Not much.
So why not kill some time sending astro marines to defend Earth from Martians. On Mars.
Mars Commando by Vidroid Studio is another defense game with charming 8 bit tunes and pixelated weird Aliens that look like retarded doughmen. Lo-Res Awesomness!
While recruiting Astro Marines to take position next to your Mars probe, a scout droid under
your control gets to shoot its machine gun at the approaching derpoids.

You like?

Play it at Armor Games or Kongregate.