Crater on a crater on the Moon
Meet Krieger and its meandering lava channel.
Seen here is the 22 kilometers wide Krieger impact crater overlaid by the smaller crater Van Biesbroeck, making the feature set look like an inverted diamond ring. That's what I see anyway.
The non-circular, polygonal shape and somewhat rough rim of Krieger indicates an old age and the smoothness of the crater on top of it indicate its youth. Notice how a rille (narrow channel) starts from Rima Krieger's western rim.
This 20 kilometers long rille, called Rima Krieger, is sinuous in nature, and formed during the time of active volcanism on the Moon. The hot lava probably drained from the Krieger crater floor out to the rille. What is most surprising is the number of sharp, near 90 degree turns that the rille takes. What caused this?
Physically examining and sampling orthogonal rilles via future lunar missions is exciting because such rilles expose layers of past lava flows and underlying structure, giving us an entry point into the volcanic history of the Moon.
