L42 Marius Hills
The densest concentration of volcanic domes, cones, and sinuous rilles on the Moon, spread across a plateau in Oceanus Procellarum — over 250 low, broad shields rising just a couple hundred meters, barely visible except under the most glancing light.
L42 Marius Hills
Oceanus Procellarum · Volcanic Field📉 Vital Statistics
🔭 Field Notes
The Marius Hills represent the Moon’s most significant concentration of volcanic features. Formed primarily during the Late Imbrian, this region is crowded with over 250 individual domes and cones. These features resulted from relatively more viscous basaltic lavas and lower eruption rates compared to the fluid flows that filled the surrounding plains. The complex includes low-profile shield volcanoes, steeper cone-like volcanic constructs (cinder-cone analogues), and several sinuous rilles, most notably the Marius Rille, a likely collapsed lava conduit.
- ▶ Morphology Contrast: The variety of shapes in the field suggests differences in eruption style, lava viscosity, and volatile content over a long period of volcanic activity.
- ▶ The Marius Hole: A 65-meter-wide vertical “skylight” within a rille is likely connected to an intact lava tube, making it a primary candidate for future lunar habitats.
- ▶ Observation Timing: These are subtle topographic features; they vanish under a high sun and are only visible when the lunar terminator is near 50°–55° West longitude.
📍 Nearby L100 Targets
- L22 Aristarchus Plateau: A volcanic uplift to the north containing the Moon’s brightest crater and the massive sinuous rille, Schröter’s Valley.
- L57 Reiner Gamma: A striking albedo “swirl” to the southwest (USGS ID: 4987), associated with a localized magnetic anomaly.
- L95 Procellarum Basin: The Moon’s largest basalt-filled plain (USGS ID: 4395). Modern GRAIL gravity data suggests its origin is tectonic rather than a single giant impact.
🚀 Mission Log
Target Acquisition — L42 Marius Hills
Start at Aristarchus, then work south into the dome field
Start at Aristarchus, the Moon’s brightest crater and an easy naked-eye-adjacent target in Oceanus Procellarum. Work south from there and the terrain roughens into a broad, low swarm of hundreds of small hills — this is the Marius Hills, centered near the modest 41 km crater Marius. There’s no single dramatic landmark to lock onto here; the field itself, an unusually dense cluster of small domes and cones on otherwise flat mare, is the target.
Chase the terminator to roughly 50°–55°W
These domes are subtle, low-relief volcanic constructs, typically only a few hundred meters tall — they vanish almost entirely under a high Sun and blend into the surrounding mare. Time your session for when the terminator falls near 50°–55° West longitude, which puts this stretch of Procellarum under the raking light it needs. Away from that window, don’t expect to find much here at all.
Work up in power to sort domes from cones, then hunt the rille
At 75x–100x, scan the field and note the variety: low, broad shield-like domes alongside steeper, more conical constructs, a mix that reflects real differences in lava viscosity and eruption style built up over a long volcanic history. Push to 150x+ and try to trace the Marius Rille, a sinuous channel likely marking a collapsed lava conduit. Deep photographic and orbital surveys have picked out a roughly 50-meter-wide pit within one of these rilles — the “Marius Hills Hole” — a probable lava-tube skylight, though it’s far too small to resolve visually from Earth.
Pair it with the rest of this volcanic corner of Procellarum
Aristarchus Plateau (L22), your starting anchor, adds the Moon’s brightest crater and the giant sinuous Schröter’s Valley to the north. Reiner Gamma (L57), the striking bright albedo swirl tied to a localized magnetic anomaly, sits to the southwest. Zoom out further and all of it sits within Oceanus Procellarum (L95) itself, whose rectangular gravitational boundary GRAIL data has linked to ancient rifting rather than a single giant impact — though the debate over its origin hasn’t fully settled.
📝 Observation Log — L42 Marius Hills
0/4 CompleteAre the Marius Hills visible tonight?
Because the Marius Hills domes rise only 200–500 m above Oceanus Procellarum, they need the low, glancing light of the terminator more than most targets — at this longitude (~54°W), the best window is fairly narrow: aim for Waxing Gibbous (roughly Day 11–12), right as the terminator passes through the field. Outside that window the domes flatten quickly as the sun climbs, though under excellent seeing faint traces can sometimes still be picked out a little beyond it. The opposite crossing falls close to New Moon and is much harder to use. Libration isn’t a major factor here, but steady seeing matters, since the shadows involved are subtle.
