DC-8a Altimeter Provides Polar Ice Breakthrough

NASA’s DC-8 Icebridge mission has used a laser altimeter to provide high-fidelity measurements of snow thickness on sea ice and to penetrate ice sheets covering the South Pole to a depth of 10 km. (6.2 mi.). The flights, which wound up for the season last weekend, are staged from Punta Arenas, Chile, and bridge a data gap in Antarctic ice studies that opened up when NASA lost its Ice, Cloud and Land Elevation Satellite (ICEsat-1) last year after six years of operation. ICEsat-2 is not due for launch until 2015. “We wanted to avoid any ‘Oh my God’ moments when we came back in six years,” said Seelye Martin, a sea ice specialist from the University of Washington-Seattle. “We wanted continuity.” Depending on what it is mapping, the DC-8 may fly ground tracks ranging from 1,500 ft. to 39,000 ft. While ground-based ice penetrating radar and core samples have indicated how deep the South Pole’s ice sheet cover is, the laser altimeter’s ability to penetrate and profile the ice from 39,000 ft. is a major breakthrough, says Icebridge project scientist Michael Studinger of the NASA’s Goddard Space Flight Institute and the University of Maryland-Baltimore. Installed on the DC-8 this year, the laser altimeter was able to map ice sheets to 3 km. beneath the surface and show bedrock 10 km. deep. Satellites do not carry instruments that can match that penetration, he says. “Very little is known about the ice sheet and how thick it is,” Studinger says. The Icebridge images “help us understand ice flow and how to model the behavior of the ice sheets over time.” But, he added, every flight produced several terabytes of data and results thus far are from “quick looks.” Full analysis will take another six months. The altimeter also was able to provide thickness profiles for snow on surface ice for the first time. Some was a kilometer deep. The Icebridge operation is budgeted for 170 flight hr. a year but 70 of it is eaten up in instrument testing and transit to Punta Arenas. Only about 100 hr. are left for missions, which last 10-11 hr., about half of which is lost getting to and from Antarctica. Visual flight rules are followed and weather is a constant menace. Last year’s weather was good, this year’s worse than normal. So far, Icebridge has conducted more than 70 flights total, says pilot Bill Brockett. About 20% of the data-taking hours are at high altitude. Scientists have been tracking steady declines in sea ice and glacier ice in Antarctica as a probable indication of the effect climate change is having on Antarctica. Changes of as much as 80 meters a year in ice levels are common and losses sometimes are as high as 100 meters, says Martin. Their best current satellite resource is the European Space Agency’s CryoSat-2 polar orbiting ice studies satellite, which was launched in April. Its synthetic aperture radar/interferometric radar altimeter measures the elevation of ice. The DC-8 flights follow ground tracks of ICEsat-1 and CryoSat-2 so their data can be correlated.