L48 Cauchy Region
A modest crater in Mare Tranquillitatis flanked by two very different scars — a straight fault scarp on one side, a sinuous rille on the other — with a pair of low domes nearby, a compact field guide to lunar surface tectonics.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L48 Cauchy Region
Mare Tranquillitatis · Fault, Rille & Domes📉 Vital Statistics
🔭 Field Notes
The Cauchy region takes its name from the small, bright crater Cauchy — the crater itself is just a landmark, barely 12 km across, unrelated in origin to the tectonic features that surround it. What you’re actually looking for is a compact patch of eastern Mare Tranquillitatis where three completely different processes left their mark within a single field of view: a scarp where the mare surface simply dropped, a rille where a strip of crust sank between two faults, and a pair of low volcanic domes rising quietly to the south.
- ▶ Rupes Cauchy, the Giant Step: Just southwest of the crater runs Rupes Cauchy, a roughly 120 km normal-fault scarp facing southwest and standing 200–300 m tall. The mare plain on its low side sits measurably lower than the plain on its high side — one of the Moon’s few large normal faults, rather than the far more common wrinkle-ridge thrust faults found almost everywhere else on the maria.
- ▶ Rima Cauchy, the Sunken Strip: On the opposite side of the crater, roughly parallel to the Rupes and about 40–50 km further northeast, runs Rima Cauchy — a graben formed not by one fault but by two facing faults, with the narrow block between them dropping down. Mapped in places as a chain of offset segments rather than one continuous crack, it shares the same extensional origin as the Rupes despite looking, at the eyepiece, like an entirely different kind of feature.
- ▶ Omega and Tau, the Quiet Volcanoes: South and southwest of the Rupes sit two low shield-like domes, Omega Cauchy and Tau Cauchy, each capped by a tiny summit pit thought to mark a volcanic vent rather than an impact scar. Built by lava thought to be somewhat more viscous than typical mare basalt, and likely related to the same regional extensional system that produced the fault and the rille, they round out the region’s tectonic-plus-volcanic story in one small stretch of mare.
📍 Nearby L100 Targets
- L12 Proclus Crater: A bright, 28 km oblique-impact crater roughly 350 km northeast of Cauchy, just across the Tranquillitatis border on the western shore of Mare Crisium. Its lopsided “butterfly” ray system is the sharpest possible contrast to the quiet, ray-free tectonics of the Cauchy region — one field is about ejecta geometry, the other about crustal stretching.
- L25 Messier & Messier A: A grazing-impact ricochet pair roughly 300 km southeast of Cauchy on the floor of Mare Fecunditatis, trailing a pair of near-parallel westward rays. Having an oblique-impact showcase this close to a showcase of pure extensional tectonics makes the short hop between them one of the more instructive stretches of the near side.
- L31 Taruntius: A 57 km floor-fractured crater roughly 300 km southeast of Cauchy, on the northwestern floor of Mare Fecunditatis. Its concentric floor fractures come from magma doming a crater floor from below — a useful companion to the Cauchy region’s fault and graben, which instead show the mare’s open surface pulling apart directly.
🚀 Mission Log
Target Acquisition — L48 Cauchy Region
Anchor on Cauchy, then split your attention two ways
Start at the small, bright crater Cauchy itself — sharply defined on the eastern plains of Mare Tranquillitatis, northwest of Taruntius. It sits right between the region’s two tectonic features: Rupes Cauchy, a fault scarp to the southwest, and Rima Cauchy, a graben roughly 40–50 km to the northeast. This is a rare two-for-one — related extensional stress expressed in two completely different shapes, close enough to share one field of view.
Time it for a low sun on both features
Both the Rupes and the Rima need a low sun angle to throw a visible shadow — the classic window is around four to five days after new moon, when the terminator sits nearby (libration will shift the ideal timing slightly, so let the terminator itself be your real guide). Rupes Cauchy’s exposed slope today averages only about 15–20°, far shallower than the roughly 60° dip typical of a fresh normal fault, so don’t expect a dramatic cliff — you’re looking for a subtle step in the mare, not a sharp edge.
Trace the fault, then compare it to the rille
Follow Rupes Cauchy along its roughly 120 km length and see if you can spot where its height varies, or where it briefly splits into two parallel steps. Then shift to Rima Cauchy and look for its offset, en echelon segments — overlapping dashes rather than one continuous line — a useful side-by-side study in how the same regional extensional stress field can produce a single throughgoing scarp in one place and a segmented graben just tens of kilometers away.
