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June 11, 2007
NASA SCIENTIST FINDS A NEW WAY TO
THE CENTER OF THE EARTH
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 and images:
http://www.nasa.gov/vision/earth/lookingatearth/earth-20070611.html
More
information on Lageos:
http://www.earth.nasa.gov/history/lageos/lageos.html
##
Contact:
Alan Buis
NASA Jet Propulsion Laboratory
818-354-0474
This text is
derived from:
http://www.nasa.gov/vision/earth/lookingatearth/earth-20070611.html
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