Satellite to measure Antarctic changes

Bentley

February 24, 1999
Web posted at: 9:45 AM EST

Charles Bentley, a University of Wisconsin-Madison glaciologist and preeminent authority on the West Antarctic Ice Sheet, has been making trips to the Antarctic since 1957 to study the rapidly moving ice streams that carry ice from the interior to the edge of that continent.

Soon he will have some help from above. ICESat, a small satellite equipped with a sensitive laser altimeter that will be able to precisely measure the height of things like clouds, ice sheets and swelling volcanoes, is scheduled for launch in 2001.

"My focus would be on using the laser altimeter to see what's happening on the West Antarctic Ice Sheet, to look for evidence of rapid shrinking," said Bentley. "We're more interested in how it changes with time than how high it is."

From its near-polar orbit 420 miles above the Earth, ICESat will emit pulses of infrared light at a rate of 40 times a second, each pulse illuminating an area 70 meters in diameter. As the light reflects back into space, it will be collected by a telescope aboard the satellite. The distance from the satellite to the reflecting surface will be determined by measuring the time it took for the light to make the round trip.

The altimeter, known as GLAS for Geoscience Laser Altimeter System, is being developed under the direction of Bob Schutz at the University of Texas at Austin. Bentley is a member of the GLAS science team. The ICESat satellite is a future member of NASA's Earth Observing System and is being built at Goddard Space Flight Center in Maryland.

Interest is high in the West Antarctic Ice Sheet because it is a dynamic and unstable system that contains 3 million cubic kilometers of ice. Each year it dumps huge amounts of ice into the ocean, sometimes spawning icebergs the size of Rhode Island or larger.

At the margins of the ice sheet, marine ice shelves that protrude over the ocean may be susceptible to a warming sea. Some scientists think that if the ice shelves were thinned enough, an important brake on the ice streams that shuttle ice from the interior of Antarctica would be removed, leading to an accelerated discharge of ice into the ocean and a rise in sea level.

The West Antarctic is also the only ice sheet on the planet that sits atop slippery marine sediments that are well below sea level. Because of this, some scientists think the ice sheet may slowly collapse into the sea. Scientists disagree about how long such a slow-motion cataclysm might take, Bentley said. Estimates range from a few hundred to a few thousand years or longer. Some don't believe the collapse will occur at all.

The main drawback to the ICESat project for Bentley is its short three- to five-year mission life.

Noticing change may take longer than the projected mission of ICESat, said Bentley. While the satellite can measure new accumulations of ice and snow, annual fluctuations in the amount of new ice and snow could mask any small but telling changes in the ice sheet's mass balance, he said. The ICESat mission may not be long enough to pin down trends in annual precipitation in the Antarctic, he said.

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