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ESA's GOCE Satellite Provides Gravity Map of Earth
kaulike writes "The European Space Agency's GOCE satellite, launched in March 2009, has provided a spectacular, highly detailed map of our favorite gravity well. This map shows the normalized surface of the earth as defined by gravity, showing the relative altitude differences from the average for each surveyed point. The article provides the helpful metaphor that a ball resting on this surface would not roll anywhere, even though there would be visual slopes, as gravity is equalized across the globe. There is a fascinating deep area in the Indian ocean (-100M) and a high area near Iceland (+80M), proving conclusively that our world is not homogeneous in terms of density (or practically any other measure). Does anyone know whether these anomalies correspond to known geographic phenomena? Deposits of heavy metals perhaps, or hotspots where the mantle is thinner? I know little about geodetic stuff, but I'm curious about the reasons for wrinkles in the data set."
The map is showing the deviation from an ideal spheroid that would result in the observed gravity variation. So, positive meters basically means that if the Earth were made of stuff of a uniform density, the surface of the earth would be this many meters above the ideal surface (gravity is weaker here than expected). Conversely, negative meters means gravity is stronger here than expected, and so correlates to a "low" (low elevation being closer to the center of mass of the earth, meaning stronger gravity).
The map is essentially showing what the surface of the earth would look like if all variation in gravity (what they observed) was due to variation in the shape of the earth, rather than density. At least, I think that is what they are showing - I don't think the article actually states if this is raw data or if it has been processed (to apply a free-air correction, for example, which would remove variability due to the actual variations in elevation of the earth's surface).
Make sense? It isn't as complicated as my half-assed explanation might make it seem (well, it is complicated, but the concept is simple).
It's worth taking a read of the satellite itself. Apparently, the accelerometers themselves (3 pairs of them) are mounted to within one picometre (that is micro-micro-metre). Gravity measurements are to within 10^-13 G. All pushed ahead by a cool xenon ion engine :)
That's some serious engineering precision. A bit more than your average accelerometer in your iPhone.
There's a bit more on how it works in this article.
Of course, the raw data looks a lots uglier than the beautiful image of the final result, but if the research is for climate change, then manipulating raw data is what they do best ;)