Most detailed map of Earth's gravity revealed

30 June 2010 by Tamera Jones

Scientists have unveiled the most detailed map yet of the Earth's gravity at a European Space Agency conference in Bergen, Norway this week.

GOCE gravity field map

The first global gravity model based on GOCE satellite data from November and December 2009

The map is based on just two months' worth of data generated by ESA's GOCE satellite - launched in March 2009 - and shows the tiny differences in gravity that exist around the Earth.

"The satellite is performing extremely well. In fact it's doing better than ESA expected," says Dr Helen Snaith from the National Oceanography Centre in Southampton, who can't wait to get her hands on the data.

GOCE stands for Gravity field and Ocean Circulation Explorer and was the first of ESA's Earth Explorer missions to be launched. And as its name suggests, the satellite was designed to give scientists unprecedented information about the Earth's gravity field, which will tell them about ocean circulation patterns around the globe.

"It's doing so well that ESA has decided to scrap a planned hibernation phase and keep the satellite going until 2013," says Snaith.

Its long operation means that by the end of the mission scientists may have an even more detailed map than they expected.

This map is based on just two months' data. As time goes on, it'll get revised and the resolution will just get better and better.


- Dr Helen Snaith, National Oceanography Centre

The satellite flies in the edge of the Earth's atmosphere at a height of just 254&middod;9 kilometres and measures miniscule differences in gravity at many points around the Earth using three pairs of highly accurate gradiometers.

"These Earth Explorer missions are based on new concepts. Gradiometers are not quite a new concept, but it's an amazing bit of kit," says Snaith.

Technically speaking the map shows the Earth's 'geoid' - or which parts of our planet have a greater gravitational pull than other parts because of the different rocks it's made of. Stronger gravitational pull is depicted by red, while a weaker pull is depicted by blue.

If you turned this map into a globe, it would look like a partially blown-up football, where peaks represent strong gravity and the troughs show slightly weaker gravity. If you placed a much smaller ball anywhere on this squashy football, it wouldn't move - even if it was on a slope - because gravity would be exactly the same all over it.

Although during school physics, you probably learnt that acceleration due to gravity at the Earth's surface is 9·8 metres per second squared, this is not strictly true. In fact, gravity differs very slightly all around the planet.

For example, because the Earth is the shape of a squashed ball, gravity is stronger at the poles than at the equator.

Before GOCE was launched, scientists knew that gravity is stronger around Greenland than around the Indian Ocean for example.

South American section map

Section of the geoid showing the South America region in more detail.

But the new map is telling them much more than they ever knew about the Himalayas, the Andes and Antarctica, all traditionally very difficult places to do research.

"The current geoid models are largely based on ground measurements, which of course is difficult in inaccessible parts of the planet," adds Snaith.

This includes the oceans. The geoid model that GOCE has generated is the shape the world's seas would be if there were no winds, tides or currents. Scientists can then subtract the geoid from real measurements of sea surface height to work out how winds, tides and currents affect ocean circulation.

"Until now, the best maps we had were on the 400 to 500 kilometre scale. GOCE's resolution is focused down to 150 kilometres. Most ocean current are around this width or smaller, so we're going to get a lot more detail about currents with this map," explains Snaith.

"This map is based on just two months' data. As time goes on, it'll get revised and the resolution will just get better and better," she adds.