L47 Alphonsus Dark Spots
Scattered across the floor of this ancient walled plain, a handful of dark patches mark low-energy eruptions of fine volcanic ash along buried fracture lines — among the clearest evidence anywhere on the Moon for genuine explosive volcanism.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L47 Alphonsus Dark Spots
Highlands East of Mare Nubium · Pyroclastic Vents📉 Vital Statistics
🔭 Field Notes
Alphonsus itself is just the setting — a large, ancient, 119 km highland crater with a flat, fractured floor. What you’re actually hunting for are the dark smudges scattered along its floor rilles: small, symmetric dark halos surrounding tiny craterlets, arranged where fractures cut across the crater floor rather than anywhere at random.
- ▶ Eleven Dark Vents: Alphonsus hosts eleven identified dark-halo craters — among them the named vents Ravi, José, Soraya, and Monira — one of the largest known concentrations of discrete pyroclastic vents on the Moon. Most are interpreted as vents formed when volatile-bearing magma erupted explosively along the floor’s fractures, showering the surrounding surface in fine, dark glass beads rather than building up a lava dome.
- ▶ A History of Suspicion: Ground-based observers flagged something odd here well before spacecraft arrived — in 1958, Soviet astronomer Nikolai Kozyrev reported a brief brightening and unusual spectral emission from the central peak, which he interpreted as a gas release from the lunar interior. The claim was controversial, but it helped make Alphonsus a prime target once probes could look closer.
- ▶ Volcanic, Not Impact: The dark-halo craters possess distinctive volcanic morphology and are surrounded by glass-rich pyroclastic mantling material compositionally distinct from the surrounding highland regolith, unlike typical impact ejecta. That, combined with their tight alignment along the floor’s fractures, is why they’re read as small volcanic vents rather than ordinary impact craters that happened to excavate darker material.
📍 Nearby L100 Targets
- L15 Straight Wall (Rupes Recta): The Moon’s most famous linear fault, roughly 200 km to the southwest across the highlands and onto the floor of Mare Nubium. Where Alphonsus shows volcanic vents erupting along tension fractures, Rupes Recta shows the fractures themselves taking center stage — a useful side-by-side lesson in what a lunar fracture network can produce.
- L46 Regiomontanus Central Peak: The oval, heavily degraded crater Regiomontanus, roughly 300 km to the south, whose central peak carries a small summit crater once cited as proof of lunar volcanism. That claim didn’t hold up the way Alphonsus’s did — the summit crater turned out to be an ordinary, if suspiciously well-aimed, impact rather than a vent.
- L51 Davy Crater Chain: Catena Davy, a neat row of small craters roughly 400 km to the west, often interpreted as the result of a fragmented impactor — a comet or rubble-pile asteroid pulled apart by tidal forces — striking the Moon in rapid succession, much like Comet Shoemaker-Levy 9’s “string of pearls” impact on Jupiter. A helpful contrast to Alphonsus: one field of dark spots is homegrown volcanism, the other is a visitor from outside arriving in pieces.
🚀 Mission Log
Target Acquisition — L47 Alphonsus Dark Spots
Find the middle of a famous three-crater chain
Alphonsus sits in the middle of a well-known north-south trio — Ptolemaeus, Alphonsus, and Arzachel — running along the eastern edge of Mare Nubium, just west of the lunar disk’s center. The crater itself is just the setting: a large, ancient, 119 km highland crater with a flat, fractured floor. What you’re actually hunting for are the dark smudges scattered along its floor rilles.
You’ll need two different lighting conditions
This target asks for opposite lighting on two separate visits. Around first quarter, with the terminator nearby, low sun angle throws long shadows from the central peak and rim, revealing the floor’s fracture network in relief. But for the dark-halo craters themselves, come back near full moon — this is the rare Lunar 100 target where high sun works better, since shallow shadows fall away and the tonal contrast between the dark ash and the brighter floor is at its strongest.
Hunt the vents, then trace the rilles that fed them
Of the roughly dozen dark-halo craters identified across the floor (the exact count varies by mapping study), a small telescope under good conditions will usually pick out four or five of the most prominent — most only a few kilometers across, with an irregular outline quite unlike a normal impact crater. They aren’t scattered randomly: look for them strung along the narrow rilles that cut across the floor, widely interpreted as fractures where rising, volatile-rich magma broke through and scattered fine dark ash for several kilometers around each vent.
Trace the observing history, then pair it with its neighbors
Locate the area just northeast of the central peak where Ranger 9 came down in 1965, and compare it with where Kozyrev’s 1958 glow was reported at the peak itself — a useful way to hold two of the Moon’s most famous transient-activity claims in the same field of view. For contrast, Rupes Recta (L15) roughly 200 km southwest shows a lunar fracture network with the fractures themselves taking center stage rather than venting anything, while Davy Crater Chain (L51) roughly 400 km west offers a field of dark spots with an entirely different origin — an outside impactor arriving in pieces rather than homegrown volcanism.
