One in a Million Mounds

Boulders, most 10 to 20 meters across, pepper the flanks of a cratered mound in the floor of Copernicus crater. The scene is illuminated from the east (right); LROC NAC M1339037292R; image width ~1.1 km [NASA/GSFC/Arizona State University].

A domical mound protrudes from the floor of Copernicus crater (340.117°E, 10.332°N), which is dominated by a solidified sea of impact melt rocks. These rocks host a variety of positive- and negative-relief features including collapse features, central peaks, and blocky cratered mounds like the one above. Mounds with craters near their peaks can superficially resemble volcanoes, which often have a summit crater. Previously, several examples of cratered mounds, usually located along the mare-highlands boundaries, were suggested as possible newly discovered volcanoes (Volcanoes in Lacus Mortis, Bull's Eye Crater or Volcanic Vent, Another Small Volcano).

This mound remains unexplored, thus we don't know for sure how it formed. Below is a collection of lunar mounds that are similar in appearance. For each example, try to think about how (or why not) these features might shed light on the formation of the cratered mound in Copernicus.

1. Volcanic Dome?

The summit crater of a volcano is expected to lack a raised rim and is usually less circular than an impact crater. Some small domes at the Compton-Belkovich silicic volcanic complex have craters that are thought to be summit vents (Example 1).

Example 1: A cratered and blocky dome at Compton-Belkovich, a silicic volcanic complex in the lunar highlands; image width 510 m [NASA/GSFC/Arizona State University].


Low basaltic shield volcanoes like those in the Hortensius region also have summit craters (Example 2). The crater on the Copernicus mound lacks a prominent rim crest, but high-resolution NAC-derived topography would better help us differentiate between a degraded impact crater and a volcanic crater in this case.

Example 2: The mare domes of Hortensius, low basaltic shield volcanoes; LROC WAC mosaic [NASA/GSFC/Arizona State University].


2. Cratered Impact Debris?

Impact debris often appears dome-shaped and sometimes lacks craters (Example 3), although these mounds are frequently cratered.

Example 3: A bouldery mound in Anaxagoras crater; image width 910 m [NASA/GSFC/Arizona State University].


Larger mounds to the north of Today's Featured Image also exhibit craters near their peaks (Example 4). This type of cratered mound is fairly common on the floor of large impact craters.


Example 4: Similar mounds observed to the north (yellow arrow) of the Copernicus mound (blue arrow) are comprised of impact debris and wall materials. Portion of LROC NAC controlled mosaic COPERNICLOB [NASA/GSFC/Arizona State University].


3. Blocks as Remnants of Impact Melt that Once Draped the Mound?

Lumpy mounds on the floor of Rutherfurd crater (Example 5) are littered with the broken fragments of impact melt rocks. These rocks formed as sheets of impact melt draped over the crater floor. Over time, this sheet of rock slowly breaks up and dis-aggregates.

Example 5: Lumpy floor of Rutherfurd crater; image width 1050 m [NASA/GSFC/Arizona State University].


A cracked mound on the floor of Anaxagoras crater (Example 6) illustrates what this sheet of melt might look like before dispersion through mass wasting. The boulders on the Copernicus mound could represent degraded bits of draping impact melt rocks; however, there is little evidence for a coherent layer of solidified melt as in the Anaxagoras example.

Example 6: Cracked mound on the floor of Anaxagoras crater, image width 600 m [NASA/GSFC/Arizona State University].


4. Squeeze-ups of Impact Melt?

Squeeze-ups of molten rock are one possible form of pseudo-volcanism that might occur in a molten sea of impact melt (Example 7).

Example 7: A possible squeeze-up of impact melt in Tycho crater; image width 730 m [NASA/GSFC/Arizona State University].


However, squeeze-ups tend to experience tensional stress across their peak (Example 8), but in the Copernicus example, we do not see any evidence of stress fracturing across the mound. Thus, when all the evidence is considered together, the cratered mound in Copernicus is most consistent with a block of impact debris with a small impact crater near its summit -- not volcanic at all!

Example 8: Fractured mound in Stevinus crater; mound is roughly 3 km across [NASA/GSFC/Arizona State University].


Explore the entire NAC image and look for evidence of other explanations for this cratered mound.

More related posts:

That's a Relief

Shiny Mound

Kagami-mochi on the Moon!

Fall Out

The Domes of Stevinus Crater

Anomalous Mounds on the King Crater Floor

Published by J. Stopar on 10 April 2014