Dings and cracks on Curiositys wheel.

NASA's Mars rover Curiosity faces ongoing wheel wear and tear as it continues its trek across the rock-strewn Red Planet. The car-size Curiosity rover has been on duty since landing on Mars in August 2012. Curiosity has six aluminum wheels, each with its own individual motor. The rover has a top speed on flat, hard ground of a little over 1.5 inches (4 centimeters) per second. But dealing with the rocky Martian landscape has become somewhat of an unanticipated wheel of misfortune for the Curiosity crew. Back here on Earth, mission engineers are watching the wheels turn, keeping an eye on the dings and cracks that have begun to appear.

Source: NASA. Space.Com.