L49 Gruithuisen Delta & Gamma
Two steep-sided domes rising abruptly from the mare near the crater Gruithuisen, built from lava thick enough to pile up rather than spread flat — likely evidence of a more evolved, silica-rich magma than typical mare basalt.
Source: IAU/Gazetteer & Sky & Telescope Lunar 100 (Wood, 2004)
L49 Gruithuisen Delta & Gamma
Imbrium / Procellarum Border · Silicic “Red Spot” Domes📉 Vital Statistics
🔭 Field Notes
The Gruithuisen domes take their name from the small crater Gruithuisen — the crater itself is just a landmark, unrelated to the volcanic story unfolding beside it. What you’re actually looking for is a pair of unusually steep, tall mountains rising abruptly from the flat lava plain where Mare Imbrium meets Oceanus Procellarum: Delta to the southeast and Gamma to the northwest, with a third, smaller dome (informally “NW”) completing the trio.
- ▶ The “Red Spots”: Long before spacecraft visited, Earth-based observers noticed that Gruithuisen Delta and Gamma stood out from ordinary lunar domes — unusually high albedo, a strong absorption in the ultraviolet, and local slopes reaching roughly 18–20°, far steeper than the broad, gently-sloped domes found in mare regions like Hortensius. That combination earned them the nickname “red spots” long before their composition was understood.
- ▶ Granite Without Water: LRO’s Diviner instrument found the domes have a short-wavelength thermal signature indicating a highly silicic composition, low in iron and titanium but enriched in thorium — chemically closer to terrestrial rhyolite or dacite than to any nearby mare basalt. On Earth, silicic magmas like these form with help from oceans of water and plate tectonics; the Moon has neither, which is exactly what makes the domes a standing puzzle.
- ▶ A Story Still Being Written: Gravity data from the GRAIL mission has since turned up several additional low-density masses buried beneath the surrounding plains — possible magma chambers or wholly buried silicic domes suggesting multiple phases of eruption rather than one isolated event. Competing models propose remelted crustal material or fractional crystallization of a deep magma body, but no Apollo or Luna mission ever sampled a silicic target, so the debate has stayed unresolved from orbit alone.
📍 Nearby L100 Targets
- L14 Sinus Iridum: The ~249 km flooded impact bay whose Montes Jura arc lies to the north-northeast of the domes, on the opposite side of the Imbrium/Procellarum shoreline. Its “missing rim” is a lesson in a crater drowned by mare lava; the Gruithuisen domes, by contrast, are volcanic mountains that rose after the flooding — two very different ways the same lava flood left its mark on the landscape.
- L86 Prinz Rilles: A complex fan of sinuous rilles originating on the flanks of the flooded crater Prinz, well to the south near the Aristarchus Plateau. Both sites involve lava-related volcanism, but the rilles mark thin, fast-flowing basaltic lava channels — the fluid opposite of the thick, slow-moving silicic lava that built the domes.
- L98 Imbrium Lava Flows: The subtle flow-front boundaries within Mare Imbrium itself, best traced under low sun in the plains the domes overlook. These mark ordinary basaltic flooding events spread across a long span of time — the same general mare-building process the Gruithuisen domes stood apart from, both compositionally and in the shape they took as they erupted.
🚀 Mission Log
Target Acquisition — L49 Gruithuisen Delta & Gamma
Find the peninsula, then the mountains at its tip
Locate Sinus Iridum first, then look south along the highland peninsula that projects into Oceanus Procellarum, on the western rim of the Imbrium basin. Gruithuisen Delta and Gruithuisen Gamma sit at the tip of that peninsula, named for the nearby crater Gruithuisen — Gamma to the west and smaller, Delta the larger dome to its east, with a third, smaller dome (informally “NW”) completing the trio.
Catch them near the terminator, not under high sun
Seen from Earth, these domes appear foreshortened — Rükl described Gamma as looking like an “upturned bathtub.” Under high sun they flatten out and lose their character; low, grazing light near the terminator reveals the real steepness of their slopes, with local grades reaching roughly 18–20° — far steeper than the broad, gentle mare domes found elsewhere, like Hortensius.
Work up in aperture for the summit crater
At moderate-to-large aperture, you should be able to separate the two domes cleanly and judge their relative size and steepness by eye — Delta may actually be three smaller domes grown together into one larger mass. On your best night of seeing, try for the small summit crater near the top of Gamma, roughly 900 m across; a much smaller crater near that same summit was formally named Mareta by the IAU in 2025.
Pair it with the Imbrium/Procellarum shoreline
Sinus Iridum (L14) lies to the north-northeast, on the opposite side of the same lava shoreline — its flooded, “missing rim” is a lesson in a crater drowned by mare lava, a useful contrast to these volcanic mountains that rose after the flooding. Farther south near the Aristarchus Plateau, the Prinz Rilles (L86) mark thin, fast-flowing basaltic lava channels — the fluid opposite of the thick, slow-moving silicic lava that built the domes. Closer at hand, the subtle flow fronts of the Imbrium lava flows (L98) trace the ordinary basaltic flooding these domes stood apart from, both in composition and in the shape they took.
