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Astronomy - Planet Mercury
Mass: 3.303 x 1023 kg (0.0553 Earths)
Radius (equatorial): 2 439 km
Mean density: 5.42 g cm-3
Distance from Sun: 57 910 000 km
Rotational period: 58.6 days
Orbital period: 88 days
Escape velocity: 4.25 km s-1
Apparent magnitude: -1.90
Surface temperature: 179°C (427deg;C, max. -173°C,
min.)
Atmospheric composition: Helium (42%), sodium (42%),
oxygen (15%).
Number of satellites: None
Characteristics
Mercury is the second smallest planets (smaller than Saturn's satellite Titan). Its
orbit is highly elliptical. It is only 46 million km from the Sun at its perihelion but
70 million km at its aphelion. There are only three 'Mercury days' in two 'Mercury
years'. In orther words, it rotates three times as it completes two orbits round the
Sun.
Mercury has a very thin atmosphere. Because Mercury is so close to the Sun, most
of the atomsphere has boiled off and escape to the space. The atmospheric
residuals are constantly replenished from materials blast off from the surface by
the solar wind. The lack of atmosphere means that the surface temperature is very
extreme: from the low -173°C to boiling hot 427°C. The hot temperature is only
surpassed by Venus but the latter temperature is very stable due to the presence
of its thick atmosphere.
The surface features on Mercury is very similar to the Moon: peppered with craters
and it has no plate tectonic. Mercury is, however, the second densest planet,
followed closely after Earth. Due to its small size, Mercury is thought to contain no
magnetic field as the core would have solidified long ago. However, it was during
the Mariner 10 mission in 1974-75 found that Mercury has a very small magnetic
field. The strength is only 1% of Earth's. This led to the conclusion that Mercury
may have a partially molten iron core. Magnetic fields are generated from the
rotation of a conductive molten core and is known as the dynamo effect.
The high density of the planet indicates that the planet is consisted of up to 70%
of metal by weight and the rest silicates. This gives a core radius of 75% of the
planet radius, or around 1850 km. Mercury is often visible to naked eyes but is
difficult to spot since the planet is located low in the twilight sky. When looking at
the projected image through a telescope Mercury may sometimes be seen as a
small black disk moving slowly across the Sun (WARNING: Looking at the Sun
directly with a telescope or binoculars only invites irreversible blindness).
1. Mercury is hot but it might have ice
Mercury is the closest planet to the Sun and is one of the hottest planets in the Solar System, but it may
contain ice. Mercury slowly rotates around the Sun, exposing all of its sides to the Sun’s relentless rays,
so it seems hard to imagine where the ice could remain solid. Scientists speculate that the ice is located in
craters near the poles of the planet. These craters are deep enough and close enough to the top of the
planet to keep the ice out of direct sunlight.