Featured Image - 07/29/2008
Isaev Crater, A Mare Puddle on the Lunar Farside
Isaev is a medium-sized crater (86.9 km in diameter) on the ancient lunar farside of the Moon, but it is unusual in that it contains low-albedo mare floor materials. Mare materials are not evenly distributed around the Moon. Maria are abundant on the nearside of the Moon, but are less common on the farside (see the previous LIOW for a discussion). The crater Isaev is Nectarian in age (3.92-3.5 Ga) and lies inside an even older (Pre-Nectarian) crater Gagarin. Isaev is unusual in that it is one of only a few mare-filled craters on the farside not associated with a basin -- it is just outside the South Pole Aitken Basin. Volcanic materials may have been trapped near the surface following faulting during the impacts that created either the South Pole Aitken Basin or Garagin crater. The Isaev impact could have later provided the trapped volcanic material access to the surface by exaggerating the previous faults or creating new ones under the floor of Isaev. The thickness of the mare materials in Isaev have been calculated to be approximately 1-km thick (Walker and El-Baz, 1982) using lunar topographic maps. The mare materials inside Isaev were dated by crater-counting methods to be about 3.7 billion years old (Walker and El-Baz, 1982). Studies by Walker and El-Baz suggest that the maria on the lunar farside are typically between 3.7 and 3.8 billion years old and erupted during a geologically narrow time period of roughly 100 million years. Maria on the lunar nearside cover a wider range of ages. Definitively determining ages for this region will require detailed fieldwork by human lunar explorers, as well as age-dating the samples they collect.
Figure 1: AS15-M-0099. Isaev crater (dashed line) and low-albedo mare floor (solid line).
(Apollo Image AS15-M-0099 [NASA/JSC/Arizona State University])
Walker, A. S. and F. El-Baz (1982) The Moon and the Planets 27: 91-106.