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Researchers Develop the Most Detailed Map of Gravitational Variations Ever

samzenpus posted about a year ago | from the he's-not-heavy dept.

Science 88

schliz writes "An Australian-German team of researchers has developed the most detailed map of gravitational variations ever, using satellite data, gravitational readings and small-scale topographical models. They say the data will help civil engineers and miners, and will be available for free online. Gravitational fields vary because the Earth isn't perfectly spherical. According to the new map, the field is 0.7% greater near the North Pole (9.83ms-2) than at Peru's Nevado Huascaran summit (9.76ms-2). The difference is 40% more than previously expected."

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direct link (5, Informative)

schneidafunk (795759) | about a year ago | (#44862655)

Here is a direct link to the map [curtin.edu.au] if you are wondering where you'll be the lightest :)

Re:direct link (0, Offtopic)

Anonymous Coward | about a year ago | (#44862771)

In related news, the Parkinson's Spirograph Project has announced that they have misplaced this year's submissions. They have officially declared it a tie.

Re:direct link (4, Funny)

Cryacin (657549) | about a year ago | (#44862961)

The important question: Are they attracting tenure with the weight of their research?

Re:direct link (0)

Anonymous Coward | about a year ago | (#44863921)

Where's my mod points when I need them?

Re:direct link (1)

edumacator (910819) | about a year ago | (#44865573)

Hey. Don't make light of the gravity of this research.

Re:direct link (1)

Platinumrat (1166135) | about a year ago | (#44866941)

Interesting you say that. The way grant funding works in Australia is different from the US. In Australia you can get grant funding becasue you've previously done good research before. Thus, the funding is along the lines of... "You've done good work, so we'll keep funding you to continue researching".

Re:direct link (1)

ArsonSmith (13997) | about a year ago | (#44867261)

It's nice to hear that another country has found a fair and balanced way to quantify "good work." because here in the US it's all about what you can convince someone as "good work" leading to requiring far more political/sales/marketing skills and the research itself is secondary to that.

So what is the fair and balanced weighting of "good work" to which you employee?

Re:direct link (2)

LWATCDR (28044) | about a year ago | (#44863069)

Probably should be most detailed map released to the public. The Military of both the US and Russia/USSR have been working on maps of gravitational variations for decades.

Re:direct link (1)

Anonymous Coward | about a year ago | (#44863197)

Civilian uses for high resolution maps include mineral prospecting.

Re:direct link (2)

rwise2112 (648849) | about a year ago | (#44863399)

Probably should be most detailed map released to the public. The Military of both the US and Russia/USSR have been working on maps of gravitational variations for decades.

It would also be only the most detailed measured using sattelites. More detailed maps can be made using measurements from airborne gravimeters or surface measurements. They are used often for oil and mineral exploration.

Re:direct link (1)

scuzzlebutt (517123) | about a year ago | (#44866163)

If all the obese people go there, wouldn't that at least begin to mitigate the gravity variance?

Your Mom's House (4, Funny)

Esion Modnar (632431) | about a year ago | (#44862657)

The gravity field spikes hard there.

Of course it does (0)

Anonymous Coward | about a year ago | (#44862739)

My wife was visiting.

Re:Your Mom's House (0)

Anonymous Coward | about a year ago | (#44862809)

Her house? You can make out each of her thighs.

Re:Your Mom's House (0)

Anonymous Coward | about a year ago | (#44862821)

Must be because of my gigantic penis. She just kinda gravitated toward it.

Re: Your Mom's House (1, Informative)

peragrin (659227) | about a year ago | (#44862879)

Actually the opposite is true. The more mass you have between the gravity source and your self the lower the gravitational pull is.

Though why Everest has higher gravity than a mountain in Peru is odd. Unless the lost city of gold is under that mountain.

The bigger the object the greater the gravitational field. However the more mass you have between yourself and said field point"source" the lower the effects of gravity you feel.

