Mercury held at least one major surprise when astronomers studied the planet with radar in 1991. At that time, Mercury's north pole was tilted slightly toward Earth. Astronomers were very surprised to find that north pole area to be "radar bright." These observations suggest ice deposits, strange as that may seem on a world so close to the Sun! But it is possible that the poles may remain cold enough for ice to survive, especially if it was tucked away in a deeper crater.
Another more recent discovery is traces of an atmosphere around Mercury. Granted, it is a very thin tenuous atmosphere, and probably does not really belong to Mercury, but is was a surprise nonetheless! This atmosphere is only a few billionths as dense as Earths, but it is composed of sodium, potassium, hydrogen, helium, oxygen and argon. There is no conceivable way that Mercury's weak gravity could hold onto an atmosphere for long, and so it is believed that this atmosphere is actually captured from the solar wind and therefore constantly replenished.
Planetary scientists have a model for Mercury's formation and history. They believe Mercury has a core much like the Earth's iron core surrounded by crust. Once the core developed, the crust would have expanded, allowing magma to flow up from beneath. As the core cooled, the crust then shrank, wrinkling and forming the scarps so common on Mercury. Solar tides also would have slowed Mercury's rotation.
Copyright © 1997 Kathy A. Miles and Charles F. Peters II