H. Meyer

LROC News System

Lunar craters less than between approximately 15-20 km in size are usually bowl-shaped, so the crater above, excavating the floor of Abel C (41.5 km; 36.72°S, 82.5°E) has a somewhat irregular morphology relative to most craters of its size (about 100 meters).

This crater is characterized by its block-strewn, hummocky floor and low-relief rim. The step-like or benched appearance most evident in the northeastern portion of the wall is due to a strength discontinuity. Such discontinuity indicates that the impact penetrated through two materials of different strengths. Lab experiments have shown that the bench crater morphology forms when the surface layer is composed of thin and poorly consolidated regolith and the subsurface is composed of harder, more coherent material.

Small relatively fresh excavation, the 100 meter crater right of center shown in context of the full, 3.6 km-wide field of view swept up in LROC NAC observation M1153673248R, May 2, 2014 [NASA/GSFC/Arizona State University].

In some cases, the underlying material is bedrock, evidenced by an abundance of boulders. So, what did the impactor punch through at the surface? Usually regolith is the culprit, but further investigation of the floor of Abel C suggests that it may not be regolith alone.

Ancient Abel C (41.5 km; 36.72°S, 82.5°E), on the northwest frontier of Mare Australe, has a smooth floor of relatively low reflectance, compared with it's equally eroded neighbors. Arrow marks the location of the 100 meter crater presented at high resolution above. LROC 100 meter Wide Angle Camera (WAC) global mosaic [NASA/GSFC/Arizona State University].

Abel C is a 36 km diameter crater on the southeastern limb of the Moon, just off the northwest edge of Mare Australe, a large region dominated by volcanic activity. Clementine (1994) color ratio images are extremely useful in identifying nonmare deposits since non basaltic compositions tend to stick out when Clementine images are placed in the proper color space. Three bands (415 nm, 750 nm, and 1000 nm) from the Clementine UVVIS camera were used to create the false-color image below.

Clementine (1994) false color image of the Mare Australe region with Abel C off the western edge. The floor of Abel C appears more red than the surrounding highlands, along with a clear indication of similar pyroclastic exposure like the multiple indications within Mare Australe   [NASA/GSFC/Arizona State University].

The three bands were ratioed to control the colors of the false-color image. The 750/415 ratio controls the red component, which is an indication of low titanium or high glass content as found in mature lunar regolith and to a greater degree pyroclastic deposits. The 750/1000 ratio controls the green component and is an indicator of the amount of iron on the surface. The 415/750 ratio controls the blue component and indicates high titanium or bright slopes and albedos. In the Clementine false-color image, Abel C stands out  from the highlands and more closely resembles the nearby mare. The strong red component, combined with the low-reflectance, mantled floor seen in Abel C is indicative of a pyroclastic deposit.

Abel C is shown in relation to the terrain of the Moon's southeastern limb, between Humboldt crater and Mare Australe. LROC WAC-derived digital elevation model and 100 meter global mosaic data [NASA/GSFC/Arizona State University].

This pyroclastic deposit encompasses approximately 190 km2 and has been identified in studies of pyroclastic deposits across the Moon (Compositional analyses of lunar pyroclastic deposits by Gaddis et al. 2003). Because pyroclastic deposits have a mantled appearance,  they must be relatively thin. It's possible that the thin upper layer the impactor penetrated was partially composed of pyroclastics.

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Fresh Bench Crater in Oceanus Procellarum

Lavoisier Pyroclastics

Brett Denevi
LROC News System

The lunar maria were once "seas" of highly fluid lava, and within their margins small islands and shorelines can be found. Today's featured image highlights a classic example of a partially flooded impact crater within Mare Imbrium. The western wall of the crater, a low point in the rim, was breached by the flowing lava, and the crater was filled nearly to its rim. What remains of the rim is known as a kipuka, the Hawaiian word describing an island of older land surrounded by younger lava flows.

