Ancient Island
Explanation:
Lunar maria are great piles of basaltic lava that were emplaced over hundreds of millions of years as thousands to perhaps millions of individual flows. Later flows often completely buried earlier ones, but sometimes an island of a higher elevation of an earlier flow remains, surrounded by younger lavas. A wonderful example of such an ancient island - called a kipuka - occurs just north of the crater Menelaus near the southern shore of Mare Serenitatis. The Menelaus Rilles occur along the crest of an elongated low ridge or bulge. This is the same family of ridges as the Plinius Rilles and the Littrow Rilles further east along the Serenitatis shore. I speculate that the lavas that overflowed from the rilles built the local high spots that preserved them. I strongly enhanced Zachs wonderful image to bring out the edges of the Menelaus Rille bulge. On the mouseover you can see where the bulge rises above the smoother later lavas to the east (right), along the north (top), and especially at the west end where the darker young lava curves around the older rise. A well-known dome occurs near the western end of the bulge and this image shows that the rilles make a Vee-shaped cut across it. Possible other lower domes can be imagined to the east along the bulge. Immediately east of the dome the bulge is embayed by younger lava - this appears to be where an impact crater dug into the older lava surface but was completely submerged by the younger lavas.
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Chuck Wood
Technical Details:
April 29, 2005; 9.10 UT. 31 cm Newtonian @ f/28 + Phillips ToUcam Pro Webcam + Wratten 25A filter. Frames from 2 minute avi stacked with Registax. Mosaic of 3 images assembled using AutoStitch.
Related Links:
Rukl Plates 33 & 34.
