16.04.2020

The coral reefs of Bikini Atoll can be mapped by a NASA satellite’s green laser, which penetrates up to 40 meters underwater.

Late in 2018, just after its arrival in orbit, NASA’s ICESat-2 satellite passed over an iconic site from the atomic age. By chance, its laser altimeter, used mostly to measure the changing height of polar ice, bounced light off the exposed rocks of Bikini Atoll in the South Pacific Ocean, home to 23 nuclear weapons tests. Then, mission scientists looked closer: To their surprise, the laser was also generating underwater reflections. “We were not just seeing the atoll,” recalls Adrian Borsa, a geodesist at the Scripps Institution of Oceanography. “We were seeing this huge reef system underneath it.”

Sparked by this discovery, last week mission scientists began to map the shallow sea floors that hug the world’s coastlines. The satellite’s green laser, penetrating up to 40 meters into the ocean, will generate bathymetric data that could be “game changing,” especially for mapping coral reefs and monitoring their health, says Greg Asner, an ecologist at Arizona State University, Tempe.

ICESat-2 will fill a critical gap, says Christopher Parrish, a geographical engineer on the project from Oregon State University, Corvallis. It may be no surprise that much of the deep ocean remains unmapped, but sea floors less than 5 meters deep are also unexplored because they are off limits to ships and their sonar beams. That leaves what sailors call a “white ribbon” draped around coastlines on nautical charts. “It’s just blank,” Parrish says. “The fact that there’s a term for it tells you how prevalent that data gap is.”

ICESat-2 splits its laser to scan the globe along three parallel tracks, each one crossing every spot on Earth four times a year. Its laser fires 10,000 times per second, and each shot generates up to 60 reflected photons detected by the satellite’s telescope, says Tom Neumann, the mission’s project scientist at NASA’s Goddard Space Flight Center. “That’s literally trillions of new elevation measures,” he says. “The amount of data is mind boggling.” The travel time of the photons reveals the surface height—and any changes—to within millimeters. Those abilities have convinced the European Space Agency to adjust the orbit of its aging radar altimeter, CryoSat-2, in August, so its data can be compared with ICESat-2’s more easily.

ICESat-2’s engineers knew its green laser would delve deeper into the ocean than the infrared beam of its predecessor, ICESat. But they expected it to penetrate only 1 or 2 meters—not worth planning for. Then data started to come in that revealed far deeper reefs—not just near Bikini but also in St. Thomas and the Bahamas. “I would describe ICESat-2 as an accidental bathometer,” Parrish says.

The new data will help with coastal navigation, and the Coast Guard and National Geospatial-Intelligence Agency have already expressed interest, says Lori Magruder, a remote sensing scientist at the University of Texas, Austin, who will lead the new bathymetry product. “There’s also a rich opportunity to understand the before and after of natural disasters,” she says, including how hurricanes reshape sediments. The data will also capture changes to major features of the shallows, such as mangrove roots or kelp forests, that could show whether they are succumbing to invasive species and how their carbon storage capacity is changing as the climate warms.

But perhaps the biggest gain will be for coral reefs, says Asner, a leader of the Allen Coral Atlas, another mapping effort. Rising ocean temperatures are killing off corals worldwide, so identifying resilient species is important. Yet mapping them, from space or by drone or airplane, is expensive and challenging. “We don’t know the geography of the living parts of coral reefs today, and we’re unable to keep up,” Asner says.

For the coral atlas, Asner and his colleagues are analyzing millions of images from satellite company Planet, looking for the green light reflecting off underwater objects. But different viewing angles and light conditions make it hard to compare images, and they struggle to capture underwater features deeper than 10 meters. “It’s nice to know we can turn to ICESat-2 and use it for calibrating our work.”

On its own, ICESat-2 will be able to say much about reefs. It could reveal a reef’s slope and depth, indicators of the habitat it provides, and knobby textures that signal a more complex reef, with many intertwining coral species. Long term, ICESat-2 could measure which reefs are growing or receding, perhaps a sign that their corals have died. “You can compare the health of reefs across the globe,” says Jenn Dijkstra, a marine ecologist at the University of New Hampshire, Durham.

Waves could limit the precision of those measurements, cautions Ved Chirayath, a remote sensing scientist at NASA’s Ames Research Center. Chirayath developed FluidCam, which images the shallow sea floor using astronomical techniques to correct for wave distortions. The camera will be tested this month on a drone flight from New Mexico to Hawaii, and could end up mounted on the International Space Station.

While the ICESat-2 team continues to explore its unexpected capability, the coronavirus pandemic has opened a new avenue of research. Normally, active harbors and rivers are opaque to the space-borne laser because of sediment stirred up by boats. The global standstill has unveiled a new realm, Neumann says. “You can measure the canal depths of Venice from space.”

Quelle: AAAS