http://www.newsweek.com/mars-space-debris-rover-impact-future-missions-1744936

There are over 15,000 pounds of space junk on the surface of Mars, which was left over the past 50 years.

Cagri Kilic, a postdoctoral research fellow in robotics at West Virginia University, calculated a figure of 15,694 pounds of trash, according to an online article on The Conversation. Kilic, who specializes in Mars and moon rovers, added up the mass of all spacecraft that have been sent to Mars (22,000 pounds) and subtracted the weight of the currently operational craft on the surface (6,306 pounds).