Re: Your Mom's House (0)

Anonymous Coward | about a year ago | (#44862915)

Sounds like someone has a fat mom.

Re: Your Mom's House (1)

somersault (912633) | about a year ago | (#44863079)

The more mass you have between the gravity source and your self the lower the gravitational pull is

That doesn't make any sense to me.. mass is your "gravity source". Though due to the inverse square nature of gravitational pull, putting yourself on top of a mountain would probably reduce the overall gravitational force exerted on your body much more than it adds to it (owing to you getting further from the centre of the Earth). Especially if the mountain was tall, thin, and had a low density.

Re: Your Mom's House (1)

peragrin (659227) | about a year ago | (#44867931)

No mass isn't the gravitational source by itself. There is another link between the two. Yes the higher the mass the higher the gravity. but this map shows you can get fluctuations in gravity's acceleration not only based on altitude as expected, but densities or other sources.

As a mountain that is 7,000' shorter than everest has less gravitational pull. Look at that map. areas that are volcanic have greater gravitational pull.

Something else is affected the expected numbers. density of the mantle is a good candidate.

Re: Your Mom's House (0)

Anonymous Coward | about a year ago | (#44869765)

"but densities or other sources." and "density of the mantle is a good candidate."

So in other words, mass is the gravitational source, it just comes down to the distribution...

Re: Your Mom's House (1)

Skapare (16644) | about a year ago | (#44863121)

Being on top of a higher mountain places you further from the all that mass of the whole planet, reducing your gravity. How it is that they are showing higher gravity for some mountains is not understood.

Re: Your Mom's House (0)

Anonymous Coward | about a year ago | (#44863191)

Uh, no, that's not how it works at all. Mountains are massive.

Re: Your Mom's House (3, Interesting)

camperdave (969942) | about a year ago | (#44863337)

True, but the real question is whether the increase in gravitational pull due to the extra mass of the mountain is greater than or less than the decrease in gravitational pull you experience by being further away from the center of the Earth due to the height of the mountain.

Re: Your Mom's House (1)

lymond01 (314120) | about a year ago | (#44864029)

Agreed. From the map they're obviously not just measuring altitude and running it through Newton's equation with R being the distance to the center of the Earth. Or maybe they are if the world isn't perfectly elliptical (as noted in the article). If the Peruvian mountains are tall but overall set in a portion of Earth closer to the center of mass, then their gravity at the peak might be stronger than other equally high mountains. Or maybe they are taking into account on site measurements as well. A satellite can't really tell how close to the Earth's center of mass it is, can it? I suppose I should just read their methodology.

And I'm still hoping for an anti-gravity cloak. Or perhaps a good explanation of gravitons or what is warping spacetime.

Re: Your Mom's House (2, Interesting)

Anonymous Coward | about a year ago | (#44864543)

If you RTFA [curtin.edu.au] ...

All quantities are given at the Earth’s surface as defined through the SRTM (Shuttle Radar
421 Topography Mission) topography. Users wishing to use geoid heights instead of quasigeoid
422 heights can do so by applying standard conversion as described, e.g., Rapp [1997].

If you want a more generic explanation of gravity anomalies? Perhaps this will help... http://www.cage.curtin.edu.au/~will/grav_anoms.htm [curtin.edu.au]

Re: Your Mom's House (1)

somersault (912633) | about a year ago | (#44870855)

I'm still hoping for an anti-gravity cloak

It could cause problems for you to not be attracted to the sun, or the centre of our galaxy, etc :D

Re: Your Mom's House (1)

K. S. Kyosuke (729550) | about a year ago | (#44864637)

the extra mass of the mountain...

...is at least partly compensated for. It's not just "some extra stuff on top of something that exactly the same as places without extra stuff on top": the mountains float on the denser mantle with the rest of the Earth's crust, with a greater portion of the lighter crust material reaching deeper into the mantle, creating a "negative mascon" under the "extra mountain". It's somewhat like ice on sea - there's light stuff under the surface, too.