The shoreline can be seen within the crater as a terrace-like ring just inside the crater rim, particularly on the eastern side. This terrace is akin to the high-water mark of a flood, and marks the high-lava point along the crater wall. As the lava cooled, it contracted and subsided to a somewhat lower level. Similar features are seen in areas like Bowditch, within Lacus Solitudinis.

The kipuka of interest, out on the vast plains of Mare Imbrium, in the 48 km-wide field of view of LROC Wide Angle Camera (WAC) monochrome (643 nm) observation M177798520C, spacecraft orbit 11338, December 6, 2011; 76.25° angle of incidence, resolution 60.03 meters from 43.97 km [NASA/GSFC/Arizona State University].

The flooded crater in today's image is approximately 2.7 km in diameter, and was likely originally around 500 m deep. That gives a maximum lava thickness of a little less than 500 meters in this spot, though that does not require a single 500-meter thick flow. Lava likely pooled in the low of the crater floor from multiple individual flows, rather than one massive influx of lava. Layering exposed within sinuous rilles,  mare pits, and impact craters suggests individual lava flows were much thinner (on the order of 10 meters).

LROC WAC context mosaic showing the location of the flooded crater (arrow) within an outline of the footprint of NAC M193132768L [NASA/GSFC/Arizona State University

Note the small (250 meters in diameter) high reflectance crater nearly in the center of this flooded crater. An astronaut could descend its interior and inspect a cross-section of about the top 25 meters of the basalts and determine the thickness and frequency of the lava flows that filled the host crater. The rim of this impact crater is the only kipuka preserved in the area, and is the last local remnant of the surface before it was drowned in the lavas of Mare Imbrium several billion years ago.

Browse the full-resolution NAC image HERE.

Related LROC Featured Images:
The Swirls of Mare Ingenii
Remnants of the Imbrium Impact

Lillian Ostrach
LROC News System

Regolith covers the lunar surface, and the thickness of regolith on the surface is related to the age of the surface. Older surfaces have thicker regolith layers than younger surfaces, and observations of crater morphologies are used to learn about the regolith for a specific area. Bench craters form in layered targets when there are variations in strength between the layers because different strength targets require different amounts of energy during the excavation phase of impact cratering. On the Moon, bench crater formation is usually interpreted to result when a bolide punches through an unconsolidated regolith layer to excavate a more cohesive layer such as mare basalt bedrock. The 75 m diameter bench crater in the opening image (61.504°S, 346.728°E) is a prime example of a bench crater that formed in an impact melt pond that is covered by a thin layer of regolith. However, observations of LROC NAC images show some bench craters like the one above to be self secondary craters, formed during the last stages of the impact process. It may be that the bench crater above was one of the last secondary craters formed during the Rutherfurd impact event, soon after the melt was emplaced, but without further study, we cannot be certain.

LROC WAC monochrome 64 meter resolution mosaic of Rutherfurd crater (61.186°S, 347.683°E, ~47 km diameter), from LROC QuickMap. Featured Image field of view noted by plot point on the southwestern crater wall [NASA/GSFC/Arizona State University].

The smoothed, softened texture of the pond surface, absence of cracks and fractures in the melt, and presence of superposed impact craters of various sizes and degradational states provide evidence of a layer of regolith in this area. If the 75 m diameter bench crater is not a self secondary crater, the projectile that formed the crater likely excavated roughly 7-8 m into the melt rock. Meter-sized boulders distributed within and around the eastern portion of the bench crater support an impact into a consolidated target and the formation of these boulders during excavation of the crater. Besides confirming the results of experiments conducted in the 1960s with layered targets, today's bench crater might be used to help constrain the depth of the impact melt pond. If there are other craters of similar degradational state in the pond, the morphology of these craters could be studied to help constrain not only the regolith thickness but also perhaps the thickness of the melt pond in this region. Unfortunately, it looks like the ~40 m diameter crater to the right of the bench crater may too degraded or affected by the boulders outcropping toward the upper right of the image. Additionally, finding these craters may prove difficult because the Featured Image may be the location of the only small melt pond with a bench crater in this portion of the Rutherfurd crater wall and any bench craters occurring elsewhere may reflect the strength contrast between the impact melt veneer on Rutherfurd's wall and the crater wall material.

