'Potato' Earth Shows How Gravity Takes Shape: Big Pic

April 5, 2011 — With the recent release of the new gravitational map of the planet -- showing the lumps and bumps in exaggerated proportions and comparing it to a tuber -- we at Discovery News would love to see "Potato Earth" launched as a new action figure for Earth Day on April 22.

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Though the European Space Agency’s GOCE satellite team jokes about the spud comparison, if the Earth didn’t have a single blemish, mountain range or dip in the sea floor; or if the planet stood perfectly still in orbit with only an ocean and no winds, and had a homogeneous interior, Earth would be a "boring sphere," said gravity modeler Roland Pail of the Technical University in Munich, Germany, during a press conference last week.

It’s due to the dynamic nature of the planet that Earth’s gravity is not the exact textbook 9.8 meters per second squared (32.2 feet per second squared) that we learned in school. That's just the average, and a close enough estimate for most daily needs.

The launch of the low-flying GOCE satellite in 2009 to an atmosphere-skimming altitude of 255 kilometers (158 miles) above the surface was intended to determine minute details in the gravitational variation.

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"GOCE has wings, an indication that we have spilled some atmospheric molecules where we fly at the moment," said Volker Liebig Director of ESA's Earth observation program. "The closer we are to Earth, the mass we want to measure, the higher is the signal."

Gravity changes are influenced for example by mass and rotation of the planet and Liebig compared the slight changes that GOCE has identified to finding out how one snowflake landing on a supertanker can change the mass of that supertanker.

Those slight variations when compared to the boring sphere scenario where gravity is constant, an ideal geoid, can pinpoint extremes in topography, warmer and cooler ocean currents, and even density changes in Earth's interior as a result of plate tectonics. Wherever cold, dense material sinks deep into the mantle, it leaves a trace on the gravitational field that GOCE can measure.

Just taking the daily spin around the axis adds a decimal point to the gravitational field, flatting the Earth into an ellipsoid, said Pail. Earthquakes rattle the way Earth balances its gravity as well.

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The Gravity field and steady state Ocean Circulation Explorer (GOCE) mission complements the earlier data collected by NASA's Gravity Recovery and Climate Experiment (GRACE) satellites launched in 2002.

GOCE takes a high resolution view of the globe, whereas GRACE takes a lower resolution but has a better perspective of changes over time. Together the missions both provide a means of better analyzing the data, said aerospace engineer Byron Tapley of the University of Texas and principal investigator for the GRACE mission in an email to Discovery News.

IMAGE: Gravitational model of Earth over the Atlantic Ocean. The red color represents 100-meter-high deviations in height from an ideal geoid, where gravity is constant, and the blue areas represent 100-meter-low deviations. (ESA/HPF/DLR)