Re: Your Mom's House (1)

OldSoldier (168889) | about a year ago | (#44864917)

They say it's a "gravitational map" but is it with or without the effects of the earth's rotation? How much less do you weigh because the earth's rotation is trying to fling you off it on a mountain top (longer radius arm) near the equator (faster rotational speed)?

Re: Your Mom's House (0)

Anonymous Coward | about a year ago | (#44865913)

It's accounted for. http://en.wikipedia.org/wiki/Physical_geodesy [wikipedia.org] . Next up, let's debate quantum mechanics on Slashdot... first post should be on whether we should call the electron the positively or the negatively charged particle? :facepalm:

Re: Your Mom's House (1)

HybridST (894157) | about a year ago | (#44863643)

Look up Newtons Theorem and try again.

Re: Your Mom's House (1)

peragrin (659227) | about a year ago | (#44864371)

The survey shows that is wrong. Everest has greater gravity acceleration than a mountain 7,000 ' shorter.

Re:Your Mom's House (1)

wbr1 (2538558) | about a year ago | (#44863843)

Screw you man, screw you!

I am not mad about your joke at my mothers expense, just that I was going to make the same joke about my ex-wife!

this is just the first step. (4, Funny)

nimbius (983462) | about a year ago | (#44862703)

the more challenging and involved effort will be the calculus of integrating this detailed graviton map into future "your mother is so fat" jokes.

Re:this is just the first step. (1)

drinkypoo (153816) | about a year ago | (#44863701)

the more challenging and involved effort will be the calculus of integrating this detailed graviton map into future "your mother is so fat" jokes.

Your mother is so fat, she has a uniform gravitational field.

Re:this is just the first step. (1)

shiftless (410350) | about a year ago | (#44889327)

Yo mama so fat she walks to the top of a mountain and gains ten pounds.

Don't know their science (2, Informative)

Daimanta (1140543) | about a year ago | (#44862785)

From the article: That means a 100kg person weighs 700g more near the North Pole, where gravity is 9.83ms-2, than at Peru’s Nevado Huascaran summit, where gravity is 9.76ms-2.

They are implying that mass is a function of gravity. Everybody who has had the most basic fundamentals of physics knows that mass doesn't change, only weight(measured in newtons)

Re:Don't know their science (3, Insightful)

RichMan (8097) | about a year ago | (#44862851)

Grocery stores "weigh" everything in grams. Grams might be mass but the general populace uses mass interchangeably with weight.

Hmm, can we use the map to get global scale calibrations to a normal mass. It would seem to be unfair that the same amount of material might cost more or less in different places due to scale errors that measure weight and use it blindly as mass.

Re:Don't know their science (0)

Anonymous Coward | about a year ago | (#44863255)

Usually scales are calibrated by putting an object of known mass on them and making sure they show the correct mass.

Unless it's the type of scales where you compare with known masses anyway; those work independently of the local gravitation (as long as that differs substantially from zero, of course).

Re:Don't know their science (5, Interesting)

Kjella (173770) | about a year ago | (#44863329)

Hmm, can we use the map to get global scale calibrations to a normal mass. It would seem to be unfair that the same amount of material might cost more or less in different places due to scale errors that measure weight and use it blindly as mass.

For weights that are comparing against a known mass there is no problem. The 1 kg of material you want to buy will always weigh the same as the 1 kg on the other side of the scale weight, no matter if it's 9.76 or 9.83 newtons on each side. So these "global scale calibrations" just involve transferring around known masses and has been done for centuries. The only way the scale would be off would be if one arm was on the North Pole and the other in Peru.

Re:Don't know their science (1)

ArsonSmith (13997) | about a year ago | (#44867329)

but my scale is really really big. Spanning form west to east Venezuela.

Re:Don't know their science (1)

JesseMcDonald (536341) | about a year ago | (#44867377)

For weights that are comparing against a known mass there is no problem. The 1 kg of material you want to buy will always weigh the same as the 1 kg on the other side of the scale weight ...

