ICESat-2’s three paired laser beams will cover the poles more quickly than its predecessor, and at higher resolution.
Nowhere are the realities of human-driven climate change more apparent than at Earth’s thawing poles. Arctic sea ice is vanishing, while melting ice sheets in Greenland and Antarctica are driving an acceleration in sea level rise. Yet for nearly a decade, NASA has lacked a dedicated satellite to measure how high the polar ice is piled—and how it is subsiding as ice melts or slides into the oceans.
That gap is set to close with the 15 September launch of the $1 billion Ice, Cloud and land Elevation Satellite (ICESat-2) from Vandenberg Air Force Base in California. ICESat-2 will bounce laser light off Earth’s surface, gauging changes in its elevation as small as the diameter of a pencil. Although the mission is a successor in name to ICESat-1, which ended in 2010, its multibeam laser instrument puts it in a different class, says Ted Scambos, a glaciologist at the National Snow & Ice Data Center in Boulder, Colorado. “Every season we’ll get a better map than ICESat-1 ever made.”
Whereas ICESat-1 wielded a single laser beam, ICESat-2 has three pairs of parallel beams, enabling it to scan along multiple paths at once. (The pairings are needed to calculate slopes across a given track, which will help avoid misinterpretations of ice loss when a slightly offset return pass identifies changes in elevation.) Its resolution is also far higher: where ICESat-1 took readings once every 150 meters along its track, ICESat-2 will record elevations every 70 centimeters, firing its lasers 10,000 times a second. The frequent firing means each pulse is relatively weak; to capture the faint reflections, the satellite uses a small telescope to funnel light to sensitive vacuum tubes that can detect single photons. “I’m a physicist, and I’m still shocked it works,” says Thorsten Markus, the mission’s project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
ICESat-2’s advances didn’t come cheap. Crystals used to amplify its lasers cracked when their metallic mounts expanded unexpectedly. Repairing them and addressing other complications delayed the original 2016 launch date and caused costs to balloon by hundreds of millions of dollars.
During the long wait scientists kept an eye on polar ice with the European Space Agency’s (ESA’s) CryoSat-2, which uses radar to detect height. But its readings have lower resolution because of its wider radar beam. NASA also mounted an annual airplane-based campaign called IceBridge, which worked well for Greenland but could not cope with the vast expanse of Antarctica, says Beata Csatho, a remote-sensing glaciologist at the University at Buffalo, part of the State University of New York system. “For Antarctica, this gap is really huge. We really don’t know what’s happened.”
A first task for ICESat-2 will be to assess the blank white mystery that is the East Antarctic Ice Sheet, Earth’s most massive. Extreme cold and high elevation are thought to protect it from major ice loss, but scientists want to understand how snowfall, melting ice, and shifting bedrock contribute to tiny changes in elevation. The satellite will also be able to peer into the crags of the Antarctic Peninsula, which, despite its small size, is responsible for a quarter of the continent’s ice loss. “That will certainly be the first place I look,” says Andrew Shepherd, a glaciologist at the University of Leeds in the United Kingdom and principal scientific adviser for CryoSat-2. He notes that CryoSat-2’s radar beam is too wide to deliver precise ice measurements within the peninsula’s rugged mountain peaks.
Scientists will also use ICESat-2 to monitor ice sheets’ grounding lines, where glaciers draining to the ocean first float free of the bedrock and become ice shelves. These shelves are vulnerable to melting from below by warm ocean waters, causing grounding lines to retreat inland. Because of the bowl-shaped topography of Antarctica’s bedrock, glaciologists worry the retreat could accelerate and expose ice to ever more water in a feedback process that could cause rapid ice collapse.
Grounding lines reveal themselves at the surface of a glacier by subtle changes in the slope of the ice at the point where the ice starts to rise and fall with the tides. ICESat-1 could detect grounding lines, but only a few, and only for part of the year; ICESat-2 will check on them every 3 months. “We’re going to get a much better idea where warmer water is coming in underneath the ice,” says Helen Fricker, a glaciologist at the Scripps Institution of Oceanography in San Diego, California.
If an ice shelf is about to collapse into the sea, scientists will want to put together an observing campaign right away. Csatho is exploring how to use ICESat-2 data in an early warning system that would detect sudden melting events in near–real time, rather than a year or two afterward.
When ICESat-2 is not watching ice sheets, it will measure the canopy height of high-latitude forests, providing climate scientists with a proxy measure for the carbon stored in trees. It will also join forces with CryoSat-2 to measure the snow that blankets land and sea ice. Because laser light bounces off the snow, while radar reflects from the ice below, combining the two satellites’ measurements could help investigators separate the snow from the ice. In Antarctica, where drifts can stand nearly 2 meters tall, that could sharpen measurements of changes in ice thickness. NASA and ESA are already discussing whether to shift CryoSat-2’s orbit to create more overlaps, Shepherd says. The European satellite has enough fuel to make the move, he adds.
But first, ICESat-2 has to reach orbit and show that its eye on the ice is as sharp as promised.
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