German amateur Bernd Faich-Wilken captured this remarkable view using a 12-inch schief spiegler telescope and an Apogee AMi 3 CCD camera. The complex rille system near Triesnecker crater is well seen, as is the rille bisecting the crater Hyginus. Many lunar rilles are concentric with basin rims, but the Rilleland features are not.  Larger image of above

lunar notebook By Charles A. Wood

Adventures in Rilleland

Between Mare Tranquilitatis and Sinus Medii is a concentration of rilles that is remarkable for its variety and ease of viewing. In Rilleland we find three systems, each with different characteristics.

The Ariadaeus Rille (named for the small simple crater at its eastern end) is a classic example of the flat-floored, parallel-walled straight rille. It is about 220 kilometers long, 4 to 5 km wide, and roughly 0.8 km deep. The Ariadaeus Rille is an excellent example of a graben, like those found on Earth. Graben form when opposing horizontal forces pull apart with enough strength that parallel faults form, and the terrain between them drops. Where the rille crosses some ruined crater rims we can see that they also drop down a few hundred meters.

Based on the geometry of the graben's sloping walls, the two inward facing faults of the Ariadaeus Rille meet at a depth of 2 to 3 km, which agrees closely with the estimated thickness of the fractured and crumbled ejecta (called the megaregolith) that overlie the deeper, more coherent lunar rocks. This similarity of depths is probably more than a coincidence since graben faults often start at the boundaries where rock layers of different strengths meet.
 

Connected to the western end of the Ariadaeus Rille by a narrow diagonal branch is the remarkable Hyginus Rille. It consists of two sections, one paralleling the nearby Ariadaeus Rille and the other aimed toward the center of the Imbrium basin. These two segments join at the 9-km-wide Hyginus crater. What makes the Hyginus Rille unique is that it contains a series of rimless collapse pits that are best visible in the segment northwest of the crater. It is beyond belief for even the most rabid impact-crater advocate that the pits in the rille are chance alignments of impact craters; they must be of internal origin. If so, why not the rimless Hyginus crater too?
 


This Lunar Orbiter 4 image (frame H-90) shows the Ariadaeus Rule, which stretches from Mare Tranquillitatis (right side of image) to Sinus Medii - following the contour of the terrain it passes through. The prominent crater at bottom right is Dionysius.
 http://cass.jsc.nasa.gov/research/lunar_orbiter/img/4-90H1.jpg

Pete Schultz, while still a graduate student at the University of Texas, suggested that Hyginus may in fact be a volcanic caldera or collapse crater. High-resolution Lunar Orbiter photographs show the crater's flat floor contains small domes that may be volcanic in origin. Schultz also pointed out that Hyginus is in the center of a 100-km-wide, 1.5-km-deep saucerlike depression. Some volcanoes on Earth are similarly centered on broad sags that result when subterranean magma reservoirs empty during volcanic eruptions. But if Hyginus formed by subsidence where are its eruption products? One possibility is that an irregular dark patch, seen around the crater at full Moon, consists of volcanic ash deposits. B. Ray Hawke of the University of Hawaii confirms (from spectral measurements) that the dark material is ash but amounts to only a small volume. It's fair to say that despite all of our lunar exploration we still don't clearly understand how Hyginus and the rimless pits along its rille really formed.
 
 

English amateur Harold Hill used a 10-inch Newtonian reflector at 286x to make this drawing of the Triesnecker Rille system. The rilles are easiest to see at low sun angles and during moments of steady seeing.

The third rille group near the center of the Moon's face is the Triesnecker Rilles. Triesnecker itself is the archetype of a complex impact crater featuring a wreath of slump blocks inside its bright and steep rim. It is clearly younger than the nearby rille features since one of them is apparently interrupted by the crater.

The Triesnecker Rilles are an intertwined system of mostly straight and narrow (0.75- to 1.5-km-wide) rilles that extend generally north to south. An intriguing feature is the continuation of a rille southward past the shallow crater Rhaeticus. Train your telescope on it and you'll see that where the rille crosses into rough highland material it seems to be made of coalescing pits - like parts of the Hyginus Rille. This is a delicate feature that is difficult to observe. Understanding how it formed is even more difficult.

Unlike the Ariadaeus Rille, the narrow Triesnecker Rilles can't be grabens unless the depth to the underlying coherent rock layer is much shallower. In the 1940s geologist Josiah Spurr wrote that Triesnecker crater was a volcanic caldera and that the rilles were fractures formed by the upliffing of the surrounding plain as the caldera erupted. We are now certain that Triesnecker is an impact crater, and it just happens to have formed on the edge of a preexisting rille complex. But no one has yet explained how or why the rilles formed.

November 1999


CHARLES WOOD is an avid Moon watcher and a researcher specializing in planetary science at the University of North Dakota.