Mercury is one of The five classical planets and an easy naked eye object. The planets stood out from the other stars to our ancestors because of the way they all moved in regular patterns against the night sky, but Mercury was unique in its rapid motion and the way it never strayed far from the sun. If the planets were to be named for gods, then it was only natural that the flighty mercurial one should be their messenger.
The Romans were great admirers of Greek culture and they assimilated many Greek gods into their own pantheon. Mercury, messenger of the gods, was an almost direct copy of Hermes, the son of Zeus and a nymph named Maia (the eldest and most beautiful of the Pleiades). Mercury served as Zeus’s herald, delivering messages and guiding those under his protection. Mercury was clever, with a darkly mischievous personality and a relaxed moral code. Immediately after being born, he invented the Lyre (out of an old tortoise shell) and stole a herd of cattle from his brother Apollo using guile and trickery. Small wonder then that, on top of his duties as patron of travel and herdsmen, the Romans associated him with business and trade as well! His temple was on the Aventine and his festival of Mercuralia was held on the Ides (15th) of May. On this day, traders and merchants offered prayers to Mercury, for forgiveness for past and future perjuries, for profit, and for the continued ability to cheat customers!
Mercury in the Sky
Mercury, as seen by Mariner 10
But how about the planet itself? Mercury only ever appears in the sky as a bright, pinkish red star shortly after sunset or before sunrise. Because it never ventures more than about 20 degrees from the Sun, It is usually lost in the dull glare of twilight and is easily missed by casual stargazers. This is a pity as it can be a very rewarding naked eye object: Not only does its closeness to the Sun make it challenging to pick out, but it moves very rapidly, changing position dramatically from night to night.
The ancient astrologers tried their best to find methods to predict Mercury’s position, but it wasn’t until the works of Copernicus, Galileo and Kepler were put together to form the modern science of Astronomy that we were able to figure out anything about the planet. Most of what we read today in astronomy textbooks about the planets was deduced in the first few decades of modern astronomy, back in the 17th century. Those early astronomers were able to calculate that it takes only 88 Earth days to orbit the Sun, and that it is very small, with only about an eighteenth of Earth’s mass. As technology advanced and better telescopes became available, they were able to view it as a disc and measure the size: It has a diameter of about a third that of Earth. Once the maths were done to calculate Mercury’s density, it turned out to be an unusually small and heavy planet.
Hundreds of years later, the Space Age began, and we started sending unmanned spacecraft out to explore the Solar System. The first mission to reach Mercury, Mariner 10, arrived in 1974 after getting a gravitational boost from Venus. Mariner revealed a world that looked very much like the Moon, with a crater-pocked surface and vast mountain ranges, and virtually no atmosphere. Unlike the Moon, however, there are no Maria (“oceans” of solidified Lava) and Mercury has a massive core of iron which causes a weak magnetic field. This magnetic field interacts with the solar wind, trapping it to create an exosphere. This exosphere is remarkably thin, close to vacuum, but is able to produce some interesting chemistry on the planets surface. The only other mission so far, Messenger, is currently manoeuvring into orbit around Mercury and has already returned a wealth of information – detailed maps of most of It’s surface, analyses of the magnetic field and exosphere, and more.
Update: Messenger has reached Mercury, and is returning enormous amounts of data, completely revising what we thought we knew about the little world. There is a lot of data, and not much has been analysed yet, but we already know that despite the fact that it looks much like our Moon, the two bodies are about as different as you can get.
Thanks to these missions, we now know a great deal about Mercury. It is the smallest planet (since Pluto was demoted), and a very harsh place indeed. While it’s year is only 88 days long, it takes 176 days to spin around its axis – one Mercury day lasts two Mercury years! This means that if an astronaut stood on Mercury and watched the sunrise, he would have to wait an entire Mercury year before he saw sunset. He would have to wait another Mercury year before the next sunrise. It would be a very strange and disorienting experience.
It would also be extremely uncomfortable — Mercury is more than twice as close to the Sun as Earth is, and has no real atmosphere to shield its surface. Combine this with the long days and the daytime temperature gets higher than 400°C! If our astronaut can hold out for long enough, though, he’ll get a chance to cool down at night, when the temperature drops to -200°C. All in all, it seems that Mercury is staying true to character and making it as difficult as possible for us to pay a visit!