📝 Observation Log — L49 Gruithuisen Delta & Gamma
0/4 CompleteAre the Gruithuisen domes visible tonight?
The Gruithuisen domes rise from the mare just south of Mare Frigoris, near the crater Gruithuisen itself, at 36.3°N, 40.0°W — far enough west and north that they reward some patience. Best viewing comes around Waning Gibbous (roughly Day 11), when a low terminator sun brings out the domes’ steep, rounded profiles against the flatter surrounding mare. A favorable westward libration is genuinely useful here, easing the foreshortening that comes with this far-west longitude and giving the low domes a bit more relief to work with.
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When to Observe Gruithuisen Delta & Gamma
These two domes look nothing like the flat, gentle mare domes seen elsewhere on the Moon — they’re genuinely mountainous, and that shape is exactly what makes them worth tracking down.
- For Orientation: Locate Sinus Iridum first, then look south along the highland peninsula that projects into Oceanus Procellarum. The domes sit at the tip of that peninsula, on the western rim of the Imbrium basin, named for the nearby crater Gruithuisen.
- For Their True Shape: Seen from Earth, the domes appear foreshortened — Rükl described Gruithuisen Gamma as looking like an “upturned bathtub.” Catching them near the terminator, with the sun low, helps reveal the real steepness of their slopes rather than the flattened look you get under a high sun.
- Aperture and Seeing: Moderate-to-large aperture helps separate the two domes clearly and gives you a shot at the small summit crater on Gamma — a genuine test of resolution at the Moon’s distance.
What to Look For
Gamma, the western and smaller of the pair, and Delta, the larger dome to its east, both rise roughly a kilometer or more above the surrounding terrain — steep-sided and rounded in a way ordinary mare domes never are. Delta may actually be three smaller domes that grew together into one larger mass.
Near the top of Gruithuisen Gamma sits a small summit crater roughly 900 m across — a genuine resolution challenge at the Moon’s distance. A much smaller crater near that same summit was formally named Mareta by the IAU in 2025.
Close-up orbital imagery shows the domes’ slopes covered in irregular furrows running straight down-slope, a texture distinct from both the smooth highland terrain and the crater walls nearby. It won’t resolve in a backyard telescope, but it’s worth knowing you’re looking at genuinely unusual, rough terrain rather than an ordinary smooth-sided hill.
At the base of Gruithuisen Gamma, right where the dome material meets the surrounding mare, orbital images show a small rille hugging the contact line — a boundary marking where later lava flows lapped up against the older dome.
The Science: Volcanoes That Don’t Belong on the Moon
Nearly every volcanic feature on the Moon is basaltic — iron- and magnesium-rich, thin, and runny. Gruithuisen Gamma and Delta are something else entirely, and that mismatch is still not fully explained.
“Nonmare” Volcanism and Red Spots
Earth-based telescopes, Lunar Prospector gamma-ray data, and Clementine spectral data all agree that the Gruithuisen domes are made of something different from either the highlands or the mare. They show high albedo, strong absorption in the visible and ultraviolet, and notably low iron and titanium compared to mare basalt — the signature of what lunar scientists call a “red spot.”
A Lunar Analog to Earth’s Silicic Volcanoes
The domes’ steep slopes and spectral properties are consistent with silicic or evolved volcanic compositions, analogous to terrestrial rhyolite- or dacite-like systems — lavas far more viscous and slower-erupting than the basalts that built the mare. That higher viscosity is a plausible reason Gamma and Delta piled up into steep mountains instead of spreading out into the broad, flat domes seen elsewhere on the Moon, though the exact lunar rock composition remains inferred from remote sensing rather than directly sampled.
Old Domes, Younger Neighbors
Crater counts date the Gamma and Delta domes to around 3.8 billion years old, roughly contemporaneous with the nearby Iridum basin-forming impact. The mare plains that now surround and partly embay the domes are younger still, with ages ranging from about 2.3 to 3.6 billion years — meaning these silica-rich mountains were already standing before much of the basaltic mare around them had even formed.
What Hasn’t Been Resolved
On Earth, silicic magmas are often associated with water-rich magmatic systems and long-term crustal reprocessing through plate tectonics — neither of which the Moon is thought to have in comparable amounts. That leaves the details of the domes’ formation as an active area of research: some researchers suspect water may have played a larger role in the lunar interior than previously assumed, while others favor a longer, hotter period of magma evolution without water’s involvement. NASA’s Lunar-VISE mission, currently planned to land on Firefly’s Blue Ghost Mission 3 at Gruithuisen Gamma, is intended to gather the surface data needed to help narrow down which explanation fits.
Most Lunar 100 volcanic targets show you basalt behaving in familiar ways. Gruithuisen Delta and Gamma show you the Moon breaking its own rules — two genuine mountains built from lava that, by every expectation of how the Moon should work, shouldn’t have been able to form at all.