L44 Mersenius
A large crater along the southwestern shore of Oceanus Procellarum, its floor domed and fractured, then peppered by the rays of secondary craters flung out from more recent impacts nearby — a fine study in how younger events overprint older ones.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L44 Mersenius
SW Mare Humorum · Domed Floor📉 Vital Statistics
🔭 Field Notes
Mersenius sits just west of Mare Humorum, and at first glance it looks like an ordinary lava-flooded highland crater. Look closer and the floor isn’t flat at all — it swells upward into a broad, shallow dome, probably because intrusive magma uplifted the lava-flooded floor sometime after the crater was filled with basalt. That bulge, cut across by small secondary craters, is why it made the Lunar 100 list.
- ▶ A Floor That Bulges: The interior rises roughly 400–500 meters above its own rim-adjacent edges, a convex dome shape unusual for a crater this size. A handful of faint rilles cross the floor, and the original central peak is absent or no longer recognizable, possibly buried or reshaped by the later uplift.
- ▶ Secondary Craters: A row of small craters crosses the dome, generally interpreted as secondaries rather than anything home-grown, and possibly related to ejecta from the distant Imbrium basin-forming impact. They’re subtle and best caught under low, raking sunlight near the terminator.
- ▶ Rimae Mersenius: To the east, spilling onto the shore of Mare Humorum, a system of roughly parallel rilles runs north-northeast for about 230 km — a separate rille system from the ones inside the crater itself, but part of the same worn, heavily fractured neighborhood.
📍 Nearby L100 Targets
- L13 Gassendi: A 110 km flooded walled plain on the northern shore of Mare Humorum, roughly 220 km north of Mersenius. Its floor is a genuine floor-fractured crater — an extensive network of rilles, a volcanically intruded central peak, and the famous “diamond ring” formed by the fresh crater Gassendi A punched into its rim. Where Mersenius bulges upward from below, Gassendi’s floor cracked and sank under the weight of Humorum’s lava.
- L54 Hippalus Rilles: A fan of concentric, arcuate rilles on the eastern shore of Mare Humorum, roughly 200 km east of Mersenius, near the crater Hippalus. They’re generally attributed to tectonic deformation associated with loading and subsidence around the Mare Humorum basin — a different rille-forming mechanism than the secondary-impact chain that marks Mersenius’s own floor.
- L91 De Gasparis Rilles: A dense rille network converging on the small crater de Gasparis, just west of Mare Humorum and only about 100 km from Mersenius — close enough that the Rimae Mersenius system is sometimes described as spilling into the same crowded, rille-cut neighborhood. Where Mersenius shows one rille system tied to its own floor, de Gasparis marks a hub where multiple rilles converge.
🚀 Mission Log
Target Acquisition — L44 Mersenius
Find the unassuming crater west of Gassendi’s shadow
Mersenius sits just west of Mare Humorum, with Cavendish to the southwest, Liebig to the south-southeast, and the much larger, more famous Gassendi nearby to the east. At first glance it looks like an ordinary lava-flooded highland crater — look closer and the floor isn’t flat at all.
Time it for the low sun that reveals the bulge
Around four days after first quarter, with the terminator nearby, low sun angle is what reveals the floor’s gentle convex dome and the terraced walls ringing it. Watch how the shadow tends to curve across the roughly 84 km floor rather than sweeping evenly across it — a sign of the dome, bulging some 400–500 m above its own edges. Notice, too, that there’s no central peak.
Work up in power for the crater row and the rilles
At higher power, with steady seeing helping most, trace the line of small craters crossing the middle of the dome — generally interpreted as secondaries, often suggested to be distant ejecta from the Imbrium impact, though a definitive source remains hard to pin down for a chain like this. In the same session, look for the delicate Rimae Mersenius clefts threading the floor’s northern half, and compare the tall, sharply terraced eastern rampart facing Mare Humorum against the heavily worn northern rim, where the smaller crater Mersenius N bites into the southwest edge.
