Scientists Get Closer to Center of the Earth

By Jeanna Bryner, LiveScience Staff Writer

posted: 15 June 2007 10:54 am ET

: Earth

Fiery journeys to the center of the Earth occur only in the sci-fi realm, but now scientists have laid out a way to pinpoint our planet’s center of mass, providing a more accurate map of that core destination.

The results will lead to critical information for studying earthquakes, volcanoes, global sea-level rise and warming, and a post-glacial rise in some surface areas related to the melting of ice sheets.

Until now, scientists have defined Earth’s center of mass in two ways—either as the mass center of Earth as a single object or as the mass-center of Earth’s system, including ice sheets, oceans and our atmosphere in the equation.

Jell-O planet

It's tricky finding the center of a giant, squishy object, scientists say.

"By its very nature, Earth's reference frame is moderately uncertain no matter how it is defined," said Donald Argus of NASA’s Jet Propulsion Laboratory in California. “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."

If Earth were a completely solid, perfectly round object, finding its center of mass would be sweet and simple. However, as first proposed by Isaac Newton, our planet is not perfectly round.

Now scientists know Earth is somewhat of a “buckled” sphere in which the midsection bulges outward. And because mass is distributed unevenly across its surface (more mass means more gravitational tug), the point around which the planet is balanced is offset from the actual center of Earth.

Plus, mass doesn’t stay put, but instead changes over time as glaciers melt, tectonic plates move and volcanoes empty out to lay massive lava on Earth’s surface. These changes in mass atop and beneath Earth’s surface cause the center of mass to shift slightly over time.

Accurate measure

Argus developed the new center-finding technique to find the mass center of Earth as a single object, which can estimate Earth’s center of mass to within 0.04 inches a year. The center of mass is calculated as a relative measurement, and so the measurement is given as a velocity.

Past estimates calculated the center of the whole Earth system, resulting in the 2000 and 2005 estimates differing by about 0.07 inches a year.

Argus believes the mass center of solid Earth is a more accurate reference frame, because it doesn’t change from year to year.

The new technique, described in the June issue of the Geophysical Journal International, relies on data collected from a string of instruments. These include a network of global positioning system (GPS) receivers, a network of satellites called Laser Geodynamics Satellites (LAGEOS), radio telescopes and satellite-tracking instruments.

The Earth-only reference frame will improve estimates of sea-level rise made by satellite altimeters, which rely on measurements of the location and motion of the center of mass of Earth’s system. Sea-level rise is a gauge of global warming, and so the results will boost scientists’ understanding of the increase in our planet’s average temperature. The rising seas are thought to be the result of melting ice sheets in areas such as Greenland and Antarctica.

“If you changed the velocity of the Earth’s center a millimeter (0.04 inches) per year, you would change the estimate of sea level by less than one millimeter,” Argus told LiveScience, “but even half a millimeter would be important.”

On the rebound

In addition, how Earth will respond physically to past and future glacial melting has been shrouded in uncertainty. When frozen, the mammoth weight of miles-high sheets of ice causes tiny deformations of Earth’s crust. And when this ice melts, the land rebounds a bit. Earth is still on the rebound from the close of the last ice age.

“For scientists studying post-glacial rebound, this new reference frame helps them better understand how viscous Earth's solid mantle is,” Argus said, “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 is working on post-glacial-rebound models. One possibility is that these updated models will shed light on the mysterious dip in the gravity field over parts of North America.