The Lake of the Dead
By Dave Furry
Lacus Mortis (“the Lake of the Dead”) is the smallest of the circular “seas” on the Moon. Actually, it is a large crater itself, about 290 kilometers (180 miles) across, that was flooded by lava after the impact event about 3.9 billion years ago. An effect of this impact was the formation of concentric rings. Although five rings have been identified, most of their traces have disappeared or are very faint. However, parts of two rings are still readily apparent: (1) the southwest edge of the mare itself (between Fracastorius and Theophilus), and (2) along the Altai Scarp. Can you see fragments of other rings?
As with all mare regions, Mare Nectaris is a lava-flooded plain bounded by lunar highlands and large craters. Some ‘wrinkle ridges’ are visible in the mare, as are some ‘ghost’ craters – most notably the crater Daguerre to the north. There are many rays that zigzag across the plain of Mare Nectaris; such rays were formed during impacts resulting in numerous small craterlets; the crater Rosse is the largest example of this phenomenon in Mare Nectaris.
The crater Theophilus is very evident along the northwest rim of Mare Nectaris. It is about 103 kilometers (64 miles) in diameter, with a flat floor, central peaks, and terraced walls; the walls rise to between 4250 and 5500 meters (14,000 and 18,000 feet) above its floor. Theophilus is a relatively young crater, and is classified as ‘complex’ because it has an uplifted center resulting in several peaks (one of which has a crater of its own – see if you can spot it with your own telescope!).Other interesting objects in the Mare Nectaris region are Fracastorius and the Altai Scarp (also referred to as Rupes Altai). The crater Fracastorius lies on the southern edge of Mare Nectaris, and it is evident that the lavas which flooded Mare Nectaris overtopped the northern rim of this crater and flooded its floor.
The Altai Scarp rises 3.5 to 4 kilometers (about 2 miles) above the surrounding terrain. This feature was named after the Altai Mountains in Asia, and is the most prominent escarpment on the Moon.