Danish Research Center To Explore Mysteries of Earth's Interior

An anonymous reader writes "The DanSeis Centre at the University of Copenhagen has just received a grant of more than €3 million from the Danish Ministry of Science, Innovation and Higher Education to investigate and tackle one of geoscience's great mysteries: do mantle plumes, hypothetically buoyant regions of heated mantle material rising towards the earth's surface, actually exist?"

What about the discontinuity of gravity?

[UnknownSoldier]

Why is the force of gravity at the core zero?
http://en.wikipedia.org/wiki/File:Earth-G-force.png [wikipedia.org]
http://en.wikipedia.org/wiki/File:Slice_earth.svg [wikipedia.org]

And why the hell is there a Gutenberg discontinuity where gravity increases the closer you get, then drops down to zero?
i.e.
The Gutenberg Discontinuity, is the boundary, as detected by changes in seismic waves, between the Earth's lower mantle and the outer core about 1800 miles below the surface. It is also called the core-mantle boundary.
http://en.wikipedia.org/wiki/Mohorovi%C4%8Di%C4%87_discontinuity [wikipedia.org]

Re:What about the discontinuity of gravity?

[jlar]

"Why is the force of gravity at the core zero?"

Because the integral of the forces acting on some mass at the center of the Earth is zero. Or to put it differently: You are being pulled by (approximately) equal forces in all directions.

Re:What about the discontinuity of gravity?

[rgbatduke]

Gravity does not increase once you are inside the Earth's surface unless perhaps you are moving down towards a large, cow-shaped lode of pure Uranium. Newtonian gravitation satisfies a Gauss Law (like electrostatic fields) and aside from minor perturbation due to the Earth being rotationally deformed and the tides, the field starts at zero at the origin/center and smoothly increases as one moves toward the surface in any direction, then smoothly decreases (like $latex 1/r^2$) once one is outside of it. Rotation (e.g. coriolis forces) alter the perceived local acceleration by a hair as a function of latitude. The slight equatorial bulge and compressibility of the core material keep the field from increasing ideally/linearly (as it would for a perfect sphere of uniform density). Finally, the Sun and the Moon create further local acceleration perturbations that are not strictly speaking variations in the Earth's field, but that result from the non-uniformity of the Sun and Moon's fields and the fact that the center of mass of the Earth has a different acceleration than points on its surface as it interacts with them both (the tides).

None of these things are even slightly mysterious. None of them are really particularly difficult to calculate, or at least estimate. "Interesting" discoveries from the systematic study of near-Earth and inner-Earth gravity are entirely possible, but one would ordinarily consider the discover of a fifth force, or a short range modulation of the gravitational force, to be "interesting" in this context. In order to make such a discovery, however, one has to know the mass distribution and compute the net relative acceleration one should be observing to very high precision, as one is basically looking for an anomaly, and small deviations from a not-too-well-known or even well-defined base quantity are the most difficult to detect, see the entire (somewhat humorous) debate about global warming for an example).

rgb

P.S. -- Off-topic general query: Wordpress lets one embed latex in comments, and it isn't even particularly focussed on technical subject matters. Is there an equally simple way to embed latex in /. comments? I'd put the ideal form of gravitation inside a sphere inline into this reply, except that nobody wants to read things like \vec{g} = -\frac{GMr}{R^3} \hat{r} or the derivation of same in latex unless they know latex well enough to read it as rendered...