APOD: The Potsdam Gravity Potato (2014 Dec 15)

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Expand view Topic review: APOD: The Potsdam Gravity Potato (2014 Dec 15)

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Nitpicker » Wed Dec 17, 2014 2:16 am

BDanielMayfield wrote: We learn things like water "flows downhill" and that water "seeks its own level". These truisms are still true, it's just that with gravity anomalies down is not always straight down, and the level of equal gravitational attraction differs from place to place.
The terms "uphill", "downhill", "straight down" and even "level" are most commonly used in relation to a gravitational field. Most builders still use spirit-levels and plumb-bobs to measure the direction of the gravity vector. But of course, these simple devices aren't precise enough to demonstrate that gravity doesn't always point to the exact centre of the Earth. (And, quite independent of any gravity anomalies, in some circumstances, water can indeed flow uphill. A hydraulic jump is a very common example one can observe in a kitchen sink.)

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by BDanielMayfield » Wed Dec 17, 2014 12:09 am

And many thanks to all helping to resolve the question. I hope I didn't give too much offense, but I wished to have the issue actually and factually addressed, and so I used humor to prompt discussion.
Chris Peterson wrote:
DavidLeodis wrote: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.
We learn things like water "flows downhill" and that water "seeks its own level". These truisms are still true, it's just that with gravity anomalies down is not always straight down, and the level of equal gravitational attraction differs from place to place.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by geckzilla » Tue Dec 16, 2014 11:17 pm

DavidLeodis wrote:Oh, just to make things clear geckzilla I did not know which interpretation was right and to say in response to my query that "You can applaud bulwynkl all you want" is not what I was suggesting was correct.
Yeah, that was for Bruce.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by DavidLeodis » Tue Dec 16, 2014 9:11 pm

Thanks geckzilla and Chris for your help. :)

Oh, just to make things clear geckzilla I did not know which interpretation was right and to say in response to my query that "You can applaud bulwynkl all you want" is not what I was suggesting was correct.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Chris Peterson » Tue Dec 16, 2014 8:52 pm

DavidLeodis wrote: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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by geckzilla » Tue Dec 16, 2014 8:40 pm

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

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by DavidLeodis » Tue Dec 16, 2014 8:24 pm

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? :?

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by BDanielMayfield » Tue Dec 16, 2014 6:47 pm

bulwynkl wrote: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. :clap:

Bruce (rhymes with moose)

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by tehbeefer » Tue Dec 16, 2014 6:27 am

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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by aristata@tds.net » Tue Dec 16, 2014 4:48 am

"...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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by BDanielMayfield » Tue Dec 16, 2014 2:22 am

BDanielMayfield wrote: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.
Chris Peterson wrote:
JohnD wrote: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 ...
bulwynkl wrote: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?

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by bulwynkl » Tue Dec 16, 2014 12:49 am

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."

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by BennyH » Tue Dec 16, 2014 12:05 am

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

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Ron-Astro Pharmacist » Mon Dec 15, 2014 5:37 pm

Chris Peterson wrote: 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. :lol2: Thanks!

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Chris Peterson » Mon Dec 15, 2014 5:16 pm

Ron-Astro Pharmacist wrote: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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Ron-Astro Pharmacist » Mon Dec 15, 2014 4:57 pm

Chris Peterson wrote:
Ron-Astro Pharmacist wrote: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 ?

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Chris Peterson » Mon Dec 15, 2014 3:34 pm

Ron-Astro Pharmacist wrote: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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Ron-Astro Pharmacist » Mon Dec 15, 2014 3:21 pm

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

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Chris Peterson » Mon Dec 15, 2014 2:49 pm

JohnD wrote: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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by JohnD » Mon Dec 15, 2014 2:39 pm

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

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Chris Peterson » Mon Dec 15, 2014 2:30 pm

JohnD wrote: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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by JohnD » Mon Dec 15, 2014 2:19 pm

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

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by BDanielMayfield » Mon Dec 15, 2014 1:15 pm

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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by Climbmateconned » Mon Dec 15, 2014 11:26 am

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.

Re: APOD: The Potsdam Gravity Potato (2014 Dec 15)

by JohnD » Mon Dec 15, 2014 10:49 am

"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

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