Barren Mercury

As mentioned before, Mercury is a planet.  But it is unique in our Solar System in that it is the only planet without any type of atmosphere.

There may be a couple of reasons for it:

1. It may be too small to hole an atmosphere.  Mars is the second smallest planet but has a very thin atmosphere.  Because of their small sizes, the escape velocity for the planets are relatively small.  Most gas molecules have a relatively fast speed when moving about in the atmosphere, so they could escape easily.  However, this explanation may not be true. Callisto, as you recall, is similar in size to Mercury. But it does have a thin atmosphere.  The next bullet point is probably the correct explanation.
2. Mercury is extremely close to the Sun, approximately 0.4 AU out.  The Sun has a very active solar wind and that solar wind is extremely hot.  Possibly in the early formation of the Solar System, the primordial solar wind stripped Mercury of its atmosphere, which would have likely been very tenuous to begin with.  If Mercury had an atmosphere, it would have been similar to that of Callisto, with probably and composition similar to that of Mars, mostly carbon dioxide.

Next time, we will learn about Mercury's strange orbit.