L23 Mons Pico
A solitary 2.4 km peak standing alone on the flat basalt of Mare Imbrium, south of the crater Plato — a surviving fragment of the basin’s buried inner mountain ring, now an island in lava with nothing around it to share its shadow. At low sun the isolated massif throws a long, knife-edged silhouette clear across the mare, looking taller than its true height suggests.
L23 Mons Pico
Northern Mare Imbrium📉 Vital Statistics
🔭 Field Notes
Mons Pico stands as one of the most striking isolated peaks on the Moon — a solitary massif rising nearly 2.4 km above the flat dark lava of northern Mare Imbrium, casting a long, sharp shadow across the plains at low Sun angles. It is not a volcano, but a fragment of one of the Imbrium basin’s inner mountainous rings: ancient highland crust that was tall enough to project above the mare basalt that flooded and buried the ring in the billions of years following the Imbrium impact. Most of that ring is now invisible beneath the lava. Mons Pico is one of the few places where it still breaks the surface. Near lunar sunrise, it casts one of the Moon’s longest mountain shadows, stretching far across the floor of Mare Imbrium before shrinking rapidly as the Sun climbs — making it one of the most rewarding targets to catch near the terminator.
- ▶ Inner Ring Fragment: Mons Pico is generally interpreted as part of an inner mountainous ring of the Imbrium multi-ring basin, approximately 650 km in diameter as estimated from surviving exposures, formed ~3.86 Ga ago. The ring is interpreted as continuing through Montes Teneriffe and Montes Recti to the northwest, with Montes Spitzbergen probably representing another exposed segment to the southeast. Between those ranges the ring is submerged below the mare lava to depths of several kilometres, leaving Mons Pico and a handful of comparable massifs as the only surface evidence of its arc.
- ▶ Ghost Crater Setting: Mons Pico appears to lie along the rim of an ancient, lava-flooded ghost crater — an impact structure so thoroughly buried by Imbrium lavas that only the faintest arc of its rim survives as a low, curved ridge detectable in excellent seeing under very oblique illumination. This structure is occasionally referred to in historical observing literature as “Ancient Newton,” a name Schröter applied to it before the designation Newton was moved to its current location.
- ▶ Mons Pico β: A secondary peak roughly 25 km to the south rises approximately 1,900 m above the mare — somewhat lower than Pico itself, but still a conspicuous target in a moderate telescope. Together the two peaks dominate the northern Imbrium plain between Plato and the wrinkle ridges that furrow the mare around them. Under low terminator light, the area surrounding both peaks is crossed by subtle compressional wrinkle ridges caused by the cooling and contraction of the lava fill.
📍 Nearby L100 Targets
- L14 Sinus Iridum (Bay of Rainbows): A vast semicircular bay roughly 250 km across, opening onto the northwestern shore of Mare Imbrium. It is the interior of an ancient impact crater — Crater Iridum — whose southern wall was breached and completely erased by the Imbrium lava floods, leaving only Montes Jura to mark the surviving arc of the original rim. At the two horns of the bay, Promontoria Heraclides (northwest) and Laplace (northeast) jut into the mare. Sinus Iridum is most spectacular near the day 10–11 terminator, when the arc of Montes Jura rises in brilliant illumination while the floor of the bay remains in darkness, creating the famous “jewelled handle” effect visible even in binoculars.
- L27 Archimedes: A large, flat-floored crater roughly 83 km across, lying on the southeastern floor of Mare Imbrium. Unlike geologically younger craters of comparable size, Archimedes has no central peak and lacks the prominent terracing typical of younger craters — its interior was completely flooded by Imbrium mare basalts not long after the crater formed, smoothing the floor to a level plain and burying whatever central rebound structure existed. Its clean, sharp rim contrasts dramatically with the featureless dark interior visible through even a small telescope. Montes Spitzbergen — another interpreted fragment of the same Imbrium inner ring as Mons Pico — lies roughly 80 km to its north.
