L17 Schröter’s Valley (Vallis Schröteri)
The Moon’s largest sinuous rille — over 160 km of winding volcanic channel carved across the Aristarchus Plateau by a catastrophic Late Imbrian lava outpouring, beginning at the iconic Cobra Head vent and ending in a 1 km-high precipice above Oceanus Procellarum.
L17 Vallis Schröteri — Schröter’s Valley
Aristarchus Plateau📉 Vital Statistics
🔭 Field Notes
Schröter’s Valley is the largest sinuous rille on the Moon — a meandering lava channel carved by enormous volumes of low-viscosity basalt that erupted, flowed, and thermally eroded the underlying surface over millions of years. The valley originates at a distinctive 6 km bowl known as the Cobra Head, located about 25 km north of the flooded crater Herodotus; the feature earned that name for its resemblance to a rearing snake when viewed from orbital photographs.
- ▶ Optimal Timing: Best seen around lunar day 12 and day 23, when low-angle illumination throws the rille walls into shadow and brings out the full sinuous profile. At full Moon the valley washes out but the Cobra Head remains visible as a dark oval notch.
- ▶ Inner Rille: A slender secondary rille runs along the flat floor of the main valley — visible from Earth under excellent seeing with a moderate aperture (6–10 inch). It indicates a renewed or continuing phase of lava drainage after the main valley formed.
- ▶ TLP Hotspot: Schröter’s Valley is one of the top three sites for reported transient lunar phenomena. The Cobra Head region in particular has accumulated more than 20 TLP reports — suggesting residual outgassing from depth, consistent with the area’s volcanic character.
📍 Nearby L100 Targets
- L22 Aristarchus Plateau: The mysterious 170 × 220 km uplifted crustal block on which the valley sits, mantled in dark pyroclastic ash and hosting the Moon’s densest concentration of sinuous rilles. Its elevated southern edge rises 2 km above the surrounding Oceanus Procellarum and contrasts strikingly in albedo with the dark mare below.
- L11 Aristarchus: The brightest crater on the Moon (40 km, 2.7 km deep), roughly 50 km to the southeast of the Cobra Head. At nearly double the albedo of typical lunar terrain, it blazes out even under Earthshine. Its inner walls display alternating dark and light bands of pyroclastic ash and excavated anorthosite — a geological cross-section readable at 150×.
- L86 Prinz Rilles: A fan of sinuous rilles extending north and west of the heavily eroded 46 km crater Prinz, located roughly 120 km to the southeast on the border of the plateau and the mare. Several individual rilles originate at small volcanic vent craters and wind across the basalt plain for up to 80 km — a compact parallel to Schröter’s Valley on a smaller scale.
🚀 Mission Log
Target acquisition
Anchor on Aristarchus
Find Aristarchus — it finds itself. The brightest crater on the Moon blazes out of the northwestern Oceanus Procellarum and is visible to the naked eye under Earthshine. In any telescope at low power it is immediately obvious. From there, look directly west, roughly 40 km, for its darker companion Herodotus — a similar-sized crater (35 km) but lava-flooded and unremarkable, with no central peak and a much lower albedo. The contrast between the two is striking and makes the pair easy to identify as a unit.
Find the Cobra Head north of Herodotus
From Herodotus, nudge north approximately 25 km. Under good illumination around Day 10–12 you will see a distinct dark oval bowl — the Cobra Head, a 6 km volcanic source vent. This is where Schröter’s Valley begins. The Cobra Head is the key landmark: once you have it, the valley unfolds clearly to the northwest from that point.
Trace the valley at 60x – 100x
From the Cobra Head, increase to 60x – 100x and follow the valley as it curves north, then northwest, then bends back south before terminating at a 1 km high escarpment at the edge of the raised Aristarchus Plateau shelf — a total run of roughly 160 km. The valley widens to nearly 10 km at its broadest point before gradually narrowing toward the terminus. The sinuous, river-like course is unmistakable at this magnification and distinguishes it from any linear graben or fault in the area.
Hunt the inner rille
Under steady seeing at 150x+, look along the flat floor of the main valley for a slender secondary rille running roughly parallel to the valley walls. It is a demanding target — requiring moderate aperture (6–10 inch minimum) and excellent transparency — but its presence confirms a second phase of lava drainage after the main valley formed. If conditions allow, also sweep the floor for subtle brightness variations indicating the pyroclastic ash mantle of the surrounding Aristarchus Plateau encroaching on the valley rim.
📝 Observation Log — L17 Schröter’s Valley
0/4 CompleteIs Schröter’s Valley visible tonight?
Schröter’s Valley shows best under a low-angle sun. Look around Day 10–12 after First Quarter when the valley walls cast strong shadows and the Cobra Head reads as a distinct dark oval, or around Day 23 after Last Quarter for an equally rewarding view from the opposite direction.
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When to Observe Schröter’s Valley
Schröter’s Valley sits on the Aristarchus Plateau at roughly 50.8°W longitude — in the northwestern quadrant of the near side, well placed in small and medium telescopes at moderate lunar ages. Like all sinuous rilles it is entirely lighting-dependent: the gently sloping valley walls disappear under a high sun but become dramatically readable under oblique illumination. Unlike Rupes Recta, the valley rewards observation on both the waxing and waning Moon, and the two appearances are more complementary than contrasting.
- Waxing Appearance (Day 10–12): As the terminator sweeps westward past the Aristarchus Plateau, the rising sun casts the valley walls into shadow from the east. The sinuous course is cleanly readable at 60x–100x, and the Cobra Head source vent is at its most distinct — a dark oval bowl sitting clearly north of Herodotus. The contrast between the pyroclastic-mantled plateau surface and the valley’s flat lava-resurfaced floor is also strongest at this phase.
