Mercury - The Legendary Prankster
For many people, the most exciting part of Astronomy is the planets. Mercury (called Hermes by the Greeks) was the swiftest of the pantheon of gods and goddesses, wearing winged sandals (and frequently little else). Mercury was always represented as a youth much given to pranks. He is the patron of messengers, elegance, commerce and thieves alike. When I teach mythology, Mercury is a favorite simply because he has the charm and grace that so many of the other stories lack.Without doubt my favorite tale of Mercury and Jupiter is in Jean Giradoux's play "Amphitryon 38". According to Giradoux, a twentieth century playwright, this story of the perfect husband [Amphitryon] and the perfect wife [Alkmena] had been told by 37 authors before him. Essentially Jupiter desires to becomes Alkmena's lover. However totally virtuous Alkmema spurns any advances except from her husband. Although Jupiter can ravish her, her vow to suicide if he does stops him. After trying one ploy after another, Jupiter turns to his trickster son Mercury for help. Mercury comes up with the solution - Jupiter must become even more Amphitryon-like than Amphitryon himself. Jupiter does this and in the process fathers Hercules on the same night that Amphitryon begets Hercules' half-twin (and mortal) brother. Jupiter is totally successful in fooling Alkmena, but in doing so he is totally defeated, because in becoming more like Amphitrion than Amphitrion, he robs himself of all godlike pleasures. Generally the play is done staged with twins playing the roles of Jupiter and Amphitrion (because they appear on stage frequently together). However the most outrageous production I have seen picked two totally different actors which made the farce all the more beguiling.Mercury is the innermost member of the Solar System. It is a small planet parched by a Sun which is more than two and a half times as intense what we see here on Earth. Mercury is certainly as swift as its namesake god. Not only does this planet have the shortest path around the Sun (less than 40% of the length of the Earth's orbit) but Mercury moves more swiftly than any other planet (more than 7000 MPH faster than Earth). The result is a short Mercurian year of just about 87 of our days.Mercury turns very slowly on its axis. It takes Mercury 58 days to turn around once (a Mercurian Day). Three Mercurial days is two Mercurian years. For most of the Nineteen and Twentieth Centuries, astronomers though that Mercury's day and year were both 87 Earth days long. If this had been the case, Mercury would have had the same side facing the Sun, just as our Moon's familiar face is always turned towards the Earth. It wasn't until late in the Twentieth Century that this was discovered to be incorrect. You will come across old science fiction stories that are based on the incorrect assumption that Mercury always has the same side facing the Sun. These stories usually talk about a liveable ring in the twilight zone. Very romantic and exciting but alas we now now they never can be.Relative to its total size, Mercury has the largest iron and nickle core of any planet. Only its small mass keeps it from being compressed more densely than any other planet. If Mercury spun as rapidly, like the Earth, Mercury's iron core would generate an intense magnetic field. This would react with the intense Solar Wind of charged particles, causing high level aurora and unique surface effects. An intense field similar to Earth's would cause bluish surface electric discharges that would look much like the St. Elmo's Fire seen on sailing ships in stormy weather.Mercury's field is extremely weak. So the Solar Wind which is largely blocked by Earth's magnetic field hits the surface. This creates an effect called sputtering. Over time the "sleet" of charged particles abrades the surface resulting in tiny divots. The old radio tubes wore out in large part because of sputtering. The lifetime of the Hubble Space Telescope is similarly limited. In spite of precautions such as never turning the telescope anywhere near facing the Sun, micrometer sixed divots are slowly by inexorably wearing away the surface of the mirror.