True, but not all scales work that way. Many use a spring or pressure sensor rather than comparing weights directly, and thus wouldn't take variations in the gravitational field into account. These scales would need to be calibrated against a known mass if they were used in a different location than they were manufactured in. (Of course, they probably needed to be calibrated anyway...)

Re:Don't know their science (1)

Charliemopps (1157495) | about a year ago | (#44863425)

Yes, I'm immediately taking this map to my drug dealer. I've been wondering for years why he will only let me make buys on Easter island... now it's become rather clear that I'm being ripped off.

Re:Don't know their science (1)

Bengie (1121981) | about a year ago | (#44865111)

New scales will need GPS build in so they can use the new gravity map to determine how many grams something is.

In other news, purchasing drugs in Peru and selling at the North Pole for profit! Someone has to keep those elves peppy.

Re:Don't know their science (2)

i kan reed (749298) | about a year ago | (#44862861)

Secretly, regular people who live in SI countries don't use or understand Newtons unless they're scientists.

Sad (0)

Anonymous Coward | about a year ago | (#44863797)

They fail grade school science as well.

Re:Don't know their science (1)

Anonymous Coward | about a year ago | (#44862875)

Yep. I regularly tell people I weigh 885 Newtons

Re:Don't know their science (1)

Anonymous Coward | about a year ago | (#44862979)

Fig or strawberry?

Re:Don't know their science (0)

Anonymous Coward | about a year ago | (#44863067)

Mixed berry.

Re:Don't know their science (1)

sjwt (161428) | about a year ago | (#44863081)

Its 2013, how many IPADs is that!

The Newton is so 90's.

Re:Don't know their science (0)

Anonymous Coward | about a year ago | (#44866625)

How many oldtons is that? Or longtons, shorttons, ye olde tonnes?

Re:Don't know their science (2)

Dunbal (464142) | about a year ago | (#44863035)

No, they are implying that here on earth, scales measure weight not mass, and the unit for weight and mass is one and the same on this planet. Tell me who actually uses the Newton to express their weight, outside a physics/engineering context?

Re:Don't know their science (1)

camperdave (969942) | about a year ago | (#44863621)

You're supposed to use balance scales, not spring scales when measuring mass. With a balance scale, the force on the test mass and the force on the counterbalancing mass are the same, so it reads properly regardless of the strength of the gravitational pull.

Re:Don't know their science (2)

rwise2112 (648849) | about a year ago | (#44863519)

From the article: That means a 100kg person weighs 700g more near the North Pole, where gravity is 9.83ms-2, than at Peru’s Nevado Huascaran summit, where gravity is 9.76ms-2.

They are implying that mass is a function of gravity

Not really. They specifically say "weighs", which is weight, not "masses".

Re:Don't know their science (2)

jalopezp (2622345) | about a year ago | (#44863839)

Actually, they are not. No one thinks mass depends on gravity. What the article actually says is:

That means a 100kg person weighs 700g more near the North Pole

Weighs 700g more. Weighs. They are merely saying that weight is a function of gravity, which is of course, true.

You are confused by the units they are using. This is actually the kilogram-force [wikipedia.org] , a non-SI unit of weight, which converts to about 9.8N or 2.2lb. Cool, no?

Re:Don't know their science (1)

JesseMcDonald (536341) | about a year ago | (#44867535)

That means a 100kg person weighs 700g more near the North Pole

You are confused by the units they are using. This is actually the kilogram-force [wikipedia.org], a non-SI unit of weight...

Anyone would be confused by their units. Even if they did mean to refer to the kilogram-force (why?) they still got it wrong. The shorthand for kilogram-force is "kgf" or "kp", not "kg" or "g". The article's "700g" is a measure of mass, not weight.

Better: A 100.0kg person at the North Pole weighs the same as a 100.7kg person at Peru's Nevado Huascaran summit.

