L58 Rheita Valley
A 445 km chain of overlapping secondary craters gouged out by ejecta blocks flung from the Nectaris impact, radial to the basin across hundreds of kilometers of highland — generally regarded as the second-longest named valley on the near side.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L58 Rheita Valley – Vallis Rheita
Southeastern Highlands · Basin Secondary-Crater Chain📉 Vital Statistics
🔭 Field Notes
Vallis Rheita cuts a long, straight gash across the southeastern highlands, running from just northwest of the crater Rheita itself, across the craters Young and Mallet, to end southeast of Reimarus. It’s generally regarded as the second-longest such valley on the near side, exceeded only by Vallis Snellius further northeast — and like Snellius, it is radial to the Nectaris Basin, hundreds of kilometers away. Because it sits close to the limb, foreshortening gives the whole feature an unusually three-dimensional look as it curves away toward the edge of the disk, an effect that makes it one of the more visually striking targets in this part of the highlands.
- ▶ Not a Valley at All: Despite its name and its river-like appearance in a telescope, Rheita Valley isn’t a rift or a collapsed lava channel — it’s a chain of more than a dozen overlapping secondary craters, gouged out when large blocks of ejecta from the Nectaris basin impact landed one after another along the same trajectory, each new crater biting into the last.
- ▶ Radial Evidence for Nectaris: Rheita Valley shares its radial alignment toward the Nectaris Basin with the longer Vallis Snellius to the northeast, and both are generally interpreted as sharing a common origin in the same basin-forming event. Together they offer some of the clearest surface evidence tying a specific secondary-crater chain directly back to the impact that produced it.
- ▶ Settling an Old Question: Lunar Orbiter IV’s photographic coverage of the Rheita Quadrangle enabled the first detailed geologic mapping of the valley, firmly establishing its interpretation as a secondary-crater chain rather than a tectonic rift — a distinction that simple telescopic observation of the surface alone couldn’t fully resolve.
📍 Nearby L100 Targets
- L40 Janssen Rille: A rare highland graben roughly 270 km southwest, cutting across the floor of the ancient, battered walled plain Janssen. Both features owe their fracturing to stresses tied to the Nectaris basin, but where Rheita Valley is a chain of impact craters, Rimae Janssen is a genuine tectonic crack — two very different mechanisms responding to the same distant event.
- L21 Fracastorius: A large crater roughly 800 km northwest, on the southern shore of Mare Nectaris itself, its northern wall entirely erased by the same basin’s mare lavas. Following Rheita Valley’s alignment back toward its source leads almost directly to Fracastorius’s doorstep.
- L7 Altai Scarp: The Moon’s most prominent escarpment, roughly 920 km northwest, forming the best-preserved rim segment of the Nectaris basin itself. It’s the basin-rim bookend to Rheita Valley’s ejecta trail — one feature marking where the impact’s rim survived, the other marking where its flying debris eventually landed.
🚀 Mission Log
Target Acquisition
Find Mare Nectaris, then follow the ejecta trail southeast
Start at Mare Nectaris, easily visible to the naked eye as a rounded dark patch southeast of Mare Tranquillitatis, and locate Fracastorius on its southern shore. From there, follow the radial trail of Nectaris ejecta roughly 800 km southeast, across increasingly rugged highland, until you reach a long, straight gash running from just northwest of the crater Rheita, across Young and Mallet, to end southeast of Reimarus. That gash is Rheita Valley.
Chase the terminator for the valley’s dramatic limb profile
Rheita Valley sits close enough to the limb that foreshortening gives it an unusually three-dimensional look as it curves toward the disk’s edge — but that same low viewing angle also means it depends heavily on a low, raking terminator to stand out clearly from the surrounding cratered highland. Time your session for morning or evening light crossing the southeastern highlands, and the valley’s full 445 km length becomes far easier to trace in one continuous sweep.
Work up in power to see the overlapping craters, not a single channel
At 100x–150x, look closely at the valley’s walls and you’ll start to notice it isn’t one continuous trench at all, but a chain of more than a dozen overlapping secondary craters, each merging into the next. Following the chain from the Rheita end southeast toward Reimarus is the best way to appreciate this — a smooth, valley-like impression from a distance gives way to a distinctly lumpy, cratered reality up close.