Finish with the domes, then compare the neighborhood
Omega and Tau Cauchy sit roughly 50 km south of Rupes Cauchy — subtler and worth more aperture, each containing a small summit pit interpreted as a volcanic vent (Omega’s is even individually named, Donna). For contrast elsewhere in eastern Tranquillitatis, Proclus (L12) to the northeast shows off ejecta geometry instead of tectonics, while Taruntius (L31) to the southeast has a fractured floor widely interpreted as the result of magmatic uplift — a useful counterpoint to Cauchy’s surface pulling directly apart.
📝 Observation Log — L48 Cauchy Region
0/4 CompleteIs the Cauchy region visible tonight?
The Cauchy region sits in Mare Tranquillitatis between Taruntius and Colombo, at 10.5°N, 38.0°E — far enough east that it comes under the terminator early in the lunation. Best viewing is around Waxing Crescent (roughly Day 4), when grazing dawn light picks out the straight scarp of Rupes Cauchy and the sinuous line of Rima Cauchy running past the small crater itself. Since the region sits at a modest eastern longitude rather than near a limb, libration has little practical effect on this one — timing the terminator is what matters.
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When to Observe the Cauchy Region
This is a rare two-for-one on the Lunar 100 — a fault and a graben, formed by related extensional stress but expressed in two completely different ways, sitting close enough together to fit in one field of view.
- For Orientation: Start with the crater Cauchy itself — small, bright, and sharply defined on the eastern plains of Mare Tranquillitatis, northwest of Taruntius. It sits right between the region’s two tectonic features.
- For the Fault and the Rille: Both Rupes Cauchy (to the southwest) and Rima Cauchy (to the northeast) need a low sun angle to show their relief. The classic window is around four to five days after new moon, when the sunrise terminator is nearby and both features throw a visible shadow.
- For the Domes: Omega and Tau Cauchy lie roughly 50 km south of Rupes Cauchy — worth a look once you’ve picked out the fault and rille, though they’re subtler and reward a bit more aperture.
What to Look For
Most lunar tectonic features are wrinkle ridges — compressional thrust faults, where the crust has been squeezed and buckled upward. Rupes Cauchy is the opposite: a normal fault, where one block of crust has simply dropped relative to its neighbor. It cuts roughly 200 km across the mare, with the southwestern plains sitting about 300 m lower than the plains to the northeast. Genuine normal faults of this scale are uncommon on the Moon — Rupes Recta is one of the only other well-known examples.
About 40–50 km northeast of Rupes Cauchy runs Rima Cauchy, a graben made of offset segments arranged in an en echelon pattern — imagine a row of overlapping dashes rather than one continuous line. Where Rupes Cauchy formed from a single fault, Rima Cauchy formed from two facing faults that dropped a narrow strip of crust down between them.
Rupes Cauchy’s slope today is only about 15°, far shallower than the roughly 60° dip typical of a fresh normal fault — a sign that billions of years of micrometeorite bombardment have worn the face down and spread material off the slope. Boulders eroding out of the old mare bedrock dot the scarp, but curiously few pile up at its base — one interpretation is that they get ground away by micrometeorite bombardment before they have a chance to roll downhill.
South of Rupes Cauchy sit Omega and Tau Cauchy, low volcanic domes each capped with a small summit depression widely interpreted as a volcanic vent rather than an impact scar. The vent atop Omega Cauchy even has its own name, Donna.
The Science: When the Moon Gets Stretched Instead of Squeezed
The Moon’s crust is overwhelmingly a story of compression — mare basins sagging under their own weight and buckling into wrinkle ridges. Cauchy is one of the places where the story runs the other way.
Two Faces of Related Extensional Stress
Both Rupes Cauchy and Rima Cauchy formed from tensional stress pulling the crust apart, but they resolved that stress differently. Rupes Cauchy released it along a single throughgoing fault. Rima Cauchy, just tens of kilometers away, released related extensional stress by dropping a narrow block of crust down between two facing faults — a graben rather than a simple scarp.
Reading the Scarp’s Age in Its Shape
A fresh normal fault scarp should be steep, close to the 60° dip typical of this kind of fault on Earth. Rupes Cauchy’s present-day slope of around 15°, combined with the heavily degraded small craters found along its crest and base, points to a fault that’s been quietly eroding for a very long time — its original, sharper profile smoothed out by eons of micrometeorite gardening rather than any single dramatic event.
What Hasn’t Been Resolved
The precise regional stress field responsible for pulling this particular patch of Mare Tranquillitatis apart isn’t fully pinned down. Extensional features like these are thought to reflect broader tectonic adjustments in the lunar lithosphere, but connecting Cauchy’s fault and graben to a specific, well-dated regional cause remains an area of ongoing study rather than a settled result.
Most of the Moon’s visible tectonics tell a story of things pushing together. Cauchy is a rare place where you can watch the crust pulling apart instead — and see, in the same field of view, two different ways that pulling can play out.