How can parts of Canada be "missing" gravity?

How can parts of Canada be "missing" gravity?

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I have always thought that gravity had a constant value but according to this article it doesn’t

What do you think?

From the LINK in the O.P.:

In other words, gravity in the Hudson Bay area and surrounding regions is lower
than it is in other parts of the world, a phenomenon first identified in the 1960s
when the Earth's global gravity fields were being charted.

Good article, by the way. Very cool!

Salt domes or granitic batholiths create gravity anomalies.

I wonder what's under Lake Wahnapitae....

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Mud.

Gravity is a function of mass, and mass is not distributed equally across the surface of our planet. Missing gravity just means it's less than expected, and the article did a good job explaining things like glacial rebound. When the data is put together from the twin GRACE satellites, the planet is not smooth at all. In fact it's very irregular:



That darkish spot to the West of the North Atlantic Ridge (1) and to the Northwest of the Puerto Rico Trench/Lesser Antilles (2) is the Hudson Bay. The Darkish spot .

This map is a map of gravity anomaly, that is the departure of the actual gravity field from what is "normal", normal being a simple mathematical model of the gravity on a planet with no topographical features whatsoever.

So missing gravity is not really missing gravity...in fact it's entirely what should be expected, since the lithosphere of our planet is not fixed and has very prominent features.

The isostatic theory is partial, under the bay is extremly dense granite as well as less mass on that portion of the globe. If it were basaltic there potentially could be postive gravity anomalies.

Very, very cool article. Easy to understand (for someone who is not very science minded like myself)

So if we packed those places with airheads, would they float away based upon a 30 K head wind or not. Serious question that needs a scientific answer.

Bear in mind that the variations are pretty small, on the order of a few millionths of the so called "standard" acceleration due to gravity. They're measured in units called milligals, a thousandth of a gal, which is the unit of acceleration in the Centimeter-Gram-Second system of units. High school physics (at least when I went) generally uses a value for the acceleration due to gravity near the earth's surface of 9.8 meters/sec/sec, which in the C.G.S. system is 980 cm/sec/sec. It actually varies with latitude because of earth's rotation making it not quite a sphere, from 978.049 at the equator to 983.221 at the poles, so a milligal is about a millionth of the normal field. Anomalies due to things like salt domes, oil-bearing structures, undulating strata, topographic variations, and ore bodies, rarely exceed a few milligals and are often only a fraction of a milligal, so gravity measurements need instruments accurate to a few parts in a hundred million.

No doubt a few of you are now thinking of bad puns about gals, tiny women, thousandths of a tiny woman. Try to restrain yourselves or I'll tell you all about the Bouguer anomaly and you can switch to bad puns about French boogers.

Gravitational force is directly proportial to the distance from the centre of the earth, so if you are on top of Mt. Logan you might be missing a lb. or two.

It depends on two things- the size of the body and the distance from the centre. That's why the earth's pull is stronger than the moon's pull for the same distance.

Gravity is not constant in any case.

Only when the size of the body and the distance from the centre is constant.

if a hole was dug all the way through the earth, where would an object end up if it were dropped in it?

If you drop a stone into a bucket of three toed sloaths do the ripples go outward or inward?

if there is water in the bucket i assume that the ripples would go outward first then bounce off the bucket and the 3 toed sloths.

Here's a nice pic of the planets lumpy gravity:


From the European Space Agency's GOCE satellite.

Missed that one before. I'm not going to try the calculation to figure it out precisely, because I'm lazy and there are no exam marks to be earned, but it'll hit the east side of the hole at a fairly shallow depth that'll vary with latitude and the diameter of the hole. If it doesn't stick to the wall, it'll continue downward, bouncing and scraping along, inertia will probably carry it past the centre at least a few times, so it'll bounce and scrape and oscillate back and forth around the centre until eventually its kinetic energy is absorbed by the friction and it comes to rest at the centre of the earth. If, and only if, the hole is directly along the axis of rotation, it won't hit the wall, but air resistance will slow it down and it'll again come to rest at the centre eventually. In the absence of anything to slow it down, and no rotation to make it hit the wall, it'll oscillate back and forth from end to end of the hole pretty much indefinitely with a period of about 88 minutes.

Whatever you drop would vapourize at around 50km. Less if the hole is on the ocean floor.

And rotational effects. By the by, you couldnt drop a rock freely through the hole unless it were pole to pole.

Depends what it's made of. The geothermal gradient's about 22 degrees C per km on average, so it'll be around 1100 degrees at 50 km, at least in tectonically stable areas, well below the melting point of many things. The core temperature's thought to be around 5500 degrees, tungsten boils at 5660 at atmospheric pressure, core pressure would much higher, we could probably make something that'd survive, assuming it were possible to engineer such a tunnel in the first place. It probably isn't, in the absence of something like the force field beloved of science fiction, but I think such considerations violate the spirit of the original question. I actually had that question on a physics exam once, with appropriate caveats to force us to do the calculation instead of saying it can't be done. The periodicity is the same as that of a satellite in low earth orbit.

The same as whats in Downtown Sudbury-nothing. Kinda neat to look at satallite pictures and seeing Sudbury is right in the middle of a giant crater. Appropriate.

Thanks for the info and picture guys, it is very interesting.

Pretty much ... considering Ottawa's on the lip of a giant crack

I really was fascinated with the gravity pic of Earth. Now that science has been drug kicking and screaming to the altar of an extinction event of 12,900 years ago, we can go forward with how advanced, how wide-spread, etc.
Let's suppose that we were more advanced than now but that fewer of us enjoyed the benefits. Let's further say that our gravity was such that there were pockets of anti gravity ala Pandora's floating mountains. So far this does not wander too far from myth. Let's assume that the elite few were able to enjoy the benefits of anti-gravity primarily at the Equator somewhere like the Cayman Trench which could have been land from the Yucatan to Cuba. Let's stop there.
Science done by Miles Mathis says that energy (photons) enter the Milky Way along the axis and are spun up to matter. I say it is then deposited into the mass of the equatorial rim of the Milky Way in a non-locality event as the galactic capacitor overloads.
As for the 12,900 year old extinction event, I propose that our heliosphere has been the subject of compression since the creation of Fluff, our local gas cloud, NASA says is 10 million years old. We are about 5. Since that time our heliosphere has been compressed and there have been various and sundry theories to explain our unexplainable history. I suggest the compression of Earth is due to the incursion of a small bit of the densest matter possible (George Gammow). I suggest that this bit of dense anti matter was caught between the matter of Sol and the matter of Centauri and has been repelled back and forth. I suggest that only briefly does this anti matter enter through the ORT shell and force the heliosphere of Sol down to Mercury, exposing all the other orbiting bodies to cosmic rays, heating, and expansion.
I further suggest that prior to the extinction event of 12,900 the elite of Earth were aware of and anticipated this antimatter's arrival. We call it G1.9 today. I suggest the time frame of its effect on E lessened each appearance and the elite manipulated the anti matter of E to last from appearance to appearance. This changed to lessening the heating effects due to core heating when the sky rivers supported by anti matter collapsed 12,900 years ago. G1.9's appearance now suggests that it has burped the quantum gas that allowed it to be an LT (light terminus) starbit and became a neutron shedding neutrinos heavily. It is now inside the ORT but will depart at the end of 2012 and the heliosphere will rebound to what we think of as normal with extremely heated gas planets now included. Hence the heat problem.
I further suggest the Second Law of Thermodynamics rules and our home Earth has lost anti matter and become heavily matter, a gravity well.
Velikovsky was correct in many things, Venus was nearly all molten 600,000 y ago. Miles Mathis has much info to share on current physics updates.