How many bench craters can you find in the full LROC NAC frame? Are the bench craters located in small melt ponds or in the impact melt veneer on Rutherfurd's wall? If you find bench craters in the melt veneer, what two layers do you think might be responsible for forming the bench (hint: think about what the melt veneer covered) if the craters are not self secondaries?

Related Posts:
Not so Simple!
Fresh Bench Crater in Oceanus Procellarum
Bench Crater in Plato

Bench craters form in terrains where two layers exist with substantially different strengths. On the Moon this is normally interpreted as a loose regolith covering a more cohesive bedrock. Because less energy is needed to penetrate the regolith than the bedrock, the crater develops a bench at the boundary between regolith and bedrock. Using this interpretation, we can estimate the depth of regolith. In the case of today's Featured Image we can interpret that a thin layer of regolith is covering the layered mare deposits within Oceanus Procellarum.

But how meaningful is a regolith thickness estimate from one crater? Regolith is created as small impacts churn up the top layer of a surface. As more and larger impacts occur, the regolith grows in thickness. However, impact events are not evenly distributed, and regolith thicknesses can vary in a small area. One way to more accurately determine the regolith thickness is to then document all the bench craters in a given area. From this data an isopach map can be made, showing the thickness of the regolith for that area!

What other types of simple craters can you find in the full NAC frame?

At the southeastern edge of Mare Imbrium, about 25 km west of Rima Hadley, there is a small shiny mound on a dark and flat mare basalt plain which looks like a white sand island in the middle of a black ocean. This mound is about 2.7 by 2.2 km across. 

Normally fresh slopes and fresh ejecta have high reflectance due to less space weathering but this mound is brightest at its highest elevations and not down the slopes, brighter than nearby ejecta implying the mound is composed of higher-reflectance materials than mare basalts. Then how was this shiny island was formed?

Most likely, the mound is a remnant of highlands sticking through the mare, a hummock of plagioclase-rich highlands materials was embayed by mare basalt volcanism, burying all except its summit. If so, mare basalt is overlapping the mound's skirt. 

Unfortunately, the contact is not clear or sharp. Over time such sharp contacts are blurred by micrometeorite bombardment. If we are lucky, in the future, a small impact may occur right at the contact once again revealing the sharp contact. Or perhaps a future explorer might take a shovel to this spot and settle the question!

Impact craters of the size shown above are often bowl-shaped, but can also present flat bottoms and concentric or 'bench' features like those seen here.

This small (~140 m diameter) crater is characterized by a low-relief rim, shallow and hummocky floor containing a small central crater, and a large population of associated blocks or boulders. Blocky and irregular craters are often the result of low-velocity secondary impacts, but can result from high velocity as well. The circularity of this crater suggests that it is the result of a high-velocity, primary impact.

Experiments were conducted in the late 1960's using a high-speed gun to fire projectiles at targets in an attempt to understand the process of small crater formation. Loose sand and epoxy resin-bonded sand was used to simulate lunar soil (regolith) over a hard bedrock substrate. These experiments determined that different types of small crater morphologies result from different thicknesses of lunar soil. The bench crater morphology shown in today's Featured Image forms when the regolith is thin with respect to the crater's final diameter.

Our featured impact had enough energy to penetrate the lunar regolith layer to the hard basaltic bedrock beneath. But because the lunar soil is unconsolidated, this energy was more effective in displacing the soil than the bedrock. Hence, we see a wide impact feature with a shallow bottom instead of a bowl. The substrate does not have to be bedrock to produce a bench crater, but there must be a contrast in target strength. The clear presence of boulders in the case of today's featured crater does, however, indicate that bedrock fragmentation was involved in its production.