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 1 Mercury solar day = 2 Mercury years!] Rotation axis is perpendicular to orbit.

Exploration

• Past missions: There have only been flybys by the Mariner 10 probe in 1974, most of the time 10,000 km from the surface; the spacecraft took pictures of half of the planet, so part of it has not been seen yet.
• Future missions: NASA has launched the Messenger mission in 2004, that will fly by Mercury starting in 2008 and orbit in 2011; The ESA togeher with Japan's ISAS is 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], possibly 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).]