Crater Bürg drawn by Harry Roberts
Expert Moon observer and member of the Sydney City Skywatchers, Harry Roberts reports:
In an earlier piece we visited the “Lake of Death” at the eastern end of Mare Frigoris, with its centrally placed crater Bürg. I mentioned that Mädler (and Beer) named the crater for selenologist Johann Bürg when publishing their famous map shortly after Bürg died in 1834. I also suggested making a sketch of the whole of Lacus Mortis when conditions permitted, and that occurred on 27 September.
Under “morning” lighting of a four day old Moon the terminator lay only 4º west of the site, and the Lacus was half filled with shadow. The western rim of the Lacus was reduced to a series of dim-lit patches of high ground dotted with bright peaks (refer to the sketch) which gave a nice impression of the whole sweep of the western wall of the Lacus Mortis impact basin. The sketch shows central crater Bürg mostly filled with shadow, and looking quite regular in shape under this particular libration; but as noted in the earlier article its outline will vary dramatically under different librations: it’s a real chameleon!
Conditions were good, and at 333 times the view was rich in detail. Several wave-like wrinkle ridges crossed the Lacus floor from north to south, and one very “soft” ridge east of and concentric with Bürg may be structural doming associated with Bürg’s ejecta blanket. NE of Bürg we see a broad dome-like ridge that rises out of the mare, a bit like Uluru. South of Bürg at the edge of the sketch we see ancient crater Plana which, three times older than Bürg, is largely ruined, but has a distinctive central peak.
A series of wire-thin bright outlines on the west edge of Plana indicated secondary crater Plana C, and part of the thin south rim of Lacus Mortis that seemed to terminate in another crater. Rükl’s “Atlas of the Moon” however, showed only a semi-circular remnant at the site. Impressive shadows were cast by part of the Plana glacis westwards to the terminator, a distance of about fifty kilometres.
Northwest of Bürg bits of high ground caught the morning light and glowed dimly, while here and there a peak stood out dramatically. These elongated patches form part of a large wrinkle ridge that is prominent under higher lighting. Harold Hill’s “Portfolio of Lunar Drawings” (still available through the Net, but only buy the hardcover version) shows the Lacus floor between Bürg and the west wall of the impact basin (see page 19). Hill shows curved shadows cast by the Lacus wall across the floor, an effect that can arise when long shadows fall on ground that is crossed by shallow curved ridges.
“Virtual Moon” states that the initial impact that formed Lacus Mortis occurred very early in the Moon’s life, about 4.5 to 3.8 billion years ago. On the NE side the rim has been mostly buried by recent lava flows, perhaps spreading from Mare Frigoris. The visible remains of the rim give Lacus Mortis an hexagonal look, and on the west side material from subsequent impacts may have invaded the site. All told, it’s an impressive part of the Moon and a great site to explore with sketch pad or webcam when the Moon is four days old.