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When to Observe the Marius Hills
This is one of the subtler targets on the Lunar 100. The domes here are broad and gently sloped rather than tall and sharp, so the single most important variable isn’t aperture — it’s how low the sun is.
- For the Domes: You need grazing, near-terminator light. Descriptions of the region’s shield volcanoes note they’re only briefly well-seen around local sunrise or sunset — practically, that means catching this patch of Oceanus Procellarum right at the terminator, roughly four days either side of First or Last Quarter. Under a high sun, the domes essentially vanish into the surrounding mare.
- For Orientation: Use Marius crater itself (41 km, sharp-rimmed) as your anchor — the dome field sits to its northwest, so find Marius first and then hunt for texture in the mare beyond it.
- Aperture and Seeing: Larger aperture helps resolve individual domes, but steady seeing matters just as much, since you’re chasing subtle shadow relief on features only a few hundred meters tall.
What to Look For
Don’t expect one obvious landform — the Marius Hills complex is a roughly 200 x 250 km plateau holding the densest concentration of volcanic domes and cones anywhere on the Moon, over 250 of them. At the eyepiece this reads as a patch of unusually rough, mottled texture within the otherwise smooth mare, rather than any single dramatic shape.
The domes themselves are broad, shield-like rises, typically 200–500 m tall, thought to have built up from lava more viscous than the thin, far-spreading flows that formed the mare around them. Mixed in among the domes are steeper, more conical hills, generally interpreted as small cinder-cone-style constructs from more explosive eruptions rather than quiet effusion.
Look for narrow, winding channels threading between the domes — these are lava channels, cut by the same kind of flowing eruptions that built the sinuous rilles found elsewhere on the Moon, and they’re one of the clearest signs that this whole plateau was built by repeated, long-lived volcanic activity rather than a single event.
Somewhere in this field, invisible from Earth, sits the Marius Hills Hole: a roughly 50–65 m wide pit discovered by Japan’s Kaguya orbiter in 2009, sitting inside one of the sinuous rilles. It’s one of the best candidates anywhere on the Moon for a skylight into an intact lava tube — well worth knowing about even though no backyard telescope will ever resolve it.
The Science: The Moon’s Busiest Volcanic Field
The Marius Hills aren’t just the densest cluster of volcanic landforms on the Moon — recent gravity and radar data suggest they’re fed by a genuinely substantial plumbing system underneath, and they may be sitting on top of one of the best-documented lava tubes anywhere off Earth.
Same Lava, Different Behavior
Spectral data indicates the Marius Hills domes and cones are compositionally similar to the ordinary mare basalt surrounding them — unlike the Gruithuisen and Mairan domes elsewhere on the Moon, which have a distinct, more feldspar-rich signature. That means the unusual dome-building behavior here likely isn’t about different magma chemistry, but about how the lava erupted: current thinking favors differences in ascent rate or crystal content over the eruption’s course, rather than a fundamentally different source material.
A Lava Tube With Actual Radar Evidence
The Marius Hills Hole isn’t just a hole — Kaguya’s radar sounder later detected a subsurface echo pattern near the pit consistent with a cave-like void roughly 200 m down, adding genuine evidence for an intact lava tube beneath the surface rather than just a surface collapse feature. Because lava tubes could shield future explorers from radiation, micrometeorites, and the Moon’s extreme temperature swings, this specific pit has become one of the most closely studied potential outpost sites on the Moon.
What Hasn’t Been Resolved
The precise magma plumbing beneath the whole complex is still being actively mapped — a 2024 gravity study identified narrow, sheeted conduits tens of kilometers long linking separate intrusions under the northern and southern Marius Hills, but connecting that subsurface picture cleanly to the surface history of any individual dome or cone remains a work in progress. Similarly, dating individual domes precisely enough to build a detailed eruption timeline for the whole field is still an ongoing effort rather than a settled result.
Most Lunar 100 volcanic targets are one dome, one caldera, one rille. The Marius Hills are the rare case where the reward is quantity and density — a whole province of overlapping shields, cones, and channels that adds up to the Moon’s single richest record of how basaltic volcanism actually built its surface.