LROC: Trapped at the Bottom

A mound of debris has piled up at the bottom of a small impact crater (26.9°N, 56.37°E) in the lava deposits of the 129 kilometer-wide "walled plain" Cleomedes, north of Mare Crisium. Field of view is approximately 500 meters in LROC Narrow Angle Camera (NAC) observation M121844756R, orbit 3090, February 26, 2010; resolution 0.5 meters from 45.08 km [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Examine the larger craters on the Moon and you will find that many of them have a tall mountain in their centers. These are known as "central peaks" in the vernacular of impact crater morphology, and are the result of a rebound effect that can accompany an impact after compression and excavation of the central region.

However, rebound peaks do not form in small craters where the strength of the target rock does not permit that kind of plastic deformation.

Smaller craters tend to be more bowl-shaped without prominent structures on their floors.

Early in November 2011, in the course of a series of low orbital passes, LROC captured the subject crater a second time 614 days later, from only 24 kilometers overhead. LROC NAC frame M174901788R, LRO orbit 10909, November 2, 2011 [NASA/GSFC/Arizona State University].

A wider context view for the Featured Image (white box), showing the entire impact feature from the October 26, 2010 NAC frame. Image field of view is roughly 1400 meters [NASA/GSFC/Arizona State University].

The small subject crater is dwarfed in an even wider context of the plain-like floor of Cleomedes, featuring the Rimae Cleomedes network. The 1-km-wide crater is situated halfway between 13 km-wide Cleomedes B and the older 12 km-wide Cleomedes J, inundated nearly to the point of becoming a "ghost crater," J was overrun by lava at some point after Cleomedes originally formed. LROC WAC M177256912C, orbit 11258, November 30, 2011; incidence angle 71.21° at 52 meters resolution from 37.93 kilometers [NASA/GSFC/Arizona State University].

Why then do we see a positive relief feature at the bottom of this small (1.4 km diameter) crater (26.697°N, 56.352°E) inside Cleomedes crater near the Sea of Crises? There are several processes that a planetary scientist will consider when tackling a problem like this. In addition to its role in the formation of large impact structures, NAC images show that impact melt is created in much smaller impact events than previously thought. If enough impact melt is generated during impact, it can accumulate on the floor to form a pond. Subsequent mass wasting and regolith development can hide the original melt deposit. However dry debris alone is unlikely to produce a positive relief structure, and is unlikely to flow radially and smoothly so that the deposit has even contributions from the full circumference of the crater walls.

The morphology of impact craters can be influenced by the target material and properties. For example, when a weak layer of material (a thin regolith layer, for example) overlies a stronger layer (mare basalt flows), a central mound can form due to the strength discontinuity between target materials. So instead of a bowl-shaped crater, the crater that forms has a positive relief mound on the crater floor. Many crater morphologies observed during the pre-Apollo and Apollo eras were reproduced by impact experiments completed in the 1960s and 1970s!

As is common on many planets, our small crater can be found hiding within a much larger one. LROC WAC monochrome mosaic from Planetary Data Base interface, image width is ~105 kilometers [NASA/GSFC/Arizona State University].

Ultimately, the mound on the crater floor may have formed from a combination of several factors and processes, not all of which are fully understood. Certainly once at the bottom of such a closed depression, any loose debris will find it difficult to escape unless ejected by another impact! What additional clues would you look for to learn the true cause of this structure?

In a near hemispheric, regional context, the small crater is still visible in this LROC WAC global mosaic overlaid upon laser altimetry (LOLA) based topography in the NASA LMMP ILIADS application. [NASA/GSFC/LMMP/Arizona State University].

Several additional similar features can be found when examining the full NAC frame, HERE. The following posts show examples of other small craters: Brayley G, Fresh Bench Crater in Oceanus Procellarum and Crater Covered with Boulders!