- Diameter: 4,879 km
- Mass: 0.055 of Earth
- Density: 5.427 g/cm³
- Surface Gravity: 0.38 g
- Semi-major Axis: 0.39 AU
- Orbital Period: 88 Days
- Rotational Period: 58 Days (3:2 Spin-Orbit Resonance)
- Axial Tilt: 0.034°
- Average Surface Temperature: 167° C
- Notable Features: Caloris Basin, Caloris Antipodal Terrain
- Number of Total Missions: 3
The smallest of the planets (smaller even than the moons Ganymede and Titan in the outer solar system by volume, although not by mass, as it outweighs the two of them combined), 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.
It is named after the swift Roman messenger god because it moves the fastest across the sky of all the planets. The surface gravity on Mercury is 0.38 g, which is 38% of Earth's gravity. Interestingly, it shares roughly the same surface gravity as Mars despite being smaller than it because it's more dense.
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 than Earth's. Some activity may be due to the contraction of the planet over time. Evidence that Mercury shrunk lies in the presence of tall and long cliffs spread relatively evenly over its surface. These cliffs are called scarps, and are widely held to be the result of subduction zones. If Mercury were tectonically active, the presence of subduction zones on one side of the planet should be an indicator of rift zones on the other side. Instead, astronomers found more scarps, consistent with the hypothesis that the crust contracted planet-wide.
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 ratio means that at times the Sun appears to move backwards in the Mercurian sky, and the orbit itself is unusually eccentric, which produces two equatorial "hot" poles (which always face the Sun at perihelion) and two "cold" poles (which never do). The orbit itself also precesses at a higher-than-expected rate; this 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 is a planet of massive extremes: the side facing the Sun is incredibly hot, but the side facing away from the Sun is incredibly cold, up to -200 degrees. In fact, the only hot parts of Mercury are the parts directly in the path of the Sun's radiation. Since the planet's wisp of an atmosphere can't hold or convect any significant amount of heat, anything in shadow is very cold. How cold? There's ice in the nooks and crannies of the planet where the Sun doesn't shine. It's still not livable by any means, but it makes the planet far more interesting to astronomers.
One particularly unique set of features on Mercury is the Caloris Basin, its largest impact crater, and the hilly Weird Terrain (that is the official name) found on the opposite side of the planet from it. The leading hypothesis is that the impact that created the Caloris Basin was so strong that it sent ripples through the entire planet, creating the Weird Terrain when the energy had nowhere else to go.
Only two probes have made it to Mercury so far, both from NASA (Mariner 10 in 1974, and MESSENGER in 2008) despite being close to Earth compared to the outer planets, it takes a lot of energy to cancel our home's orbital momentum and "fall in" towards the Sun.note As a side note, features on Mercury are generally named after intellectuals artist, painters, writers, and so forth. A third probe, the ESA and JAXA's BepiColombo, is on the way there right now.
Mercury's future is a bleak one, as most projections have it flinging itself outwards away from the Sun, with a strong possibility Earth will be in its cross-sightsnote .
Mercury being the innermost planet of course lacks the equivalent of an evening/morning star and lacking a natural satellite has no friendly moon shining on its skies. However this is compensated having Venus, that appears as a dazzlingly bright white star considerably brighter than from Earth as from there is seen as a full disk and not as a crescent, and Earth as a blue star as bright as we see Venus accompanied by the Moon that would appear as a very bright star.
Mercury in Fiction
- 1320's The Divine Comedy, specifically the fifth, sixth, and seventh cantos of Paradiso, portrays Mercury as a planet inhabited by the souls of good rulers who were too focused on politics, like Emperor Justinian. During the time Dante travels to Mercury, the planet is covered by Earth's shadow.
- "The Coldest Place", a Known Space story by Larry Niven, 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.
- Isaac Asimov:
- 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.
- Wonder Woman (1942): The denizens of Mercury are somewhat brutish giants, and Diana and the Holliday Girls fight the queen of Mercury alongside the Queen of Venus to eradicate the somewhat recent development of enslaving all the local men.
- 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.
- In Destiny, Mercury is somewhat visitable. A couple of Crucible maps take place there, and the Lighthouse social space is avaliable if you are lucky to go flawless on trials of Osiris, it might as well be the Butt-Monkey of the game. It was converted into a machine planet by the Vex prior to the first game. In Destiny 2 it winds up being used as fuel for the Almighty untill you put a stop to it, however it's still been partially destroyed in the process.
- On Invader Zim, it's 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.
- In Nebula, Mercury is portrayed as a rather arrogant anthropomorphic personification who dresses like a White Collar Worker and who gets into arguments with Venus about whether being close to Sun gives him any authority over the other planets. (It does not.)
- In Star Control 2, Starbase Commander Hayes suggests you to go to Mercury to mine it for the radioactive elements Earth Starbase needs, warning you that it's a pretty unhospitable place (better look for them elsewhere in the Solar System).