Solar energy hit the ice head on, evaporating large quantities of water, and monsoon-like winds carried the water vapour south. On the slopes of huge volcanoes, the vapour cooled, condensed and fell as snow. It turned to ice, forming glaciers that carried huge boulders down the mountainsides. That debris is what can be seen today.

The researchers used a climate model that assumed the 45-degree tilt of Mars millions of years ago. “The findings are important because they tell us that Mars has experienced big climate changes,” Dr Head said.

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