Earth's Gravitational Shape In Detail

RobHart writes "The European Space Agency (ESA) has released detailed information about the Earth's gravitational shape, based on data from the ESA's GOCE satellite (Gravity Field and Steady-State Ocean Circulation Explorer). The link includes an interesting animation of the data, using an appropriately distorted Earth."

Comments

Gravitational hole in the Indian Ocean?

[G3ckoG33k]

Cool to see how the gravitation patter largely ignores the contours of the continent.

But, is there a gravitational hole in the Indian Ocean? Could it have been an asteroid? Perhaps leading to the "fast split" of Africa and India?

Just speculating.

Re:Gravitational hole in the Indian Ocean?

[ThunderBird89]

Neutronium is extremely dense, so it would be a bulge.

Apart from that, I thought the ocean floor was mapped there. Pointers to a crater would have been spotted already, at the very least on the magnetic maps, no?

Re:Gravitational hole in the Indian Ocean?

[condition-label-red]

Some more animations a la YouTube:

• Higher res copy of animation in article
• View of northern/southern hemispheres

Re:Gravitational hole in the Indian Ocean?

[Alef]

Nope. Higher gravity attracts more water, so the sea level is higher (compared to the surface of a perfect sphere the size of the Earth).

Or you can think about it in this way: The sea level forms a surface of equal gravity (otherwise there would be a "slope" somewhere, and the water would move down it). Where there is higher gravity, the sea level needs to be farther from the Earth's center to be on that surface and experience the same gravity.

Re:Gravitational hole in the Indian Ocean?

[kvvbassboy]

Cool to see how the gravitation patter largely ignores the contours of the continent.

On the contrary, this is not entirely true. Looking at the complete 2D contour you can see that the contour lines of either high or low gravitational areas are almost always centered in the oceans, whereas the continents and landmasses almost always in the middle of the gravitational scale.

My completely uninformed gut feeling tells me that this data could go a long way in explaining why continents are located (or drifted to) where they are, and could possibly also make predictions about continental drifting in the future.

Re:Gravitational hole in the Indian Ocean?

[c0lo]

The US has less gravitational force than Europe... does that mean there are giant cavernous regions below the US?

Maybe it explains why there are more fat people in the US because they don't have to work as hard on a daily basis to keep themselves standing. (/joke)

T'is caused by the trade deficit and foreign debt, both come with a "negative weight".

Didn't it use to be highly classified data?

[tibit]

Wait a minute, didn't accurate geoid use to be highly classified information? As in "used for missle inertial navigation" kind of classified? I wouldn't be surprised if the German data could be imported into the U.S., but couldn't be re-exported, for example... Does anyone know more about this?

Re:Didn't it use to be highly classified data?

[tibit]

I think that a lot of U.S. nuke missile arsenal predateds GPS -- maybe they upgraded avionics to take advantage of it, I don't know. If there was to be any sort of a nuclear showdown, then the GPS would either go down or the clear data would be turned off, only encrypted one remaining. I think that if you're after long range weapons, you really need INS, and for that accurate geoid is a must. I would presume that any sort of a ballistic or cruise missle guidance system would have targeting accuracy specified without GPS augmentation (inertial only), with augmentation providing a "free" improvement if available.

Apart from GPS and GLONASS, there is no "other" sat-based navigation available yet. Getting any sort of a satnav receiver through its paces of military QA, you can't really add support for other systems on a whim. I think that all satnav receivers installed in U.S. weapons support GPS, and won't support anything else in the next decade or two.

You don't use the geoid to plan any sort of a trajectory. You use it for inertial navigation -- for converting outputs of your inertial reference sensors (gyros and accelerometers) into a position fix. To do this accurately, you need accurate, low-drift and low-noise sensors. Once your sensors get good enough, improving their accuracy doesn't improve the accuracy of your fix! To get any further improvement, you need to improve the accuracy and resolution of your geoid data. By my back-of-the-envelope calculations, GOCE's geoid is supposedly (based on published details) good enough to match the best inertial reference sensors out there, and would allow you to obtain the best inertial fix that's possible with current technology.

Re:What's the scale?

[pgn674]

They used elevation and colors to indicate gravity strength. Are the radii supposed to be linearly comparable? The differences look too big.

I was wondering that too, and I found an answer: "The differences have been magnified nearly 10,000 times to show up as they do in the new model.": BBC News - Gravity satellite yields 'Potato Earth' view. The article also gives further explanation of what the model represents.

Re:Use of data?

[dotbot]

(Based on my highly limited knowledge of the subject) it enables observations about the earth to be compensated for non-uniform gravitational pull, so you can get a better idea of what is really happening and stand a better chance of explaining why. For example, now we know where water is effectively flowing uphill and downhill, we can better estimate the actual ocean current forces from the observed currents, so start to guess at what is causing them.

Re:Measurable effect?

[Mt._Honkey]

Depends on how you weigh it. If you truly measure weight, then yes. If you are really measuring mass, then no. For example: a spring scale will show a difference because the gravitational force is different. If you use a pan balance you will not see a difference, because both the subject and reference masses change their weights by the same fraction. Same goes for any true measurement of mass, such as penning traps or RFQ's.

You would need a good scale, but not extraordinarily good. A 1 kg weight would weigh ~100 mg weight different between the max and min.