Measuring Small Variations in the Earth’s Gravity

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Scientists can create higher resolution maps showing small variations in the earth’s gravity gradient by combining data from two different satellites.

The NASA–German Grace satellites were launched in May of 2002 in order to map variations in Earth’s gravity field.  The two satellites fly about 220 kilometers (137 miles) apart in a polar orbit 500 kilometers (310 miles) above Earth.  Using GPS and a microwave ranging system, the GRACE system measures changes in the speed and distance between the two satellites which in turn reflects changes in the gravitational pull of the Earth.  The result is a highly accurate and detailed map of Earth’s gravity anomalies.  In addition to measuring changes in the Earth’s gravitational pull, data from GRACE has been used in other areas, such as measuring changes in groundwater storage in the United States.

While measurements from GRACE are highly accurate, the resolution is too coarse to measure small features such as Antarctica’s smaller catchment basins. 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 have found that combining data from GRACE’s mission with data pulled from the European Space Agency’s GOCE gravity mapping mission lets them see more fine grained changes in the Earth’s gravity gradient.

By combining data from the GRACE and GOCE missions, these scientists were able to see how ice mass loss changed the gravity gradient over West Antarctica.  Ice loss between 2009 and 2012 caused a dip in the gravity field over the region as shown in the map below.


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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’.

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’. Source: DGFI/Planetary Visions

How the different datasets from the GRACE and GOCE missions were used to understand the effects of ice loss on gravity in West Antarctica between 2009 and 2012 has been illustrated in this animation produced by the ESA:

Combining gravity data from GOCE and GRACE was also used to map out changes in the Earth’s gravity field as a result of the earthquake that hit Japan in March of 2011.  The 9.0 magnitude earthquake that struck east of Japan’s Honshu Island changed the earth’s gravity field in the area as measured by data from GOCE and GRACE.

Changes in Earth’s gravity field resulting from the earthquake that hit Japan on 11 March 2011 (mE=10-12s-2).  A combination of data from ESA’s GOCE mission and the NASA–German Grace satellite, shows the ‘vertical gravity gradient change’. The 'beachball' marks the epicentre.

Changes in Earth’s gravity field resulting from the earthquake that hit Japan on 11 March 2011 (mE=10-12s-2). A combination of data from ESA’s GOCE mission and the NASA–German Grace satellite, shows the ‘vertical gravity gradient change’. The ‘beachball’ marks the epicenter. Source: DGFI/TU Delft.

References

GOCE Reveals Gravity Dip from Ice Loss. September 26, 2014.  European Space Agency. Web. 29 Sept. 2014.

Earth’s Gravity Scarred by Earthquake.  December 3, 2013.  European Space Agency. Web. 29 Sept. 2014.


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