Peering beneath the ice at the north pole of Mars has now revealed the Red Planet may be surprisingly colder than was thought.
Any liquid water that might exist on Mars therefore might be hidden deeper than once suspected, closer to that world's warm heart, researchers suggested.
An international team of scientists used the Mars Reconnaissance Orbiter to probe the north pole of the Red Planet with radar. The ice cap there goes about 1.2 miles deep and is roughly the size of Pakistan at 310,000 square miles large.
These scans revealed the polar cap has up to four layers of ice rich in sand and dust, each separated by clearer sheets of nearly pure ice. Each dirty and clean layer is some 1,000 feet thick.
These dirty and clean layers were created by ages of intense dust storms followed by icy eras. This five-million-year-long cycle was likely driven by wobbles in Mars' tilt and fluctuations in the shape of its orbit around the sun. The more sunlight the Red Planet saw because of these changes, the more the polar ice caps retreated and the more dust storms Mars saw.
"All this layering is key evidence for theoretical models that predict that changes in Mars' climate are coupled with orbital changes," said researcher Roger Phillips, a geophysicist at Southwest Research Institute in Boulder.
Unexpectedly, the radar scans also revealed the massive weight of the ice cap does not deform any underlying sediment. This implies the crust beneath the cap is strong — more than 180 miles thick.
To have such a thick crust, "Mars might be colder than we thought," Phillips told SPACE.com. As a result, any liquid water that might be underground has to be buried even deeper than once speculated. "If one thought that liquid water was 5 kilometers deep (3 miles), it's now at least 30 percent deeper than that," he said.
Philips and his colleagues detailed their findings online May 15 in the journal Science.
As to why Mars might be so cold, "perhaps it was robbed of its fair share of heat-producing elements, such as uranium and thorium, when it was first born," Phillips said. Or perhaps the way heat flows on Mars is quite variable, with the crust being colder and thicker at the poles and hotter elsewhere — "like Mars' volcanic provinces of Tharsis and Elysium."
The upcoming Phoenix Lander mission, which will explore the water ice just underneath the surface soil of the Martian arctic plains, might help shed light on the layering at Mars' north pole. To solve the mystery of the heat of Mars, heat flow probes are likely needed all over that planet, as is suggested by the Mars Network Mission under proposal, Phillips said.