Geological Chronicle
The Rupes Altai is an arcuate mountain cliff in the southeastern highlands of the lunar near side, centered at 24.3° S and 22.6° E. Stretching across roughly 545 kilometers of rugged terrain, it is the most prominent fault scarp on the Moon. The feature originated as the outer topographic ring of the Nectaris impact basin during the Nectarian period roughly 3.9 billion years ago, when the lunar crust fractured and collapsed along concentric faults following the basin impact.
The cliff displays an asymmetric profile with a vertical drop of nearly 1,000 meters facing the interior basin floor to the northeast. Its southeastern end terminates along the western rim of the crater Piccolomini. Moving northward, the cliff follows an irregular curve before losing definition near the crater Tacitus and the prominent crater group of Theophilus, Cyrillus, and Catharina. The exposed scarp face reveals ancient highland crust uplifted during the basin excavation.
Rupes Altai changes appearance dramatically across the lunar phase cycle. It is difficult to locate during the full moon when the sun is overhead, but appears as a bright, winding ridge five days after the new moon under morning illumination. About four days after the full moon, low evening sunlight from the west casts long shadows across the lower plains, throwing the entire escarpment into high relief.