L24 Hyginus Rille
A 220 km graben that splits open across Sinus Medii, bisecting what is widely interpreted as a volcanic collapse crater rather than an impact scar — Rima Hyginus branches northwest and east-southeast from Hyginus crater, its length strung with small collapse pits where an underlying lava conduit gave way, the whole system one of the Moon’s clearest windows into volcanic and tectonic processes working together.
L24 Rima Hyginus
Central Near Side · Sinus Medii📉 Vital Statistics
🔭 Field Notes
Rima Hyginus is one of the most scientifically provocative features on the Moon’s near side — a 220 km graben rille that bends sharply at its midpoint around a 9 km central pit crater, Hyginus, and is studded along its length with a series of smaller collapse pits widely interpreted as having formed through volcanic or tectonic subsidence rather than impact. The rille consists of two arms meeting at Hyginus at roughly a 120° angle: the western arm extending toward Sinus Aestuum, the eastern arm running toward Triesnecker. Unlike sinuous rilles cut by flowing lava, Rima Hyginus is a straight-walled graben — a block of crust that dropped between two parallel fractures as the region’s underlying magma withdrew — and the pit craters along its floor are generally interpreted as collapse features above this same sub-surface conduit as the roof failed inward. Hyginus itself may be a maar-type volcanic crater rather than an impact structure, making it one of the very few features on the Moon where a volcanic origin is widely considered plausible. Under low Sun conditions near the terminator, the rille and its pit chain cast dramatic shadows that make the system detectable in small telescopes under favorable illumination.
- ▶ A Crater with No Rim: The central crater Hyginus lacks the raised ejecta rim that characterizes even modest impact craters. Its walls slope inward without a raised lip, and its floor shows no central peak — both strongly suggesting it formed by collapse of the surface into a void left by the withdrawal of magmatic material, rather than by the excavation of an impactor. This makes Hyginus one of the Moon’s strongest candidates for a volcanic pit crater, an exceptionally rare designation on a world overwhelmingly shaped by impact.
- ▶ The Pit Chain: Both arms of Rima Hyginus are lined with a string of smaller pits — roughly circular depressions 1–3 km across — that are spaced unevenly along the rille floor. Their alignment with the graben strongly implies they mark the trace of a sub-surface dike or magmatic conduit whose roof progressively failed. Similar pit chains are known from basaltic volcanic terrains on Earth. LRO imagery shows several pits along the eastern arm with unusually steep, sharp-edged walls, indicating comparatively limited degradation.
- ▶ Graben Geometry: The rille’s two straight arms meet at Hyginus at a distinct angle rather than forming a smooth curve — an arrangement that reflects the intersection of two separate fault systems meeting at a common structural node. The graben floor lies several hundred metres below the surrounding surface, and the rille walls are steep and largely unmodified, indicating the feature appears geologically younger than much of the surrounding highland terrain. It sits in the southeastern corner of Sinus Medii, where the rille itself is detectable in small telescopes under favorable illumination, though resolving the individual pit chain requires more aperture.
📍 Nearby L100 Targets
- L79 Sinus Aestuum (Seething Bay): A ~290 km dark volcanic plain to the west of Rima Hyginus, distinguished not by topography but by its strikingly dark surface — a blanket of pyroclastic glass beads ejected in ancient fire-fountain eruptions that coat the terrain and give it an unmistakably charcoal appearance against the surrounding highlands. One of the Lunar 100’s premier examples of a dark mantling deposit, Sinus Aestuum is most conspicuous near Full Moon, when high Sun angles maximize albedo contrast. The western arm of Rima Hyginus points directly toward it, and the two features share the same regional structural setting on the central near side.
- L50 Cayley Plains: A small but scientifically pivotal smooth plain roughly 50 km across, lying between the craters Cayley and De Morgan to the northeast of Rima Hyginus. The Cayley Formation — light-toned, relatively smooth material that fills low ground across much of the central near side — was long interpreted as ancient volcanic flood deposits before Apollo 16 landed here in April 1972 and returned samples that proved the material is impact melt and ejecta from large basin-forming events, not volcanic in origin. This overturned a major hypothesis and remains one of Apollo’s most significant geological revelations. The plains are not dramatic through a telescope but reward close attention for their unusual smoothness relative to surrounding highland terrain.
