What Is the Core of Mercury Made of?

Significance

• NASA researchers working with radio telescopes found definitive evidence in 2007 that Mercury has at least a partially molten core. Astronomers had been working to figure out the composition of Mercury's core since 1974, when the spacecraft Mariner discovered that Mercury has a weak magnetic field, an unexpected finding. A magnetic field indicated the presence of liquid iron, and since small planets cool off quickly after forming, scientists had always theorized that the core was solid iron, with a silicate mantle surrounding it.

  For the core to remain molten in such a small planet, there must be impurities in the iron--large amounts of a light element that lowers the iron's melting temperature. Scientists are still working to determine how this occurred, because those light elements condensed further out in the solar system. They speculate that small protoplanets left sulfur behind, which Mercury picked up while it was still forming.

Types

• Venus and Mars do not have a magnetic field, while Earth has a magnetic field because of its molten outer core. Mercury's magnetic field is very weak, only about one percent of the Earth's. These are the only four rocky planets in the solar system; the others are gaseous. The core composition of Venus has yet to be determined, but scientists theorize it has a partially molten core since it is similar in size and mass to Earth.

Size

• Mercury is even smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Since the discovery of Pluto, Mercury had been classified as the second-smallest planet until 2006, when Pluto was re-categorized as a dwarf planet, which is actually a large asteroid body.

  At a diameter of 4,879 kilometers, Mercury is not much larger than Earth's moon, whose diameter is 3,475 kilometers. Mercury is the second-most dense object in the solar system, other than the Earth. Mercury's core apparently is larger than that of Earth, and may take up most of the planet.

  Mercury has a very thin atmosphere, and looks a lot like Earth's moon with a very old surface that is heavily cratered.

Geography

• Mercury is the closest planet to the sun, about 58 million kilometers away. Earth is about 150 million kilometers from the sun. Because of Mercury's proximity to the sun, we can only see Mercury with the naked eye when it appears as a bright star close to the horizon, just before sunrise or just after sunset.

Time Frame

• On Mercury, it is daytime for one year, and night for one year. This is because the planet orbits the sun quickly while it rotates on its axis very slowly. Mercury takes about 88 Earth days to orbit the sun, compared with Earth's 365 days. On the other hand, it takes about 176 Earth days for Mercury to rotate on its axis, while Earth takes only 24 hours.

  Because Mercury has an extremely elliptical orbit around the sun, rather than a circular orbit like Earth's, and because Mercury rotates so slowly, this makes the sun seem to move in ways that people on Earth would find very strange. If viewing the sun from Mercury's surface, sometimes the sun would appear to stop completely, then back up for awhile, then move forward again to continue past the point where it previously stopped.

  Also, since the elliptical orbit takes the planet further from and closer to the sun, the sun sometimes looks larger and sometimes looks smaller.