Mercury: closest planet to the Sun

Mercury is the closest planet to the Sun and is also the smallest of the currently recognised planets, now that Pluto has been demoted from planetary status. It is therefore difficult to see, and attempts to do so should be made with the greatest care, as training a telescope in the direction of the Sun is a very unwise thing to do. It is best observed at dawn or dusk, but only if you really know what you are doing. If it’s any consolation, Copernicus never managed to see it, although its existence had been known about since classical times.

Mercury orbits the Sun at an average distance of 58 million kilometres (36 million miles). It completes an orbit every 88 days at a speed of 50Km a second, which is the fastest of any of the planets. This is why Mercury has the name that it does, Mercury being the swift-winged messenger of the gods in Roman mythology.

Mercury revolves on its axis three times for every two orbits round the Sun. This means that a day on Mercury is not much shorter than a Mercury year, and lasts for 59 Earth days.

The surface of Mercury is deeply cratered but it also has some smooth plains. This suggests that the planet was volcanically active at one time, but that this activity ceased a long time ago and meteor impacts have pockmarked most of the surface. Mercury appears to have cooled down and may have shrunk in size as it has done so. The fact that a magnetic field has been detected suggests that it still has a molten core.

Daytime temperatures on Mercury are extremely high, at around 350 degrees Celsius, due to the planet’s proximity to the Sun and the fact that it only has a very thin atmosphere. However, the nights are long enough for the temperature to drop as low as minus 170 degrees Celsius. Mercury does not have a tilted axis, unlike Earth, and therefore has no seasons. This also leaves open the possibility that there could be water on Mercury, existing as ice deep within polar craters into which the Sun never shines.

The atmosphere mentioned above is only as dense as the very outer reaches of Earth’s atmosphere. It comprises mainly helium, hydrogen and oxygen, but traces of potassium and sodium have also been discovered. Although atoms of these elements are constantly being lost into space, they are replaced by the action of Mercury’s magnetosphere in capturing ions from the solar wind that batters the planet.

© John Welford