The French novelist Jules Verne's 1869 novel "Twenty Thousand Leagues Under the Sea" vividly captures the unknown underwater world. However, even 150 years later, scientists believe that the Earth's deep-sea floor is less understood than the Moon, Mars, and Venus explored by humanity. Recently, scientists from the U.S. and France succeeded in mapping previously unknown underwater hills.
Researchers from the Scripps Institution of Oceanography and the French National Centre for Space Studies (CNES) announced in the journal Science on the 12th (local time) that they have discovered over 200,000 underwater hills in the most detailed map of the seafloor created using satellite radar information.
◇About 70% of the world is unexplored territory
According to scientists, the deep-sea floors of the Pacific and Atlantic Oceans are not only home to plains dozens of times larger than the Korean Peninsula but also feature rugged mountain ranges similar to the Himalayas and countless smaller hills. About 85% of the Pacific Ocean floor and 50% of the Atlantic Ocean floor are covered by underwater hills (topographic features) that rise hundreds of meters.
The ocean occupies a vast area that corresponds to 71% of the Earth, but only a small portion has been precisely mapped. Various countries have been operating submarines and conducting marine drilling to create maps, but considerable areas still remain unexplored. Larry Mayer, director of the Center for Coastal and Ocean Mapping at the University of New Hampshire, noted, "Only about 25% of the Earth's seafloor has been mapped using sonar signals so far," adding that while recent developments in unmanned drone vessels and autonomous underwater vehicles are expanding mapping efforts, there is still a long way to go. In fact, the U.S., which has the most advanced seafloor mapping technology, only completed mapping the continental shelf around North America last year.
Traditionally, seafloor maps have been created by ships directly measuring depths in certain areas or by calculating the return signals of radar emitted from satellites. However, the ship-based approach is slow, and satellites have limitations in precision.
The National Aeronautics and Space Administration (NASA) and CNES launched a radar satellite called the Surface Water and Ocean Topography (SWOT) satellite in 2022. This satellite, which cost a total of $1.2 billion (about 1.7253 trillion won), captures more precise sea surface images by utilizing the positional differences between two radar satellite images (Synthetic Aperture Radar) collected from different locations over the same surface.
The satellite determines the height of sea surface in a region measuring 150 kilometers wide and 120 kilometers long in just 20 seconds, similar to a camera photo. For oceans, it represents each point with a 1-kilometer by 1-kilometer section, while for rivers or lakes, it uses 50 meters by 50 meters, recognizing depth differences of 3 to 10 centimeters.
The SWOT satellite was originally launched to observe changes in "water resources" between land and sea. The radar attached to the satellite accurately detects small changes in sea surface height that occur in the presence of underwater mountains and trenches (canyons). When there is a mountain on the seafloor, gravity increases, raising the sea surface. Conversely, in deep trenches, reduced gravity lowers the sea surface.
Since its launch, the SWOT satellite has secured a significantly more precise global seafloor map over the course of a year. This map, which represents the seafloor every 8 kilometers in both horizontal and vertical dimensions with one point (pixel), is regarded as more detailed than any map created in the last 30 years. It depicts the underwater terrain twice as precisely as previous methods.
◇200,000 underwater hills discovered
Scientists have previously identified underwater mountains using satellites, with the number of confirmed underwater mountains reaching 40,000 so far.
The SWOT satellite has newly discovered hills that rise hundreds of meters and span several kilometers in diameter. This is double the number previously found. Mayer stated, "The new high-resolution map covers a much wider area," and added, "This map truly represents a significant leap forward."
Scientists expect the newly created seafloor map to aid in the study of "plate tectonics" that explains continental movements. In particular, it is anticipated to bring groundbreaking changes in the study of deep-sea hills, which cover about 70% of the seafloor while remaining largely unexplored. These underwater hills form as magma erupts along mountain ranges where tectonic plates move apart. Generally, the furthest from the ridge is the oldest. The new map has revealed sudden changes in expansion directions in areas like the southwestern Indian Ocean. Scientists evaluated that this information would be useful in reconstructing existing plate tectonic theories. The frequency and size of hill formations can help understand how magma eruption rates have changed over thousands of years.
It is expected to also be useful in observing changes in seafloor topography at polar regions. Researchers have identified unique geological patterns in shallow seas between islands in northern Canada, where hills and valleys alternate in cracks. In the Weddell Sea off Antarctica, they discovered a herringbone structure exhibiting vertical patterns resembling the bones of a herring at the meeting point of the South American, African, and Antarctic tectonic plates. Mayer stated, "This map can precisely pinpoint areas where actual anomalies and strange phenomena are found," and expressed hope that the newly secured detailed seafloor map would invigorate plate tectonic theory, adding, "I am optimistic that new theories will emerge."
◇Plate tectonics, climate change, and submarine route research applications
The new seafloor map is expected to set a new benchmark for ocean current research. Underwater mountains serve to push the deep-sea water to sea surface. Underwater mountains and hills are crucial mechanisms in the conveyor belt of ocean currents that transport heat, nutrients, and carbon dioxide. The Scripps Institution of Oceanography is analyzing how underwater mountains mix ocean currents and produce small whirlpools that trap large icebergs.
The U.S. Department of Defense, which operates the world's most powerful submarine fleet, plays a major role as a primary sponsor of this seafloor mapping project. Precise underwater maps are essential for safely navigating submarines. This will help submarines avoid dangers while traversing the deep ocean. It is expected to provide useful information during seabed drilling and the laying of underwater cables. Lirong Pu, a researcher at NASA's Jet Propulsion Laboratory, stated, "The satellites have been functioning perfectly since launch," adding that "there will be groundbreaking results in the coming years."
Scientists believe the seafloor map will become more precise. Researchers are using overlapping sonar data and satellite data to train artificial intelligence (AI) models, which are then employed to improve the resolution of the maps. The research team noted, "The information produced by AI is expected to provide much better terrain data than past maps."
Reference
Science (2024), DOI: http://doi.org/10.1126/science.ads4472
Science (2024), DOI: http://doi.org/10.1126/science.zoam474