The smallest of the planets (smaller even than the moons Ganymede
in the outer solar system by volume, although not by mass.), and the closest to the Sun
. The planet is mostly made of rock and metal, with a larger proportion of metal than the other inner planets, and as a result is the second densest of the large moons and planets. (Earth
is more dense only because the large size compresses materials more.) A number of theories exist for the extra concentration of metal: The heat near the early sun may not have allowed as many silicates to condense, the Sun may have blown off the other rocks early on, or an impact may have blown off most of the outer rocks.
The planet is geologically inactive today, mostly covered in craters, although a few remains of possible volcanoes and faults are found on the surface. Despite the inactivity, the planet has a magnetic field, although one much weaker the Earth
's. Some activity may be due to the contraction of the planet over time.
It was expected that the planet would be tidally locked to the Sun, with one side always facing the Sun and the other always facing away, but the planet actually rotates in a 2:3 ratio with its orbital time. The orbit itself is unusually eccentric, and the movement of this orbit was an unexplained mystery that was finally solved by general relativity. Because of this longstanding belief, most depictions of Mercury can be summed up as pre-1965 (Mercury as a tidally locked planet with permanent "hot" and "cold" sides) and post-1965.
Mercury in Fiction
- The Coldest Place, which was a Bait and Switch in that until the end the location wasn't referred to by name, and was just called "the coldest place in the solar system"- which made readers think it was Pluto until the dramatic reveal that it was actually Mercury's dark side. The story had the misfortune, however, in being published (not written) just after it was found to not be tidally locked.
- Several of Isaac Asimov's short stories, including
- The Dying Night has its plot twist of Who Dunnit hinge on the then-accepted idea that Mercury has a constant day and a constant night on one side or the other.
- Lucky Starr and the Big Sun of Mercury
- In Tama of the Light Country (and its sequels), in addition to the atmosphere and alien life, Mercury is tidally locked.
- Mission to Mercury by Hugh Walters has the heroes end up trapped in Mercury's dark side and need to escape before they freeze to death in the near total-zero conditions.
- Iceworld by Hal Clement has a gang of aliens set up base on the hot side of Mercury for their drug operation. Since they're aliens, they still need to set up a system of mirrors to concentrate the sunlight to keep things warm enough for them.
- Kim Stanley Robinson has several stories take place on Mercury, including 2312, where civilization depends on a Mercurial Base in the form of the city of Terminator, which survives the extreme temperature changes by traveling around the planet on a giant pair of tracks, keeping itself in the survivable zone through keeping just ahead of the Sun.
- On Invader Zim its been turned into a giant spaceship by the (who else?) Martians. Zim and Dib proceed to have a space battle using Mars (which has also been turned into a ship) and Mercury, respectively.