Satellites reveal hidden features at the bottom of Earth's seas

A new marine gravity model of the central Indian Ocean, which is poorly charted and includes the presumed crash site of Malaysia Airlines Flight 370. The red dots represent strong earthquakes, which together outline the locations of current seafloor spreading ridges.

Oceanographers have a saying: Scientists know more about the surface of Mars than they do about the landscape at the bottom of our oceans. But that may soon change. Using data from satellites that measure variations in Earth’s gravitational field, researchers have found a new and more accurate way to map the sea floor. The improved resolution has already allowed them to identify previously hidden features—including thousands of extinct volcanoes more than 1000 meters tall—as well as piece together some lingering uncertainties in Earth’s ancient history.

Roughly 90% of the deep-ocean sea floor remains unmapped, a fact that’s been thrown into sharp relief by searchers’ inability to locate Malaysia Airlines Flight 370, which is thought to have crashed somewhere in the southern Indian Ocean in March. What maps do exist have been largely generated by instruments such as ship-based multibeam sonar systems, which take soundings of the depth to the sea floor as they crisscross select areas of ocean. That leaves “big holes hundreds of kilometers across with no data,” says David Sandwell, a geophysicist at the Scripps Institution of Oceanography in San Diego, California.

Satellite-based radar altimeters can help fill in those gaps. These instruments bounce signals off the surface of the ocean as they pass over it multiple times a day. When corrected for wave heights and tides, the repeated pinging of the ocean surface builds up a picture of its overall topography, its bumps and valleys. Those bumps at the surface of the ocean, Sandwell says, reflect features—such as seamounts or extinct volcanoes—on the sea floor below. “A seamount, for example, exerts a gravitational pull, and warps the sea surface outward,” he says. “So we can map the bottom of the ocean indirectly, using sea-surface topography.”

Right now, four satellites have high-resolution radar altimeter data sets available to scientists: the U.S. Navy’s aging Geosat and Geosat Follow-On missions; the European Space Agency’s (ESA’s) ERS-1; Jason-1, a joint mission between NASA and CNES, the French national space research center; and CryoSat-2, the youngest of the four, an ESA satellite launched in 2010 to study the polar ice caps. Until recently, seafloor gravity maps relied on declassified data from the Geosat missions (launched in 1985 and 1998, respectively) and on ERS-1 data. But those instruments were unable to resolve some features of tectonic plate boundaries, particularly if they were covered by sediment layers. “We could see ridges and transform [faults] that weren’t buried by sediment”—but anything under sediments was basically blurry.

But now, new data from Jason-1 and CryoSat-2 are sharpening the focus. The difference, Sandwell says, is like the difference between ordinary and high-definition television. In particular, CryoSat-2’s high-range precision altimeter—as well as its 4 years of amassed data (each of the other missions collected data for only a couple of years)—makes it the star of the show. Using those data—as well as significantly improved analysis techniques—Sandwell and his colleagues built a new marine gravity model that is at least twice as accurate as existing models, they report online today in Science.

Among the new features they’re now able to detect, Sandwell says, are thousands of previously unknown seamounts between 1000 and 2000 meters tall dotting the ocean floor. They also discovered an 800-kilometer-long now-extinct (i.e., no longer actively spreading) ridge in the South Atlantic Ocean that formed after Africa and North America rifted apart. The team also reports the exact location of a now-extinct seafloor spreading ridge, a zone where two tectonic plates began pulling apart 180 million years ago to form the deep basin that became the Gulf of Mexico. “That was a surprise to me—you’d think everyone would know everything about the Gulf because it’s so well-studied,” he says. “Of course, people knew it opened from seafloor spreading, but they didn’t know exactly where the ridge and transform faults were.” Those features were so deeply buried by sediment that the gravity signals were extremely faint.

The authors “have dramatically improved the signal-to-noise ratio in their marine gravity grid, allowing very subtle features to be resolved,” says geophysicist Paul Wessel, a geophysicist at the University of Hawaii, M­anoa, who was not involved in the study. “This work brings home the importance of collecting new data, as well as applying expert processing to older data—squeezing out more information than was thought possible.”

Oil companies—and, Sandwell notes, the Chinese government, which is exploring the South China Sea—are keenly interested in the new gravity maps as a reconnaissance tool. Oil and gas exploration focuses along continental margins where the sea floor is relatively flat under thick layers of accumulated sediment, and the new gravity maps make it possible to “see” the sediment basins that are possible reservoirs.

One thing the new maps won’t do, Sandwell notes, is locate the missing Malaysia Airlines jet. Satellite-based instruments—even the high-range precision radar altimeter aboard CryoSat-2—just don’t have high enough resolution to spot a plane-sized object in the vast, 60,000-square-kilometer swath of southern Indian Ocean now identified as the priority search area. “If you want to find an airplane, you’ve got to resolve to maybe 10 meters,” he says. “Even the best a ship’s multibeam sonar can do is about 100 meters [resolution].” The Australian Transport Safety Bureau, which is leading the search for the lost plane, last week released new data from its bathymetric survey of the priority search area, which used shipboard multibeam sonar; when the survey is complete, searchers will use towed deep-sea submersibles to continue the search, scanning the sea floor at even greater resolutions.

But there are more than enough new seafloor data from Jason-1 and CryoSat-2 to keep the marine science community busy for the next 5 years, Sandwell says. “It’s a lot of new information, and it’s all very detailed”—and buried in there, somewhere, are probably hundreds of new discoveries waiting to be uncovered.

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