Planetary core

The planetary core consists of the innermost layer(s) of a planet.

The cores of terrestrial planets tend to be mainly composed of iron and can include a solid and/or a liquid layer. Earth's core is partially liquid, whilst the cores of Mars and Venus are thought to be completely solid, due to their lack of an internally generated magnetic field.^[1] In our solar system, core size can range from about 20% (the Moon) to 75% of a planet's radius (Mercury).

Comparison of the inner core of the extrasolar planet HD 149026b and Jupiter

Gas giants also have iron-rich cores. Although these cores are proportionately much smaller than those of terrestrial planets, gas giants are so large that their cores can actually be larger than Earth. Jupiter's core is thought to be approximately 12 times the mass of Earth (3% of its total mass), and the exoplanet HD 149026 b is thought to have a core approximately 70 times the mass of Earth.

It is thought that some gas giants orbiting very close to their primaries may have their atmospheres stripped away, leaving only their core behind. This as-yet hypothetical class of planets are called "Chthonian."

Some moons, asteroids and other minor planets may also have well-differentiated cores depending on their size and history. Jupiter's moons Io and Europa are in many ways sisters of the terrestrial planets and have very substantial cores comprising about a third of their radii. The large asteroid 4 Vesta is likewise believed to have a differentiated structure with a distinct core.

[edit] Power source of the Earth's core

The core of the Earth is still hot because it contains radioactive uranium and thorium atoms. Although the core is mostly iron, it also contains most of the Earth's uranium. Uranium atoms in the Earth's core themselves are "powered" by the supernova explosions that created them more than 4.6 billion years ago. Without the heat produced over time by their radioactive breakdown, the Earth's core would long since have cooled and solidified. See age of the Earth.

[edit] References

1. ^ Luhmann, J. G.; Russell, C. T. (1997). "Mars: Magnetic Field and Magnetosphere". Encyclopedia of Planetary Sciences: 454–6. Chapman and Hall.