Astronomical Society of Coonabarabran

Lunar Volcanism: The Enigma of the Rimless Craters

by Harry Roberts

Historically the extent of lunar volcanism has been hotly debated, from the earliest days when all the craters were thought to be volcanic, up to the present when only a few landforms are known to have volcanic origins (apart from the extensive mare lava flows). The 20C probes showed that few lunar craters had endogenous (i.e. internal) origins, and most are too small for amateur scopes. It seems certain that the Moon never had a molten core capable of generating earth-like volcanic cones. But there are some lunar features that probably resulted from activity INSIDE the moon - and these include the rimless craters.

All impact craters have distinctive turned-up rims caused by the explosive impact of a piece of solar system debris. Such rims result from both tilting and compression of lunar crust surrounding the impact site. Even the most degraded impact sites show raised rims.

But there are some craters with no rims - and they tell a very different story; such a crater is Hyginus in Sinus Medii. My first look at Hyginus occurred years ago as I was drawing the fantastic network of rilles associated with nearby crater Triesnecker. Sketching the rilles led me to Hyginus, a most distinctive 10km diameter crater sited on a bend in the large fracture named Rima Hyginus. I had a more detailed view of Hyginus on last August 9 [Ed. 2008] when seeing briefly allowed 400 magnification.

Hyginus showed no sign of a raised rim, and no rim shadow was cast across the lunar surface. The crater walls though seemed bright, fresh and very steep. There were no bright impact rays emanating from Hyginus - instead two fans of dark material (pyroclastics ?) that perhaps vented from Hyginus and the twin small craters just west (left) of Hyginus spread about 20km over the lunar surface. In the large scale regional sketch these dark fans were seen to spread fifty km from the craters. Ray-like streaks were seen on the detail sketch.

Crater Hyginus sits on Rima Hyginus, a 3km wide rift valley over 200 km long, shown in the regional sketch. In the detail sketch we see that many small rimless craters are sited along the Rima - very like Catena Davy. But these can't be accidental impacts squarely on the rift, so the rimless craters must have developed there, a result perhaps of surface material collapsing into a fissure below. The resulting craters are conical and rimless, much like an ant-lion's trap. And it's possible that dark ash or dust vented from some of these pits to coat the surroundings in an earlier time of low grade volcanism.

Crater Hyginus looks much like a terrestrial sink-hole, where surface crust drops into a cavity below - often caused on Earth by water erosion. No water on the Moon, so perhaps lava flowing through the rift below created a cavity into which surface crust collapsed. A short branch of the rima emerges from the north side of Hyginus for 5km, suggesting complex windings of the rift below the surface. Several authors point out that the west part of Rima Hyginus is radial with the recent Imbrium impact, while the eastern part of the Rima is radial with older Mare Tranquillitatis; so both rifts perhaps arose from basin impact events. And if lavas further excavated the Rima and caused the collapse craters such lavas may have been caused by the Imbrium event. Certainly Imbrium debris has "sculpted" the whole site - but the rimless craters post-date that holocaust.

Take a close look at Hyginus - I think it's the largest rimless crater on the Moon, and be on the lookout for other craters without rims. Enjoy moonwatching.

Figure 1 - Hyginus Region

Figure 2 - Hyginus Detail