Mercury: The Forgotten Planet; November 1997; Scientific American Magazine; by Nelson; 8 Page(s)

The planet closest to the sun, Mercury is a world of extremes. Of all the objects that condensed from the presolar nebula, it formed at the highest temperatures. The planet¿s dawn-to-dusk day, equal to 176 Earthdays, is the longest in the solar system, longer in fact than its own year. When Mercury is at perihelion (the point in its orbit closest to the sun), it moves so swiftly that, from the vantage of someone on the surface, the sun would appear to stop in the sky and go backward--until the planet¿s rotation catches up and makes the sun go forward again. During daytime, its ground temperature reaches 700 kelvins, the highest of any planetary surface (and more than enough to melt lead); at night, it plunges to a mere 100 kelvins (enough to freeze krypton).

Such oddities make Mercury exceptionally intriguing to astronomers. The planet, in fact, poses special challenges to scientific investigation. Its extreme properties make Mercury difficult to fit into any general scheme for the evolution of the solar system. In a sense, Mercury¿s unusual attributes provide an exacting and sensitive test for astronomers¿ theories. Yet even though Mercury ranks after Mars and Venus as one of Earth¿s nearest neighbors, distant Pluto is the only planet we know less about. Much about Mercury--its origins and evolution, its puzzling magnetic field, its tenuous atmosphere, its possibly liquid core and its remarkably high density--remains obscure.