L40 Janssen Rille
A rare rille system arcing across the floor of one of the Moon’s largest, most battered ancient craters — Rimae Janssen cuts through terrain heavily overprinted by later impacts, its fracturing likely tied to stresses from the distant Nectaris basin.
L40 Janssen Rille
Southeastern Highlands · Highland Graben System📉 Vital Statistics
🔭 Field Notes
Janssen is one of the Moon’s oldest and most battered walled plains, a 201 km hexagonal ruin near the southeastern limb whose outer wall is breached in multiple places. Buried in its southern two-thirds are the traces of an even older, larger crater, overlaid by the younger 78 km crater Fabricius in the northeast quadrant. The floor of that ancient inner depression hosts Rimae Janssen, curving away from Fabricius’s rim toward the southeastern outer wall of Janssen itself. What makes it worth seeking out is where it sits: most well-known lunar rilles occur in or near mare regions, carved either by a collapsed lava tube or by extension over a buried dike, tied to the volcanic flooding that fills the low ground. Rimae Janssen sits instead in unflooded highland crust, a graben formed by crustal extension with no obvious surface evidence of mare volcanism nearby.
- ▶ A Highland Outlier: Most graben-type rilles trace stress patterns tied to mare loading and buried volcanic plumbing. Rimae Janssen shows that the same extensional fracturing can occur in ancient, unflooded terrain, without an obvious source of magma at the surface.
- ▶ The Buried Crater Beneath It: The rille traces the floor of a large, degraded crater that predates Fabricius and much of Janssen’s own visible structure, a faint record of just how many impacts have stacked up in this stretch of highland.
- ▶ A Modest-Scope Target: Under favorable illumination near the terminator, the rille can be picked out in modest amateur telescopes, though it’s a subtler catch than showpiece rilles like Hyginus or Ariadaeus.
📍 Nearby L100 Targets
- L45 Maurolycus: A 115 km complex crater in the southern highlands with tall, terraced walls, most dramatic on its eastern side, and a towering central peak. It’s joined at its southeast rim by the smaller Barocius, forming a distinctive paired silhouette near the terminator.
- L7 Altai Scarp: The Moon’s most prominent escarpment, arcing roughly 427 km along the southwestern rim of the Nectaris impact basin before terminating near the crater Piccolomini, a fragment of the same basin-forming shockwave that shaped this entire quadrant of highlands.
- L58 Rheita Valley: A 445 km chain of overlapping secondary craters lying northeast of Janssen, aligned radially with the Nectaris basin and thought to have been gouged out by debris flung from that same ancient impact.
🚀 Mission Log
Target Acquisition — L40 Janssen Rille
Find Rheita Valley, then work southwest into Janssen’s ruined hexagon
Start at Rheita Valley, the distinctive 445 km chain of overlapping craters near the southeastern limb — easy to pick out even at low power. Work southwest from there and you’ll run into Janssen, a battered 201 km walled plain whose hexagonal outline is still traceable despite a rim breached in multiple places. The crisp, much younger crater Fabricius sitting in its northeast quadrant is the giveaway that you’ve found the right ruin — nothing else in the neighborhood looks quite like an old crater with a newer one punched into its floor.
Catch it near the terminator, when the ruin looks its age
Janssen rewards low sun angles — one longtime observer likened a terminator visit here to walking through the “ruins of an old castle,” with the worn walls, buried inner rim, and scattered smaller craters all thrown into relief. Under a high Sun the whole structure flattens out and loses most of its readability, so this is very much a terminator target, not a full-Moon one.
Work up in power to trace the rille itself
At 75x–100x, map out the floor: Fabricius with its central peaks, the connected crater Metius on the northeast rim, worn Brenner to the north, the joined pair Steinheil and Watt to the southeast, and Lockyer astride the southwest wall. Push to 150x+ and follow Rimae Janssen as it curves away from Fabricius’s rim across the floor of an older, heavily degraded crater-like structure toward Janssen’s own southeastern outer wall — a subtler catch than showpiece rilles like Hyginus or Ariadaeus, but visible in modest amateur telescopes under the right light.
Pair it with the rest of this corner of the highlands
Maurolycus (L45), with its tall terraced walls and central peak, sits to the northwest and pairs naturally with Janssen’s own low-sun session. Rheita Valley (L58), your starting anchor, and the Altai Scarp (L7) further northwest — arcing along the rim of the Nectaris basin — round out a stretch of highlands largely shaped by that same ancient basin-forming impact.
📝 Observation Log — L40 Janssen Rille
0/4 CompleteIs the Janssen Rille visible tonight?
