Not so long ago the ocean floor was as unknown as the far side of the Moon. Now, an international team of scientists is using satellite data to chart the deep ocean by measuring the Earth's gravitational field. The result is a new, highly-detailed map that covers the three-quarters of the Earth's surface that lies underwater. The map is already providing new insights into global geology.

Up until about a hundred years ago, the only way to learn anything about the bottom of the sea was to toss a piece of lead overboard on the end of a line. Measuring the line gave the depth and if you were really sophisticated, you stuck a wad of wax on a depression in the lead to bring up a sample of the bottom. More sophisticated techniques were developed, but at the end of the day, they were just weights and grabs on the end of a very long string.

Things took a step forward with the invention of sonar, but even with decades of intense work by the world's great navies and scientific institutions, only a very small fraction of the seafloor was mapped at a resolution that missed whole mountains. What was needed was a way of measuring the bottom features without having a ship passing over every point to take direct measurements.

Using data from ESA's CryoSat-2 and NASA/CNES' Jason-1 satellites as well as older space missions, scientists from the University of Sydney have now taken a more lateral approach. They used the data to build a map of the Earth's gravitational field by studying how the orbits of the satellites changed, and from this deduced what the seafloor looked like.

The idea was based on the fact that the Earth's gravitational field is far from uniform. Because of the Earth's irregular shape and the presence of areas of denser materials in the crust, the field is very uneven, with its pull varying from place to place. This can be measured in units called milligals, which is a change in the pull of gravity by one thousandth of one centimeter per second squared.

This change in the Earth's surface distorts sea level heights above them. By combining the orbital data with sea level heights and comparing them against direct bottom measurements using computer analysis, a map of the sea floor can be drawn with resolutions down to objects 5 km (3 mi) wide. The result is a world map in shades of orange and red, which represent denser areas corresponding to underwater ridges, seamounts, and the edges of Earth's tectonic plates, and darker blue areas for less dense areas showing the deepest troughs and trenches.

This map is already producing new insights into the ocean floor, as noted by a team from the Scripps Institution of Oceanography and the US National Oceanic and Atmospheric Administration when studying an area of the Indian Ocean floor the size of West Virginia or Tasmania. This area broke off from the Antarctic tectonic plate when it rammed into Asia at 15 cm (6 in) per year to create the Himalayas some 47 million years ago. Called the Mammerickx Microplate after seafloor mapping pioneer Jacqueline Mammerickx, it's the first found on the Indian Ocean floor. The impact was so hard that the microplate spun like a ball bearing – though at a geological pace.

NASA says that the new map will provide a better understanding of how the Earth's plates move, which may lead to better earthquake prediction. It will also prove valuable to prospectors, as well as surface ship and submarine captains.

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