Any Possible Mars Water or Life Is Deep Below Surface
Richard A. Lovett
for National Geographic News
|May 15, 2008|
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.
"This is a different beast than the north polar deposits," Phillips said.
The expected layers appear in some places there, he pointed out, but elsewhere the structure looks completely different.
Kathryn Fishbaugh is a planetary scientist at the Smithsonian National Air and Space Museum who was not involved with the new study.
"The new radar results are fascinating and shed an invaluable light on parts of the polar deposits invisible to other instruments without actually drilling into them," she said.
"It will be interesting to see if interpretations of other data yield the same climate change story."
In Deep Water
The study's most surprising finding, though, came in areas where the radar was able to penetrate through 10,000 feet (3,000 meters) of ice all the way to bedrock.
Even in the weak Martian gravity, that much ice should have caused the bedrock to buckle under the load. But it does not, Phillips said, indicating that the crust is thicker or more rigid than previously believed.
This means that the crust is also colder, Phillips added, indicating that liquid water, if it exists, is buried much more deeply than previously suspected. (Related: "Mars's Water Could Be Below Surface, Experts Say" [January 25, 2007].)
"That has implications for all kinds of things, including the depth at which you might find extant life," he said.
The finding also poses problems for astronomers modeling the early years of the solar system. The only way the Martian crust can be that cold and thick is if it's unexpectedly low in heat-generating radioactive elements such as potassium 40, thorium, and uranium.
To date, Phillips said, nobody knows why the planet might have been cheated of such materials when it was forming.
"This tells us that we don't understand [Mars] as well as we thought we did," he said. "This is an amazing thing to say from the fact that the surface doesn't deflect under the ice load, but those are the implications."
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