Moonwatch
August’s top lunar feature to observe
Archimedes is a large, lava-flooded feature located in the eastern region of Mare Imbrium. It’s also the largest feature within the boundary of this immense 1,250km-diameter sea of solidified lava. Archimedes has a curious appearance with a rim that looks more like a raised ring surrounding a remarkably flat base. The crater’s floor lacks any relief and there is no sign of any central mountain complex. Larger apertures or high-resolution imaging setups may be able to detect some of the multitude of craterlets that pockmark the smooth lava floor. The most notable of these is 3km Archimedes T, which sits close to the inner northeast edge of Archimedes. If you can see this feature, look for another 2.6km unlabelled craterlet 8.5km to its southeast. A pair of even smaller features lie 10km to the east of this. Archimedes’s floor appears striped with lighter material. This seems aligned with the younger crater Autolycus, the formation of which may have something to do with at least some of the stripes.
The appearance of Archimedes suggests its smooth lava floor was formed when cracks in the original crater floor allowed lava to well up inside the boundary rim. Despite being totally flooded, the walls of Archimedes are completely intact. They are also highly detailed, with steep inner terraces rising to a height of around 2km above the crater floor. The outside profile rises in a gentler but still steep fashion toward the rim’s edge. The boundary material appears to connect to a bumpy, almost rectangular, mountainous region to the south known as Montes Archimedes.
The surrounding region is equally as fascinating and contains varied examples of lunar geology. To the southeast is the flat region known as Palus Putredinus, an unwelcoming name meaning the ‘marsh of decay’. This stretches into the foothills of the spectacular Montes Apenninus. A small enclave of lava close to the Apennines’ edge contains the superb Rima Hadley, a sinuous rille which looks like a winding river engraved into the lunar surface.
The region south of Palus Putredinus, running from Montes Archimedes to Montes Apenninus, is known as the Apennine Bench. This region is defined by rough terrain cut by linear rilles and has been something of a puzzle: although it appears to be as old as the Imbrium Basin, it was probably formed from volcanic activity not associated with Mare Imbrium. Analysis of Apollo samples and via remote sensing satellites suggests the region has a unique chemical component called KREEP – potassium (chemical symbol K), rare Earth elements (REE) and phosphorous (chemical symbol P).
Located 150km north of Archimedes’s centre is the impressive form of the isolated mountain chain called Montes Spitzbergen. The peaks rise to a height of around 1.4km above the surrounding surface of Mare Imbrium, which frames them perfectly. There are also some superb neighbouring craters: 40km Autolycus 150km to the east and 55km Aristillus, 100km north of Autolycus. These are well worth spending time investigating when the lunar terminator is nearby.