- Waning Appearance (Day 23): Return after Last Quarter. The setting sun illuminates the opposite valley walls, reversing the shadow geometry and revealing western slopes that were in darkness two weeks earlier. The Cobra Head takes on a subtly different profile. Experienced observers use the two views together to build a more complete three-dimensional picture of the valley’s cross-section.
- Avoid Full Moon: Under a high sun the shallow valley walls produce neither shadow nor highlight. The valley effectively vanishes. The Cobra Head remains faintly detectable as a dark spot under full illumination — its lower albedo pyroclastic material persists — but the valley itself requires oblique lighting to read.
What to Look For
Begin with Aristarchus blazing in the northwestern mare. Shift west ~40 km to Herodotus, then north ~25 km to pick up the Cobra Head. From there, follow the valley as it curves north, bends northwest, then turns back south before terminating at the edge of the plateau shelf — a total run of over 160 km (primary rille ~155 km by recent measurement). The maximum width is about 10 km near the source, narrowing progressively toward the terminus to less than 1 km. Nothing else on the plateau resembles this sinuous, river-like course, and at 60x–100x the full shape fits comfortably in the field of view.
At 80x–120x, the Cobra Head deserves attention as an object in its own right. It is not simply the top of the valley — it is a discrete volcanic source crater roughly 6 km across, noticeably wider and deeper in tone than the valley downstream, sitting 25 km north of Herodotus. Its dark interior reflects the pyroclastic and basaltic material concentrated at the eruption site. The Cobra Head is one of the best-defined and most accessible volcanic source vents on the near side, and understanding it as the eruption origin transforms how the entire valley reads.
A slender secondary rille runs along the flat floor of the main valley, roughly parallel to the walls. It originates at the same Cobra Head vent as the primary rille and most likely reflects a separate, later eruption that sent a smaller, more focused flow through the already-formed valley. It is most visible along the widest central section of the valley where the floor offers the highest contrast. This inner rille is catchable with moderate aperture (6–10 inch minimum) under excellent, steady seeing, and can be photographed from Earth under near-perfect conditions.
Return at Last Quarter and revisit each feature. The valley walls cast shadows in the opposite direction, the Cobra Head’s western interior slope is now lit where it was previously dark, and the narrowing terminus section may show detail invisible two weeks earlier. This comparison is not just aesthetically interesting — it is the same technique planetary geologists use to extract topographic information from oblique-illumination imagery. Noting the differences between your Day 10–12 and Day 23 views is a genuine observational exercise, not a repeat visit.
The Science: How Was the Largest Sinuous Rille Formed?
Schröter’s Valley is the largest sinuous rille on the Moon, and the mechanism that carved it — though clearly volcanic — has been an active area of scientific debate for over fifty years. The question is not whether lava was involved, but precisely how: open-channel flow cutting thermally into the surface, a collapsed lava tube, or some combination of both. The answer has broader implications for understanding lunar volcanic eruption rates and lava volumes.
Theory 1 — Thermal Erosion by Turbulent Lava Flow
The dominant model in planetary geology holds that Schröter’s Valley was carved by high-volume, low-viscosity basaltic lava erupting from the Cobra Head vent and flowing downslope across the plateau surface. At sufficiently high eruption rates, lava flows turbulently and erodes the substrate both mechanically and thermally — melting and incorporating the underlying rock as it flows. Modelling by Hurwitz et al. (2012) applied to Rima Prinz (a smaller nearby rille) and Wilson and Head’s broader work on sinuous rilles support thermal erosion as the dominant process at the gentle slopes characteristic of the Aristarchus Plateau. The valley’s progressive narrowing from ~10 km near the Cobra Head to less than 1 km at the terminus is consistent with a flow losing erosive power as it travels further from the vent.
Theory 2 — Collapsed Lava Tube
An alternative view, noted by Phil Harrington and supported by terrestrial analogy, holds that the valley originated when a roof collapsed above a subsurface lava tube — a roofed conduit through which lava flowed and subsequently drained, leaving a hollow tunnel that later fell in. Terrestrial lava tubes on Hawaii and the Canary Islands collapse in exactly this way. On the Moon, where tubes can be far larger than on Earth due to lower gravity and slower cooling, this mechanism is plausible and has gained support from LROC imagery identifying pit craters and skylights elsewhere on the plateau that may be intact tube entrances. The “U-to-V” cross-sectional shape seen in some terrestrial collapse trenches has been proposed as diagnostic for this origin versus pure thermal erosion.
Theory 3 — Combined Constructional and Erosional Origin
The most nuanced current view, informed by work on Vallis Schröteri itself (Hurwitz et al., Warner & Garry) and similar rilles, holds that the two mechanisms are not mutually exclusive and may have operated sequentially. In this model, lava erupting from the Cobra Head initially spreads as a broad sheet flow controlled by pre-existing topography. A channel develops within that sheet flow and deepens through thermomechanical erosion. The inner rille — originating at the same Cobra Head vent but extending even beyond the primary rille’s terminus — is best explained as a later, separate eruption whose flow backed up, overflowed, and cut a new narrower channel through the primary rille’s floor, analogous to behavior observed during the 1984 Mauna Loa eruption in Hawaii. In this picture, the inner rille is not simply a late drainage feature, but the product of a distinct eruptive episode following a different path once the primary channel was established.
All three models agree on the broad sequence: a large volcanic eruption from the Cobra Head vent, dated to roughly 3.2–3.5 billion years ago during the Late Imbrian period, produced an enormous outpouring of low-viscosity basalt that carved the valley, flooded part of Oceanus Procellarum, and then subsided — leaving the valley, the inner rille, and the Cobra Head as a preserved record of one of the most dramatic volcanic events accessible in an amateur telescope.