The Planet Mercury
The ancient astronomers named the planet that seemed to them to be moving fastest "Mercury". It makes a revolution of the sun in a mere 88 days. As the messenger god, Mercury was reputed to be among the swiftest of the gods, and as the ancients associated their polytheistic gods with the heavens it was an easy correlation to make.
Mercury is not an easy planet to see because its orbit lies so close to the sun. From Earth then it is usually above the horizon in daytime. To see it when the sun is over the horizon requires brief observations be made just after sunset or just before dawn and at an elevation off the horizon where the Earth's atmosphere is thickest. For this reason Mercury, before the advent of extraterrestrial telescopes, remained difficult to see.
Until recently astronomers thought that Mercury's orbit was tidally locked to the sun, meaning that the same side always faced old Sol. This led to conjectures that half the planet was very hot and the other half very cold. However, the Mariner 10 spacecraft sent near the planet in the 1970's dispelled these ideas. It was found out that Mercury's rotation is actually 1 and 1/2 cycles in the course of a year, meaning that though temperatures are still extreme, periodically every part the planet has the oportunity to bask in the sun's glow.
The Mariner 10 orbiter had another surprise in store for astronomers. Mercury has a magnetic field. It had been conjectured that only planets with a molten metal core could possess a magnetic field. Mercury was thought to be old enough that this core should have cooled by now. Nevertheless, the magnetic field is weak, equaling only about one percent of the Earth's magnetic field. Because of its relatively low strength some scientists believe that Mercury's core has indeed cooled and that the magnetic field is a residue of a stronger past magnetic field imprinted on its dense metallic content. For Mercury is dense. 65% of the planet is comprised of its metallic core making it far more dense than the Earth.
The surface of Mercury largely resembles the surface of the moon. It is pockmarked by craters and has a seemingly dusty, barren surface. Some of the craters, observed from Mariner 10, are double-craters, one crater centered directly in the middle of another crater. There is evidence of volcanic activity, with areas of magma evidently strewn over the surface. There are long ridges or cliffs running for hundreds of kilometers scarring the surface. These may have formed when the planet cooled; it shrank and subsequently the surface cracked.
Perhaps the most interesting surface feature is the reflective craters near the north pole. It seems that these craters reflect radar waves in a way that indicates that there may be frozen water in their depths. As the planet's orbit is nearly perpendicular to its path around the sun, it does not have seasons in the way the Earth does. The light and heat of the sun can never reach into the craters; so water escaping the surface might be trapped in these sheltered areas.
Mercury has what is called an "eccentric" orbit. At perihelion (closest approach to the sun) it is 46 million kilometers distant. At Aphelion it is 69.8 million kilometers from the sun. The nearness of Mercury to the sun means that it gets lashed by solar winds which limits its atmosphere. Yet it does have a very thin layer of gasses which is sometimes referred to as the exosphere. It is comprised of hydrogen, helium, oxygen, sodium, potassium and calcium. From the surface the sun appears two and a half times larger than it does from Earth. The diameter of Mercury is 4878 kilometers (about 3030 miles) compared to the Earth's 12,756 Kilometers.
The planet Mercury, then, embodies many of the attributes the ancients gave to the personality of Mercury, it is close to the power of the sun, very much influenced by its dictates. It is reliable in its movements, weighty, but compact. The Mercurial aspects of the messenger god has also been discerned in one of the basic elements.