Some guy put them there.
The Mars watchers over at UFO Sightings Daily say that a new photo appears to show the shadow of a man, perhaps with an oxygen tank, tinkering with the outside of NASA's Curisoity Rover.
The site claims this is proof of one of three things:

  1. The rover is on Earth, not Mars;
  2. Humans are living on Mars (perhaps in abandoned alien bases);
  3. Aliens are messing with our rovers.

We're kind of betting on the fourth option: the weird placement of a rover light created a weird shadow from a weird part on the rover.
Still, you've got to admit – the shadow does look vageuly humanoid.