6. Mercury: a battered world

    • Because of its close proximity to the Sun, the innermost planet Mercury cannot be studied from Earth against the dark night sky; many astronomers and most people have never seen the elusive planet.

    • During the daytime, Mercury's ground temperature reaches a blistering 700 kelvin; at night it plunges to a freezing 90 kelvin.

    • Although Mercury is one of the Earthís nearest planetary neighbors, only two spacecraft have ventured near Mercury. They are the Mariner 10 spacecraft in 1974-75 and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging, abbreviated MESSENGER, spacecraft in 2008-2011.

    • There is a simple three-to-two resonance between Mercury's rotation period of 58.646 Earth days and its orbital year of 87.969 Earth days. This spin-orbit coupling is produced by solar tides in the solid planet,

    • The interval from sunrise to sunset at a given location on Mercury is 87.969 Earth days, and the night lasts 87.969 Earth days more, so the day on Mercury lasts 175.938 Earth days and twice Mercury's year.

    • Mercury's rotation axis is aligned perpendicular to its orbital plane, so there are no seasons on the planet, and its polar regions never receive the direct rays of sunlight. Radar echoes suggest that water ice may reside in permanently shaded regions within deep craters near Mercuryís poles.

    • Mercury has highland craters and impact basins that resemble those found on the Moon. The craters and basins on both objects were most likely formed during a late heavy bombardment by meteorites 3.9 billion years ago.

    • An ancient period of volcanic flow, during the late heavy bombardment of Mercury 3.9 billion years ago, obliterated small craters, partially filled larger craters, and created intercrater plains that are not found on the Moon.

    • Smooth volcanic plains have filled old craters and impact basins after they formed, and covered approximately 40 percent of the surface of Mercury.

    • Irregularly shaped depressions surrounded by bright material have been attributed to volcanic vents on Mercury.

    • Long, winding cliffs, or rupes, are found on Mercury, and not on the Moon. They are attributed to the contraction of the young planet as it cooled.

    • Relative to its size, Mercury has the biggest iron core of all the terrestrial planets, and Mercury's core is much larger than the core of the Moon.

    • Mercury may have been blown apart by an ancient collision with a planet-sized object, removing its low-density rocky mantle.

    • Mercury has a dipolar magnetic field with a magnetic axis closely aligned with the planetís rotation axis.

    • The magnetosphere of Mercury can be opened on its dayside by magnetic reconnection during interaction with the magnetic fields emanating from the Sun.

    • Rotational twists discovered by radar observations of Mercury suggest that it has a liquid core in which the planetís magnetic field might be generated.

    • More than a century ago, astronomers found that Mercury did not appear in its expected place, leading Einstein to develop a new theory of gravity in which the Sun curves nearby space.

Copyright 2010, Professor Kenneth R. Lang, Tufts University