APOD: The Potsdam Gravity Potato (2014 Dec 15)

[APOD Robot]

The Potsdam Gravity Potato

Explanation: Why do some places on Earth have higher gravity than others? Sometimes the reason is unknown. To help better understand the Earth's surface, sensitive measurments by the orbiting satellites GRACE and CHAMP were used to create a map of Earth's gravitational field. Since a center for studying this data is in Potsdam, Germany, and since the result makes the Earth look somewhat like a potato, the resulting geoid has been referred to as the Potsdam Gravity Potato. High areas on this map, colored red, indicate areas where gravity is slightly stronger than usual, while in blue areas gravity is slightly weaker. Many bumps and valleys on the Potsdam Gravity Potato can be attributed to surface features, such as the North Mid-Atlantic Ridge and the Himalayan Mountains, but others cannot, and so might relate to unusually high or low sub-surface densities. Maps like this also help calibrate changes in the Earth's surface including variable ocean currents and the melting of glaciers. The above map was made in 2005, but more recent and more sensitive gravity maps of Earth was produced in 2011.

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[Beyond]

A lumpy potato smothered in gravity, instead of gravy.

[Chris Peterson]

Need to fix that last link, which has a malformed URL.

[Boomer12k]

I guess it is not a homogenous "potato"....

:---[===] *

[JohnD]

"Many bumps and valleys on the Potsdam Gravity Potato can be attributed to surface features, such as the North Mid-Atlantic Ridge and the Himalayan Mountains, but others cannot, and so might relate to unusually high or low sub-surface densities."

You can say that again! The blurb writer seems to have misspoke (polite for saying made a complete ass (ass=donkey!) of themselves!), as this interpretation is at odds with the model. Yes, the North Atlantic Ridge is a high gravity area, but not the South, and the 'Line of Fire' along the west coast of America is a high gravity area, in South America, not North. In Africa, high/low gravity seems to bear no relationship to geography, even of the Great Africa Rift Valley, and in Asia the Himalayas are a distinctly ordinary gravity area!

John

[Climbmateconned]

The percentage difference from the red to blue areas would have been helpful. I suspect is is not nearly as much as the exaggerated potato shape would suggest.

[BDanielMayfield]

Granted, most of Earth's land area is on the shown hemisphere, but it would have been nice to have had a view of the western hemisphere too, if only for comparison.

[JohnD]

Use the "gravity maps" link in the OP for an animated globe of the model.
And see what I meant.
John

[Chris Peterson]

Use the "gravity maps" link in the OP for an animated globe of the model.
And see what I meant.

Sorry, I can't figure out what you meant or what your complaint is with the caption, which seems just fine.

[JohnD]

Chris,
The Op said " Many bumps and valleys on the Potsdam Gravity Potato can be attributed to surface features, such as the North Mid-Atlantic Ridge and the Himalayan Mountains, but others cannot, and so might relate to unusually high or low sub-surface densities. " But as I pointed out, that does not match the model, esp. the Himalayas!

John

[Chris Peterson]

Chris,
The Op said " Many bumps and valleys on the Potsdam Gravity Potato can be attributed to surface features, such as the North Mid-Atlantic Ridge and the Himalayan Mountains, but others cannot, and so might relate to unusually high or low sub-surface densities. " But as I pointed out, that does not match the model, esp. the Himalayas!

Gravity anomaly maps like this show the Himalayas just fine. You can't judge that from this particular visualization because topographic surface features are overlaid on the anomaly map, hiding the geoid. You'd need to go to the raw data to view the actual geoid and see how it compares with topography.

