Triesnecker and Rimae on the Shores of the Central Bay

Triesnecker and Rimae on the shores of the Central Bay
By Richard Handy

Sinus Medii or Central Bay, is aptly named for its relation to the geographic midpoint of the Moon (the point on the surface where the Earth would appear at the zenith at all times). On the northeastern shore of Sinus Medii lies the Copernican Period crater Triesnecker, the 26 km archetypical complex crater. Only hints of its distinguishing characteristics are apparent is my sketch because most of the crater’s inner walls are cloaked in shadow in this view, awaiting lunar sunrise to reveal their contours. The intense highlights off the scalloped western wall is the only evidence of considerable mass wasting that the crater has experienced. Hidden in this view is a floor whose entire western half is buried in wall debris. The deviation from circular (polygonal) form that Triesnecker displays is apparent all around the periphery of the crater. Terrace collapse has resulted in a reduction in floor depth and a annular ring of hills that are hidden from our view here. A mid morning Triesnecker sports a central peak and a small mare like area of what appears to be impact melt. The larger the crater the higher the peak, a product of the rebound energy of a surface that is impacted. Triesnecker sized craters lie at the lowest energy level necessary to produce central peaks, consequently the peak heights hover around 300 to 500 meters, large craters between 80 and 100 km have central peaks that average 2 km high. Immediately to the east, the complex rille network of Rimae Triesnecker dominates the right side of my sketch. Several things intrigue me in this area. Despite Triesnecker’s obvious superposition on top of these features, it seems amazing to me that these rilles survived being buried by its ejecta blanket. However this sketch is probably telling for what it does not reveal, filled in rilles which might be responsible the isolated rimae segments I’ve rendered here. The biggest mystery to me is why these rilles are located in this area of the Moon in the first place. Wilhelm suggested extensional forces. Perhaps Procellarum/Imbrium and Serenitatis/Tranquillitatis mascons are responsible for their appearance here, meaning these rilles are interpreted as grabens. A quick look to the northeast and we are amidst the obviously volcanic domain of the Hyginus crater and Rima. The surface elevation in this region has subsided and that may be due to extensional forces and subsequent outflows of mare lavas and terminal eruptive episodes

Here are the sketch details:

Subject: Triesnecker and Rimae Rukl: 33, 34
Date: 1-26-07 Started- 4:38 UT End- 5:55 UT
Seeing: Antoniadi III to IV Weather: Clear
Telescope: 12″ Meade SCT f/10
Binoviewer: W.O. Bino-P with 1.6X Nosepiece.
Eyepieces: W.O. WA 20mm Plossls
Barlow: 2X Televue Powermate
Magnification: 396X
Lunation: 7.03 days Phase: 86.8 deg Illumination: 52.8%
Colongitude: 359.7 deg Lib in Lat.: -4 deg 33 min Lib in Long.: +03 deg 35 min
Sketch medium: White and black Conte’ Crayons on black textured Strathmore paper.
Sketch size: 18″ x 24″.