Spaceborne Lasers Detect Thinning Ice Sheets
NASA's ICESat-I Measures Ice Thickness and Extent
Sep 30, 2009
The National Aeronautics and Space Administration has a mission of understanding and protecting our home planet. For this, NASA uses its robust Earth Observing System (EOS) – an army of satellites orbiting the Earth gathering critical Earth science information – to better understand the atmospheric, oceanic and geologic processes of our planet.
Recently, scientists have realized the importance of studying ice sheets and how changes in the Earth's atmosphere and climate will affect polar ice masses and global sea level. To do this, NASA designed a satellite specifically engineered to track changes in polar ice sheets called the Ice, Cloud and Land Elevations Satellite, or ICESat. The flight path of this satellite is polar orbiting, meaning it travels along longitudinal lines from North Pole to South Pole repeatedly through out the year.
Geoscience Laser Altimeter System
The sole instrument on board the ICESat spacecraft is the Geoscience Laser Altimeter System (GLAS). The GLAS uses laser light pulses at 532 nanometer and 1064 nanometer wavelengths that illuminates the Earth and measures the surface elevation of the polar ice sheets and profiles the vertical distribution of clouds and aerosols on a global scale. The pulse of the laser will measure the vertical distance from orbit to the Earth's surface 40 times per second, and characteristics of the Earth's atmosphere in between the surface and the satellite can also be detected. Laser signals returning to the satellite are collected in a one meter diameter telescope. Since February 2003, over 1.9 billion measurements have been taken using this technique.
Measuring Ice Sheets
The GLAS instrument determines the distance from satellite to the Earth's surface and intervening clouds by precisely measuring the time it takes for a laser pulse to travel to the reflecting object and return to the satellite. The one diameter telescope gathers the returning laser signal and photon detectors tell computers inside GLAS that the laser pulse has returned. Moreover, a Global Positioning System (GPS) receiver on board ICESat records exactly where laser pulses are sent out in relation to the Earth. The data on the distance the laser traveled and the position of the satellite are combined to calculate an elevation and position for each point measured. This is very valuable for determining the seasonal or annual variability of the thickness of ice sheets.
Thinning Ice Sheets
A comprehensive analysis of millions of ICESat measurements taken over the past few years shows that ice sheets are thinning both on Greenland and the Antarctic. A September 23, 2009, press release by the British Antarctic Survey summarizes a study done by the British Antarctic Survey and Bristol University and published in the journal Nature. The scientists involved with this study used GLAS data from ICESat to determine how recent increases in ocean temperatures and climate change were affecting ice sheets on Greenland and Antarctica. Using GLAS data, they were able to conclude that the elevation of these ice sheets showed thinning not only where they entered the ocean, but in the interior as well.
ICESat-I is currently offline, but a new satellite, ICESat-II will be launching sometime during 2014-2015. In the meantime, a NASA airborne science mission, called Operation ICEBridge, will use instruments, similar to the GLAS instrument, flown on board aircraft to continue taking ice sheet measurements over Antarctica and Greenland and bridge the data gap between the two ICESats. The continuity of these measurements are critical for scientists to keep track of how climate change is affecting polar environments.
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Sources:
British Antarctic Survey - Press Release: Lasers from space show thinning of Greenland and Antarctic ice sheets
NASA Goddard Space Flight Center: ICESat Science Mission
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