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SATELLITES
REVEAL A MYSTERY OF LARGE CHANGE IN EARTH'S GRAVITY FIELD
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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.
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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 said. 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. Back
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