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."

Comments

First?

[frank_adrian314159]

Gravity map? Heavy, man!

known geographic phenomena

[nurb432]

No, but they do correspond to the location of the stargates.

Where's a 1920x1200pxl Image?

[Doc Ruby]

That's a really nice image. Where can I find a 1920x1200 pixel image file of it to use as desktop wallpaper?

Re:Where's a 1920x1200pxl Image?

[EdIII]

Why is this modded insightful?

The original image is 786x427. If you expand that up to HD, or in this case 1920x1200 it would result in a very pix-elated image. It's not lazy to ask for a higher resolution image, and that part of the post is the difference between +flamebait, and +funny.

cool

[john_uy]

this is quite a very informative article.

my question though from the image produced is that the metres/meters scale shows how "strong" or "weak" the gravity is from the normalized sphere? how is it in the unit of metres/meters? i would appreciate if someone could explain the map more detailed (i probably need another explanation from the article to understand it more.)

the goce satellite is cool. i mean i didn't realize that we have technology such as xenon ion thrusters. i thought they were limited to star trek. my ignorance. :((

thanks in advance. :)

Re:cool

[demonbug]

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).

Re:cool

[maeka]

No, because that image shows one how height above the ellipsoid (not spheroid as someone said above) model of the earth (as used in GPS / etc.) minus the geoid model deviation (what these satellites are, in effect, measuring) equals your orthometric height (what most people call "elevation").

I don't believe this helps the GP's query, because it doesn't explain what the geoid model is.

What most people never think about is that old-school surveying (with a conventional level) always, even before we understood that the earth's gravity varied, compensated for the variation. A surveyor's level, be it an old one where the circular plate is made level with a spirit bubble or a newer one where the recticle floats and self-compensates, is always looking perpendicular to the line between it and the center of gravity. Everyone always assumed, back in the day, that this really meant the center of the Earth, but the positive side effect is that elevation measurements "carried" with a surveyor's level were always compensating for these differences in gravitational potential.

So much so, that all the major geoid models used to this day are heavily based upon high-precision GPS observations (of height above the ellipsoid) on benchmarks possessing elevations leveled in.

Interesting Pattern Near the Ring of Fire

[BJ_Covert_Action]

If you look at the map, a lot of the high-gravity areas tend to appear near highly volcanic areas like the ring of fire (and, as the reader pointed out, Iceland). I wonder if this has something to do with more magma being closer to the surface in those areas...or something similar?

Re:Interesting Pattern Near the Ring of Fire

[BJ_Covert_Action]

Oh, I'd also like to point out, just for the sake of inflammatory goodness on the interwebs, that America is entirely green and homogenous, thus scientifically proving that we are the best at gravity as well as everything else. Also, we officially qualify as normal and neutral so take that all you backwards-ass, long-haired, rock-and-roll loving, high/low gravity hippy nutjobs on the rest of the continents! Give me back my 9.806 m/s^2 .... I mean...32.174 ft/sec^2....aww hell, I made it back to my OP.

Re:Interesting Pattern Near the Ring of Fire

[Arthur Grumbine]

Well I was hopping it explained why Americans are fatter than the rest of the world...

I fail to see how your hopping would explain anything besides minute fluctuations in the gravity between your feet and the ground, and your predilection towards hats with ear flaps.

Re:Interesting Pattern Near the Ring of Fire

[CaroKann]

It looks like there could also be a relation to continental movements in some areas. The cooler colored areas appear to be trailing behind fast moving continental plates, almost like a backwash. Look at the southern tip of India, which is quickly moving north into Asia. There is a cool area to the south of the California plate, which is moving north. There is another cool area off the east coast of North America and to the east of the Caribbean.

Re:Interesting Pattern Near the Ring of Fire

[treeves]

it corresponds really well because it is exactly an image of the same thing, just an earlier, less accurate version.

Got KML?

[BigDXLT]

I'd love to zoom around in google earth to look at this.

Re:Map of our favorite gravity well

[UnknownSoldier]

Not quite full HD but still is a higher res: 1081 x 541
  http://www.usna.edu/Users/oceano/pguth/website/so432web/ww15mgh.jpg

The U.S. military already has one of these

[tlambert]

The U.S. military already has one of these

It's used in inertial navigation for weapons systems. Interestingly, the inertial navigation software itself is available as source code for download, but the data of the map itself is classified to prevent its use by non-U.S. aggressors. Also, for what its worth, the military data resolution is far better than the 100km between data points, as it is with GOCE, but is the resolution falls off on non-projected weapons trajectory route splines.

See also the geoid from the earlier GRACE observations (animated spinning globe) which were 322km resolution, along with a more technical discussion of GOCE:

        http://www.scientificblogging.com/planetbye/grace_goce

-- Terry

Exactly

[tlambert]

Exactly... "the resolution falls off on non-projected weapons trajectory route splines".

You get very good data for the areas in which you want to fly your birds, and lesser data for where you don't expect to do that. This is necessary to, for example, use inertial guidance rather than active TFR in a cruise missile and keep it below the enemy radar.

-- Terry

Re:wonder why they duplciated US GRACE satellite?

[mbone]

Grace and GOCE are completely different experiments, which measure different (but related) things about the Earth's gravity field. Grace tracks the motions of a pair of satellites, which GOCE uses pairs of accelerometers. Different data types, different measurement errors, different types of systematic errors. I think it is well worth running both experiments.

Some background: The Satellite Itself

[highways]

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 ;)

Re:Some background: The Satellite Itself

[Zoxed]

> That's some serious engineering precision.

If that is your cup of tea, then you will be interested in the in-development https://secure.wikimedia.org/wikipedia/en/wiki/Gaia_probe : it will measure arcs of the order of the diameter of a hair from 1000 km away !!

Re:Some background: The Satellite Itself

[the_other_chewey]

Apparently, the accelerometers themselves (3 pairs of them) are mounted to within one picometre

No they are not. That is entirely impossible to do, as this is quite a bit smaller than atomar scale.
Even looking at them with too many photons would already change their position by more than that.

Their relative position is measured to that precision in service.

high points are high gravity. (Re:cool)

[darkonc]

uhm, no. the geoid is at negative height where the gravity is WEAKER. The geoid consists of a surface where a ball would never roll because the gravity differential would counteract the slope (or vice-versa)...

Think of it another way... The observed 'center' of gravity is always perpendicular to the slope of the geoid.

Thus consider a piece of slope tilted like a forward slash ( / ) The gravity would have to be stronger on the right hand side to hold the ball flat against the slope... Thus the gravity is stronger on the 'high' side, and weaker on the 'low' side.