Mercury is the closest planet to the Sun, which brings to it special qualities. It's hotter, brighter, and smaller than most of the other planets. Although it can be brighter than any star, Mercury is never seen against a dark background. So, from earth it is only visible with the naked eye when it is low in the west after sunset or low in the east before dawn. From Earth, surface details are very hard to distinguish, and nothing much can be recorded besides its phases. There are large parts that can't even be mapped because the same regions were sunlit when the spacecraft that took the pictures passed by.
During its orbit around the Sun, Mercury changes its distance for 46 million km (at perihelion) to 70 million km away from the Sun (at aphelion). At first, astronomers thought maybe an undiscovered planet (referred to as Vulcan) was rotating around Mercury, and that Vulcan's rotation would push Mercury toward and away from the Sun at different points and impact the speed of the perihelion. However, Einstein showed that the large changes in Mercury's orbit give a false impression of being even larger than they are. According to his General Theory of Relativity--his theory that everything changes when you change position--when Mercury moves it appears to be farther away from the sun than it really is. Einstein was able to predict the motions of Mercury based on this theory, making his theory more acceptable.
Mercury is only a third the diameter of the Earth (see picture at left), but its mean density is about the same. So, 65-70 % of Mercury's weight must be made up of a heavy material, probably of iron. Most of it is concentrated in the core, while the outer region is made of silicate rock similar to the Earth's mantle. Mercury is also almost a perfect sphere, with an equal diameter of 4,878 km. For comparison, the Moon is 3,476 km and Mars is 6,794 km.
There is a trace of atmosphere, consisting mainly of helium, but the ground pressure is only 10-9millibars. The thin atmosphere that is there is just a bunch of atoms that blast off its surface because of the solar wind. Since Mercury is so hot, these atoms quickly escape into space and Mercury's atmosphere must constantly be replenished. There are two 'hot poles' where the Sun is overhead when Mercury is at its closest to the Sun. One pole lies in the Caloris Basin. So, what about life on Mercury? Doubtful. With virtually no atmosphere, a huge temperature range, and rather hostile conditions, life could not survive. Mercury has no satellites because of the heat.
It is interesting to imagine what you would see if you were on Mercury and looking out at the universe. First of all, since a "Mercury day" lasts 176 "Earth days" and during that time Mercury is moving closer to and then farther away from the Sun, you would see the Sun rising and gradually increasing in size as it moves toward the zenith. When it got there, the Sun would stop and then appear to go back the way it came, getting smaller as it went.
Heraclitus (544-483 BC) postulated that Mercury and Venus orbit the Sun. The Italian astronomer Giovanni Schiaparelli (1835-1910)--the same person who called the straight lines on the surface of Mars canals--believed that Mercury was tidally locked to the Sun, just as the Moon is tidally locked to Earth. But in 1962, radio astronomers looked at radio emissions from Mercury and determined that the dark side was too warm to be tidally locked.
In 1933, E. M. Antoniadi published a detailed map of Mercury. His map was made with the help of a powerful 33 inch refractor telescope and showed dark features he named. Later, the rotation period of Mercury was investigated by Pettengill and Dyce (in 1965) and Goldstein (in 1971). Then, between 1973 and 1974, the spacecraft Mariner 10 flew by the planet three times and mapped 45% of Mercury's surface, including the Antoniadi Ridge. Unfortunately, the planet was too close to the Sun to be safely imaged by HST.
Overall, it is a lot like the Moon. Both surfaces have tons of craters and are extremely old, and neither has plate tectonics. Of course, Mercury is much more dense, as it is the second densest body in the solar system, after Earth. Thus, the planet is dominated by the iron core, leaving an outer shell that is only 500-600 km thick.
Although Mercury is heavily cratered, it has several large regions of smooth plains. Volcanoes are thought to be active there, and based on the evidence of the data from the Mariner 10, it looks like there were eruptions rather recently. Also, radar shows evidence of water ice in the north pole, which could develop in the shade of large craters.
The Caloris Basin (see below for more information) is the largest feature of Mercury. This giant crater is 1,300 km (800 miles) in diameter and was formed on impact of an asteroid. So great was the force of that impact that mountain ranges were disrupted by the event. Because of seismic activity thought to follow the formation of the basin, mountain blocks and other "weird terrain" was formed.
There are also some scarps (troughs) on the surface of Mercury. These troughs are probably the result of tectonic activity and may be several hundred kilometers in length. This tectonic activity must have occurred, based on the orientation of the troughs, when Mercury's rotation caused the crust to flex, disrupting the surface. Another cause could be from the expansion or contraction of the crust because of internal changes. Some scarps cross other types of terrain, showing they came after, while other scarps are covered by another type of terrain, showing that they came first.
Based on evidence from the Mariner 10, Mercury was affected most from meteor impacts and volcanic eruptions. About 4 to 4.5 thousand million years ago, there was heavy bombardment by the meteors. This was then followed by volcanism which formed the basins, craters, and plains. Then, there was some phase of compression which formed the planetary-scale ridges.