[Ron-Astro Pharmacist]

What a substrate for wild ideas? But not from me today. I would be curious if it was related to the similar findings on the moon? Suppose so but I thought the Earth's innards were a little more homogenous.

http://newsoffice.mit.edu/2013/an-answe ... neven-0530

[Chris Peterson]

What a substrate for wild ideas? But not from me today. I would be curious if it was related to the similar findings on the moon? Suppose so but I thought the Earth's innards were a little more homogenous.

http://newsoffice.mit.edu/2013/an-answe ... neven-0530

The Earth's innards are not homogeneous, since they vary with depth. But the variations in the Earth's gravity field are probably the result of shallow variations in constitution- the crust and upper mantle. The variations in the Moon's gravity are the result of both shallow features (such as impact remnants) and asymmetries extending throughout the body.

[Ron-Astro Pharmacist]

What a substrate for wild ideas? But not from me today. I would be curious if it was related to the similar findings on the moon? Suppose so but I thought the Earth's innards were a little more homogenous.

http://newsoffice.mit.edu/2013/an-answe ... neven-0530

The Earth's innards are not homogeneous, since they vary with depth. But the variations in the Earth's gravity field are probably the result of shallow variations in constitution- the crust and upper mantle. The variations in the Moon's gravity are the result of both shallow features (such as impact remnants) and asymmetries extending throughout the body.

I probably should have stated that the Earth's interior was "rather more homogenous than the moon's" but the question was intended to quiz if the two phenomenas were related? The mind might think density differences from below would fuel the gravity variables from above. Maybe the crust and upper mantle holds the solution to today's Earth riddle and the Moon's asymmetries are from heterogenousties throughout its body .

I am surprised density differences in crust and upper mantle would lead to the degree of lumpiness shown in today's APOD.

http://en.wikipedia.org/wiki/File:Earth_poster.svg

They (crust and upper mantle) seem too thin to hold enough density. Is the scale true or exaggerated to point out anomalies ?

[Chris Peterson]

I probably should have stated that the Earth's interior was "rather more homogenous than the moon's" but the question was intended to quiz if the two phenomenas were related? The mind might think density differences from below would fuel the gravity variables from above. Maybe the crust and upper mantle holds the solution to today's Earth riddle and the Moon's asymmetries are from heterogenousties throughout its body .

In both cases the gravity varies because of density variations, mainly shallow ones. The difference is that on the Moon they seem mostly related to a long impact history, and on the Earth to tectonics.

I am surprised density differences in crust and upper mantle would lead to the degree of lumpiness shown in today's APOD.

Why? How lumpy do you imagine it really is? The actual variation is tiny. It is exaggerated in this image to make it apparent. The smoothest ball bearing looks just as lumpy and misshapen if you exaggerate the vertical detail enough.

[Ron-Astro Pharmacist]

The smoothest ball bearing looks just as lumpy and misshapen if you exaggerate the vertical detail enough.

I think you just answered my question. Maybe a ball bearing featured for scale would have made a good comparison. Or an Idaho spud? But then all other potates would look deficent. Thanks!

[BennyH]

Looks like the iron core is out of center.
We better send somebody in there, to adjust it with a few nukes...

[bulwynkl]

High areas on this map, colored red, indicate areas where gravity is slightly stronger than usual, while in blue areas gravity is slightly weaker.


I think this is the wrong way around.... I'm pretty certain the gravity off the coast of Sri Lanka is a known anomaly such that the surface of the ocean is 100m closer to the centre of the Earth than the average... (it's one of those things I learned as a child and remember).

which means the colour map is as follows:

blue = surface of the ocean LOWER than average = HIGHER gravity
red = surface of the ocean HIGHER than average = LOWER gravity


Confirmed from the source:
http://media.gfz-potsdam.de/gfz/wv/05_M ... 0_engl.pdf
p13.
"South of India the sea surface
slopes into a 110 metre deep valley,
north of Papua New Guinea it forms a
85 metre high mound."

[BDanielMayfield]

Granted, most of Earth's land area is on the shown hemisphere, but it would have been nice to have had a view of the western hemisphere too, if only for comparison.

Use the "gravity maps" link in the OP for an animated globe of the model.
And see what I meant.

Sorry, I can't figure out what you meant or what your complaint is with the caption, which seems just fine.

Thanks JohnD. I did use the gravity maps link, and I did see what you meant.

