earthdata.nasa.gov By Laura Naranjo
A new satellite mission sheds light on Earth's gravity field and provides clues about changing sea levels. According to legend, Isaac Newton discovered gravity after watching an apple fall from a tree. Using the word "gravitas" (Latin for "weight"), he described the fundamental force that keeps objects anchored to the Earth. Since then, scientists have used maps of the Earth's gravity to design drainage systems, lay out road networks, and survey land surfaces. But Newton probably didn't imagine that gravity could reveal new information about the global hydrology cycle.
Traditionally, scientists constructed gravity maps using a combination of land measurements, ship records, and more recently, remote sensing. However, those measurements weren't accurate enough to capture the slight changes in water movement that cause gravity to vary over time. With the help of a new satellite mission, scientists can now weigh water as it circulates around the globe and relate these measurements to changes in sea level, soil moisture, and ice sheets.
To better assess these gravity variations, an international team of engineers and scientists developed the Gravity Recovery and Climate Experiment (GRACE) mission. Launched in March 2002 as a joint venture between NASA and the Deutsches Zentrum fuer Luft und Raumfahrt (German Aerospace Center), the mission was implemented through collaboration between the University of Texas Center for Space Research, the GeoforschungZentrum (Germany's National Research Centre for Geosciences), and the NASA Jet Propulsion Laboratory (JPL).
GRACE relies on two identical satellites, each about the size of a car. As the satellites fly approximately 220 kilometers (137 miles) apart, one following the other, a microwave ranging system monitors the distance between them to within a micron -- smaller than a red blood cell. Scientists can map gravity anywhere on the Earth's surface by measuring tiny changes in distance between the two satellites as each of them speeds up and slows down in response to gravitational force.
Archived at NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC) in Pasadena, California, and the GeoForschungZentrum Information System and Data Center (GFZ/ISDC), GRACE data are changing the way scientists and modelers view gravity. GRACE provides monthly maps that are at least 100 times more accurate than previous maps at detailing changes in the Earth's gravity field. "The classic idea of gravity being something that you measure once is no longer accepted. Gravity is an element that scientists must continue to monitor," said Byron Tapley, director of the Center for Space Research and principal investigator for the GRACE mission.
Because scientists can't see, feel, or directly observe gravitational forces, they map the Earth's gravity using a mathematical model that describes an imaginary spherical surface called the geoid. The geoid represents oceans as smooth, continuous surfaces unaffected by tides, winds, or currents. It creates a locally horizontal surface against which scientists can measure the downward pull of gravity.
Gravity is determined by how much mass a given material has, so the more mass an object has, the stronger its gravitational pull. For example, granite is a very dense material with a high level of mass, so it will exert a greater pull than the same volume of a less dense material, such as water. Earth's mass is distributed between various landforms and features -- such as mountain ranges, oceans, and deep sea trenches -- that all have different mass, which creates an uneven gravity field.
More:
https://earthdata.nasa.gov/user-resources/sensing-our-planet/matter-in-motion-earth-s-changing-gravity
