ESA: GOCE giving new insights into Earth’s gravity

[bystander]

GOCE giving new insights into Earth’s gravity
ESA News - 29 June 2010

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The first global gravity model based on GOCE satellite data has been presented at ESA’s Living Planet Symposium. ESA launched GOCE in March 2009 to map Earth's gravity with unprecedented accuracy and resolution.

The model, based on only two months of data, from November and December 2009, shows the excellent capability of the satellite to map tiny variations in Earth’s gravity.
...
New GOCE models are already yielding a wealth of new information that is useful for many domains of geosciences. GOCE’s final gravity map and geoid will be instrumental in advancing science and applications in a broad range of disciplines, ranging from geodesy, geophysics and surveying to oceanography and sea-level research.

[Beyond]

So if we went from a dark blue place to a red place, how much of a change would we actually pick up on?

[Chris Peterson]

So if we went from a dark blue place to a red place, how much of a change would we actually pick up on?

Although it isn't obvious in the image, the scale units are meters (above or below the mean geoid). The difference between the extremes is about 200 meters, which near sea level is enough to produce a variation of 0.0016% of a standard gravity. If you weigh 80 kg at a blue zone, you'll weigh 79.997 kg in a red zone. (It's possible that the deviation in weight will be upwards, as I'm not sure about the sign convention used on this map.)

[neufer]

So if we went from a dark blue place to a red place, how much of a change would we actually pick up on?

Although it isn't obvious in the image, the scale units are meters (above or below the mean geoid). The difference between the extremes is about 200 meters, which near sea level is enough to produce a variation of 0.0016% of a standard gravity. If you weigh 80 kg at a blue zone, you'll weigh 79.997 kg in a red zone. (It's possible that the deviation in weight will be upwards, as I'm not sure about the sign convention used on this map.)

I believe one would weigh MORE in a red zone since that
is where an open ocean would accumulate due to its higher gravity.

The geoid is a measure of the lumps and bumps in Earth's gravity, and shows
how the surface would look if an ocean covered the earth, also known
as surface of equal gravitational attraction and mean sea level.

[Beyond]

The way it sticks out of the page with 3-D glasses it looks rather pregnent.
So we really can't tell any difference in Gravity unless we see it on a fancy meter?

[Chris Peterson]

So we really can't tell any difference in Gravity unless we see it on a fancy meter?

Not really. You could probably tell the difference on a good scale between the highest and lowest points on Earth, and between the lowest and highest gravity zones. But you wouldn't sense any difference.

The actual variations across the Earth are much greater than this map shows. This is a sort of average, which defines the geoid. Locally, variations in gravity are quite large. Geologists regularly map underground structures by measuring the force of gravity over a surface grid.

[Ann]

Well, I've been putting on weight lately but maybe I can take comfort in the fact that I live in Sweden, where the local gravity seems to be just a tiny bit above the earthly average. Maybe I should try to get a green card and move to the United States, where everything seems to weigh a little less, eh? Particularly along the east coast. I guess that explains why Superman lives in Metropolis, U.S.A., and not in Sweden. It's easier to overcome gravity in the States!



Fly me to the moon... or fly me from the States!

Good thing I don't live in Iceland, though. The poor people there have been hit by something close to a national bankruptcy due to the international recession, they have been covered in ash from a recent volcanic eruption, and they appear to suffer from the second worst gravity in the world, after New Guinea and Papua New Guinea!

Ann

P.S. Hey, Art, I like you potao-shaped Earth!

[neufer]

So if we went from a dark blue place to a red place, how much of a change would we actually pick up on?

Although it isn't obvious in the image, the scale units are meters (above or below the mean geoid). The difference between the extremes is about 200 meters, which near sea level is enough to produce a variation of 0.0016% of a standard gravity. If you weigh 80 kg at a blue zone, you'll weigh 79.997 kg in a red zone. (It's possible that the deviation in weight will be upwards, as I'm not sure about the sign convention used on this map.)

I believe one would weigh MORE in a red zone since that
is where an open ocean would accumulate due to its higher gravity.

Well, I've got to correct myself once again.

The open ocean accumulates in the red zone due to its LOWER gravity allowing for
the ocean to expand out (just as it expands out at the equator [not shown] thanks to centrifugal force).

