THE PLANET MERCURY IS NAMED after the Greco-Roman messenger of the gods, because it circles the Sun faster than the other planets, completing its circuit in 88 Earth days. Because it travels so close to the Sun, Mercury is often difficult to observe. Even though its reflected light makes it one of the brightest objects in the night sky,Mercury is never far enough from the Sun to be able to shine out brightly. It is only visible as a "morning" or "evening" star, hugging the horizon just before or after the Sun rises or sets. LikeVenus, Mercury also has phases (p.20). Being so close to the Sun, temperatures during the day on Mercury are hot enough to melt many metals. At night they drop to -291°F (-180°C), making the temperature range the greatest of all the planets. The gravitational pull of the Sun has" stolen" any atmosphere that Mercury had to protect itself against these extremes.

Some of Mercury's hills and mountains were created by the impact of a huge meteorite.The impact created a crater, known as Caloris Basin, where the meteorite struck the surface and sent out seismic, or shock, waves through the semi-molten core of the planet. These waves traveled through Mercury to the other side, where the crust buckled and mountain ranges were thrown up. The surface of Mercury closely resembles our crater-covered Moon. Mercury's craters were also formed by the impact of meteorites, and the lack of atmosphere has kept the landscape unchanged. Around the edges of the craters, a series of concentric ridges record how the surface was pushed outward by the force of the impact.

Mercury has great mass for its size. Even though it is only slightly larger than Earth's Moon, its mass is four times that of the Moon. This means its density must be nearly as high as Earth's, most likely due to a very high quantity of iron. Astronomers believe that Mercury must have a massive iron core that takes up nearly three-fourths of its radius to achieve such great mass-a fact backed up by Mariner la's evidence of a magnetic field. When the densities of Mercury and Saturn, the huge gas giant (pp.52-53), are compared, Saturn would float and Mercury, whose density is seven times as great, would sink.