Pair it with the Mare Humorum neighborhood
Gassendi (L13), roughly 290 km north, shows a genuine floor-fractured crater — rilles, a volcanically intruded central peak, and the famous “diamond ring” — its floor fractured and reshaped by magma intrusion and stresses associated with the loading of the Humorum basin, where Mersenius instead bulges upward from below in one unbroken sheet. Hippalus Rilles (L54), roughly 200 km east, trace a different rille-forming mechanism tied to basin loading and subsidence rather than secondary impacts. And de Gasparis Rilles (L91), roughly 100 km south, mark a hub where multiple rilles converge, close enough that the Rimae Mersenius system is sometimes described as spilling into the same crowded, rille-cut neighborhood.
📝 Observation Log — L44 Mersenius
0/4 CompleteIs Mersenius visible tonight?
Mersenius sits on the southwestern shore of Oceanus Procellarum, southeast of Cavendish and Cruger, at 21.5°S, 49.2°W — far enough west that best viewing centers on Waning Gibbous into Last Quarter (roughly Day 11), when the low terminator light rakes across its domed, fault-cut floor and throws the secondary-crater scars into sharp relief. A favorable libration to the west is a real help here, pulling the crater further onto the visible disk and easing the foreshortening that comes with sitting this close to the limb.
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When to Observe Mersenius
Mersenius is an unassuming crater at first glance, overshadowed by its larger neighbor Gassendi — but its floor tells two separate stories at once: one of a broad, gentle uplift from below, and another of a chain of much younger craters that happened to land squarely across it.
- For Orientation: Mersenius sits just west of Mare Humorum, with Cavendish to the southwest, Liebig to the south-southeast, and the much larger crater Gassendi nearby to the east.
- For the Domed Floor: Around four days after first quarter, with the terminator nearby, low sun angle is what reveals the floor’s gentle convex bulge and the terraced walls ringing it.
- For the Crater Row: The chain of small craters crossing the floor is subtler and benefits from steady seeing and a careful look under that same low-sun lighting.
What to Look For
Mersenius is about 84 km across and roughly 2.3 km deep, with broad, terraced walls that are especially prominent on the western side. Unlike most craters this size, it has no central peak. Instead, the whole floor has been pushed upward into a gentle dome, estimated at roughly 400–500 m above the surrounding floor at its highest point.
A line of small craters runs across the middle of the floor, cutting through the dome rather than being part of it. These are generally thought to be secondary craters — debris thrown out from a distant, much larger impact — often suggested to be related to ejecta from the Imbrium basin-forming impact, although definitive source attribution remains difficult for chains like this.
Delicate clefts, cataloged as Rimae Mersenius, cross parts of the floor, mostly in the northern half. They’re part of a broader family of rilles that run roughly parallel to the nearby shore of Mare Humorum, and are generally associated with the same regional stresses that shaped that basin’s rim.
The rampart is tallest and most sharply terraced facing Mare Humorum to the east, but the northern section is heavily worn down. The smaller crater Mersenius N bites into the southwestern rim, one of several named satellite craters scattered around the perimeter.
The Science: A Floor Pushed Up From Below
Mersenius’s dome, its crater row, and its rilles are really three separate stories layered onto the same patch of ground, each pointing to a different chapter of lunar history.
Doming Without Breaking
The floor’s broad convex bulge is generally attributed to magma intruding beneath the crater at some point after it formed, pushing the floor upward into a broad dome without producing the extensive fracture networks seen in more strongly modified floor-fractured craters like Alphonsus or Gassendi. The absence of a prominent central peak, in a crater this size, is consistent with substantial post-impact modification of the crater floor rather than one preserving its original impact-formed structure.
Scars From Somewhere Else
The crater row cutting across the dome is a reminder that not every crater on the Moon formed from a nearby, isolated impact. Large basin-forming impacts like the one that created Mare Imbrium threw debris far enough to carve chains of secondary craters across huge stretches of the lunar surface, and Mersenius’s crater row is usually discussed as one candidate example of that kind of far-flung ejecta.
What Hasn’t Been Resolved
The precise timing relationship between the floor’s doming, the rilles, and the crater row isn’t fully pinned down — it’s not settled, for instance, whether the secondary craters formed before or after the dome finished rising, which would change how the sequence of events here is read. Mersenius remains a useful, if still only partly resolved, case study in how several unrelated processes can leave their mark on the same small patch of lunar crust.
Where Alphonsus vented through cracks in its floor, Mersenius’s floor rose in one broad, unbroken sheet — and then took a scattering of hits from an impact that happened somewhere else entirely.