NASA Scientist Finds a New Way to the Center of the Earth
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.
Image right: This spectacular "blue marble" image is the most detailed true-color image of the entire Earth to date. A new NASA-developed technique estimates Earth's center of mass to within 1 millimeter (.04 inches) a year by using a combination of four space-based techniques. The more accurate frame of reference has applications ranging from improving estimates of global sea level rise to improving our understanding of earthquakes and volcanoes. Image credit: NASA/GSFC + Larger view
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
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.
Media contacts: Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.