Janssen sits deep in the southeastern highlands (~40°E, 45°S) — best around Waning Gibbous (roughly Day 18–19), when the evening terminator sweeps across this heavily worn crater and rakes light across the floor-crossing Rimae Janssen. The matching morning crossing arrives only about four days after New Moon, when the crescent is thin and low for a highland target this far south, though it’s not unusable under good seeing and favorable libration. Since Janssen sits close to the limb, favorable southerly libration meaningfully improves the view.
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When to Observe Janssen Rille
Janssen is a huge, battered walled plain in the southeastern highlands, but the rille that gives this entry its name is a subtle target: a graben-like crack, not a bold volcanic channel. Catching it well means favoring the terminator over a high, flat sun.
- For the Rille and Wall Relief: Best near the terminator while the feature is still catching low, raking light. Janssen sits at roughly 41°E, so it crosses into sunlight early in the lunation — practical lunar calendars place its sunrise around Day 6–7 (waxing crescent approaching First Quarter). A similar low-angle view returns during the waning phase as the terminator sweeps back over it roughly two weeks later.
- For the Age Contrast: A higher sun brings out Janssen’s hexagonal, heavily worn outline against the sharper, fresher rim of Fabricius punched into its northeast floor — useful for appreciating just how much older Janssen is.
- Aperture: The main rille is visible in small telescopes under good seeing; steady air matters more here than raw aperture, since you’re chasing a low-relief crack rather than a bold shadow-caster.
What to Look For
Janssen’s outer wall, roughly 201 km across, forms a distinctly hexagonal outline with a slight curvature at each vertex — easier to trace near the terminator than you might expect for a rampart this degraded. Punched into its northeast floor is Fabricius, a much younger, sharp-walled crater about 78 km across with multiple central peaks; the contrast between Fabricius’s crisp rim and Janssen’s crumbling one is a textbook case of a young crater superimposed on an ancient one.
Look for a curving crack running from the southwest wall of Janssen up to the south wall of Fabricius — the official gazetteer figure for the main rille is about 114 km, though mapped branches and connected segments are cited at varying lengths up to around 140 km depending on the source. Rima Janssen is generally read as a graben, a strip of crust dropped between parallel faults, which is what earns it the Lunar 100 distinction: most lunar rilles occur on the maria, and a straight rille confined entirely to ancient highland terrain like this is genuinely uncommon.
In the southern two-thirds of Janssen’s floor, look for the faint remains of an older, concentric crater wall — a second, earlier impact structure whose rim Fabricius later overran on its northeast side. It’s subtle and easy to miss, but it’s a reminder that what looks like a single ancient scar is really at least two impacts layered on top of each other.
Once you’re oriented, work outward: Metius sits attached to the northeast rim, the heavily worn Brenner lies just north, the joined pair Steinheil and Watt sit southeast, and the smaller Lockyer straddles the southwest wall. Further east, appearing deceptively close due to foreshortening, is the much larger Vallis Rheita.
The Science: A Crack in the Highlands
What makes this rille worth a dedicated Lunar 100 entry isn’t its size, but its setting — and the ancient crater it sits in turns out to have lent its name to a marker used across all of lunar geology.
A Graben Where Grabens Rarely Form
Straight lunar rilles are generally understood as grabens: blocks of crust that subsided between parallel faults under extensional stress. Most known examples sit on or near the maria, often linked to the same crustal stresses that let basaltic lava well up. Rima Janssen cuts across ancient highland floor material with no mare flooding or visible volcanic constructs at this specific site, making it a rare case of that same graben-forming process turning up somewhere the usual mare-related explanations don’t obviously apply — which is exactly why Charles Wood singled it out as a “rare example of a highland rille” for the Lunar 100.
The Janssen Formation: A Named Marker in Lunar Time
Much of the debris covering Janssen’s floor is thought to be ejecta from the later Nectaris basin impact, material so well exposed in this region that USGS geologists formally named it the Janssen Formation. That formation now serves as the official boundary marker separating the Nectarian period from the older Pre-Nectarian period across the entire lunar geologic timescale — meaning this one worn, ancient crater lent its name to a dividing line used to date terrain all over the Moon, even though Janssen itself predates the material named after it.
What Hasn’t Been Resolved
The exact tectonic trigger for Rima Janssen’s extension at this particular spot isn’t settled in the sources available — straight-rille grabens elsewhere are sometimes tied to nearby basin-scale stresses or to subsurface intrusions, but I couldn’t confirm a specific, well-established mechanism for this one. Similarly, the precise relationship between the rille, the older concentric crater beneath Fabricius, and Fabricius’s own later impact is more inferred from the geometry than demonstrated in detail.
Janssen rewards patience the same way Schickard does: a low-relief crack that’s easy to overlook under a high sun, and a quiet reminder near the terminator that this ancient floor has been rewritten at least twice — once by a distant basin’s ejecta, and once by Fabricius punching straight through it.