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
- 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
- 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
Copyright 2010, Professor Kenneth R. Lang, Tufts University