L33 Serpentine Ridge
A winding wrinkle ridge formed as Mare Serenitatis’ basalt crust buckled and folded under compression as it cooled and contracted — three IAU-named segments, Dorsum Nicol, Dorsa Smirnov, and Dorsa Lister, trace a single ridge system across the eastern mare floor.
L33 Serpentine Ridge
Eastern Mare Serenitatis · Wrinkle Ridge📉 Vital Statistics
🔭 Field Notes
Serpentine Ridge is one lunar landmark hiding behind three IAU names: Dorsum Nicol in the north, Dorsa Smirnov through the middle, and Dorsa Lister in the south, together tracing a winding compressional fold across the eastern floor of Mare Serenitatis. First drawn and described by the German selenographer Johann Schröter in the 1790s, it was singled out even then as one of the loftiest wrinkle ridges visible on the Moon’s near side. Unlike a graben such as Rima Ariadaeus, it’s a purely compressional feature — the mare crust buckling and folding as it cooled and contracted over the underlying basalt fill.
- ▶ Origin near Posidonius: The northern end begins just southwest of Posidonius, near a small crater, before winding southward across the mare and throwing out minor branches, eventually fading into the mare near Promontorium Acherusia at the eastern tip of the Haemus range.
- ▶ The Crater Very: The small 5 km crater Very sits almost exactly on the crest of Dorsa Smirnov, offering a fixed reference point for tracing the ridge’s central segment.
- ▶ Asymmetric Profile: Like most mare wrinkle ridges, its cross-section is lopsided — a broad, gently swelling flank rising ~200 m over several kilometers, paired with a much steeper scarp gaining similar height in under a kilometer.
📍 Nearby L100 Targets
- L18 Mare Serenitatis Dark Edges: The high-titanium dark annulus rimming the basin’s southern and eastern margins, sampled by Apollo 17 at Taurus–Littrow. The ridge threads north–south through the paler eastern floor just inside this dark border zone, best caught near the terminator rather than at the full Moon that favors the dark edges.
- L20 Posidonius Crater: The 95 km floor-fractured crater anchoring the ridge’s northern end — Serpentine Ridge originates just off its southwestern flank, giving observers a natural pairing of a fractured crater against a tectonic ridge in the same field of view.
- L41 Bessel Ray: A solitary bright ejecta streak crossing the southern half of Serenitatis near the crater Bessel, of disputed origin. It’s an albedo feature best seen at full Moon — the opposite lighting condition from the low-sun terminator views that make the ridge stand out.
🚀 Mission Log
Target Acquisition
Find Posidonius, then trace the ridge south across the mare floor
Start at Posidonius, the large 95 km floor-fractured crater sitting on the northeastern rim of Mare Serenitatis — an easy naked-eye anchor even at low power. Serpentine Ridge originates just southwest of Posidonius as Dorsum Nicol, then winds south across the mare as Dorsa Smirnov through the central and widest stretch of the system, before finishing as Dorsa Lister and fading out near Promontorium Acherusia, the eastern cape of the Haemus mountains. Three IAU names for segments of the same wrinkle-ridge system — expect to follow it one named segment at a time rather than as a single sweep.
Chase the terminator — this is not a full Moon target
Like all mare wrinkle ridges, Serpentine Ridge is a low-relief feature — roughly 200–400 m of relief spread over hundreds of kilometers — so it depends entirely on raking sunlight to cast a shadow. Catch it when the morning or evening terminator lies near eastern Mare Serenitatis — exact timing shifts a bit with libration and solar colongitude — and the asymmetric profile pops: a broad, gentle flank on one side, a much steeper scarp on the other. Wait for high or full Sun and shadow relief largely disappears into the mare floor — that lighting instead favors the region’s darker basaltic edges and unrelated ray features, so don’t mistake a washed-out session for a bad instrument night.
Work up in power to pin down the central segment
At 75x–100x, follow the ridge’s winding path and pick out the minor branches it throws off as it crosses the mare. Push to 150x+ near the terminator on the Dorsa Smirnov stretch and look for the tiny 5 km crater Very, sitting almost exactly on the ridge’s crest — a handy fixed marker for confirming you’re tracking the right segment rather than a neighboring fold. Compare the two flanks side by side: the long, subtly swelling rise versus the short, steep scarp is the classic profile of a compressional wrinkle ridge, distinct from a graben like Rima Ariadaeus.
Pair it with the crater nearby, and the features it hides beside
Right where the ridge begins, Posidonius (L20) offers a natural same-field pairing — the floor-fractured crater alongside one of Serenitatis’ major wrinkle ridges, both modified during the basin’s later tectonic evolution, though Posidonius itself formed much earlier. To the south and east, the Mare Serenitatis Dark Edges (L18) — the high-titanium basalt sampled by Apollo 17 at Taurus–Littrow — rim the basin just outside the paler floor the ridge threads through; it’s a useful reminder that the two features want opposite lighting to reveal themselves. And crossing the southern half of the basin near the crater Bessel, the solitary Bessel Ray (L41) is best seen at full Moon — precisely the lighting that erases the ridge itself.
