October 25, 2012
Half a Mare ...
image by Jocelyn Serot, France
...is better than none. This great image of the western half of Nectaris joins the growing LPOD collection high resolution view of maria - Serenitatis, Tranquillitatis and then Humorum. In this case the half mare exhibits no volcanic features other than the lava surface itself. The mare is peppered by secondary craters formed by ejecta from Theophilus. Of course, Theophilus itself is the star of western Nectaris. Its youthful appearance - no craters of significant size formed on it - and its massive central peaks, terraced walls, and flat, impact melt-filled floor are classic features of large complex craters. Even better are the Moon's most visible impact melt ponds on the northern flanks of the crater - neither Tycho nor Copernicus have such conspicuous external melts. Now move southward from the perfect complex crater to one essentially the same size but quite different in looks. When it first formed Fracastorius probably looked nearly identical to Theophilus, but it has been severely modified. It tilted into the subsiding Nectaris basin, with lava rushing in to inundate and bury its peaks. Shaking from nearby impacts - especially the huge moonquakes caused by the Theophius impact - caused the terraced walls of Fracastorius to come tumbling down too. Because the era of mare volcanism is long passed, and the rate of impact bombardment has dropped immensely, Theophilus will never look like Fracastorius. It will be fresh and complete nearly forever, with small impacts slowly sandpapering away its asperities and slowly rounding its topography.
Oct 5, 2012, 2h45 UT, 12" Dall-Kirkham refl @ prime focus (F=5m), red filter, DMK 31 camera. Processing : Autostakkert2 + Registax 6. This is a mosaic of two images.
Rükl plate 46
Yesterday's LPOD: Stepping Down
Tomorrow's LPOD: Slow Motion X