For the first time, the melting of glaciers in Greenland could now be measured with high accuracy from space. Just in time for the tenth anniversary of the twin satellites GRACE (Gravity Recovery and Climate Experiment) a sharp image has surface, which also renders the spatial distribution of the glacial melt more precisely.
The Greenland ice shield had to cope with up to 240 gigatons of mass loss between 2002 and 2011. This corresponds to a sea level rise of about 0.7 mm per year. These statements were made possible by the high-precision measurements of the GRACE mission, whose data records result in a hitherto unequaled accurate picture of the earth's gravity.
The GRACE gravity field measurements therefore give us information on mass changes, including climate-related ones." But there's more. The uneven distribution of mass on and within the planet causes, due the resulting variability of gravity, the earth to have an irregular shape, which deviates significantly from sphericity.
Known as the "Potsdam Gravity Potato, the geoid has achieved global notoriety. But this potato shape is equally subject to temporal changes. During the last Ice Age, a mile-thick ice sheet covered North America and Scandinavia. Since the ice melted, the crust, now liberated from its load, continues to rise to this day. This causes material flow in the earth's interior, in the mantle, to replenish. With GRACE, a joint project of the U.S. space agency NASA and the German Aerospace Center (DLR), this glacial-isostatic adjustment can for the first time be accurately detected globally as a change in the geoid height: the ice ages continue to have an effect, which is especially evident in North America and Scandinavia.
On 17 March 2012, the two GRACE twin satellites will have been in orbit for exactly 10 years. The scientists named them "Tom and Jerry", because they chase each other on exactly the same orbit around the earth. Since their launch from the Russian cosmodrome in Plesetsk, the two satellites have circled the Earth more than 55 000 times on a near polar orbit at about 450 to 500 km altitude and a distance of 220 km, and continuously collected data.
The primary scientific goal of the GRACE satellite mission is to measure the gravitational field of the earth and its changes over time on a global scale with unprecedented accuracy. If the earth were a homogeneous sphere, the two satellites would orbit at exact elliptical orbits around the Earth. But the uneven distribution of mass causes perturbations in the trajectory.
"Their analysis allows us to derive the irregular structure of the Earth's gravity," explains Dr. Frank Flechtner. "This, however, requires the satellites' orbits to be measured with high precision. Each of the two GRACE satellites is therefore equipped with a GPS receiver for positioning, an accelerometer to correct for disturbing forces due to the residual atmosphere and solar radiation, and two star trackers to determine the satellites' position in space."
But the core is the ultra-precise distance measurement system developed by NASA / JPL, which allows the separartion of the two satellites to be continuously measured with a precision of one tenth of a hair's breadth.
From the varying distance between the two satellites, GFZ scientists can determine the gravitational field of the earth. Approximately every 30 days, the satellite pair has collected enough data for a complete global map. This monthly survey of gravity is at least 100 times more accurate than any previous model, and thus invaluable for the research at the GFZ and the international user community.
"Many processes in the climate of our planet are accompanied by large-scale water mass redistributions, which are made visible in the gravitational field," adds Flechtner. This enabled, as the name of the mission suggests, the first observation and analysis of homogeneous and globally numerous climate-related processes from the monthly gravity field models over the last 10 years.
The 'Potsdam Gravity Potato', originally developed in 1995, is now much more precise thanks to GRACE. This is not a gimmick, but is required, for example, to improve the trajectories of geodetic satellites and derive accurate global reference systems from them - a prerequisite for the combination and evaluation of various global sensor systems such as the Global Positioning System (GPS), Satellite Laser Ranging (SLR), the satellite altimetry or local gauge measurements such as for the observation of sea level rise.
Another scientific objective of the GRACE mission is to derive about 150 globally distributed vertical temperature and water vapor profiles from GPS data on a daily basis. These data reach the GFZ via its own receiving station in Ny Ålesund (Spitsbergen) and are delivered to the global weather centers within 2 hours to improve global forecasts. In addition, these data are used for studies of climate induced changes in the earth's atmosphere.
The Daily Galaxy via Helmholtz Association of German Research Centres
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