The ocean floor is a vast scope on our planet that remains largely unexplored. Although oceans cover 71% of Earth, we have clearer pictures of the Moon and Mars than we do of the landscape beneath the waves. But a newly launched satellite mission coined SWOT (Surface Water and Ocean Topography) is now giving scientists a much sharper picture of these hidden underwater worlds.
When researchers talk about “mapping the ocean floor,” they usually refer to where underwater mountains, hills, and valleys lie. These features may sound distant and abstract, but they actually influence things a lot of the things we care about: the path of undersea cables that give us internet, the routes ships can safely sail, and even how marine life finds food.
Until now, only about a quarter of the seafloor has been mapped using ships with special sonar equipment. The rest has remained a mystery. However, the SWOT satellite is changing that by doing something a little out of the box. SWOT measures tiny changes in the height of the ocean’s surface from space, even down to a few centimetres. These small changes happen because underwater mountains called seamounts have extra gravity that pulls water slightly higher above them, creating a bulge on the surface.
Older satellites detected only large bulges like larger seafloor mountains. In contrast, SWOT can pick up much smaller masses.
“The SWOT satellite was a huge jump in our ability to map the seafloor,” said David Sandwell, a geophysicist at Scripps Institution of Oceanography in La Jolla, California. He’s used satellite data to chart the bottom of the ocean since the 1990s and was one of the researchers responsible for the SWOT-based seafloor map, which was published in the journal Science.
Previous ocean-observing satellites have detected massive versions of these bottom features, such as seamounts over roughly 3,300 feet (one kilometer) tall. The SWOT satellite can detect seamounts less than half that height, potentially increasing the number of known seamounts from 44,000 to 100,000.
We might rightfull ask ourselves why bother to map underwater hills and valleys so precisely. WQell, one reason is that these features affect how ocean currents carry heat and nutrients around the globe. They also influence the movement of marine life. Additionally, accurate maps are vital for undersea construction, laying internet cables, and guiding submarine navigation.
“Seafloor mapping is key in both established and emerging economic opportunities, including rare-mineral seabed mining, optimizing shipping routes, hazard detection, and seabed warfare operations,” said Nadya Vinogradova Shiffer, head of physical oceanography programs at NASA Headquarters in Washington
At SWOT’s core is a specialized instrument called the Ka-band Radar Interferometer (KaRIn). It works by sending out radar pulses toward the Earth’s surface and recording the return signal with two antenna arrays. Each antenna receives a signal slightly out of sync, or “out of phase,” compared to the other. By comparing these phase differences, researchers can precisely calculate the height of the water column below.
Unlike traditional altimeters, which measure only a narrow strip of the ocean directly beneath a satellite, SWOT’s interferometer sweeps out a wide swath ten of miles across on both sides of the satellite’s ground track. As the satellite orbits Earth, these overlapping swaths create a two-dimensional map of the planet’s water surface, covering nearly all the world’s oceans as well as large rivers, lakes, and reservoirs.
Through repeated passes, approximately once every 21 days, SWOT can trace how water heights change over time. This allows researchers to see how ocean currents flow, where floods occur, how droughts progress, and what parts of the ocean floor might lie hidden under slight bulges in the sea surface caused by underwater mountains and hills.
SWOT team members are also excited to learn more about abyssal hills covering the seafloors. These geological structures are the most abundant on Earth, covering more than half of the seabeds.
“Abyssal hills are the most abundant landform on Earth, covering about 70% of the ocean floor,” said Yao Yu, an oceanographer at Scripps and lead author on the paper. “These hills are only a few kilometers wide, which makes them hard to observe from space. We were surprised that SWOT could see them so well.”
SWOT’s mapping complements an effort by the international scientific community to chart the entire seafloor using ship-based sonar by 2030.
“We won’t get the full ship-based mapping done by then,” said Sandwell. “But SWOT will help us fill it in, getting us close to achieving the 2030 objective.”