Mercury: First planet from the Sun

Mercury is the closest planet to the Sun. Because of this, it's often difficult to spot in the night sky. On occasion, though, the planet can be seen as a "morning star" just before the Sun rises, or as an "evening star" just after the Sun sets. Because it is the innermost planet, it is also the quickest -- it orbits the Sun once every 88 days. The ancient Romans noticed its speed and named it after the speedy Mercury, the messenger of the gods.

Mercury is also the second smallest planet in the solar system. It's barely larger than Earth's moon, and Jupiter's moons Ganymede and Titan are actually larger than Mercury.

With a telescope, you can see that Mercury goes through phases just as the Moon does, sometimes appearing as a thin crescent and at other times more round. The phases appear because we see different sides of Mercury lit up as the planet orbits the Sun.

Mercury is very hot on its daytime side, reaching 427°C (800°F) on the surface facing the Sun. However, since it has very little atmosphere to trap heat, it also gets very cold on its nighttime side -- sometimes as low as -183°C (-297°F). In fact, Mercury is not even the hottest planet in the Solar System. Venus, with its thick, heat-trapping atmosphere, is the hottest. However, Mercury has the greatest temperature extremes in the solar system with the day and night temperatures being different by around 590°C (1100°F). On Earth, the coldest winter temperatures and the warmest summer temperatures are only about 80°C (180°F) apart.

It is so difficult to see detail on the surface of Mercury that it wasn't until the 1960's before astronomers determined how fast the planet rotates. In 1974 the American spacecraft Mariner 10 flew past Mercury, sending back about 10,000 images of the surface. The images confirmed that Mercury is a barren world covered with craters, similar to Earth's moon. It also found a very thin - almost nonexistent - atmosphere on Mercury, as well as a weak magnetic field.

Mariner 10 did leave a large portion of the planet unexplored, and no spacecraft has visited since. In 1991, astronomers used radio waves to investigate some of the unexplored regions of the surface. By bouncing radio signals off the planet, they discovered a bright spot near the north pole which might be an ice cap, similar to those on Earth and Mars. There are areas at the poles of Mercury which would be cold enough for the ice caps to exist indefinitely. Mercury's equator directly faces the Sun, so the poles receive only a tiny amount of the Sun's rays and heat. There may in fact be craters at the poles deep enough to stay perpetually in shadow.