Subject: How does a planet's gravity affect its atmospheric pressure?

I'm working on designing a solar system for a novel, and I'm trying to develop a 
scenario where both a planet and its moon would have terrestrial atmospheres.  
What I need to know is how to determine the surface pressure of a body based on 
its gravitational attraction.  The moon would have slightly more than 0.5g.  
Would it be able to maintain enough of an atmosphere to be breathable?  Also, 
would there be enough greenhouse effect to keep the temperature above freezing?

Re: How does a planet's gravity affect its atmospheric pressure?