Hundreds of valleys meander across the Moon. Many of them look like river valleys on Earth. And some of them might have been formed in the same way – by flowing liquid. There’s no water on the Moon, though, so the channels were carved by lava.

The channels are known as rilles – from a German word that means “grooves.” They come in three basic forms. One form probably took shape as pieces of crust pulled apart, leaving a wide, straight gap between them.

The second category follows a gently curving path. These rilles probably formed when a river of lava cooled and condensed, sinking into the ground.

The final group looks most like Earthly riverbeds. “Sinuous” rilles twist and turn across the surface – sometimes dramatically. They probably formed from flowing lava. The lava either carved a channel on the surface or tunneled below the surface, and the empty tube later collapsed. Sinuous rilles often begin at a crater, which probably is the “vent” where lava poured onto the surface.

Apollo 15 landed near one of these rilles. Known as Hadley Rille, it’s about 75 miles long, a mile wide, and a thousand feet deep. Astronauts looked into its depths and gathered samples from its rim – the edge of a “river valley” on the Moon.

The almost-full Moon is in the east at nightfall. Regulus, the brightest star of Leo, is close below it. They move closer during the night, and are almost touching as they set, before dawn.

Script by Damond Benningfield