Chapter 2: Energy at the Lunar Surface - Lunar and Planetary Institute

Rock fragments more abundant
West than typical mare
- - East
Outcrop.,.~p- ,
Horizontal distance, m
GRAVITATIONAL ENERGY
Fig. 2.23. Diagram to illustrate winnowing of lunar soil into a rille. Impacts on the rille rim eject
material in all directions but the rim receives ejecta from only one side. This results in a net movement
of soil into the rille (after Swann et al., 1972).
vicinity of the rim. In the narrow zone along the lip all craters greater than
0.5 m or so in diameter penetrate the fine-grained material, and therefore the
ejecta consist primarily of rock fragments. In the areas of normal soil
thickness, only those craters greater that 20 to 25 m in diameter penetrate
the regolith, and even then most of the ejecta are fine-grained materials from
craters approximately 100 m in diameter. Therefore, the blocky nature of
the 200 to 300 m zone along the'rille is due to the nearby source of rocks in
the area of very thin soil along the rille rim.
At the bottom of Hadley Rille, the fragment-size has a bimodal
distribution (Fig. 2.24). Numerous large boulders have broken from the
outcrops at the rille rim and were large enough to roll over the fines to the
bottom. The rest of the material is mainly fine grained and has probably
been winnowed into the rille by cratering processes. Where the rille meets
the pre-mare massif, the rille wall is made of fine-grained debris, and the
bottom of the rille is .shallower and flatter than elsewhere. This situation
indicates a considerable fill of fine-grained debris derived from the massif.
Mass movement is thus filling the rille and rounding the v-shape.
Talus Slopes
The Hadley Rille at the Apollo 15 landing site also offers the best
information about talus slopes on the moon. The talus slopes that form the
main walls of the rill& are blocky compared to most of the lunar surface (Fig.
2.24). Loose debris is approximately at the angle of repose. Boulder tracks
visible on the slope of the rille wall indicate recent instability of the blocks.