L20 Posidonius Crater

📉 Vital Statistics

🔭 Field Notes

Posidonius sits on the northeastern rim of Mare Serenitatis, its western wall so heavily eroded and breached by lava that it barely reads as a rim at all, while the southern and eastern ramparts stay sharp and terraced. There’s no central peak — instead the floor is hilly, slightly bulged from long-ago lava intrusion, and threaded by the Rimae Posidonius rille system. Researchers regard it as one of the Moon’s best-studied floor-fractured craters, with its formation still debated: most current evidence points to both magma intrusion beneath the floor and later mare volcanism from Serenitatis having shaped its present appearance.

• ▶ Rimae Posidonius — The Real Highlight: This sinuous rille runs over 130 km, hugging the base of the north wall before veering off on a southerly course. Its squiggly, river-like path makes it a hybrid of two rille types — the straight fractures typical of floor-fractured craters and the winding lava channels typical of sinuous rilles — and its eroded banks expose layers of the mare lava that once flowed through it.
• ▶ The “Double Wall” Effect: A smaller, semi-circular inner rim sits offset toward the eastern edge of the main wall. It isn’t a second impact — it’s a huge slab of the original rim that slumped and separated, creating the terraced “crater within a crater” look that’s easiest to see where the eastern wall is still intact.
• ▶ Posidonius A & B: A sharp, well-formed crater sits just west of center on the floor, while a second, smaller Posidonius B interrupts the northeast rim. Posidonius A’s crisp round shape makes it a favorite target for testing telescope resolution and a given night’s seeing conditions.
• ▶ Posidonius Gamma: A small peak on the neighboring Dorsa Smirnov ridge, topped by a summit crater with a faint halo of bright ejecta. Nineteenth-century cartographer Julius Schmidt first logged it for its resemblance to the brighter, better-known halo around the crater Linné.

📍 Nearby L100 Targets

• L18 Mare Serenitatis: The basin’s dark lava edges border Posidonius directly along its breached western wall — the mare essentially spills into the crater floor there, which is a large part of why that wall barely survives as a rim.
• L33 Dorsa Smirnov (Serpentine Ridge): A wrinkle-ridge system roughly 180 km southwest, tracing the eastern edge of Mare Serenitatis. Older accounts even trace its winding path back to a small crater tucked under Posidonius’s own northwest wall. It’s a useful contrast in mechanism: Posidonius’s rilles come from magma intruding beneath its floor, while the ridge is simply mare crust buckling under compression as it cooled.
• L34 Lacus Mortis: A 159 km flooded basin roughly 410 km north-northwest, sitting between Mare Frigoris and Lacus Somniorum. It’s the closest relative on the Lunar 100 list — also a floor-fractured crater later flooded by mare basalt — though it expresses the uplift as a graben system and a candidate lava-tube skylight rather than Posidonius’s single dominant rille.