So Chris was right the first time (once again ).

The geoid is a measure of the lumps and bumps in Earth's gravity, and shows
how the surface would look if an ocean covered the earth, also known
as surface of equal gravitational attraction[?] and mean sea level.

[Chris Peterson]

So Chris was right the first time (once again :( ).

Just luck. I didn't give the sign of the result much thought, since it was the magnitude that was interesting.

[alter-ego]

I believe one would weigh MORE in a red zone since that
is where an open ocean would accumulate due to its higher gravity.
Well, I've got to correct myself once again.

The open ocean accumulates in the red zone due to its LOWER gravity allowing for
the ocean to expand out (just as it expands out at the equator [not shown] thanks to centrifugal force).

So Chris was right the first time (once again ).

Not so fast Art, I'm quite sure you were right the first time When I first read about the Indian Ocean depression years ago I had the same reasoning, but I remember something counter intuitive about the logic so I did some digging. Turns out it's mostly it's local mass uniformity differences that affect variation in the ocean surface, so the higher local gravity "pulls" the ocean up and vice versa. The red regions mean hight gravity delta (in milligals). So when you travel into the Indian Ocean you actually weigh less. If you find definitive interpretation to the contrary, please pass it on -- I think this description is accurate.

http://www.newscientist.com/article/dn1 ... z_53_133_c (May 2010)
<<Ancient pieces of continental crust that are falling to the bottom of the Earth's mantle could explain mysterious dents in our planet's gravitational field.
Sail towards the centre of the Indian Ocean and you will find yourself losing weight because the Earth's gravitational field is weaker in this region. Similar dents in field strength are found in the north-east Pacific Ocean and the Ross Sea.>>


http://rst.gsfc.nasa.gov/Intro/Part2_1b.html
http://www.universetoday.com/2007/08/02 ... ming-soon/

[Chris Peterson]

The red regions mean hight gravity delta (in milligals).

Note, however, that for the map shown at the beginning of this discussion, the units appear to be meters, not millgals.

[neufer]

Not so fast Art, I'm quite sure you were right the first time

That would shock no one more than me, alter-ego.



Mountain ranges are clearly high points in the geopotential such that on either side of mountains there are horizontal geopotential gradients (i.e., horizontal gravitational forces) pulling in plumb bobs and/or ocean waters in the direction of the mountain itself. On that we can agree.



However, any high geopotential bulge must also correspond to weaker vertical geopotential gradients (or vertical gravitational forces) in the immediate vicinity of a mountain range which permits the ocean water to rise higher above underwater mountains. (Otherwise the water pulled in from the sides would then be pulled down into a kind of giant maelstrom.) In other words, mountains represent not a monopole but a quadrupole gravitational distortion: pulling in from the sides while simultaneously pushing out at the top and bottom. This would all be consistent with the concept of mountains being big "light icebergs" with deep roots floating on heavy basaltic "seas."

Hence, one weighs less above mountains even if
1) those mountains lie under the sea and even if
2) the sea level didn't also bulge up to further distant you from the center of the earth.

[alter-ego]

Not so fast Art, I'm quite sure you were right the first time

That would shock no one more than me, alter-ego.

Mountain ranges are clearly high points in the geopotential such that on either side of mountains there are horizontal geopotential gradients (i.e., horizontal gravitational forces) pulling in plumb bobs and/or ocean waters in the direction of the mountain itself. On that we can agree.

However, any high geopotential bulge must also correspond to weaker vertical geopotential gradients (or vertical gravitational forces) in the immediate vicinity of a mountain range which permits the ocean water to rise higher above underwater mountains. (Otherwise the water pulled in from the sides would then be pulled down into a kind of giant maelstrom.) In other words, mountains represent not a monopole but a quadrupole gravitational distortion: pulling in from the sides while simultaneously pushing out at the top and bottom. This would all be consistent with the concept of mountains being big "light icebergs" with deep roots floating on heavy basaltic "seas."

Hence, one weighs less above mountains even if
1) those mountains lie under the sea and even if
2) the sea level didn't also bulge up to further distant you from the center of the earth.

