Almost our smallest planet, Mercury is just a bit larger than the
Moon. It's a LOT heavier, though. The mostly iron core makes up
nearly the entire planet. Almost all of our information about
Mercury comes from
one spacecraft, Mariner 10. It flew by three times in 1974 and
1975. Only 45% of the surface was mapped. A new mission to Mercury
has been recently approved. MESSENGER will launch in 2004
and orbit Mercury in 2009.
Orbit of Mercury
Mercury's orbit is highly eccentric; at perihelion it is
only 46 million km from the Sun but at aphelion it is 70 million. The
perihelion of its orbit precesses around the Sun at a very slow rate.
Mercury rotates three times in two of its years (a Mercury year
is 88 earth days). The time between subsequent sunsets is TWO
Mercury years! Mercury is the only body in the solar system
known to have an orbital/rotational resonance with a ratio other
than 1:1. This resonance is due to the tidal force of the Sun
on Mercury (like the tidal-lock Earth has on the Moon).
Atmosphere of Mercury
Mercury actually has a very thin atmosphere consisting of
atoms blasted off its surface by the solar wind. Because
Mercury is so hot, these atoms quickly escape into space.
Because of its lack of atmosphere and close proximity to the
Sun, temperature variations on Mercury are the most extreme
in the solar system ranging from 90 K to 700 K.
Surface of Mercury
The surface of Mercury exhibits enormous escarpments,
called lobate scarps,
some up to hundreds of kilometers in length and as
much as three kilometers high. Some cut thru the rings
of craters and other features in such a way as to indicate
that they were formed by compression, wrinkling the
surface. It is estimated
that the surface area of Mercury shrank by about 0.1%
(about 1 km in the planet's radius) when the planet cooled.
Caloris Basin is the largest known feature on Mercury's surface (seen at the shadow line at left). It is thought to be similar to the large basins (maria) on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system. That impact was probably also responsible for the odd terrain on the exact opposite side of the planet. The Caloris impact may have caused some of the smoother (and presumably younger) terrain to have formed in lava flows.
The Mystery of Mercury's Interior
Scientists were shocked to find a moderate magnetic field on
Mercury! This suggests it's core must be partially molten
(liquid). Also, nearly the entire planet is made of the core.
It is suggested that a large impact, well after Mercury
differentiated, may have driven off most of its lighter
crust and mantle material.