Bouncing, Bounding Boulders!

Many boulder trails are found on the lunar crater walls and basin massifs. Some of the trails are smooth and nearly straight while others are curvy or gouge into the surface. In many cases by following a boulder's trail you are led back to its point of origin, useful information for a future geologist sampling otherwise unreachable points. Image width is 600-m across, north is up, and the boulder in the lower left-hand corner is 15-m across, unnamed 15-km diamater crater southwest of Rowland crater [NASA/GSFC/Arizona State University].

The 50-cm resolution of the LROC NAC cameras in the mapping orbit provides many opportunities to discover boulders and boulder trails. Boulder trails are formed when a boulder rolls downslope due to the effects of gravity, mass wasting, and perhaps perturbation from impact events. Today's image features several boulder trails formed by boulders of different sizes, tracing a path downslope from the higher topography of the walls to the crater floor. Some of the trails are quite smooth with few disruptions to the boulder path while others have discontinuous trails, as if the boulders bounced from their original position all the way down the crater wall. Some trails appear straight while others appear curvy. While many investigators agree that gravity, mass wasting, and impact events likely promote boulder movement downslope, some tracks might be formed by boulders ejected from nearby impact sites that skid and bounce across the surface because the boulders do not have enough energy to form secondary craters. The LROC NAC data can be used to identify boulder trails, map their occurrence and locations, and possibly help estimate regolith properties, such as thickness.

Astronaut Harrison Schmitt sampling a large boulder at Station 6 during the final Apollo 17 traverse [NASA].

Boulder trails were first identified in Lunar Orbiter V images and again in Apollo Panoramic Camera frames. Apollo 17 Astronauts Schmitt and Cernan sampled a large boulder at the termination of an impressive boulder trail at North Massif, the "Station 6 Boulder", on their third traverse at Taurus-Littrow. Since lunar geologists were able to trace the original position of the Station 6 Boulder, investigators could correlate the Station 6 samples returned from the Moon with hypotheses about local and regional lunar geology and examine differences between this sample and others collected elsewhere in the valley. Observing boulder trails in the LROC NAC images will allow scientists to discover the point of origin for many large boulders on the lunar surface and, over the whole LROC mission, will allow boulder movement and changes to boulder trails to be examined on short timescales. For example, has the Station 6 Boulder moved since the astronauts sampled it over 30 years ago, and if so, what can scientists say about erosional processes on the Moon over this timescale (since boulder movement is largely affected by gravity and mass wasting)?

Browse the whole uncalibrated NAC image!

Published by Lillian Ostrach on 15 October 2009