Comment Re:Ho Hum (Score 5, Informative) 181
I knew the core was supposed to be the cause of our magnetosphere
That's a large part of the answer right there! The magnetosphere acts as a shield to keep a lot of harmful particles from the solar wind away, things which would work to strip away the atmosphere. Mars is an example of what can happen to planets that lack this. (Obviously, Mars' lower gravity works against it in this regard as well)
http://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/