I had thought I had figured out how to skin this tater too, but then this comment came out ...

High areas on this map, colored red, indicate areas where gravity is slightly stronger than usual, while in blue areas gravity is slightly weaker.

I think this is the wrong way around.... I'm pretty certain the gravity off the coast of Sri Lanka is a known anomaly such that the surface of the ocean is 100m closer to the centre of the Earth than the average... (it's one of those things I learned as a child and remember).

which means the colour map is as follows:

blue = surface of the ocean LOWER than average = HIGHER gravity
red = surface of the ocean HIGHER than average = LOWER gravity

Confirmed from the source:
http://media.gfz-potsdam.de/gfz/wv/05_M ... 0_engl.pdf
p13.
"South of India the sea surface
slopes into a 110 metre deep valley,
north of Papua New Guinea it forms a
85 metre high mound."

Is bulwynkl right?

[aristata@tds.net]

"...more sensitive gravity maps of Earth was produced in 2011." Someone was asleep at the wheel. I'm a plant person, but one reason I love APOD is the intelligent commentary. That said, I could easily have made the same gaffe myself.

[tehbeefer]

Even if the gravity "depression" in the region south of India is due to a deeper sea-floor, it seems unusually circular, at least in the 2005 image. Just armchair speculation here, but could that be an indication of an impact crater? I'm also wondering if one could see the Amazon outflow in the Atlantic due to density differences.

[BDanielMayfield]

High areas on this map, colored red, indicate areas where gravity is slightly stronger than usual, while in blue areas gravity is slightly weaker.


I think this is the wrong way around.... I'm pretty certain the gravity off the coast of Sri Lanka is a known anomaly such that the surface of the ocean is 100m closer to the centre of the Earth than the average... (it's one of those things I learned as a child and remember).

which means the colour map is as follows:

blue = surface of the ocean LOWER than average = HIGHER gravity
red = surface of the ocean HIGHER than average = LOWER gravity


Confirmed from the source:
http://media.gfz-potsdam.de/gfz/wv/05_M ... 0_engl.pdf
p13.
"South of India the sea surface
slopes into a 110 metre deep valley,
north of Papua New Guinea it forms a
85 metre high mound."

By the way bulwynkl, welcome to the starship, and I must say that your first post was excellent. You've not only dared to question authority, you made made your case with convincing arguments supported with solid documentation. While Borus and Natasha ignore your logic inverting insight, I joyfully applaud it.

Bruce (rhymes with moose)

[DavidLeodis]

I wonder if there is a consensus as to which is correct? Is it that red shows stronger gravity and blue weaker gravity (as given in the explanation) or is that wrong and it should be the other way round?

[geckzilla]

The APOD description is correct. You can applaud bulwynkl all you want but this is not a topographical map of Earth. It's a way of visualizing gravitational anomalies.
http://earthobservatory.nasa.gov/Featur ... /page3.php

[Chris Peterson]

I wonder if there is a consensus as to which is correct? Is it that red shows stronger gravity and blue weaker gravity (as given in the explanation) or is that wrong and it should be the other way round? :?

Blue shows regions where the geoid is below the reference ellipsoid, meaning the gravity anomaly is negative (there is less mass there). Red shows regions where the geoid is above the reference ellipsoid, meaning the gravity anomaly is positive (there is more mass there). Although many gravity anomaly maps show the entire surface of the Earth, this Potsdam map only shows the ocean areas.

The darkest blue areas represent a height difference between the reference ellipsoid and the geoid of about -100 m (that is, in the absence of other forces like winds and tides, the ocean surface would be below mean sea level there). The darkest red areas represent a height difference of a little less than 100 m. The ideal ocean there would be above mean sea level. These correspond to gravity anomalies of about -50 and about +50 milligals. A milligal is a bit more than one millionth of a standard gravity (9.8 m/s²).

In summary, blue means less mass, less gravity, and a lower equipotential surface. Red means more mass, more gravity, and a higher equipotential surface.