A tour and commentary of the southern part of Mare Humorum …..

Mare Humorum (425-km-diameter) is a small impact basin/mare (“basalt sea”) on the southwest quadrant of the visible side of the lunar surface. On the southern ‘shores’ lie the 64-km-wide Doppelmayer crater with its intact southwest rim and a northeast rim that descends beneath the mare. To the SSE of Doppelmayer lies the flooded Lee crater with its wide gap on the northeast side where mare basalt lavas have breached or over-spilled the crater wall and flooded the crater. Northeast of Doppelmayer lies Puiseux crater which is almost buried by the mare except for the uppermost rim of the outer rampart, forming an almost perfect hoola-hoop. Moving east from Lee, we see the arresting Vitello crater (once erronously thought to be a lunar caldera). The floor of Vitello is irregular and hummocky, and the craters' most eye-catching feature is a semi-circular rille that encircles the central group of rebound peaks on the crater floor, making Vitello one of the stranger craters on the lunar surface. East of Vitello lies the 16-km-wide Dunthorne crater, near the edge of a small mare called Palus Epidarium (Marsh of Epidemics) and the Ramsden family of Rima.

Moving on further north we see Promontorium (Cape) Kelvin and also Rupes Kelvin, a relatively linear 78-km-long fault escarpment (named after William Thomson, 1st Baron Kelvin – inventor of the Kelvin scale of absolute temperature measurement). The Rupes Kelvin escarpment (fault line) trends northeast towards the flooded 58-km-wide Hippalus crater, whose crater floor is punctured by the 5.3-km-wide Hippalus B crater. The southwest rim of Hippalus is missing, likely buried beneath the mare. Actually, sub-surface basalt injection below Mare Humorum is thought to have resulted in tilting of existing craters, allowing their sides to be flooded by the molten basalt.

Other fascinating features are the 3 large-scale arcuate (concave-to-west) rille systems labelled Hippalus Rille, Rima Hippalus II and Rima Hippalus III, with the Hippalus Rille transecting the Hippalus crater as it skirts the east edge of Hippalus B. (Rille is German for ‘groove’ – used to describe any long narrow depressions on the lunar surface). These broad arcuate linear rilles/rima are considered to be arcuate tension features that develop concentrically around the margin of some mare basins, in this case around the east side of Mare Humorum. The general picture is where the central parts of the basin have subsided under the weight of several kilometres of impact-generated basalt (lunar crust that melted upon impact). As the basalt within the basin cooled, central parts of the impact basin subsided, causing contraction and folding that formed the anticlinal flexures (low ridges) that can be seen wandering across the surface of Mare Humorum (see the ridge which was later impacted by Puiseux D). As the mare subsided, elevation of the rims of the basin created tension features that opened up in the lunar crust to form the concentric rima of the Hippalus group. The Agarthachides and Campanus A impacts both overprint Rima Hippalus III, so a chronological timeline of events can be constructed using these various impact/tectonic features.