📝 Observation Log — L33 Serpentine Ridge
0/4 CompleteIs Serpentine Ridge visible tonight?
The low relief of Serpentine Ridge only separates from the mare floor under raking sunlight — aim for Waxing Gibbous (Day 6) or Waning Gibbous (Day 20–21) when the terminator crosses eastern Mare Serenitatis near the ridge’s longitude (~26°E). Under high or full Sun, shadow relief largely disappears and the ridge all but vanishes into the mare. The asymmetric flank-and-scarp profile is most rewarding near the terminator.
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When to Observe Serpentine Ridge
With only 200–400 m of relief spread across 300+ km, Serpentine Ridge is entirely dependent on raking light. This is one of the clearest cases in the Lunar 100 of a feature that needs the opposite lighting from its own near neighbors.
- Best Window: Roughly Day 5–7 after New Moon (morning terminator, waxing) or the mirror window around Day 19–21 (evening terminator, waning), when the shadow line lies near eastern Mare Serenitatis. Treat these as rough guides — exact timing shifts with libration and solar colongitude.
- What to Avoid: Full Moon. Under high Sun the ridge’s shadow relief disappears almost entirely into the flat mare floor.
- The Trade-Off: Full Moon is exactly when the nearby Mare Serenitatis Dark Edges (L18) and Bessel Ray (L41) are at their best. You genuinely can’t have all three at once — plan separate sessions rather than expecting one night to deliver everything in this part of the basin.
What to Look For
Start at Posidonius, the large 95 km floor-fractured crater on Mare Serenitatis’s northeastern rim — an easy anchor even at low power. Serpentine Ridge begins just southwest of it as Dorsum Nicol, then winds south across the mare as Dorsa Smirnov through the widest central stretch, before finishing as Dorsa Lister and fading out near Promontorium Acherusia. Three separate IAU names for one continuous fold — expect to follow it segment by segment rather than in a single sweep.
At 75x–100x near the terminator, compare the ridge’s two flanks directly: a broad, gently swelling rise on one side climbing roughly 200 m over several kilometers, paired with a much steeper scarp gaining similar height in under a kilometer on the other. That lopsided cross-section is the signature shape of a compressional wrinkle ridge, distinct from the straight-sided graben of a rille like Rima Ariadaeus.
Push to 150x+ on the Dorsa Smirnov stretch and look for the tiny 5 km crater Very, sitting almost exactly on the ridge’s crest. It’s a handy fixed point for confirming you’re tracking the correct segment rather than drifting onto a neighboring fold in the mare.
Widen out and note how differently the surrounding L100 targets behave under the same illumination. Posidonius (L20), right where the ridge originates, pairs a much older floor-fractured crater with a comparatively young tectonic fold in one field of view. The Mare Serenitatis Dark Edges (L18) and Bessel Ray (L41) both sit nearby but want the opposite lighting entirely — full Sun rather than a low terminator.
The Science: A Fold From a Cooling, Settling Mare
Serpentine Ridge is a purely compressional structure, formed as Mare Serenitatis’s basalt fill cooled, contracted, and settled under its own weight — a very different process from the crustal extension that produces a rille like Rima Ariadaeus or the Triesnecker network.
Cooling, Loading, and Buckling
Wrinkle ridges like this one are generally understood to form from a combination of thermal contraction as thick lava fill cools, and the load of that same basalt causing the basin floor to flex and subside. Together these processes put the upper layers of mare crust under horizontal compression, and the crust responds by buckling into a low fold — commonly interpreted as overlying shallow thrust faulting at depth, though the exact fault geometry varies from ridge to ridge and isn’t always a single clean plane.
A Long Observational History
Johann Schröter was already recording the prominent ridges of Mare Serenitatis in the 1790s, and singled this one out even then as among the loftiest wrinkle ridges visible on the near side. That makes Serpentine Ridge a target amateur observers have been tracking with modest optics for well over two centuries, long before any spacecraft imaged it directly.
What Orbital Missions Added — and Didn’t
Lunar Orbiter 4 captured the full run of the ridge system in 1967, and Apollo 15’s orbital cameras photographed it in 1971, though neither mission targeted it directly. Apollo 17 landed at Taurus–Littrow in 1972, close enough to help place the ridge within the broader volcanic and tectonic history of Serenitatis through its orbital photography and returned samples, but still roughly 150–200 km away — too far for any surface-level study of the ridge itself. No mission has set instruments directly on it, so its structure below the visible surface is still inferred from orbital data rather than sampled on site.
Serpentine Ridge is a reminder that not every dramatic-looking lunar feature comes from an impact or from magma pushing upward — sometimes it’s the slow, unglamorous process of a lava plain settling and folding under its own weight, still visible today as a low ridge that a telescope-era observer was already sketching more than two hundred years ago.