A long, winding valley, usually with steep sides, on a planetary surface. Sinuous rilles were first seen on the Moon. They have similar characteristics to lava channels and tubes on Earth, but are distinctly different from rivers. For example, they usually start in a crater, become narrower instead of larger as they flow downslope, have abrupt breaks, and sometimes have distributaries (where the main channel splits in two, as in a river delta) but not tributaries. Results from the Apollo 15 mission to Hadley Rille showed that the sinuous rilles are indeed vast lava channels, formed during the eruption of the mare lavas 2.0–3.9 billion years ago. Sinuous rilles have since been identified on Mars and Venus.
Subjects: Astronomy and Astrophysics.