📝 Observation Log — L47 Alphonsus Dark Spots
0/4 CompleteIs Alphonsus’ dark-halo spots visible tonight?
Alphonsus sits between Ptolemaeus to the north and Arzachel to the south, at 13.7°S, 3.2°W — close enough to the meridian that timing here is less about geometry and more about lighting angle. The dark-halo spots on its floor show up best under low, oblique light around First Quarter (roughly Day 8), when shadow contrast is strongest, with a second, subtler viewing opportunity near Full Moon when the ash deposits’ low albedo stands out against the brighter floor. Libration barely matters at this near-central position; getting the sun angle right is what counts.
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When to Observe the Alphonsus Dark Spots
Alphonsus is one of the Moon’s best case studies in small-scale explosive volcanism — a floor-fractured crater whose fissures once vented dark ash across an otherwise ordinary highland floor. It also carries some of the most debated observing history of any Lunar 100 target.
- For Orientation: Alphonsus sits in the middle of a famous three-crater chain — Ptolemaeus, Alphonsus, and Arzachel — running north to south along the eastern edge of Mare Nubium, just west of the lunar disk’s center.
- For the Crater’s Structure: Around first quarter, when the terminator is nearby, low sun angle throws long shadows from the central peak and rim, and this is the best time to see the floor’s fracture network in relief.
- For the Dark Spots Themselves: This is the opposite of most Lunar 100 relief targets — the dark-halo craters show up best under high sun, near full moon, when shallow shadows fall away and the tonal contrast between the dark ash and the brighter floor is at its strongest.
What to Look For
Alphonsus’s floor holds a family of small, low-rimmed pits, each ringed by a dark halo of ash rather than typical crater ejecta. About a dozen of these dark-halo craters have been identified across the floor, though the exact count varies by mapping study, and a small telescope under good conditions will usually pick out four or five of the most prominent. Most are only a few kilometers across, with an irregular, sometimes non-circular outline quite unlike a normal impact crater.
The dark-halo craters aren’t scattered randomly — they sit strung along narrow rilles that cut across the crater floor. These rilles are widely interpreted as fractures associated with magma intrusion beneath the crater floor, where rising magma occasionally broke through, scattering fine dark ash for several kilometers around each vent.
Alphonsus has an unusually dramatic observing history. In 1956, Dinsmore Alter photographed a blurring of the floor’s rilles in violet light that was absent in infrared, hinting at a temporary haze. In 1958, Soviet astronomer Nikolai Kozyrev reported a reddish glow at the central peak and captured a spectrum showing emission he attributed to escaping gas — one of the most famous claimed detections of transient activity on the Moon, though it was never independently confirmed and remains disputed. In 1965, NASA deliberately crashed the Ranger 9 probe into the floor just northeast of the central peak, sending back the first live television images of an approaching lunar impact, but without turning up obvious evidence of ongoing volcanism.
Given Alphonsus’s reputation, it’s worth noting that its roughly 2 km-high central peak is made of anorthosite — ancient crust from the Moon’s early magma ocean, not volcanic rock. Whatever fed the dark-halo eruptions on the floor, the peak itself is an uplifted piece of primordial highland crust rather than a volcanic edifice.
The Science: Small Eruptions, Big Questions
Alphonsus is the type locality for a whole class of lunar features — small pyroclastic vents tied to floor fractures — and it has done as much as any single crater to shape how planetary geologists think about explosive lunar volcanism.
Small-Scale Explosive Volcanism
The dark-halo craters are interpreted as volcanic vents where volatile-rich magma drove explosive eruptions that deposited dark pyroclastic material along the floor fractures, in eruptions far smaller than the vast lava floods that filled the neighboring maria. The pyroclastic deposits are thought to be roughly contemporary with the ancient lavas of nearby Mare Nubium, dated broadly within the range of about 3–3.5 billion years, though the Alphonsus vents themselves aren’t independently dated to that precision.
The Kozyrev Controversy
Kozyrev’s 1958 spectrum is still cited as one of the best-documented claims of transient activity on the Moon, but the interpretation has never settled. Most lunar scientists remain skeptical that it represented genuine ongoing volcanic outgassing, and alternative explanations — from observational artifacts to non-volcanic gas release along old fractures — have never been fully ruled in or out either. Ranger 9’s 1965 impact photographs, taken from the same general area, showed no obvious sign of fresh eruptive activity.
What Hasn’t Been Resolved
Whether Alphonsus has experienced any geologically recent gas release remains disputed, but no convincing evidence for ongoing volcanic activity has been confirmed. The eruptions that built the dark-halo craters are ancient, and claims of more recent transient activity have proven difficult to either confirm or definitively rule out with the data collected so far.
Where Cauchy shows the Moon’s crust pulling apart, Alphonsus shows it venting from below — a quiet catalogue of small, ancient explosions, wrapped in one of the Moon’s longest-running observational mysteries.