NASA Scientist Finds a New Way to the Center of the Earth

http://www.jpl.nasa.gov/news/news.cfm?release=2007-064

NASA Scientist Finds a New Way to the Center of the Earth
June 11, 2007

PASADENA, Calif. -- Humans have yet to see Earth's center, as did the
characters in Jules Verne's science fiction classic, "Journey to the
Center of the Earth." But a new NASA study proposes a novel technique
to
pinpoint more precisely the location of Earth's center of mass and how
it moves through space.

Knowing the location of the center of mass, determined using
measurements from sites on Earth's surface, is important because it
provides the reference frame through which scientists determine the
relative motions of positions on Earth's surface, in its atmosphere
and
in space. This information is vital to the study of global sea level
change, earthquakes, volcanoes and Earth's response to the retreat of
ice sheets after the last ice age.

The accuracy of estimates of the motion of Earth's center of mass is
uncertain, but likely ranges from 2 to 5 millimeters (.08 to .20
inches)
a year. Donald Argus of NASA's Jet Propulsion Laboratory, Pasadena,
Calif., developed the new technique, which estimates Earth's center of
mass to within 1 millimeter (.04 inches) a year by precisely
positioning
sites on Earth's surface using a combination of four space-based
techniques. The four techniques were developed and/or operated by NASA
in partnership with other national and international agencies. Results
of the new study appear in the June issue of Geophysical Journal
International.

Scientists currently define Earth's center in two ways: as the mass
center of solid Earth or as the mass center of Earth's entire system,
which combines solid Earth, ice sheets, oceans and atmosphere. Argus
says there is room for improvement in these estimates.

"The past two international estimates of the motion of the Earth
system's mass center, made in 2000 and 2005, differ by 1.8 millimeters
(.07 inches) a year," he said. "This discrepancy suggests the motion
of
Earth's mass center is not as well known as we'd like."

Argus argues that movements in the mass of Earth's atmosphere and
oceans
are seasonal and do not accumulate enough to change Earth's mass
center.
He therefore believes the mass center of solid Earth provides a more
accurate reference frame.

"By its very nature, Earth's reference frame is moderately uncertain
no
matter how it is defined," Argus said. "The problem is very much akin
to
measuring the center of mass of a glob of Jell-O, because Earth is
constantly changing shape due to tectonic and climatic forces. This
new
reference frame takes us a step closer to pinpointing Earth's exact
center."

Argus says this new reference frame could make important contributions
to understanding global climate change. The inference that Earth is
warming comes partly from observations of global sea level rise,
believed to be due to ice sheets melting in Greenland, Antarctica and
elsewhere. In recent years, global sea level has been rising faster,
with the current rate at about 3 millimeters (.12 inches) a year.
Uncertainties in the accuracy of the motion of Earth's center of mass
result in significant uncertainties in measuring this rate of change.

"Knowing the relative motions of the mass center of Earth's system and
the mass center of the solid Earth can help scientists better
determine
the rate at which ice in Greenland and Antarctica is melting into the
ocean," Argus explained. He said the new frame of reference will
improve
estimates of sea level rise from satellite altimeters like the
NASA/French Space Agency Jason satellite, which rely on measurements
of
the location and motion of the mass center of Earth's system.

"For scientists studying post-glacial rebound, this new reference
frame
helps them better understand how viscous [gooey or sticky] Earth's
solid
mantle is, which affects how fast Earth's crust rises in response to
the
retreat of the massive ice sheets that covered areas such as Canada
20,000 years ago," he said. "As a result, they'll be able to make more
accurate estimates of these vertical motions and can improve model
predictions."

Scientists can also use the new information to more accurately
determine
plate motions along fault zones, improving our understanding of
earthquake and volcanic processes.

The new technique combines data from a high-precision network of
global
positioning system receivers; a network of laser stations that track
high-orbiting geodetic satellites called Laser Geodynamics Satellites,
or Lageos; a network of radio telescopes that measure the position of
Earth with respect to quasars at the edge of the universe, known as
very
long baseline interferometry; and a French network of precise
satellite
tracking instruments called Doppler Orbit and Radiopositioning
Integrated by Satellite, or DORIS.

More information on Lageos is at
http://www.earth.nasa.gov/history/lageos/lageos.html .

More information on NASA's global positioning system research is at
http://sideshow.jpl.nasa.gov/mbh/series.html .

JPL is managed for NASA by the California Institute of Technology in
Pasadena.



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Media contact: Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.

2007-064

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