Mercury - closest plant to the Sun
Mercury is the closest planet to the Sun but not the hottest (Venus is the
hottest). It is a small rocky planet that looks similar to our Moon because its
surface is heavily cratered. However, Mercury's craters are slightly flatter than
the Moon's - its stronger gravity may have caused the crater rims to slump more;
or erosion may have occurred long ago, if Mercury had a thicker atmosphere in the
past. The craters are believed to be mainly from impacts of interplanetary debris
during a much earlier stage in the life of our solar system.
Currently, Mercury has an almost negligible atmosphere - only a few billionths
of the density of Earth's. The atmosphere consists mainly of sodium vapour,
perhaps ejected from the surface by meteorite impacts. There are also small amounts
of helium and hydrogen from the solar wind. As for the body of Mercury itself, data
from the Mariner 10 spacecraft indicate that this small planet has an unexpectedly
large metallic core (probably based on iron) and a rocky crust.
Mercury is much nearer to the Sun than the Earth is, and travels in an
oval-shaped orbit. It is normally too close to the Sun to be easily seen from
Earth. Even when it is visible, it is not far above the horizon - so the light
coming from Mercury has to pass obliquely through the Earth's atmosphere and it
is impossible to obtain a clear image. Also, Mercury's surface is made of dark
material and reflects only 6% of the sunlight that falls on it. From infrared
observations, we believe that the surface is covered in very fine dust.
Our knowledge of Mercury was very sketchy until 1974, when the American space
probe Mariner 10 photographed the surface. It passed as close as 300km above Mercury
and the on-board TV cameras took some 4000 pictures which were then transmitted to
Earth. The Mariner pictures show a large meteorite impact crater called the Caloris
Basin, covering nearly one-tenth of the planet's surface. Another feature that
stands out is lines of cliffs (scarps) hundreds of kilometres long.
| Diameter | 4,878km |
| Surface temperature | -173°C to +425°C |
| Distance to Sun | 45.9 - 69.7 million km |
| Rotation period | 58 days 15 hours |
| Period of orbit | 88 days |
| Number of moons | Nil |
