Public release date: 1-Aug-2002
Contact: Lynn Chandler
NASA/Goddard Space Flight
Center--EOS Project Science Office
reveal a mystery of large change in earth's gravity field
Satellite data since 1998 indicates the bulge in the Earth's gravity
field at the equator is growing, and scientists think that the ocean
may hold the answer to the mystery of how the changes in the trend of
Earth's gravity are occurring.
Before 1998, Earth's equatorial bulge in the gravity field was getting
smaller because of the post-glacial rebound, or PGR, that occurred as a
result of the melting of the ice sheets after the last Ice Age. When
the ice sheets melted, land that was underneath the ice started rising.
As the ground rebounded in this fashion, the gravity field changed.
"The Earth behaved much like putting your finger into a sponge ball and
watching it slowly bounce back," said Christopher Cox, a research
scientist supporting the Space Geodesy Branch at NASA's Goddard Space
Flight Center, Greenbelt, Md.
Currently, the Earth has a significant upward bulge at the equator, and
a downward bulge at the poles. "Observations of the Earth's gravity
field show that some phenomena are counteracting the gravitational
effects of PGR. Whereas PGR has been decreasing the bulge in the
Earth's gravity field at the equator, this recent phenomena is causing
the bulge to increase," Cox said. Such changes in the gravity field can
be sensed using ultra precise laser tracking of satellites to observe
tiny changes in the orbits of those satellites and by tracking changes
in the length of day or rotation of the Earth.
Scientists believe movements of mass cause this recent change from the
high latitudes to the equator. Such large changes may be caused by
climate change, but could also be part of normal long-period climatic
variation. "The three areas that can trigger large changes in the
Earth's gravitational field are oceans, polar and glacial ice, and
atmosphere," Cox said.
Cox and colleague Dr. Benjamin Chao have ruled out the atmosphere as
the cause. Instead, they suggest a significant amount of Ice or water
must be moving from high latitude regions to the equator, and oceans
could be the vehicles of this movement.
Estimates of today's glacier and polar ice melting are too small to
explain the recent changes in the gravity field. If melting ice were
the cause of the recent changes in the gravitational field, it would
require melting a block of ice 10 km (6.2 miles) on each side by 5 km
(3.1 miles) high every year since 1997 and pouring it into the oceans.
"The recent reports of large icebergs calving in Antarctica can't
explain this, because they were already floating in the ocean," Cox
Further, radar altimeter observations of the average sea level rise
provided by the TOPEX/POSEIDON satellite show no corresponding change
in the rate of the global sea level increase.
Consequently mass must have been redistributed within the oceans.
That's where the ocean circulation theory comes in. Ocean currents can
redistribute mass quickly, such as the 5-year time frame that these
changes were first observed. The TOPEX/POSEIDON observations of sea
level height do show an increase in the equatorial bulge of the oceans
corresponding to the observed gravity changes, but the data are not yet
conclusive. One critical factor is the temperature of the world's
oceans, and its salinity, for which detailed data are not yet available.
In 2002 NASA also launched the GRACE and JASON missions, missions that
will help to more precisely track these sorts of changes in Earth's
geodesy, and will launch the ICESAT mission this winter.
An article on this NASA-funded study appears in the August 2 issue of
the journal Science.
For more information and images, go to:
Return to the
Originating Document of The M+G+R
The web site for the International Laser Ranging Service can be found
For more about the TOPEX/Poseidon mission, go to: