CONSTRUCTION AND EROSION IN THE FORMATION OF LUNAR SINUOUS RILLES

Detailed morphometric analyses of sinuous rilles in Oceanus Procellarum (specifically within Lunar Quadrangle 10 [LQ10], informally known as the “Marius Quadrangle” suggests that large rilles, such as Rima Marius, formed through a combination of constructional and erosional processes. Rille length, width, radius of curvature, number of curves comprising each rille, sinuosity and fractal dimension were measured for 90 rilles in this region. In addition, new images from the Lunar Reconnaissance Orbiter Camera were used to create detailed morphologic maps of portions of Rima Marius.

Rilles were grouped by geography to determine if location influenced rille morphology: 40 rilles in the Aristarchus plateau region; 41 rilles in the Marius hills region; 3 rilles near the Struve-Eddington craters; 3 rilles in highland mare ponds; and 3 “outlier” rilles, located to the east of LQ10 (within Oceanus Procellarum) were measured.

Vallis Schröteri is an extreme morphologic end-member, and is not discussed here. The remaining rilles show no clear trends with geography. Compared with values for terrestrial rivers, lunar sinuous rilles display a narrow range of fractal dimensions and sinuosities. Unlike terrestrial rivers, lunar sinuous rilles do not become increasingly complex as they lengthen, probably reflecting limits of lava behaviors. Observations of tight meanders, meander cutoffs and nick-points suggest that the lunar lava was able to erode its substrate.

However, detailed mapping of reaches of Rima Marius reveal previously unobserved “rillettes:” narrow (<100 m), shallow (tens of meters) rilles that are locally sub-parallel to, or cross-cut by, Rima Marius. We propose that these rillettes represent an early phase of the Rima Marius eruption, where the lava was initially emplaced as a sheet flow that developed preferred pathways (represented by the rillettes). As the eruption continued, a single preferred pathway—Rima Marius—dominated, and with time, lava flowing through Rima Marius eroded the pre-existing surface. The presence of rillettes around Rima Marius suggests that this rille may be among the youngest on the Moon.