By DAVID PERLMAN
San Francisco Chronicle
April 05, 2007
Researchers studying images of the Martian surface taken by generations of orbiting spacecraft have found that the planet's most prominent features have darkened and lightened in recent decades, altering the way sunlight reflects from its sandy soils, its rocky heights and its deep craters.
The result, say the scientists, is that the average global ground temperatures on Mars have risen by more than 1.5 degrees Fahrenheit during the past two decades, while the surface air temperature has risen by a little more than a degree in the same period.
That level of climate change, extended over a century, is well within the ballpark of the 5 degree global-warming trend on Earth that has been forecast by the International Panel on Climate Change with more and more certainty for the end of this century.
A report on Mars and its global warming was published Thursday in the journal Nature, by Lori Fenton, a planetary geologist with the SETI Institute's Carl Sagan Center in Mountain View, Calif., together with Robert M. Haberle, a climate model expert at NASA's Ames Research Center in Sunnyvale, Calif., and Paul Geissler, a Mars specialist at the U.S. Geological Survey in Flagstaff, Ariz.
On Earth, the recent decades of increasing global temperatures have been caused by increased emissions of industrial heat-trapping gases like carbon dioxide and methane, while on Mars, it's the darkening of many surface regions that allows the sun to warm the planet, Fenton says.
"Albedo" is the technical term for a planet's ability to reflect sunlight, and on Mars its overall albedo has decreased since it was first measured between 1976 to 1978 by infrared detectors aboard the two orbiting Viking spacecraft whose landers had descended to the surface in a search for signs of Martian life.
More than 20 years later, infrared heat detectors aboard the orbiting Mars Global Surveyor measured the planet's albedo and found that it had darkened significantly.
The cause, according to Fenton, is a combination of high winds that periodically scour bright sand and dust from many rocky surface regions, fierce "dust devils" similar to those that whip desert sands on Earth, and truly major dust storms like the one in 2001 that began inside a crater called Hellas Basin and then literally wrapped itself around the planet before subsiding.
The causes of global warming on Mars - still not entirely understood, Fenton said - are not only far different from those on Earth, but the temperatures involved are totally different.
According to Fenton and her colleagues, the global average ground temperature on the Martian surface as measured by Viking's instruments was about 97 degrees below zero more than 20 years ago, and by the time Mars Global Surveyor took the planet's temperature 22 years later, it had heated to a more comfortable 95 degrees below zero.
Surface air temperatures rose dramatically, too, during the period, Fenton's team calculated: from a frigid 108 degrees below zero during the Viking orbits to a sultry 106 degrees below 22 years later when Mars Global Surveyor took the readings.
Cold as those numbers are, on frozen Mars the changes mean that global warming is continuing, and whether it persists depends on where and how the winds whirl the sands around and change the planet's albedo from bright to dark or back again, Fenton said.
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