Pair it with the rille, the crater, and the basin rim nearby
Roughly 270 km southwest, Janssen Rille (L40) cuts across the floor of the ancient walled plain Janssen — a genuine tectonic crack sitting alongside Rheita Valley’s very different, impact-derived origin. Back at your starting point, roughly 800 km northwest, Fracastorius (L21) marks close to where this ejecta trail originated, its own northern wall erased by the same basin’s mare lavas. And roughly 920 km northwest, Altai Scarp (L7), the Nectaris basin’s best-preserved rim segment, is the bookend to Rheita Valley’s story — one feature marking where the impact’s rim survived, the other marking where its flying debris eventually landed.
📝 Observation Log — L58 Rheita Valley
0/4 CompleteIs Rheita Valley visible tonight?
Rheita Valley runs through the southeastern highlands at 42.5°S, 51.5°E, radial to the Nectaris Basin hundreds of kilometers northwest. Best viewing is around Waning Gibbous (roughly Day 3, or its lunar-month equivalent), when the terminator crosses near 51°E and low light gives the valley’s limb-hugging profile an unusually three-dimensional look. Its position close to the eastern limb means favorable libration is worth checking alongside terminator timing.
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When to Observe Rheita Valley
Rheita Valley looks like a river or rift cut across the southeastern highlands, but its true origin only becomes clear up close — and its position near the limb gives it a dramatic profile that rewards careful timing.
- For the Full Length: You need raking light. Rheita Valley sits around 51.5°E in the southeastern highlands, so its best light comes when the terminator is crossing that longitude. Track the terminator’s position rather than counting on a fixed day, since exact timing shifts with libration and solar colongitude.
- For Orientation: Anchor on Mare Nectaris and Fracastorius on its southern shore, then follow the radial ejecta trail southeast roughly 800 km to the long, straight gash running from northwest of Rheita, across Young and Mallet, to southeast of Reimarus.
- Aperture and Seeing: Moderate power traces the full length; higher power near the terminator reveals the overlapping individual craters that make up the chain, rather than a single continuous trench.
What to Look For
Trace the valley’s full length from just northwest of Rheita to southeast of Reimarus. Its proximity to the limb causes foreshortening that gives the whole feature an unusually three-dimensional appearance as it curves toward the disk’s edge.
Look closely at the valley’s walls and you’ll notice it isn’t one continuous trench at all, but a chain of more than a dozen overlapping secondary craters, each merging into the next.
Rheita Valley is radial to the Nectaris Basin, hundreds of kilometers to the northwest — a direct line connecting this valley back to the impact that produced it, shared with the longer Vallis Snellius further northeast along the same trajectory.
Rheita Valley is generally considered the second-longest named valley on the near side, exceeded only by Vallis Snellius. Both trace back to the same Nectaris basin-forming impact, offering a rare case of two separate secondary-crater chains from a single, identifiable event.
The Science: Ejecta Mistaken for a Rift
Rheita Valley looks so much like a channel or tectonic rift that it took years of orbital photography and geologic mapping to work out what it actually is — a chain of impact craters wearing the disguise of a valley.
Secondary Craters From a Basin-Forming Impact
Rheita Valley formed when large blocks of ejecta thrown from the Nectaris basin impact landed one after another along the same trajectory, each new crater merging into the last. It’s one of the clearest examples anywhere on the Moon of a basin’s ejecta carving a recognizable, named feature far from the impact site itself.
Confirming the Chain
Rheita Valley looks so much like a channel or rift that it took Lunar Orbiter IV photography of the Rheita Quadrangle, combined with later geologic mapping, to help confirm its real identity as a secondary-crater chain rather than a tectonic feature — a distinction simple telescopic observation alone couldn’t fully resolve.
What Hasn’t Been Resolved
Later orbital imaging and altimetry have refined measurements of the valley’s length, width, and depth, and clarified the morphology of its overlapping secondary craters and their relationship to other Nectaris ejecta features. Questions remain about the detailed emplacement mechanics of the largest secondary crater chains, and how variations in ejecta velocity, impact angle, and target terrain produced the differences seen between Vallis Rheita and Vallis Snellius.
Most Lunar 100 targets are exactly what they look like. Rheita Valley is a case of mistaken identity resolved by careful science — a feature that reads as a river-like gash from a distance, but reveals itself, up close, as a string of impact craters telling the story of an ancient basin-forming collision hundreds of kilometers away.