Mmmmm You said some pretty interesting stuff . However, the data correlates positve surface deviations from the geoid with postive deviations from g (accelaration). Unless there is convoluted definition of +/- mGal, gravity is higher in the red regions (higher surface elevation) and lesser than in the blue (surface depressions). Granted, the details of how mass is distorted locally may be oversimplified, but I believe your conclusion(s) errs from the data. If you think otherwise, send me the links that show me how I am misinterpreting the data (i.e. show me that accelaration deviation data is reversed, red = less gravity, etc). I get you have a different interpretation, but make it consistent with the data. There is enough info published that support the description I've given that I question your reasoning (although it sounds convincing )
(Edit: Specifically I'm referring to the Indian Ocean anomally. The correlation between surface elevation and acceleration anomally does not appear to be 100%.)

If I have incorrectly interpreted the data, no problem, I just want to get it right.. but I still think you were right the first time ae

[neufer]

If I have incorrectly interpreted the data, no problem, I just want to get it right.. but I still think you were right the first time ae

OK...try this:

The oblate spheroidal earth has a matching oblate spheroidal equipotential surface and hence is in overall equilibrium.

However, if the earth were to stop rotating it would, of course, NOT be in equilibrium. That is because without centrifugal force the equipotential surface of an oblate spheroidal is actually prolate and it will remain prolate until the earth relaxes into a hydrostatic sphere.

A rigid non-rotating prolate asteroid likewise has an oblate equipotential surface. All mountains on that asteroid will also have their own mini oblate equipotential surfaces which result in local dips in the overall oblate equipotential surface. Oceans on a rigid non rotating prolate asteroid will thus concentrate around the equator and dip over local mountains.

In contrast, mountains on the earth are local blobs of light floating material that have mini prolate equipotential surfaces that result in corresponding local peaks in the overall equipotential surface. That is because like the earth itself mountains are in hydrostatic equilibrium with their environment such that the equipotential surface emulates (and is thus in harmony with) the actual surface.

[alter-ego]

If I have incorrectly interpreted the data, no problem, I just want to get it right.. but I still think you were right the first time ae

OK...try this:

The oblate spheroidal earth has a matching oblate spheroidal equipotential surface and hence is in overall equilibrium.

However, if the earth were to stop rotating it would, of course, NOT be in equilibrium. That is because without centrifugal force the equipotential surface of an oblate spheroidal is actually prolate and it will remain prolate until the earth relaxes into a hydrostatic sphere.

A rigid non-rotating prolate asteroid likewise has an oblate equipotential surface. All mountains on that asteroid will also have their own mini oblate equipotential surfaces which result in local dips in the overall oblate equipotential surface. Oceans on a rigid non rotating prolate asteroid will thus concentrate around the equator and dip over local mountains.

In contrast, mountains on the earth are local blobs of light floating material that have mini prolate equipotential surfaces that result in corresponding local peaks in the overall equipotential surface. That is because like the earth itself mountains are in hydrostatic equilibrium with their environment such that the equipotential surface emulates (and is thus in harmony with) the actual surface.

AEN1.jpg (17.22 KiB) Viewed 1276 times

[Chris Peterson]

The red regions mean hight gravity delta (in milligals). So when you travel into the Indian Ocean you actually weigh less.

I think this is the correct interpretation. When you look at the GOCE data, with its units in meters, it isn't so obvious what this means. But when compared with the other data you linked, where the data is in mgal, it becomes fairly clear that a negative deviation in meters from the mean radius of the Earth corresponds to a lower gravitational potential, which does make sense.

While the two maps are very similar in structure, they are quite different in value. That surprises me. The GOCE map has a total range of about -100 meters to +80 meters, which is -31 mgal to +25 mgal. The earlier data seem to show a range of about -50 mgal to +50 mgal. That seems like a lot more difference than I'd expect.

[neufer]

The red regions mean hight gravity delta (in milligals). So when you travel into the Indian Ocean you actually weigh less.

I think this is the correct interpretation. When you look at the GOCE data, with its units in meters, it isn't so obvious what this means. But when compared with the other data you linked, where the data is in mgal, it becomes fairly clear that a negative deviation in meters from the mean radius of the Earth corresponds to a lower gravitational potential, which does make sense.

While the two maps are very similar in structure, they are quite different in value. That surprises me. The GOCE map has a total range of about -100 meters to +80 meters, which is -31 mgal to +25 mgal. The earlier data seem to show a range of about -50 mgal to +50 mgal. That seems like a lot more difference than I'd expect.

