Lunar Day Eighteen

Abstract

Tonight the Moon gives a wonderful opportunity to revisit ancient landmark crater Posidonius (Figs. 19.1 and 19.2). Did you notice crater Charconac to Posidonius’ southeast? Under this lighting you can see that it is far older than Posidonius itself and its steep slopes are obliterated by Posidonius to the northwest. Charonac is just one of those features we do not usually study - although we should. At an estimated 4 billion years old, it sprang to life in the Pre-Imbrian period, and one look at its distressed floor tells of a violent life. Look for craterlet Chacornac A and Rimae Chacornac inside of its 50 km diameter expanse! Now, continue southward from Posidonius and Charconac along the edge of Mare Serenitatis to catch partially open crater Le Monnier. This ruined ring greatly resembles a smaller version of Sinus Iridum, but only measures around 60 km in diameter. Far younger than anything around it, Le Monnier was formed in the Nectarian geological period. What happened to the west wall? Who knows, but what we can see spells of collapse, and the lava of Mare Serenitatis has flowed in. What is so special about Le Monnier? It contains the remains of the Luna 21 mission - forever awaiting salvage in the gray sands along Le Monnier’s southern edge. Be sure to look again for Dorsa Smirnov - better known as the “Serpentine Ridge” nearby.