earth’s gravity field
It is one of the most exquisite views we have ever had of the Earth.
by jonathan amos of the bbc.
This colourful new map traces the subtle but all pervasive influence the pull of gravity has across the globe.
Known as a geoid, it essentially defines where the level surface is on our planet; it tells us which way is “up” and which way is “down”.
It is drawn from delicate measurements made by Europe’s Goce satellite, which flies so low it comes perilously close to falling out of the sky.
Scientists say the data gathered by the spacecraft will have numerous applications.
One key beneficiary will be climate studies because the geoid can help researchers understand better how the great mass of ocean water is moving heat around the world.
The new map was presented here in Norway’s second city at a special Earth observation (EO) symposium dedicated to the data being acquired by Goce and other European Space Agency (Esa) missions.
Europe is currently in the midst of a huge programme of EO development which will see it launch some 20 missions worth nearly eight billion euros before the decade’s end.
The Gravity Field and Steady-State Ocean Circulation Explorer (Goce) is at the front of this armada of scientific and environmental monitoring spacecraft.
Launched in 2009, the sleek satellite flies pole to pole at an altitude of just 254.9km – the lowest orbit of any research satellite in operation today.
The spacecraft carries three pairs of precision-built platinum blocks inside its gradiometer instrument that sense accelerations which are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth.
This has allowed it to map the almost imperceptible differences in the pull exerted by the mass of the planet from one place to the next – from the great mountain ranges to the deepest ocean trenches.
Two months of observations have now been fashioned into what scientists call the geoid.
“I think everyone knows what a level is in relation to construction work, and a geoid is nothing but a level that extends over the entire Earth,” explained Professor Reiner Rummel, the chairman of the Goce scientific consortium.
“So with the geoid, I can take two arbitrary points on the globe and decide which one is ‘up’ and which one is ‘down’,” the Technische Universitaet Muenchen researcher told BBC News.
In other words, the map on this page defines the horizontal – a surface on which, at any point, the pull of gravity is perpendicular to it.
Put a ball on this hypothetical surface and it will not roll – even though it appears to have “slopes”. These slopes can be seen in the colours which mark how the global level diverges from the generalised (an ellipsoid) shape of the Earth.
In the North Atlantic, around Iceland, the level sits about 80m above the surface of the ellipsoid; in the Indian Ocean it sits about 100m below.
The geoid is of paramount interest to oceanographers because it is the shape the world’s seas would adopt if there were no tides, no winds and no currents.If researchers then subtract the geoid from the actual observed behaviour of the oceans, the scale of these other influences becomes apparent.
This is information critical to climate modellers who try to represent the way the oceans manage the transfer of energy around the planet.
But a geoid has many other uses, too. Having a global level underpins a universal system to compare heights anywhere on Earth.
In construction, for example, it tells engineers which way a fluid would naturally want to flow through a pipeline.
Geophysicists will also want to use the Goce data to try to probe what’s happening deep within the Earth, especially in those places that are prone to quakes and volcanic eruptions.
“The Goce data is showing up new information in the Himalayas, central Africa, and the Andes, and in Antarctica,” explained Dr Rune Floberghagen, Esa’s Goce mission manager.
“This is, in one sense, not so surprising. These are places that are fairly inaccessible. It is not easy to measure high frequency variations in the gravity field in Antarctica with an aeroplane because there are so few airfields from which to operate.”
Goce’s extremely low operating altitude was expected to limit its mission to a couple of years at most. But Esa now thinks it may be able to continue flying the satellite until perhaps 2014.
Unusually quiet solar activity has produced very calm atmospheric conditions, meaning Goce has used far less xenon “fuel” in its ion engine to maintain its orbit.
Ultimately, though, that fuel will run out and the residual air molecules at 255km will slow the satellite, forcing it from the sky.