Overview, Main Properties

  • Origin of name: In Roman mythology, the god of commerce, travel and thievery; In Greek mythology Hermes, the messenger of the Gods (in the evening; Apollo in the morning); Probably received this name because it moves quickly across the sky.
  • Viewing: Best viewed at dawn or sunset; Always close to the Sun; Has a full cycle of phases; Can be seen occasionally transiting the Sun [will do so 14 times in the 21st century, the first one was in 2003, and then parallax will give us its distance].
  • Size and mass: Radius about 40% of Earth's [Mass about 5.5% of Earth's]; From its mass and volume we know that its average density is high, almost as high as the Earth's.
  • Motion: Orbit 0.4 au from the Sun, so 1 Mercury year = 88 days, considerably elliptical; Slow rotation with period = 59 days = 2/3 year, in resonance with the orbit [so on Mercury, 1 solar day = 2 years!]. Rotation axis is perpendicular to orbit.


  • Past missions: Until this year, only flybys by Mariner 10 probe in 1974, mostly 10,000 km from the surface; The spacecraft only took pictures of half of the planet.
  • Current missions: NASA's Messenger mission, launched in 2004, flew by Mercury in 2008 in preparation to enter into orbit in 2011.
  • Future missions: The ESA together with Japan's ISAS are planning the Bepi-Colombo orbiter mission for 2011 launch.

  Surface and Atmosphere

  • Surface: Mercury's appearance is similar to the Moon's, but there are fewer craters; Solidified volcanic lava from past activity fills many craters and plains [but there are no maria].
  • Special features: Very large Caloris basin [1300 km diameter] due to impact, and "weird/wild terrain"; Cliffs or scarps (hundreds of km long) due to tectonic stresses from planet shrinking, after most of the cratering.
  • Atmosphere: Almost none; Gas comes out of the hot planet, but gravity is too weak to hold much of it near the surface; Even the oldest remaining craters are mostly sharp and uneroded.
  • Consequences: No sounds, rain, clouds, blue sky; Wide temperature swings [from –150°C, about –250°F, to 400°C, about 800°F]; One of the hottest places in the Solar System but may also have ice inside dark craters (Messenger will look).

Interior and Evolution

  • Structure: It must have a large, heavy, iron-rich core [Mercury is not as compressed as the Earth, and yet its density is similar], probably molten but relatively cold, with a thinner rock mantle; There is a weak magnetic field, but no geological activity in the mantle and crust for 3 Gyr.
  • Formation: Formed about 4.5 billion years ago with the rest of the Solar System; A giant impact may have vaporized most of the original envelope, made of lighter material.
  • Evolution: Mercury cooled slowly because of its closeness to the Sun; Intense volcanic activity erased early craters, but the crust was compressed as the core cooled and prevented later volcanic flows [after the first 700 million years or so].

[The number of past impacts can be taken to be indirect evidence for asteroid-like objects near the Sun (Vulcanoids).]

page by luca bombelli <bombelli at>, modified 29 sep 2012