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Wednesday, 21 November, 2001, 17:23 GMT
New gravity map released
Gravity map, Nasa
Satellites will update the gravity map of Earth
By BBC News Online science editor Dr David Whitehouse

A new gravity map of the Earth suggests that if you want to lose weight you should go to India, where the pull of gravity is slightly less than it is elsewhere on the planet.

You would be slightly less than 1% lighter there.

The gravity map has been prepared to help scientists plan the forthcoming Grace (Gravity Recovery And Climatic Experiment) satellites, to be launched in a few weeks.

Earth is lumpy and so is its gravitational field. The variations are tiny but important for understanding what the Earth is made of and how it is recovering from the last Ice Age.

Ocean effects

Grace will allow scientists to monitor the oceans in a new way, helping them to determine if sea level is rising due to more water being added to the oceans or to the water already there expanding due to global warming.

Grace satellite, Nasa
A new satellite will map the Earth's magnetic field
In the new map, the lower the elevation the weaker is the pull of gravity. The vertical scale is highly exaggerated.

The low gravity off the coast of India is thought to be due to the remains of some old tectonic feature, which was left over from the collision of the Indian sub-continent into the Eurasian tectonic plate that gave rise to the Himalayas.

There is also a strong region of gravity in the South Pacific, which is thought to be due to structures in the Earth's mantle.

Grace is actually two satellites that will follow each other in orbit around the Earth at a separation of 220 kilometres (137 miles).

Better than before

A radio beacon will allow the distance between them to be measured to less than one-fiftieth of the thickness of a human hair.

In their orbits, each Grace satellite will respond to the subtle differences in the Earth's pull at slightly different times.

Measuring this effect will allow scientists to construct a new gravity map of the Earth about a hundred times more accurate than the one used for planning.

"Every month during Grace's five-year expected lifetime, we will get a map of the Earth's gravitational field," says Michael Watkins of the American space agency's Jet Propulsion Laboratory.

"We will be able to see various phenomena that involve transporting mass around. These things are not easy to see with any other type of measurement."

New dimension

Grace's gravity map will have a spatial resolution of about 300 km (186 miles) so it should reveal gravity fluctuations caused by mountain ranges, tectonic plates and land still uplifting after the ice from the last Ice Age melted.

M Watkins, Nasa
Nasa's Michael Watkins: A new map every month
The gravity data will also allow a new look at the oceans. When free from other influences, the ocean surface takes up the shape of the Earth's geoid - the surface on which the pull of gravity is equal everywhere.

The problem is that sea level can also be affected by currents and sea temperature. By combining their data, Grace and other ocean-monitoring satellites will separate the various effects.

By sensing worldwide fluctuations in gravity, Grace should add a new dimension to our understanding of the planet.

See also:

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