European Space Agency satellite reveals gravity dip from ice loss

A press release from the European Space Agency: Although not designed to map changes in Earth’s gravity over time, ESA’s extraordinary satellite has shown that the ice lost from West Antarctica over the last few years has left its signature.

More than doubling its planned life in orbit, GOCE spent four years measuring Earth’s gravity in unprecedented detail.  Scientists are now armed with the most accurate gravity model ever produced. This is leading to a much better understanding of many facets of our planet – from the boundary between Earth’s crust and upper mantle to the density of the upper atmosphere.

The strength of gravity at Earth’s surface varies subtly from place to place owing to factors such as the planet’s rotation and the position of mountains and ocean trenches.

Recently, the high-resolution measurements from GOCE over Antarctica between November 2009 and June 2012 have been analysed by scientists from the German Geodetic Research Institute, Delft University of Technology in the Netherlands, the Jet Propulsion Lab in USA and the Technical University of Munich in Germany.

Remarkably, they found that the decrease in the mass of ice during this period was mirrored in GOCE’s measurements, even though the mission was not designed to detect changes over time.

Using gravity data to assess changes in ice mass is not new. The NASA–German Grace satellite, which was designed to measure change, has been providing this information for over 10 years. However, measurements from Grace are much coarser than those of GOCE, so they cannot be used to look at features such as Antarctica’s smaller ‘catchment basins’....

Changes in Earth’s gravity field resulting from loss of ice from West Antarctica between November 2009 and June 2012 (mE = 10–12 s–2).  A combination of data from ESA’s GOCE mission and NASA’s Grace satellites shows the ‘vertical gravity gradient change’

Gravity is climate

The Daily Galaxy: 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.

One of Newton's laws states that the gravity of an object depends directly on its mass. "When the mass of the Greenland ice sheet changes, so does the gravity there," explains Dr. Frank Flechtner from the GFZ German Research Centre for Geosciences. "

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....

Image of the Potsdam gravity potato from the Daily Galaxy website