Here's some of the litter we're leaving on Mars

Yes, it's proof of intelligence life on Mars - the only problem is that it's just some of our old crap. Specifically, it's the heat shield jettisoned by the Opportunity rover when it landed on Mars a few years back.

This photo was snapped back in 2005 as the rover paid a visit to its discarded heat shield. As you can see, it's about the only thing breaking the topographic monotony of the vast, empty, and flat Meridiani Planum. Because Mars does have a thin atmosphere, the heat shield (and the rover itself, for that matter) probably won't last as long as some of the junk we've left on the completely airless Moon. Still, it's an interesting reminder that, if humanity were to suddenly vanish, the last vestiges of our civilization in the distant future might not actually be found on Earth at all.

Also of interest in this photo is the small, metallic rock on the bottom left. A NASA astronomer explains:

The surprise is the rock visible on the lower left, found to be made mostly of dense metals iron and nickel. Scientists do not think that the basketball-sized metal "Heat Shield Rock" originated on Mars, but rather is likely an ancient metallic meteorite. In hindsight, finding a meteorite in a vast empty dust plain on Mars might be considered similar to Earth meteorites found on the vast empty ice plains of Antarctica. The finding raises speculations about the general abundance of rocks on Mars that have fallen there from outer space.

Via NASA.