|
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 is a small, rocky planet that is closest to the
Sun. Mercury is the smallest of the principal planets (with
the exception of Pluto) and, although it can become brighter than any star,
it is not very easy to see with the naked eye. Before the flight of Mariner
10 in 1974 little was known of its surface. Mariner 10, which made three
active passes of Mercury between 1974 and 1975, showed that the surface is
heavily cratered, looking superficially very much like that of the Moon. There are mountains; valleys; scarps and ridges;
gently rolling plains; smooth, flat plains; long, steep cliffs called
scarps; and hills. There are also depressed
basins, of which one the Caloris Basin is 1,300 kilometres in diameter. Mercury is also a very heavy world, made up of 70% iron with the rest silica
rock similar to Earth or the Moon.
However, the planet spins so slowly on its axis
that one day on Mercury would last 59 Earth days. That means it rotates
completely on its axis every 59 Earth days. It makes one revolution around
the Sun every 88 Earth days. However, the time between sunrises on Mercury
is 176 Earth days.
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. At one time, the thought
that there was another planet still closer to the Sun was used to explain
some odd motions in Mercury's orbit. 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.
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). Mercury's surface is covered with regolith, a layer of scattered
rubble created by the same impacts that caused its craters. There are large
sheets of ice on the planet's polar regions. Mercury has a very thin
atmosphere that is made of hydrogen, helium, potassium, and sulfur.
It receives about seven times as much heat and light as the Earth
does. Mercury is the smallest inner planet;
it is the smallest planet. It is also a very dense
planet. The planet's diameter is about two-fifths that of the Earth. Mercury's force of gravity is about one-third as strong as Earth's.
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 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 (around 1800 to 1900 km) is relatively larger than Earth's, probably
comprising the majority of the planet. Mercury therefore has only a
relatively thin silicate mantle and crust. 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.
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 no known satellites.
It is hard to see the planet from Earth without a telescope because it is
so close to the Sun. 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.
Mercury has a small magnetic field whose strength is about 1% of Earth's.
At maximum elongation, Mercury is only 28° from the Sun,
making observers only able to see it at daylight hours and short periods of
time at dawn and dusk. Because of the difficulty in observing Mercury from
Earth's telescopes, spacecraft were necessary to understand the planet in
much greater detail. The Mariner 10 (launched in November 3, 1973) flew by
Mercury three times in 1973 and 1974. On March 29, 1974, it flew by the
planet at a distance of 705 kilometers from the surface. On September 21,
1974 and March 16, 1975, Mariner 10 flew by Mercury for its second and third
visits, mapping out around 45% of the planet's surface. The rest of the
planet remains uncharted due to the fact that the Hubble Space Telescope
cannot safely image the planet due to its close range to the Sun.
On the visits to Mercury, Mariner 10 unexpectedly found a
magnetic field. This indicates that the iron core inside of the planet is at
least partially molten. It is important to understand the dynamo effect,
which is "the generation of magnetic fields through the rotation of a
conductive molten core the basis of many of the complex interactions such as
the aurora that take place in our Solar System and throughout the Universe.
Physical data
Equatorial diameter: 4868 km
Mass: 1/20 earth's
Mass: 0.3302 x 1024 kg
Mean density: 5.43 g/cubic cm
Density: 5.42 g/cm3
Maximum surface temperature:
427° C
Minimum surface temperature:
-180° C
Strength of surface gravity: less
than half earth's
V.O. (Objects visible magnitude
at opposition): -1.90
Radius: 2,439.70 km
Mean density: 5427 kg/m3
Escape Velocity: 4.2507 km/s
Volume: 6.085 x 1010
km3
Surface gravity (eq.): 3.70 m/s2
Bond albedo: 0.056
Visual geometric albedo: 0.11
Visual magnitude V(1,0): -0.42
Orbital data
Obital Period: 87.969 days
Orbital inclination:
7.004°
Tilt of Axis or obliquity: 0.00°
Orbital Eccentricity: 0.2056
(Mean Orbital Velocity): 47.87
Semimajor axis: 57.9 x 106
km
Distance to the Sun or to the
planet center: 57,910,000 km
Sidereal orbit period: 87.969
days
Sidereal rotation period: 1407.6
hrs
Tropical orbit period: 87.968
days
Perihelion: 46.0 x 106
km
Aphelion: 69.8 x 106
km
Synodic period: 115.88 days
Mean orbital velocity: 47.87 km/s
Time from sunrise to sunrise: 176
days
Length of day: 58.7 earth days
Length of year: 88 earth days
Speed around sun: 172,000 km/h
Obliquity to
orbit (deg): ~0.1
Atmosphere
Surface Pressure:
~10-15 bar (0.001 picobar)
Average
temperature: 440 K (590-725 K, sunward side)
Atmospheric
composition:
| 42% Oxygen(O2) |
Carbon Dioxide(CO2) |
| 29% Sodium(Na) |
Water(H2O) |
| 22% Hydrogen(H2) |
Nitrogen(N2) |
| 6% Helium(He) |
Xenon(Xe) |
| 0.5% Potassium(K) |
Krypton(Kr) |
| Possible trace amounts of Argon(Ar) |
Neon (Ne) |
|
