Space games - I have to play them :-) You have a nice idea here, and I felt the presure to protect Mars as best as I could. The controls needed some practice, but I got it decently right at some point.
Some ideas/suggestions: Let a colony grow on Mars - starting small, growing bigger and around the planet over time. That way, you give players a chance to get used to the mechanic (only a small fraction needs protection in the beginning, growing larger over time). The more evolved parts of the colony, the more valuable and protactable they become.