Researchers from Amherst College and The University of Texas at Austin have described a new technique that might one day reveal in higher detail than ever before the composition and characteristics of the deep Earth.
There's just one catch: The technique relies on a fifth force of nature (in addition to gravity, the weak and strong nuclear forces and electromagnetism) that has not yet been detected, but which some particle physicists think might exist. Physicists call this type of force a long-range spin-spin interaction. If it does exist, this exotic new force would connect matter at Earth's surface with matter hundreds or even thousands of kilometers below, deep in Earth's mantle. In other words, the building blocks of atoms—electrons, protons, and neutrons—separated over vast distances would "feel" each other's presence. The way these particles interact could provide new information about the composition and characteristics of the mantle, which is poorly understood because of its inaccessibility.
"The most rewarding and surprising thing about this project was realizing that particle physics could actually be used to study the deep Earth," says Jung-Fu "Afu" Lin, associate professor at The University of Texas at Austin's Jackson School of Geosciences and co-author of the study appearing this week in the journal Science.
This new force could help settle a scientific quandary. When earth scientists have tried to model how factors such as iron concentration and physical and chemical properties of matter vary with depth—for example, using the way earthquake rumbles travel through the Earth or through laboratory experiments designed to mimic the intense temperatures and pressures of the deep Earth—they get different answers. The fifth force, assuming it exists, might help reconcile these conflicting lines of evidence.
Earth's mantle is a thick geological layer sandwiched between the thin outer crust and central core, made up mostly of iron-bearing minerals. The atoms in these minerals and the subatomic particles making up the atoms have a property called spin. Spin can be thought of as an arrow that points in a particular direction. It is thought that Earth's magnetic field causes some of the electrons in these mantle minerals to become slightly spin-polarized, meaning the directions in which they spin are no longer completely random, but have some preferred orientation. These electrons have been dubbed geoelectrons.