- L35 Triesnecker Rilles: A complex radial network of tectonic graben rilles spreading outward from the crater Triesnecker, roughly 40 km east of Rima Hyginus. While Rima Hyginus is a relatively simple two-armed system with a volcanic signature, the Triesnecker rilles form a branching, irregular web of intersecting grabens covering an area roughly 200 km across — a far more structurally complex system reflecting multiple phases of crustal extension. Together, Rima Hyginus and the Triesnecker rilles form one of the finest paired examples of graben tectonics on the Moon, close enough to observe in the same low-power field near the terminator.
🚀 Mission Log
Target Acquisition
Find the Moon’s central meridian, then drop to Sinus Medii
Rima Hyginus sits almost dead-centre on the lunar near side — which makes it easy to locate in principle and easy to overlook in practice, since the central region lacks the dramatic contrasts of the mare edges. Start by identifying Sinus Medii, the small dark plain near the center of the lunar near side. It’s modest naked-eye but straightforward in binoculars. The rille system lies just off the southeastern corner of Sinus Medii, where the plain gives way to older highland terrain.
Anchor on Triesnecker, then step northwest
The most reliable route in is through Triesnecker, a sharp 26 km crater that is easy to pick up at low power just east of Sinus Medii. Triesnecker is also the centre of its own rille network (L35), so if you’re already familiar with that region you’re already most of the way there. From Triesnecker, move the field about one low-power field to the northwest and the Hyginus rille system will come into view — a bent, V-shaped dark groove with the central pit crater Hyginus sitting at the angle where the arms meet.
Resolve the rille at 100x – 150x near the terminator
The rille proper — the graben trench — is detectable in small telescopes under favourable illumination, but 100x – 150x near the day 7–8 or day 21–22 terminator is where the system starts to give up its detail. At this magnification and lighting, the two arms cast sharp linear shadows, the walls show their steep profile, and the central crater Hyginus becomes clearly distinct from a normal impact structure — it appears unusually subdued, lacking the sharply raised rim seen around most craters of similar size. The overall shape of the system, two straight arms meeting at an angle, is unlike anything else in the central near side.
Hunt the pit chain along the eastern arm
With the rille in view and seeing steady, push magnification higher and work along the eastern arm toward Triesnecker. A string of smaller pits lines the rille floor — roughly circular depressions spaced unevenly along the trench. These require good seeing and sufficient aperture; they are not a small-telescope target. Their alignment with the graben is the key thing to notice: they track the fault trace rather than appearing randomly scattered, which is the observational basis for interpreting them as collapse features above a sub-surface conduit rather than a simple chain of secondary impacts.
📝 Observation Log — L24 Rima Hyginus
0/4 CompleteIs Rima Hyginus visible tonight?
Rima Hyginus shows best near the terminator and becomes much harder to see under high Sun. Catch it when the terminator crosses the central near side around First Quarter (Day 7–8) or Last Quarter (Day 21–22). The pit chain along the eastern arm requires the steadiest seeing of the session — check Triesnecker’s rilles first as a benchmark.
Check Moon Phase TodayReturn to the List
Ready to find the next target? Go back to the full map to see what else is visible tonight.
← Back to Lunar 100 Map
When to Observe Rima Hyginus
Rima Hyginus is a terminator target throughout. The graben walls that define both arms of the rille cast the shadows that make the system visible, and those shadows vanish under high Sun — the rille becomes much harder to detect as illumination approaches Full Moon. The pit chain along the eastern arm and the subdued appearance of Hyginus crater itself are also terminator features, requiring low raking light and steady seeing to show at all. Unlike some Lunar 100 targets, there is no complementary high-Sun observation to fall back on here.
- The Rille System: Best around Day 7–8 (First Quarter) or Day 21–22 (Last Quarter), when the terminator crosses the central near side. At this phase both arms cast clear linear shadows and the bend at Hyginus crater is readily apparent at 100x–150x in a moderate telescope.
- The Pit Chain and Hyginus Crater Detail: Same terminator window, but requiring the best available seeing within that session. Use the Triesnecker rille network to the east as a seeing benchmark — if those rilles are showing cleanly, push magnification and work the eastern arm of Hyginus for the pit chain.
