Humans haven't been exploring Mars very long, and already we've left some litter on the planet. Several robots have explored the planet's surface, from the 1970s Viking Missions to the ongoing Mars Exploration Rovers, Spirit and Opportunity. Our newest spacecraft, the Mars Reconnaissance Orbiter, has a strong enough camera that it can see the bits and pieces of these robots on the surface. For example, here is a satellite picture of the rover Opportunity's landing platform, forever standing in the middle of Eagle Crater on Mars. This picture was taken from a distance of about 160 miles above the surface of Mars, and the lander is only 10 or 12 feet across.
In coming months, the Marso Reconnaissance Orbiter will look for the Viking 1 and 2 landers, the Mars Sojourner Rover, and maybe even some of the failed landers, like the Mars Polar Lander that shut off its rockets while still about 150 feet above Mars and crashed to the surface. Such a picture might clear up the mystery of what happened to the lander. Further, pictures of now-defunct spacecraft can help us to learn about the weather on Mars. How fast does dust get blown around the surface? Can we see any other changes since the last pictures were taken? And so on.