L53 Lamont
A ghostly ring barely traceable on the floor of Mare Tranquillitatis, outlined by faint ridges rather than any raised rim — very likely the buried remains of an ancient basin drowned beneath later lava flows.
Source: USGS Gazetteer of Planetary Nomenclature & Sky & Telescope Lunar 100 (Wood, 2004)
L53 Lamont
Mare Tranquillitatis · Buried Basin📉 Vital Statistics
🔭 Field Notes
Lamont sits quietly in Mare Tranquillitatis southeast of Arago, and at first glance there’s nothing there to see — no rim, no bowl, no shadow-casting wall. What you’re actually looking for is a faint system of low wrinkle ridges tracing out two incomplete, roughly concentric rings, an inner one about 60 km across and an outer one closer to 120 km. Catching them takes the right lighting and more than a little patience: under a low sun near the terminator, the ridge network can resolve into a spidery, radial pattern reaching out from the center in every direction except due east and west, but at almost any other illumination it disappears into the general clutter of mare ripples around it.
- ▶ A Crater That Isn’t: Lamont has no true topographic bowl at all — it’s the ghost of one. An ancient impact basin, likely somewhere in the 70–80 km range originally, was buried under Tranquillitatis’ lava flows so completely that only a ring of wrinkle ridges standing a few hundred meters high, tracing the old rim’s outline, survives on the surface today.
- ▶ The Mascon Underneath: Lamont carries its own mascon — a concentration of denser material beneath the surface, left over from the impact and later lava fill — though on a much smaller scale than the mascons under giants like Imbrium or Serenitatis. Doppler tracking of orbiting spacecraft first revealed the region’s gravity anomaly, and the 2012 GRAIL mission later mapped it with far greater precision, providing strong support for reading Lamont as a genuine buried basin rather than just a coincidental ridge pattern.
- ▶ A Color Anomaly: Clementine color-ratio imagery showed something odd — Lamont’s center reads the same reddish hue as the rest of Tranquillitatis, but the ridge ring around it shows up distinctly blue. The contrast itself is well documented; what it means is less settled, with the leading interpretation pointing to a compositional or textural difference in the lava that settled along the old buried rim versus the basin interior.
📍 Nearby L100 Targets
- L32 Arago α and β: A pair of low volcanic domes beside the crater Arago, roughly 65 km northwest of Lamont’s ring. Arago itself is the obvious reference point for locating Lamont in the eyepiece — find the crater, then look southeast for the faint ridges.
- L38 Sabine and Ritter: A matched pair of similarly sized, flat-floored craters roughly 140 km southwest, sitting at the western edge of Tranquillitatis near the Apollo 11 landing region. They make a useful staging point before hunting for Lamont’s much subtler ring further east.
- L90 Armstrong, Aldrin, Collins: Three tiny craterlets named for the Apollo 11 crew, clustered near Tranquility Base a little further southwest of Sabine. Like Lamont, they’re a test of patience — small, low-relief features that reward careful observing under the right light rather than casual glancing.
🚀 Mission Log
Target Acquisition — L53 Lamont
Anchor on Arago, then drop southeast into open mare
Start at Arago, a sharp-rimmed 27 km crater in southwestern Mare Tranquillitatis — easy to hold at low power. Lamont’s ring system sits just southeast of it, with no crater, rim, or wall of its own to catch your eye; you’re dropping into what looks like ordinary open mare. Sabine and Maskelyne sit further southwest and southeast respectively and make useful secondary markers if you lose your place, but Arago is the anchor to keep coming back to.
Time it for the terminator crossing ~23°E, and be ready to try twice
Lamont has no rim to throw a shadow, so its faint ridge network only shows up with the sun almost skimming the surface. Track the terminator itself rather than counting days — you want it crossing roughly 23°E in southwestern Tranquillitatis, which happens once on the morning terminator during the waxing phase and once on the evening terminator during the waning phase. Exact timing shifts with libration, and the two passes don’t always show the ridges equally well, so treat a first attempt as a scouting run and plan on a second session under the opposite terminator.
Work up in power to trace the rings and spokes
At 100x–150x, look for two incomplete, roughly concentric rings of low wrinkle ridges — an inner one around 60 km across, an outer one closer to 120 km, both approximate. Push further and watch for ridges radiating outward from the center like spokes, with a noticeable gap in the east and west quadrants and some thickening to the southeast. Steady seeing matters more here than raw aperture, since you’re resolving relief only a few hundred meters high standing against a sea of mare ripples.
Pair it with the rest of the Tranquillitatis neighborhood
Arago α and β (L32), a pair of low volcanic domes right beside your anchor crater, make a natural first or last stop on the same session. Further southwest, Sabine and Ritter (L38), a matched pair of flat-floored craters near the Apollo 11 landing region, and the tiny Apollo 11 craterlets Armstrong, Aldrin, and Collins (L90) round out the region — all low-relief, patience-testing targets that share this same corner of the Moon.
