Frosty layers at Mars's north pole show that the planet is emerging from an ice age.
Mars experiences big climate shifts because of the way it tilts on its axis and orbits the Sun. Isaac Smith of the Southwest Research Institute in Boulder, Colorado, and his colleagues used a radar instrument aboard the Mars Reconnaissance Orbiter spacecraft to hunt for signs of these changes at the north pole. The geometry of the ice layers — sometimes flat, sometimes cutting across one another — allowed the scientists to work out the history of the ice. Some 87,000 cubic kilometres have built up since the end of the last ice age about 370,000 years ago.
The researchers conclude that the ice age is ending because most of this ice accumulated at the north pole, which, unlike on Earth, is warmer than the rest of the planet during an ice age.