L18 - Mare Serenitatis Dark Edges
All mare are formed when magma has risen up through the crust and flooded an impact crater, but there is no rule that says this flooding can happen only once. Look at Mare Serenitatis (Sea of Serenity) and notice that the outer edge of the mare material is darker than the central area. Thus, at least two magma flows have happened here, and with two different types of material. Geologically, this is not as serene a place as one might think!
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Images © Doug Anderson
This image was taken on 10-Jan-2006 through a 203mm Celestron C-8 Schmidt-Cassegrain reflector with a Canon Digital Rebel XT (350D) camera using eyepiece projection through a 26mm eyepiece.