Re:Don't know their science (1)

Bigby (659157) | about a year ago | (#44863873)

How much of an effect does centripetal force play here? Gravity is the centripetal force at the equator that keeps people from flying off the earth due to their tangential velocity. So the perceived gravity on the equator should be less, right?

Re:Don't know their science (1)

K. S. Kyosuke (729550) | about a year ago | (#44864103)

You see, that's what you get when you measure mass AND force both in pounds. It took me some time to understand why Americans measure specific impulse of rocket fuels in "seconds" when I knew since high school that the actual unit is Ns/kg.

Re:Don't know their science (1)

AJWM (19027) | about a year ago | (#44868035)

Seconds as a unit of Isp is nice because it tells you how many seconds of e.g. 1 lb force thrust you get for 1 lb mass of fuel. (Or conversely, how many pounds force you get from that 1 lb mass of fuel if you burn it all in one second.) It's a handy comparison number.

Granted it's kind of an odd unit if you're trying to work out trajectories and mass ratios and delta-vees and such, but hey, it's not rocket science.

Oh, wait...

Re:Don't know their science (1)

synaptik (125) | about a year ago | (#44865169)

The gram is both a unit of mass, and a unit of weight. The intended meaning is dependent on context. This is why your bathroom scale will happily express your weight in either pounds or kilograms.

"most detailed" too strong a claim (0)

Anonymous Coward | about a year ago | (#44862991)

The statement that this is the "most detailed ever" is likely false -- US intelligence agencies did this work during the cold war to improve inertial guidance systems of ICBMs. Although, a direct comparison of the data is likely impossible due to classification.

Re:"most detailed" too strong a claim (2)

Voice of satan (1553177) | about a year ago | (#44863563)

Indeed,

The US, the French and probably the Russians have their own classified gravity maps. Essential to improve inertial guidance of ICBMs so the ones launched by submarines match the precision of ground based ones. I had a colleague whose works on acoustics where classified by NATO. It means he was forbidden to publish his work. I Guess this hasn't changed a lot and that a too precise gravity map could not be released even today.

Re:"most detailed" too strong a claim (1)

K. S. Kyosuke (729550) | about a year ago | (#44864069)

Are you sure this wasn't to increase the precision of the sub navigation itself, as opposed to more precise calculations of missile trajectories? Despite the fact that the unevenness of the Earth's gravity field has to be taken into consideration to some extent, beyond certain spatial resolution, this becomes meaningless - the small local variations essentially cancel each other out in their perturbative influence on the trajectory as the missile is passing overhead at high velocity, and from a high altitude, observing the influence of these changes on the trajectory is difficult anyway (much like a set of point lights illuminating a distant object almost uniformly, as opposed to a closer one passing along the lights). What the gravity field does for you in a submarine, however, is the option to use high-resolution gravimetric maps and an on-board precise gravimeter as another feed for the navigational system to improve the precision of the navigation and to decrease the time drift of the combined feed, which is much more important for precise launches. Without this, the submerged sub would only have INS. (You won't catch the GPS signal underwater, and you can't rely on it anyway since the enemy could either jam it of somehow disable the network before the attack.)

Re:"most detailed" too strong a claim (1)

geogob (569250) | about a year ago | (#44864285)

In absolute it may be innacurate, but it may still be the most detailed ever in the sens of published work available to the public. The publication / availability of information is a critical factor for scienctifc work.

No scientific work is complete before it has been published and peer reviewed.

Ground Truth? (1)

ka9dgx (72702) | about a year ago | (#44863031)

I wouldn't dare use this "map" for any serious purposes. It appears all they did was add the fine details from a topographic map to the rather low resolution results of other surveys. There's no high resolution direct measurement of gravity here.