📝 Observation Log — L53 Lamont
0/4 CompleteIs Lamont visible tonight?
Lamont lies in Mare Tranquillitatis west of Ritter and Sabine, not far from the Apollo 11 landing site, at 4.4°N, 23.7°E — a modest eastern longitude that keeps it well clear of any limb effects. Best viewing is around Waxing Crescent to First Quarter (roughly Day 5–6), and unusually for this list, getting a very low sun angle matters more than the exact day: Lamont’s ghostly ring of ridges is only faintly raised and needs a near-grazing terminator to show at all. Libration is not a meaningful factor at this longitude.
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When to Observe Lamont
Lamont is the single flattest target on the Lunar 100 — there’s no rim to catch the light, no crater wall to throw a shadow. What you’re hunting is a ring of ridges only a few hundred meters tall, and it only reveals itself under the most extreme grazing light.
- For the Ring System: You need the sun almost skimming the surface. Lamont sits around 23°E in southwestern Mare Tranquillitatis, so its best light comes when the terminator itself is crossing that longitude — either the morning terminator during the waxing phase or the evening terminator during the waning phase. Exactly which night that falls on shifts a little with libration, so track the terminator’s position rather than counting on a fixed day. Outside that narrow window, the ridges wash out completely into the surrounding mare.
- For Orientation: Anchor on Arago crater (27 km, sharp-rimmed) to the northwest — Lamont’s ring system sits just southeast of it. Sabine and Maskelyne are useful secondary markers nearby.
- Aperture and Seeing: This is a patience target more than an aperture target. Moderate-to-large scopes help, but steady seeing matters more, since you’re resolving a faint ring of relief that barely rises above the lava that buried it.
What to Look For
Lamont isn’t a crater in the usual sense — it’s the ghost of one. Look for two roughly concentric but incomplete rings of low ridges. Most descriptions put the inner ring around 60 km across and the outer one around 120 km, though the exact numbers shift depending on which ridge segments a given source counts as part of the system — the catalogued diameter of the core feature itself is around 75–83 km. Treat the figures as approximate rather than a precise, agreed-upon boundary.
Beyond the rings, ridges are described as radiating outward from Lamont’s center like spokes — with a notable absence in the east and west quadrants. The ridges themselves are on the order of a few hundred meters high and about 5 km wide on average, though accounts note they thicken to the southeast.
Lamont sits directly on the axis of a north-south topographic trough, roughly 1,500 m deep, running through western Mare Tranquillitatis. It’s been proposed as a synclinal fold in the lunar crust, and the ridge pattern around Lamont is thought to reflect the combined stress of that fold plus the load of whatever’s sitting underneath it.
Beneath the flat mare surface, Lamont sits within a positive free-air gravity anomaly — a mascon, a buried concentration of denser material. Nothing at the eyepiece hints at this directly, but it’s part of why Lamont made the Lunar 100 list at all: a nearly featureless patch of mare that turns out to be one of the best surface-visible clues to buried structure and tectonics anywhere on the near side.
The Science: A Buried Basin Hiding in Plain Sight
Lamont is less a landform than a case file. Every visible ridge is a clue pointing toward something that’s been erased from the surface but never quite disappeared underneath it.
A Crater Swallowed by Lava
The leading interpretation is that Lamont was once an impact crater 70–80 km across, later flooded and buried by the same basaltic lavas that filled the rest of Mare Tranquillitatis — a fate similar to Grimaldi. What survives at the surface isn’t a rim at all, but a ring of wrinkle ridges marking where the buried structure still pushes back against the crust above it. It’s the most-cited explanation, though the exact impact history behind it isn’t universally settled.
The Mascon Beneath the Mystery
Gravity modeling by Dvorak and Phillips (1979) fit both high- and low-altitude gravity data to place nearly all of Lamont’s anomalous mass in the subsurface, supporting the idea that Lamont is a genuine mascon rather than just a surface pattern. That buried mass, combined with the load it places on the lithosphere, is proposed as the driver of the radial and concentric ridge fabric seen at the surface.
What Hasn’t Been Resolved
The exact combination of forces producing Lamont’s ridge pattern is still debated — the synclinal-fold hypothesis and the mascon-loading hypothesis aren’t mutually exclusive, but pinning down how much each contributes to the specific geometry of the rings and radial ridges remains unsettled. Likewise, whether Lamont represents a single buried crater or something more complex in its impact history is still an open question.
Most Lunar 100 targets reward you with something dramatic to look at. Lamont rewards you with the opposite — proof that some of the Moon’s most significant structures left almost no visible trace at all, and that the flattest patch of mare in your eyepiece might be sitting on top of one of its best-documented buried basins.