An irregularly shaped, oblong boulder careening down a hill on Mars left behind a 500-meter long dotted line (about a third of a mile) that was visible from space.
The rolling rock came to rest in an upright position (with its long axis pointed up) at the downslope end of the trail. The track was spotted in an image from July 3, 2014 taken by the High Resolution Imaging Science Experiment (HiRISE) camera, one of six instruments onboard NASA's Mars Reconnaissance Orbiter.
The image was targeted to cover part of a small, chaos terrain with lots of steep slopes. The spacecraft was 274 kilometers (170 miles) above the surface of Mars when it snapped that shot, Discovery reports.
The shadow it cast in the mid-afternoon sunlight – combined with the known angle of light during the time of the exposure -- reveals that it’s 6 meters (20 feet) tall. Looking down at it from the top, the boulder is 3.5 meters (11.5 feet) wide. For comparison, the upright stones at Stonehenge are between 6 and 7 meters (20 to 25 feet) tall.
The odd repeating pattern of the trail suggests how the boulder couldn’t roll straight because of its shape.