Craters Frozen in Time
Around 3.85 billion years ago a massive impact formed the Imbrium basin, obliterating any pre-existing craters within the basin. Remnants of the outer rim of this basin can be seen on the lower right of the image as the Montes Apenninus, and on the upper right of the image as the Montes Caucasus.
Later impacts created Archimedes, Spurr, and Cassini, which were subsequently partially buried by basalt lava flows. North of Aristillus, a faint ring reveals the remains of a crater that was nearly lost in the lava. The volcanic flows produced the flat surfaces which embayed the mountains and isolated other peaks such as the Montes Spitzbergen.
After the volcanic activity ceased, further craters pocked the flattened terrain. A few such as Aristillus, Autolycus, and Theaetetus, exceed 25 km diameter, but most of the later craters are less than 15 km in diameter.
Aristillus is the largest fresh-appearing crater with central peaks, terraced walls, sharp rim, and hints of rays visible on this image. It is considered Copernican in age, or less than 1.1 billion years old. In other words, all of the features and events discussed occurred eons ago, and were already frozen in time before there were eyes on Earth to marvel at the Moon.
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