December 7, 2012
Sinuous rilles are frequent in the mare-filled basins on the Moon and reflect erosion caused by turbulent, very hot lava extruding from a vent. Oftentimes, sinuous rilles meander in tight twists and turns. However, there are exceptions to the "general rule" of sinuous rilles and today's Featured Image of an unnamed rille near Promontorium Heraclides (41.07°N, 326.49°E) may be one such example. Less than 10 km long, the rille is linear with one gentle twist. The opening image highlights a portion of the southern wall of this rille, where rocks outcrop from the rille walls. The rocks jut out from the wall, forming a jagged shadow with illumination from the lower right, and there is abundant debris on the floor that likely represents eroded wall material. It may be that this outcrop is the ceiling of a slight overhang into the rille. If this is the case, obtaining rock samples from beneath the overhang would be useful in order to ascertain exposure ages of rocks in the outcrop compared to those beneath. In addition, measurements of the ancient solar wind could be made from rock samples as solar wind particles are implanted onto the lunar surface. However, let's not be too hasty! The interpretation of an overhang created by the outcrop is based largely on the presence of the distinct shadow on the rille floor. This image has an incidence angle of ~40°, so the Sun is just a little more than halfway to noon in the lunar sky. Illumination often plays tricks on scientific interpretation, so just because there is a prominent shadow cast by the outcrop does not mean that an overhang truly exists. The best way to determine whether the overhang is real or an illusion is to observe the location under different illumination conditions. Unfortunately for us, the current LROC coverage contains images with incidence angles of ~40° to ~45°, so we are stuck wondering whether this outcrop overhangs the rille walls for the time being. Outcrop in the south wall of this unnamed rille may form an overhang. Image width 400 m across, LROC NAC M124790534R [NASA/GSFC/Arizona State University].