OK, I concede!

I was right (originally) and Chris was wrong (originally).
(How embarrassing! )

Sealevel equipotential is, of course, defined by constant gravity forces and
it is NOT the same as constant gravity potential (as I had assumed).

One can't push the sea down a meter at point A and have it riseup a meter at point B and gain energy
(; which is quite different than simply moving an electron horizontally from point A to point B and gaining energy).

Related to it is the sealevel equipotential, a surface on which gravity is everywhere equal to its strength at mean sea level.

However, when one travels by boat into the Indian Ocean one essential
weighs the same because one is traveling on an sealevel equipotential surface.

[Beyond]

Oh Good. Now that you guys have finally figured out what you were figuring out -- could you please say it in english for the rest of us??
You do not have to explain the picture of Alfred e. Neuman. That one is worth more than a thousand words!, and most of them are HA-HA-HA-HA

[alter-ego]

Related to it is the sealevel equipotential, a surface on which gravity is everywhere equal to its strength at mean sea level.

However, when one travels by boat into the Indian Ocean one essential
weighs the same because one is traveling on an sealevel equipotential surface.

Very good point Art.
By definition, the geoid is a sealevel equipotential surface, and not the same surface on which a satellite samples gravitational acceleration. To a good approximation, the variation in gravity for a specific location, say over an ocean, as measured by a satellite should not be measured at the surface of the ocean, but a ship with an accurate altimeter will measure a change in elevation. So I do agree with you that one should not weigh any different at high or low sealevel elevations (taking out earth's rotation)

You've helped clarify a loose end I had with interpreting the data. Thanks for that insight.

BTW, I apologize if I offended you with my humor. It was truly in jest, and I thought you might get a kick out of it. Your free-form style of posting provoked me to apply some free-form style of humor Hey, it might not be the last!
ae

[bystander]

GOCE gravity mission back in action
European Space Agency | 07 Sept 2010

ESA’s GOCE gravity mission has recovered from a glitch that prevented the satellite from sending its flow of scientific data to the ground. News of the recovery comes earlier than expected, thanks to the fervent efforts of a team of experts.

The recovery follows a serious communications malfunction on 8 July, when the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite suddenly failed to downlink scientific data to its receiving stations.

Extensive investigations by experts from ESA and industry revealed that the issue was almost certainly related to a communication link between the processor module and the telemetry modules of the main computer. These telemetry modules are situated between the processor board and the transmitters, forming a vital part of the onboard data handling and communications system.

Recovery from the situation came after software patches gained access to troubleshooting information via the slow trickle of data that was still reaching the GOCE ground stations. This new information allowed the team to develop an understanding of the state of all the onboard systems. As part of the action plan, the temperature of the floor hosting the computers was raised by some 7°C – resulting in restoration of normal communications.

[neufer]

GOCE gravity mission back in action
European Space Agency | 07 Sept 2010

Recovery from the situation came after software patches gained access to troubleshooting information via the slow trickle of data that was still reaching the GOCE ground stations. This new information allowed the team to develop an understanding of the state of all the onboard systems.

As part of the action plan, the temperature of the floor hosting the computers
was raised by some 7°C – resulting in restoration of normal communications.

As R2-D2 & C-3PO are indoctrinated into Jabba's workforce on Tatooine, we glimpse 8D8 torturing a GNK power droid with a smelting device that features a hot glowing iron. The GNK screams in agony as the iron touches its feet. =>

<<GNK power droids were effectively power generators with legs and a very simple Artificial Intelligence so they could understand rudimentary commands. They were commonly found on under-developed worlds that did not have an expansive power grid, or in mobile military operations. They often made a low honking noise that sounded like the word "gonk," resulting in the nickname gonk droid or simply gonk.

Sometime after the Battle of Endor, rumors began to spread regarding a "Cult of the Power Droids." Apparently, a pair of GNK Power Droids would come to one's door and request funding for a religious fringe group. Advice from renowned linguist Ebenn Q3 Baobab was that one should merely utter the phrase "Gonk. Gonk. Gonk ko kyenga see," a highly controversial statement that was not allowed to be legally translated>>