june: ariadaeus rille

The dramatic trench running across the middle of this Apollo 10 image is a rille. These curious features are quite common on the Moon, and fall into three types. Sinuous rilles meander like a river with tight loops, and are widely thought to be the remnants of lava flows or collapsed lava tubes. Arcuate rilles, which have smooth curves, and straight rilles are both thought by some to be grabens - sunken sections of crust between parallel faults.

Rima Ariadaeus is a particularly long straight rille, around 3-5 km (2-3 miles) wide and some 225 km (140 miles) long, and is seen here from about 14 km (9 miles) above the lunar surface.

Image: NASA.