Earth's interior structure and composition consists of a thin mostly solid rocky crust, a slowly flowing rocky mantle, a liquid metallic outer core, and a solid metallic inner core. It is possible to drill and directly sample only the top few miles of the crust. So how do scientists deduce the composition and structure of Earth's deep interior?
There are three major clues that scientists can use to probe Earth's interior. These same clues can also help scientists deduce the interior structures of other planets. The clues are:
- The planet's bulk density.
- Seismic waves from earthquakes.
- The planet's magnetic field.
This article discusses how Earth's magnetic field gives scientists clues about Earth's interior structure and composition. Magnetic fields also tell us about the interiors of other planets.
Magnetic Field
Earth's magnetic field is generated deep in the interior, so the strength and orientation of the magnetic field tells geophysicists about the conditions where the field is generated.
As Oersted discovered, electric currents induce magnetic fields. Earth's magnetic field is no exception, electric currents deep in Earth's interior induce its magnetic field. In order to have electric currents in the core, the composition must be an electrically conducting material, such as metal. Earth's density is also consistent with a metallic core, so both the density and magnetic field tell geophysicists that Earth has a metallic core.
From seismic waves scientists know that Earth's metallic outer core is liquid. The liquid metallic outer core circulates as Earth rotates and generates electric currents. These currents in turn induce Earth's magnetic field.
An isolated electric current induces magnetic fields, but the presence of a magnetic metal, such as iron, increases the magnetic field. Hence scientists think that Earth has an nickel iron core rather than some nonmagnetic metal. The nickel iron metallic composition matches both the observed magnetic field and density.
Application to Other Planets
The Jovian planets, Jupiter, Saturn, Uranus, and Neptune, all have magnetic fields. However their densities are too low for them to have a significant iron composition. Some low density material must conduct electric currents in their cores.
Jupiter and Saturn have a large percentage of hydrogen in their composition. Hydrogen is normally a nonmetallic gas, but under very high temperatures and pressures hydrogen behaves more like a metal. Jupiter and Saturn are massive enough to compress some of their interior hydrogen into this metallic state. The metallic hydrogen conducts electric currents that induce the planetary magnetic fields.
Uranus and Neptune are also not dense enough to have iron cores. In addition they are not massive enough to compress hydrogen into a metallic state as in Jupiter and Saturn. Their density is however consistent with a partially molten icy core. Water can conduct electricity, so Uranus and Neptune generate electric currents as the slushy ice portions of their interiors circulate. Their magnetic fields, combined with their densities, tell scientists that these planets must have partially molten icy layers in their interiors.
Earth's magnetic field helps geophysicists deduce Earth's interior structure and composition. Similarly magnetic fields of other planets help scientists deduce the interior structure and composition.
Further Reading
Zeilik, M. Astronomy: The Evolving Universe, 9th ed. Cambridge, 2002.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