What to Look For
At 100x–150x near the terminator, the most immediately striking thing about Rima Hyginus is its shape. Most rilles on the Moon are either roughly straight or sinuous; Hyginus is neither — two straight arms meet at an angle at the central pit crater, giving the system a bent, V-like appearance that is unlike anything else in the central near side. Confirming both arms and the angle at which they meet is the primary goal of a first observation. The western arm points roughly toward Sinus Aestuum; the eastern arm runs toward Triesnecker.
The central pit crater Hyginus, sitting at the junction of the two arms, is roughly 9 km across and appears unusually subdued compared with a typical impact crater of similar size — lacking the sharply raised rim that characterises most fresh lunar craters. This subdued appearance is the observational basis for the widely discussed interpretation that Hyginus may be a volcanic pit crater rather than a simple impact structure, making it one of the very few such candidates on the Moon where a volcanic origin is widely considered plausible. Confirming the absence of the usual bright rim ring is the observational goal here, though it requires steady seeing at high magnification to make the comparison reliably.
With the rille in view and seeing steady, push magnification higher and work along the eastern arm toward Triesnecker. A string of smaller pits lines the rille floor — roughly circular depressions spaced unevenly along the trench. They require good seeing and sufficient aperture and are not a small-telescope target. Their alignment with the graben is the key thing to notice: they track the fault trace rather than appearing randomly scattered, which is consistent with the interpretation that they formed by collapse of the surface above a sub-surface conduit rather than as a chain of secondary impact craters.
With both systems in the same general field near the terminator, compare the simple two-armed geometry of Rima Hyginus against the branching, irregular web of rilles spreading outward from Triesnecker crater roughly 100 km to the east. Hyginus is structurally simple — two grabens meeting at a point — while the Triesnecker system is a more complex network of intersecting fractures. The contrast illustrates the range of graben tectonic styles the Moon produced during its period of volcanic and tectonic activity, and the two systems together make one of the finest paired targets of this type on the near side.
The Science: Graben, Pits, and a Crater That May Not Be a Crater
Rima Hyginus sits at an intersection of two of the Moon’s most active geological debates: the origin of linear graben rilles, and whether any surface feature on the Moon can be confidently identified as a volcanic pit crater rather than an impact structure. Both questions remain open, and Hyginus is central to both.
What a Graben Is and Why Hyginus Has Two
A graben is a block of crust that has dropped between two parallel faults as the surrounding terrain pulled apart or the ground subsided. Both arms of Rima Hyginus are grabens — straight-walled trenches formed when the crust fractured and a central strip dropped. The two arms meet at Hyginus at a distinct angle rather than curving smoothly, reflecting a structural node where two fault trends converge — an arrangement characteristic of tectonic fracture networks.
The Pit Craters: Collapse or Impact?
The pits strung along the arms of Rima Hyginus are widely interpreted as collapse features — places where the roof of a sub-surface void or drained lava conduit failed and the surface dropped inward. Their alignment with the graben trace, rather than random distribution across the terrain, is the primary observational argument for a collapse rather than impact origin. Similar pit chains are known from basaltic volcanic terrains on Earth, where they form above drained lava tubes or along the traces of sub-surface dikes. The Hyginus pits are generally considered consistent with this volcanic collapse model, though the interpretation rests on morphology and alignment rather than direct sampling.
Is Hyginus Crater Volcanic?
The central crater Hyginus is one of the Moon’s most discussed ambiguous features. It lacks the raised ejecta rim that characterises even modest impact craters, and its walls slope inward without the usual bright lip — all characteristics more consistent with collapse into a void than with excavation by an impactor. These properties make it a plausible volcanic pit crater, and it is widely cited as one of the strongest candidates for a non-impact origin on the near side. However, direct sampling would be required to confirm the interpretation; morphology alone, while suggestive, cannot definitively rule out an unusual impact or a heavily modified impact structure. The question remains genuinely open.
What makes Rima Hyginus worth returning to is that the open questions are readable at the eyepiece. The absence of a raised rim on Hyginus, the alignment of the pit chain with the fault trace, the angular junction of two structurally distinct grabens — none of these require specialist knowledge to notice. They are visible in a moderate telescope under good terminator conditions, and they are the same features that have kept lunar geologists debating the origin of this system for decades.