Gravity Recovery and Climate Experiment (GRACE): Detection of Ice Mass Loss, Terrestrial Mass Changes, and Ocean Mass Gains
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The gravity field of the Earth, caused by the distribution of masses inside and on the surface of the Earth, changes in time due to the redistribution of mass. Such mass fluxes can be due both to natural processes (such as the seasonal water cycle, ocean dynamics, or atmospheric variations), as well as due to human actions, such as the systematic withdrawal of groundwater for human consumption. The ability to measure such changes globally is of great significance for understanding the environmental dimension of sustainability.
KeywordsGravity Field Indian Ocean Dipole Global Precipitation Climatology Project Glacial Isostatic Adjustment Terrestrial Water Storage
- Equivalent water thickness
Since time changes in the gravity field are caused by time changes in mass distributions, equivalent water thickness (“EWT”) is the variable thickness of a thin layer of water (thin relative to both the radius of the Earth and the horizontal scale of the signals) draping the Earth that would correspond to the observed changes in gravity. The conversion from gravitational spherical harmonics to water thickness (and vice versa) is unique and well defined, regardless of what actually causes the gravitational changes. The concept is not used when studying changes in the solid Earth, such as glacial isostatic adjustment or earthquakes.
- Glacial isostatic adjustment (GIA)
Also known as postglacial rebound, it is the viscoelastic response of the mantle and lithosphere to the removal of the great ice sheets that covered parts of the Earth and peaked 21,000 years ago . The deglaciation was essentially complete 6,000 years ago. The lithosphere rises where the ice sheets used to be, but sinks in other locations.
A set of layers at altitudes between approximately 80 and 1,000 km above the Earth’s surface, with electrons and electrically charged atoms. The ionosphere leads to a delay to electromagnetic radiation, which is frequency-dependent and changes with local time and solar activity. The GRACE KBR system uses two frequencies to correct for this path delay.
K-band microwave ranging system measures the distance between the two GRACE satellites using two frequencies, 24 and 32 GHz.
Mass concentrations. The term was coined by Muller and Sjogren  to describe mass concentrations in the lunar nearside, beneath the center of the surface features termed “mare” (pl. “maria”). Today the term “mascons” refers to an alternative method to solve the GRACE gravity fields in terms of distributed spherical caps or point masses, instead of using the spherical harmonic representation.
- Newton’s law of gravitation
It states that every point mass attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G.m1.m2/r2, where m1 and m2 are the masses, r is the distance between them, and G is the universal gravitational constant, G ≈ 6.6738 × 10−11 m3/kg/s2. This law is at the heart of the GRACE measurements, since any specific mass on the Earth is in general at a different distance from the two spacecrafts, causing a slight difference in the gravitational acceleration they impart to the spacecraft, and thus causing a slight but measurable relative acceleration between the spacecrafts.
Is an object, natural (like the Moon) or artificial (each GRACE satellite) that orbits around another large object, in this case the Earth. “Orbits” means that the centripetal acceleration due to the speed of the satellite equals the gravitational acceleration between the satellite and the larger object it orbits around; in this manner the satellite neither falls toward Earth, nor escapes its gravitational pull. In practice, the GRACE satellites do fall slightly toward the Earth while they orbit around it, whereas the Moon slowly increases its distance to the Earth.
- Spherical harmonics
Are a set of functions of latitude and longitude that form an infinite, orthogonal, normalized set of basis functions whose sum, with appropriate scale coefficients, completely describes any other function defined in terms of spherical coordinates. Spherical harmonics satisfy Laplace’s equation, as does the gravitational potential outside the Earth. Laplace’s equation states that the sum of the second derivatives of the gravitational potential with respect to each of the three directions of space at a point must add up to zero if there are no masses at that point.
This work was performed in part at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and at the Center for Space Research, University of Texas-Austin. Copyright 2011 California Institute of Technology.
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