L72 Atlas Crater
An 87 km impact crater in the Moon’s northeastern quadrant — its dark lava-flooded floor hides a central peak complex, winding volcanic rilles, and rare dark-halo pyroclastic pits, best seen paired alongside neighbour Hercules.
L72 Atlas
Northeastern Highlands · Floor-Fractured Crater📉 Vital Statistics
🔭 Field Notes
Atlas sits in the northeastern highlands southeast of Mare Frigoris, forming one of the Moon’s most recognizable crater pairs alongside Hercules, its slightly smaller neighbor just to the west. Where Hercules has a comparatively smoother floor with localized lava flooding, Atlas is one of the busiest crater interiors on the Moon: a multiply terraced inner wall with a slumped, sharp-edged lip, a hummocky floor threaded by delicate rilles, and a cluster of low central hills rather than one dramatic peak. T. W. Webb called it “a superb amphitheatre” with “its ring rich in terraces and towers” — a description that still holds up at the eyepiece today.
- ▶ Dark-Halo Craters — The Real Highlight: Along the floor’s northern and southeastern edges sit two dark patches, informally called Atlas North and Atlas South, surrounding small dark-halo craters widely interpreted as volcanic vents produced by explosive eruptions. These are believed to mark spots where eruptions, rather than impacts, deposited dark pyroclastic glass-rich material — a genuine sign of past lunar volcanism hiding inside an impact crater.
- ▶ Rimae Atlas: A cobweb-like system of slender rilles, cataloged as Rimae Atlas I through V, crosses the crater’s floor. They begin near the southeastern wall and branch outward in broad arcs, a pattern generally attributed to magmatic intrusion beneath the crater floor, with associated volcanic activity responsible for the dark-halo craters nearby.
- ▶ Floor-Fracturing From Below: Atlas is one of the Moon’s many floor-fractured craters, a class generally attributed to magma intruding beneath the original impact floor after formation, uplifting and cracking it rather than simply flooding it from above. Numerous boulders and rockfalls are especially evident along the crater’s southwest interior wall.
📍 Nearby L100 Targets
- L34 Lacus Mortis: A 159 km flooded, floor-fractured crater roughly 360 km southwest, between Mare Frigoris and Lacus Somniorum. Both targets show post-impact volcanic modification of a crater floor, though Lacus Mortis expresses it as a graben system and a candidate lava-tube skylight rather than Atlas’s dark-halo vents.
- L20 Posidonius: A 95 km floor-fractured crater roughly 565 km south, on the northeastern rim of Mare Serenitatis. It’s a useful comparison in the same broad family of features — both are craters whose floors were uplifted and cracked by magma from below, just expressed differently on the surface.
- L33 Serpentine Ridge: A winding compressional wrinkle ridge roughly 765 km south-southwest, crossing the eastern floor of Mare Serenitatis. It’s the most distant of the three, but offers a contrast in mechanism: Atlas’s rilles and dark-halo craters came from magma intruding beneath a crater floor, while the ridge is simply mare crust buckling under its own cooling weight.
🚀 Mission Log
Target Acquisition
Anchor on Mare Frigoris, then find the crater pair
Locate Mare Frigoris, the long, dark lava plain running east-west through the Moon’s northern highlands. Atlas sits just southeast of its curved southern edge, forming one of the Moon’s most recognizable crater pairs alongside Hercules, its slightly smaller neighbor immediately to the west. Atlas is the larger, more rugged, dark-floored crater of the two.
Time it to a low, oblique terminator
Atlas’s interior is a relief feature through and through, so avoid full Sun. Best viewing falls around Day 4 waxing (morning terminator) or roughly Day 19 waning (evening terminator), when the low light throws long shadows across the terraced walls and hummocky floor. Wait too close to full Moon and the floor loses much of its relief, making the rilles and terraces far harder to distinguish.
Work up in power to separate the floor’s competing features
At 100x–200x, resolve the multiply terraced inner wall and the cluster of low central hills — Atlas replaces the single central peak seen in many large craters with this cluster instead. Then look for the cobweb-like Rimae Atlas rille system branching outward from the southeastern wall, and finally the two dark patches informally called Atlas North and Atlas South, each surrounding small dark-halo craters interpreted as pyroclastic vents — the real L100 highlight here, and often the hardest of the three to pin down.
From Atlas’s rilles to Lacus Mortis, Posidonius, and the ridge
Roughly 360 km southwest, Lacus Mortis (L34) is another floor-fractured crater, but expresses its post-impact volcanic modification as a graben system and a candidate lava-tube skylight rather than Atlas’s dark-halo vents. About 565 km south, Posidonius (L20) is a useful direct comparison — a floor uplifted and cracked by magma from below in the same broad family as Atlas, just expressed differently on the surface. And roughly 765 km south-southwest, the Serpentine Ridge (L33) offers a contrast in mechanism entirely: not the result of magma intruding beneath an impact crater floor, but mare crust buckling under its own cooling weight after emplacement.
📝 Observation Log: Atlas (L72)
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Atlas (L72) – Observation & Geology Guide
Floor-Fractured Volcanic Complex🔭 Optimal Observation Windows
Atlas (87 km diameter) is located at 46.7°N, 44.4°E in the northeastern quadrant, southeast of Mare Frigoris. Its dark lava-flooded floor and subtle volcanic features require low solar incidence angles for maximum relief and contrast.
- ▶ Best Viewing: ~4–5 days after New Moon (waxing crescent to first quarter/waxing gibbous) for morning terminator illumination, or ~3–4 days after Full Moon (waning gibbous to last quarter) for evening terminator. Avoid Full Moon—features wash out under high-angle light.
- ▶ The Dynamic Duo: Forms a prominent pair with Hercules (west). Atlas shows more extensive lava flooding and volcanism; Hercules is more rugged and less modified—ideal for comparative geology.
🔍 What to Look For (100x–200x+)
- 1. Dark Lava-Flooded Floor: Basaltic infill creates near-black albedo contrast. Challenge: Resolve faint dark-halo craters (pyroclastic deposits from explosive eruptions)—subtle but diagnostic of late-stage volcanism (L72 highlight).
- 2. Rimae Atlas & Central Hills: Winding volcanic rilles (lava channels) cross the floor; complex, multi-hill central peaks (not a single summit) stand out under low Sun, casting sharp shadows.
- 3. Terraced & Slumped Walls: Multiply terraced inner rims slump inward, producing jagged, layered textures. Sunrise/sunset lighting casts dramatic saw-tooth shadows onto the floor.
🧪 The Science: Law of Superposition
Atlas exemplifies lunar stratigraphic sequencing via the Law of Superposition (younger units overlie older ones), revealing impact + volcanism history.
- 1. Oldest: Pre-existing highland crust and regional basin-forming impacts (Imbrium-related ejecta influence).
- 2. Middle: Atlas impact (~Upper Imbrian) excavates terraced walls and central peaks.
- 3. Youngest: Basaltic lava floods floor; later volcanism forms Rimae Atlas rilles and dark-halo pyroclastic deposits (fire-fountain eruptions). Floor fracturing indicates uplift/subsidence from magma intrusion.
Volcanic modification post-dates the impact, distinguishing Atlas from less-altered neighbors (e.g., Hercules). This records the transition from major basin flooding to localized explosive activity.