The Winged Messenger
Mercury is the closest planet to the Sun and the eighth largest. Mercury is
smaller in diameter than Ganymede and Titan but more massive.
orbit: 57,910,000 km (0.38 AU) from Sun
diameter: 4,880 km
mass: 3.30e23 kg
In Roman mythology Mercury is the god of commerce, travel and thievery, the
Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet
probably received this name because it moves so quickly across the sky.
Mercury has been known since at least the time of the Sumerians (3rd
millennium BC). It was given two names by the Greeks: Apollo for its apparition
as a morning star and Hermes as an evening star. Greek astronomers knew,
however, that the two names referred to the same body. Heraclitus even believed
that Mercury and Venus orbit the Sun, not the Earth.
Mercury has been visited by only one spacecraft, Mariner 10. It flew by three
times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately,
it is too close to the Sun to be safely imaged by HST).
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. 19th century astronomers made very
careful observations of Mercury's orbital parameters but could not adequately
explain them using Newtonian mechanics. The tiny differences between the
observed and predicted values were a minor but nagging problem for many decades.
It was thought that another planet (sometimes called Vulcan) might exist in an
orbit near Mercury's to account for the discrepancy. The real answer turned out
to be much more dramatic: Einstein's General Theory of Relativity! Its correct
prediction of the motions of Mercury was an important factor in the early
acceptance of the theory.
Until 1962 it was thought that Mercury's "day" was the same length as its
"year" so as to keep that same face to the Sun much as the Moon does to the
Earth. But this was shown to be false in 1965 by doppler radar observations. It
is now known that Mercury rotates three times in two of its years. Mercury is
the only body in the solar system known to have an orbital/rotational resonance
with a ratio other than 1:1 (though many have no resonances at all).
This fact and the high eccentricity of Mercury's orbit would produce very
strange effects for an observer on Mercury's surface. At some longitudes the
observer would see the Sun rise and then gradually increase in apparent size as
it slowly moved toward the zenith. At that point the Sun would stop, briefly
reverse course, and stop again before resuming its path toward the horizon and
decreasing in apparent size. All the while the stars would be moving three times
faster across the sky. Observers at other points on Mercury's surface would see
different but equally bizarre motions.
Temperature variations on Mercury are the most extreme in the solar system
ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very
Mercury is in many ways similar to the Moon: its surface is heavily cratered
and very old; it has no plate tectonics. On the other hand, Mercury is much
denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the second densest major
body in the solar system, after Earth. Actually Earth's density is due in part
to gravitational compression; if not for this, Mercury would be denser than
Earth. This indicates that Mercury's dense iron core is relatively larger than
Earth's, probably comprising the majority of the planet. Mercury therefore has
only a relatively thin silicate mantle and crust.
Mercury's interior is dominated by a large iron core whose radius is 1800 to
1900 km. The silicate outer shell (analogous to Earth's mantle and crust) is
only 500 to 600 km thick. At least some of the core is probably molten.
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. Thus in contrast to the Earth and Venus whose atmospheres are
stable, Mercury's atmosphere is constantly being replenished.
The surface of Mercury exhibits enormous escarpments, 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. It is estimated that the surface area of Mercury shrank
by about 0.1% (or a decrease of about 1 km in the planet's radius).
One of the largest features on Mercury's surface is the Caloris Basin
(right); it is about 1300 km in diameter. 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 (left).
In addition to the heavily cratered terrain, Mercury also has regions of
relatively smooth plains. Some may be the result of ancient volcanic activity
but some may be the result of the deposition of ejecta from cratering impacts.
A reanalysis of the Mariner data provides some preliminary evidence of recent
volcanism on Mercury. But more data will be needed for confirmation.
Amazingly, radar observations of Mercury's north pole (a region not mapped by
Mariner 10) show evidence of water ice in the protected shadows of some craters.
Mercury has a small magnetic field whose strength is about 1% of Earth's.
Mercury has no known satellites.
Mercury is often visible with binoculars or even the unaided eye, but it is
always very near the Sun and difficult to see in the twilight sky. There are
several Web sites that show the current position of Mercury (and the other
planets) in the sky. More detailed and customized charts can be created with a
planetarium program such as Starry Night.
Bill Arnett; last updated: 2002 Jun 27