for National Geographic News
New radar mapping of Mars's north pole shows that the planet's current ice caps are probably only about five million years old and that in the intervening years the Martian climate has undergone several major fluctuations.
The scientists also found unusually flat bedrock under the ice, suggesting that liquid water and life—if they exist at all on Mars—are a lot deeper below the surface than previously thought.
The study, to be published tomorrow in the journal Science, used an orbiting radar instrument to peer though the ice cap, in places seeing all the way through to the underlying rock.
It has long been known that the ice cap is made of many fine layers separated by bands of dust. The ice-penetrating radar revealed that the layering extends all the way across the ice cap and contains four major divisions in addition to the thin layers.
"This means that some very large-scale process is responsible for the layering," said study co-author Roger Phillips, a geophysicist at the Southwest Research Institute in Boulder, Colorado. "The obvious culprit is climate."
These climate cycles, he added, probably link to long-term changes in the planet's obliquity—the angle at which its poles tilt toward the sun. Changes in these angles affect the strength of summer and winter temperature swings, altering climate planetwide.
One well-known cycle, operating over about a 100,000-year time period, probably accounts for the fine layers. Another, operating over the course of about a million years, likely explains the major layers, Phillips said. (Related: "Mars, Like Earth, Has Cyclical Ice Ages, Study Says" [September 14, 2007].)
If this is correct, he added, the observed number of layers suggests the ice cap probably began forming about five million years ago.
There may have been an ice cap before that, he said, but an intervening cycle of high obliquity must have warmed the poles enough during the planet's long summers to turn the existing ice directly from a solid to a gas. (Related: "Mars Pole Holds Enough Ice to Flood Planet, Radar Study Shows" [March 15, 2007].)
The present ice cap then began forming again when the climate became more suitable.
The next step is to look more closely at the south polar ice cap.
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