Figure 1. One of the GRACE earth orbiters showing some of the instruments. GRACE earth orbiters use state-of-the-art instruments and complex processing to detect and translate the very small changes in distance between them into measurements of Earth’s mass.

By measuring the relative distance between GRACE's two satellites, we measure tiny changes in the gravity field of Earth [Fig. 3]. The gravitational pull of Earth varies slightly from place to place because subsurface water and rocks have different densities. This has been known for a few hundred years, and has been measured with gravimeters (gravity meters) carried on trucks, ships and airplanes. However, over much of the world, we know little about this gravity signal, and we know almost nothing about how it changes from year to year. For example, when ice sheets melt, or sea level rises, or when atmospheric pressure increases, this causes tiny changes in that areas gravitational pull. GRACE is very sensitive to small changes; for example, from the data we will be able to measure the change in gravity due to the evaporation of 1 cm of water spread over 230 km on thew side. With this powerful satellite tool we hope to see changes in the water content of aquifers, shifts in ocean currents, and variations in the mass of ice sheets.

Figure 2. Artists conception of the two GRACE earth orbiters stowed on the launch rocket.

GRACE's data will help answer one of the most challenging questions in oceanography and climate research: is sea level rising? Currently we have a lot of data from satellite altimeters, such as TOPEX/Poseidon, that can precisely track changes in the volume of our oceans. For example, if the sun heats up a small area of the ocean, it will expand upwards (it cannot easily go sideways), so the altimetry will detect this rise. But the actual mass of water in that area has not changed (only its volume has changed). Measurements from GRACE, in addition to the altimeter, would enable us to determine if the sea level rise is caused by the expansion of water, rather than added water mass [Fig. 4]. What is of great interest to climate watchers is sea level rise caused by melting of polar ice sheets. So if an actual change in mass of the oceans occurs, it will be detectable by combining altimetry and measurements from GRACE. My study site will be the oceans around the Antarctic, the Antarctic Circumpolar Current [Fig. 5] which links the Pacific, Atlantic and Indian oceans. There is no other current in the world that links all these major oceans. Recently, colleagues at the Scripps Institution of Oceanography noticed a large planetary wave that travels all around this current, taking 8 years. Because of its size, it is capable of moving as much heat as some El Niño's. By measuring changes in this current over several years with both GRACE and altimetry, we expect to better understand how the oceans move heat around the world.

Figure 3. Changes in the earth’s mass result in minute changes in the force acting on the individual earth orbiters. These changes in force result in micron sized changes in the distance between the GRACE earth orbiters.

Figure 4. GRACE data will be used with altimeter data to monitor sea level changes and mass changes.

Figure 5. The circumpolar current around the Antarctic continent.