Don't match (1)

Anonymous Coward | about a year ago | (#44863203)

Why the two images in the post don't match?

no key or legend (2)

Gravis Zero (934156) | about a year ago | (#44863439)

it's great and all that they posted a pretty picture [curtin.edu.au] but they forgot to add a key or a legend of some kind. a color gradient scale with some kind of metric is the least they could do, even the weather channel knows that! [weather.com]

i'm sure the people who made this are the same damn kids that keep walking on my! </rant>

Re:no key or legend (0)

Anonymous Coward | about a year ago | (#44863585)

Also, apparently there is no gravity over oceans...

Re:no key or legend (3, Informative)

AJWM (19027) | about a year ago | (#44868049)

Of course there isn't. That's why boats float.

Re:no key or legend (1)

AdamHaun (43173) | about a year ago | (#44864175)

They had a scale in the first article [alluremedia.com.au] . You can probably make your own using the tool.

I wonder how many countries... (0)

Anonymous Coward | about a year ago | (#44863601)

..."An Australian-German team of researchers has developed the most detailed map of gravitational variations ever, using satellite data, gravitational readings and small-scale topographical models. They say the data will help civil engineers and miners, and will be available for free online. .." ...would have charged for it?

This is really the way science should operate. 'Open-source' the findings and let entrepreneurs develop applications without patent fights...

Perhaps it would be a good idea to encourage such entrepreneurs to make a contribution to science projects if they find them useful....

torrent? (1)

vagn (2168) | about a year ago | (#44863719)

It would be fun to play with this data.
Anyone have a torrent?

Obvious improvement (0)

Anonymous Coward | about a year ago | (#44863801)

would be to use a large-scale topographic model. Too bad the researchers don't understand scale.

Bah (1)

K. S. Kyosuke (729550) | about a year ago | (#44863985)

"Gravitational fields vary because the Earth isn't perfectly spherical."

Uhm, not just that. Gravitational fields also vary because different places on Earth (or other bodies, for that matter) have materials with different density under them, forming so-called mascons.

Re:Bah (1)

Pseudonym (62607) | about a year ago | (#44869257)

Also, the variation is measured relative to the reference ellipsoid rather than a sphere.

Only Google Earth thinks that the reference shape for the Earth is a sphere. Unfortunately, everyone wants their systems to work with Google Earth, so everyone has basically copied the mistake. Lars is thus responsible for effectively undoing 400 years of geography.

Best ever? (1)

khb (266593) | about a year ago | (#44864155)

Folks who fly ICBM's need very accurate masscon (mass concentration) maps for guidance. So I'll bet that various governments militaries have more accurate maps. They do, after all, have a bevy of satellites whose orbit perturbations allow the computation of such things to any degree of accuracy desired ;>

Whether they make displays as nice, I don't know.

Re:Best ever? (1)

GodfatherofSoul (174979) | about a year ago | (#44864453)

Do people actually actively guide ICBMs? I'd figured they were completely automated. And, how accurate do you need to be with a nuke?

Re:Best ever? (1)

AJWM (19027) | about a year ago | (#44868105)

The more accurate you can make the nuke, the smaller it can be. Saves you nuclear materials and launch weight.

And against hardened targets like missile silos or command bunkers, pretty accurate (eg, Cheyenne Mountain couldn't withstand a direct hit, but it could a nearby miss. Colorado Springs would be toast either way.)

ICBMs are pretty much autonomous once launched (I would assume/hope they have an abort mode), but their targeting data is updated regularly (especially true for sub-launched missiles, of course, but also for the independently targeted warheads on a MIRV.)

Re:Best ever? (1)

K. S. Kyosuke (729550) | about a year ago | (#44864553)

I'd really like to see some numbers to support this claim. Just because mascons cause *measurable* changes in trajectory doesn't mean that the influence of any but the largest ones (or just large-ish, which sort of ruins the "very accurate maps" part) is such that it would outweigh the influence of the unguided atmospheric reentry on the actual CEP. And what about the constantly changing effects of the solar activity on the height of the atmospheric boundary itself? That's also one other variable you can't account for in advance that probably has a greater influence on the CEP than small-ish mascons, thus rendering their survey for purposes of increasing guidance precision meaningless.

