A tour of Mare Humorum …..

New image of the feature-rich southeast margin of Mare Humorum (Sea of Moisture), inserted relative to the Gassendi panel imaged on the same night and which covers the northwest margin of the mare.

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.

Moving north past Loewy we move into an area where Mare Humorum is connected to Mare Cognitum (“Sea that has Become Known”), part of the broader Oceanus Procellarum (“Latin for Ocean of Storms”). In this “seaway” on the northeast side of Mare Humorum can be seen a series of island-like areas of high-ground on the right side of the upper image. Rimae Herigonius can just be seen lying west of the Herigonius crater in Mare Cognitum.

Continuing our anticlockwise journey around Mare Humorum we encounter the stupendous 110-km-wide crater Gassendi, with its beautifully imaged central rebound peaks. How did these peaks form in the centre of Gassendi crater ? Picture a splashing drop of water that impacts in a teaspoon of water and then rebounds into a central spire milliseconds after impact, and then extend the physics of that rebounding energy wave to the scale of a bolide impacting the lunar surface. In this case the same physics occurred when the Gassendi bolide impacted and instantaneously vaporised/melted the lunar surface, a central spire of lunar crust (and molten basalt) rebounded upward at the location of impact, thus forming the central peaks of Gassendi that today reach elevations of over 1300m above the crater floor. The floor of Gassendi is dissected by numerous rilles. Gassendi is a thing of beauty to lunar observers, and was named after Pierre Gassendi, a French philosopher, priest, astronomer and mathematician.

Southwest of Gassendi lies Mersenius, also named after another French philosopher – Marin Mersenne, a polymath whose work touched on mathematics and Messene prime numbers, music theory and harmonics, theology (an ordained priest) and philosophy.

Will have to revisit this area on another phase of the lunar cycle when the sun-angle is lower and relief more accentuated.