A meandering channel on the Moon: Rima Hadley

October 12, 2007

Hadley Rima_Harry Roberts

Rima Hadley, drawn by Harry Roberts

After extensive imaging and at least one Apollo visit these winding lunar channels still await final explanation.

The Moon abounds with rilles (rimae) of two kinds, the straight (or non-sinuous) rimae, and the sinuous rimae. The latter are bright, smooth, “dried river bed” structures that meander across lava flooded plains. They are remarkably uniform in width, but can often be seen to taper subtly into invisibility. They can be very long, sometimes hundreds of kilometres. Rilles can form branches like terrestrial streams, but they branch only rarely.

Rima Hadley arises on the west side of the Apennine Mountains at an elongated crater named Běla, that Rükl says is 11 x 2 km. though it looks wider (Fig 1). Běla is a generic Slavic female name, I think, and the crater connects to an arcuate valley-like formation that curves NW, terminating in a pointed end. Near this pointed end Rima Hadley emerges, and flows generally northwards.

Various authors suggest that Běla and the arcuate feature are volcanic landforms (elongate vents) and that lava from them excavated the rille we see today. An alternative view is that volcanic gas and lava flowed through an underground lava tube across the plains of Palus Putredinis, winding its way around obstacles like the protruding headlands of the mountains, to dwindle when reaching the lower levels of the Palus. The Běla formation is known to be elevated well above the plains, so this is plausible. Subsequently, through seismic action (crater impacts) the roof of the lava tube collapsed leaving the v-shaped channel we see today.

A close up of Hadley Rille by Apollo 15 astronauts_NASA

A close up of Hadley Rille by the Apollo 15 astronauts, courtesy NASA

Perhaps both explanations are right. Apollo 15 astronauts explored the rima in their moon buggy and imaged it close-up (Fig 2). Although it looks smooth from Earth, the rim shows backwasting, i.e. rim material has collapsed into the rille forming scree-slopes. The screes rise at about thirty degrees from the floor, and the rille is 1½ km wide and 300 m deep. Boulders as big as houses lie scattered along the floor!

At the half way point Hadley Rille briefly contacts fresh looking crater Santos-Dumont C. Did the crater impact on the rille, or did the rille flow around an existing crater? It’s hard to know. Where the rille reaches the foot of Mount Hadley Delta (Δ in Fig 1) it takes a sharp turn west, becoming hard to see, perhaps due to the lighting direction. This turning point marks the Apollo 15 landing site. Rima Hadley is 80 km long, although Orbiter photos show it faintly continues much further that this.

Hadley Rille is an elegant feature, and the arcuate vents (Běla etc) are the best examples of the landform type that I know of. Also another vent-like form lies just east of Běla in the mountains. Perhaps these vents erupted shortly after the titanic Imbrium impact, as eruptive volcanism is known to have followed the more recent Mare Orientale impact on the Moon’s west limb.

Though often viewed by moon watchers the Rima Hadley region repays closer study. Clear skies.

Harry Roberts
Sydney City Skywatchers

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