Re:Best ever? (1)

AJWM (19027) | about a year ago | (#44868171)

That's a reasonable point on the atmospheric effects of reentry, but that's only in the last couple of hundred miles, so any angular deflection will have a lower effect on the CEP than would even a smaller deflection closer to the launch. Over a few thousand miles, a slight angle change makes a big difference.

And, you're assuming the reentry is unguided. That isn't necessarily the case. Even a simple cone can have some crossrange depending on the relation of center of mass vs center of aerodynamic pressure, with small thrusters, aerosurfaces, or weight-shifting to modify that. (Not that I know for sure one way or the other; if I did I probably couldn't say.)

Re:Best ever? (1)

turbidostato (878842) | about a year ago | (#44868957)

"Folks who fly ICBM's need very accurate masscon (mass concentration) maps for guidance."

Or what? The 10 Mt ICBM will boom 10 m out of its target on the other side of the world?

Hm, shouldn't one be heavier at high altitude? (0)

Anonymous Coward | about a year ago | (#44864163)

Isn't gravity defined by the mass (gravitons anyone)? Going towards the centre of the Earth would render you weightless.

At the top of the mountain, should have a stronger gravitational pull. For example, on Jupiter being a bigger object, the gravitational pull is a lot stronger on its surface. So the more mass you have under your feet, the stronger the gravity.

Re:Hm, shouldn't one be heavier at high altitude? (1)

K. S. Kyosuke (729550) | about a year ago | (#44864389)

If by "heavy", you mean mass, then that is certainly the case: by working upwards in the conservative field, your mass increases towards an asymptote as you're removing the negative binding energy.

ho hum... (1)

vettemph (540399) | about a year ago | (#44865341)

maybe I'm not understanding the situation of the gravity here.

Islands have high gravity? (0)

Anonymous Coward | about a year ago | (#44865675)

The map shows a lot of little islands in red. So islands have a higher gravity than than most other areas? And are on par with the tallest mountians? Even with less dense water surrounding the islands instead of land?

Re:Islands have high gravity? (1)

spike hay (534165) | about a year ago | (#44866989)

From Newton, the gravitational potential from a point source is mass*G/distance. With topography, there are two things acting. Because mountains are high, they are further away from the Earth's center of mass (typically, the Earth's gravitational potential map is modeled as a multipole expansion of Legendre polynomials centered at point source at the center of the earth).

However, there is also the gravity of the mountain(s) itself pulling you down. In most mountain ranges, there is lower gravity because of isostasy. The crust is floating on the mantle. Contintental crust is less dense than oceanic crust, and thicker. That is why it is higher than the floor of the ocean. It floats up more like a piece of styrofoam on water. Mountain ranges like the himalaya usually have thicker crust which displaces more of the upper mantle around it than thinner crust in places like Missouri. The crust in the Himalaya/Tibet is around 70 km thick, whereas normal continental crust is more like 30. Assuming close to isostatic equilibrium, from buoyancy the 70 km thick crust (averaging 5 km in elevation so 65 km below sea level) has roughly the same weight as 30 kilometer thick non-montane crust + 35 km of mantle.

However, in the mountains, you are further away from the center of the earth, so there is less gravity.

Seamounts are a different story. They are small, so instead of being in isostatic equilibrium the crust supports them like an elastic beam. They are also made out of heavy oceanic crust. That means they have high gravity because their mass isn't compensated. However, they do sink eventually. That is why if you look at the Hawaii/Emporer seamount chain, the Big Island is the newest and highest, while they get lower and eventually are no longer above sea level as you get older.

Re:Islands have high gravity? (1)

spike hay (534165) | about a year ago | (#44867223)

Also, looking at the world map, it is a map of Bougeur anomalies (or using some other correction), which are gravitational anomalies corrected for the elevation they are taken at and the mass of the topography. That is why a good portion of mountainous areas are shown in red.

The map of Australia is just straight-up gravity (and is different from Australia in the global anomaly map).

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