Mercury – Baked to a Crisp

Global Properties:

Radius / 38 % that of Earth
Mass / 5.5 % that of Earth
Average density / 98% of Earth.
Surface gravity / 38 % that of Earth
Length of day/night / 176 (!) Earth days
Satellites / No
Atmosphere / None, the surface is directly exposed to space.

Orbit and Rotation:

  • First planet from the Sun, average distance = 0.31 AU.
  • 88 day orbital period.
  • Never strays far from the Sun on the sky – maximum separation = 27.
  • Originally, Mercury’s rotation period was assumed = to its orbit. In 1965, radar observations showed that the true rotation period = 59 days.
  • 3 planetary rotations occur per 2orbits around the Sun, producing a “day" that is 176 Earth days long.
  • This odd rotation rate driven by a 3:2 orbit resonance, due to tidal forces produced by the Sun on Mercury.
  • Mercury’s orbit is strongly eccentric for our solar system: 20.6 %

Surface Features:

  • Difficult to observe from Earth due to its proximity to the Sun. Surface details required images from spacecraft. Little was known prior to Mariner 10.
  • Only half the surface has been imaged up close.
  • Covered with impact craters. There are no large maria, as seen on the Moon, but extensive intercrater plains are observed.
  • Mercury displays many linear features, termed scarps. These appear to be long, kilometer-high cliffs, likely caused by cooling and shrinking of the crust early in Mercury’s history.
  • A major asteroid impact formed the Caloris Basin, which appears as a classis impact structure.
  • Opposite to the Caloris Basin, the surface is strangely jumbled, a result of the seismic waves induced by the Caloris inpactor.
  • Peak surface temperatures of about 700 K occur at the subsolar point. At night, temperatures drop to around 100 K, a 600 K difference, probably the largest day-night temperature change in the solar system.

Interior:

  • Mercury’s high mean density implies a large, iron-rich core. This core may still be partially molten.
  • The ratio of core volume to planet volume is the largest in the solar system.
  • A weak magnetic field, about 1% of Earth's, has been detected. Its origin is not understood. Mercury’s rotation is too slow to produce a planetary magnetic field like that found around Earth.

Evolution:

  • At formation, Mercury was already rich in heavy elements (iron, nickel, copper, etc). Given its proximity to the Sun, and its low mass, Mercury could not efficiently retain lighter elements.
  • The planet melted and differentiated during the first 0.5 billion years, forming the dense core.
  • Possibly experienced a violent collision, which helped remove more of the lighter elements.
  • The crust formed by cooling, followed by extensive cratering during the early asteroid bombardment phase.
  • Planetary cooling produced scarps as the crust “shrank”. Vulcanism was terminated, as the crust “thickened”, and closed off avenues for molten rock to reach the surface.
  • At present, Mercury is a geologically dead planet.

Exploration:

  • Only 1 spacecraft has visited Mercury: Mariner 10, which made three flybys in 1974-75.
  • NASA’s MESSENGER probe, designed to be the first Mercury orbiter was launched in July 2004. MESSENGER will arrive at Mercury in 2009, and explore the planet’s geology and surface chemistry.
  • The European Space Agency is also planning a Mercury mission, named BepiColombo, slated for a June 2011 launch.