Currently, Mercury has an almost negligible atmosphere - only a few billionths of the density of the Earth's.
The atmosphere consists maily 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 elliptical orbit. It is normally too close to the Sun to be easily seen from the 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 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 300 km above Mercury and the on-board TV cameras took some 4 000 pictures which were transmitted to Earth.
The Mariner picture (right) show a large meteorite 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.
Physical Data:
Diameter: 4 878 km
Temperature: -173 degrees Centigrade to 425 degrees Centigrade
Minimum distance from Earth: 91.7 million km
Distance from Sun: 45.9 to 69.7 million km
Period of rotation: 58days 15hours
Period of orbit: 88 days
Number of moons: nil
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