By Jonathan Amos
Science correspondent, BBC News, Bergen
It is one of the most exquisite views we have ever had of the Earth.
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.
… Imaginary ball
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.
…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.
…
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/8767763.stm
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No one has actually explained the map in a clear way (shame on the confused BBC article).
There are those who have said that blue=lower density, red higher, but I find the opposite. Please correct me where I am going wrong!
To explain (for my own sanity, if not for others):
The Earth is an ellipsoid (a sphere squashed at the top and bottom, which is squashed again at on either side of the equator), so standing on the equator I will experience a greater gravitational force than at the poles.
At the poles I am closer to the centre of the Earth than at the equator, so some of the mass of the earth is ‘above’ me at my distance from the centre, hence a lower gravitational force.
To find the points of equal gravitational force, I would have to move into the earth at the equator (more blue), so more of the Earth’s mass is ‘above’ me, and rise above the earth at the poles (more red), so more of the Earth’s mass is below me, until I experienced the same gravitational force.
Now, since we know the Earth is an ellipsoid, it would not be useful to show the poles as red and equator blue, so this difference is removed from the map, so that the map is just showing mass differences over the planet. A uniform density would show the whole Earth as green.
At Sri Lanka the gravitational force is higher still (greater mass), so I would have to move further into the earth to compensate (more blue). At Iceland the gravitational force is lower (less mass), so I would have to move higher to compensate (more red).
This is the shape that water would settle at if it could move freely through ‘ghost’ rock and into a ‘ghost’ Earth.
Phew.
Ok, I have thought about it a bit more and now agree that blue=lower density, red=higher.
My corrected explanation is:
The Earth is an ellipsoid (a sphere squashed at the top and bottom, which is squashed again at on either side of the equator), so standing on the equator I will experience a lower gravitational force than at the poles because I am further away from the Earth’s mass. At the poles I am closer to the Earth’s mass, hence a higher gravitational force.
To find the points of equal gravitational force, at the equator I would have to move into the Earth so I am closer to the centre of the Earth (blue), but balanced with a reduced Earth’s mass as some of it is now above me. At the poles I would have to move away from the Earth to reduce the gravitational force (red).
Now, since we know the Earth is an ellipsoid, it would not be useful to show the poles as red and equator blue, so this difference is removed from the map, so that the map is just showing mass differences over the planet. A uniform density would show the whole Earth as green.
At Sri Lanka the lower density means that I would have to move into the earth to compensate (blue). At Iceland the higher density means I would have to move higher to compensate (red).
Colin W says:
June 30, 2010 at 6:17 am
[–snip for brevity–] At Sri Lanka the lower density means that I would have to move into the earth to compensate (blue). At Iceland the higher density means I would have to move higher to compensate (red).
Gee, Colin, I consider that you’ve figured out why the inhabitants of Sri Lanka always seem to be high, and the Icelanders always seem to be so ‘down to Earth’ …
😉
Yes; and considering how they believe that the whole world revolves around them, it also might explain why we appear to be going in circles? Maybe it has something to do with a political frame of inertial reference?
Just wait for successive versions of the maps and the explanations for change. Apart from the abovementioned guidance of Cruise missiles which used to rely on a selected geiod, the best we can expect from these satellites is pretty images at quite coarse resolution. I suspect that the limiting factor, also noted above, will be confusion of velocity change caused by tiny changes in atmospheric composition. There’s also a problem that Earth is plastic and its shape is changed by tides. Might impact on the necessary measurement of satellite altitudes.
It has always been difficult to measure differences in gravity. The instruments of old were hand made, fragile, horribly expensive, needed to be upright at all times, needed quite a time to stabilise at each new station and were usually transported like passengers strapped into vacant airline seats, sometimes under the name “G. Meter”. The sensitivity was often limiting and it was a good deal better than these satellites give.
BTW, I have to disagree with wayne Job
June 29, 2010 at 4:51 am when he says that gravity measurements were made from sensors under aircraft in the 1960s. Magnetism was measured, plus a variety of electrical methods where signals of various frequencies were sent to earth and responses picked up by drogues on cables – several variations, but that’s what he might have seen.
That’s geoid.