- L19 Alpine Valley (Vallis Alpes): A linear graben roughly 166 km long and up to 10 km wide that cuts straight through the Montes Alpes, connecting Mare Imbrium to Mare Frigoris. It is one of the Moon’s clearest examples of a graben — a block of crust dropped between two parallel faults as the Imbrium basin’s crust settled after the impact. Running along the valley floor is a slender sinuous rille, far narrower than the valley itself and detectable only in very good seeing at high magnification; its origin remains uncertain, with both volcanic and tectonic mechanisms proposed. The valley’s flat floor and almost perfectly straight walls give it an almost engineered appearance that made it one of the most remarked-upon features in the early history of telescopic lunar observation.
🚀 Mission Log
Target Acquisition
Start at Plato — the unmistakable black eye on the northern mare
Plato is your anchor. Its flat, near-black floor makes it one of the easiest craters on the Moon to identify at any magnification — a dark oval sitting right on the northern shore of Mare Imbrium where the highlands meet the lava plain. You can pick it up in binoculars without effort. Once Plato is in the field, you are in the right neighbourhood. Mons Pico sits on the open mare floor to Plato’s southeast, well clear of the crowded mountain terrain to the north.
Sweep southeast from Plato onto the mare floor
From Plato, move your field southeast onto the flat dark expanse of northern Mare Imbrium. Near the terminator, Mons Pico will announce itself before you’ve finished looking — the shadow it throws at low Sun is disproportionately long for its footprint, stretching far across the mare away from the rising Sun. That shadow is often the first thing you see: a dark spike driving into the lava plain from what appears to be nothing. At 50x – 75x the peak itself resolves as a bright, compact massif.
Identify Mons Pico β to the south
Once you have the main peak, look roughly 25 km to the south for the secondary summit, Mons Pico β. It is lower — around 1,900 m above the mare versus Pico’s 2,400 m — and throws a shorter shadow, but under good terminator lighting it is a distinct and separate peak rather than a shoulder of the main massif. Confirming both peaks in the same field is satisfying and also orients you correctly for the next step: the wrinkle ridges that furrow the mare between Pico and the distant Apennines.
Look for the ghost crater arc and surrounding wrinkle ridges
Under the best terminator conditions — very low Sun, steady seeing, 100x or more — scan the mare around Pico for the faint curved ridge thought to mark part of the rim of a buried ghost crater associated with the Pico massif. This is not a routine catch; it requires excellent conditions and a deliberate search rather than a casual glance. More accessible are the compressional wrinkle ridges crossing the mare between the two peaks and extending south toward the Apennine foothills — low, sinuous swells that are invisible under high Sun but cast clear shadows near the terminator.
📝 Observation Log — L23 Mons Pico
0/4 CompleteIs Mons Pico visible tonight?
Mons Pico is overwhelmingly a terminator target. The dramatic shadow it casts across northern Mare Imbrium is best caught around First Quarter (Day 7–8) or Last Quarter (Day 21–22) when low raking light brings the peak and the surrounding wrinkle ridges into sharp relief. Under high Sun it becomes a modest bright spot with little to distinguish it.
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When to Observe Mons Pico
Mons Pico is almost entirely a terminator target. Its isolation on the flat lava plain of northern Mare Imbrium means there is nothing nearby to provide context or contrast under high Sun — it becomes a modest bright spot indistinguishable in character from dozens of other small elevated features on the mare. Near the terminator, everything changes: the shadow it casts across the lava plain is one of the most dramatic produced by any isolated mountain on the near side, and the surrounding wrinkle ridges and Mons Pico β both become accessible targets in the same low-power field.
- The Shadow and Main Peak: Best around Day 7–8 (First Quarter) or Day 21–22 (Last Quarter), when the terminator crosses northern Mare Imbrium. At lunar sunrise the shadow projects dramatically across the mare away from the rising Sun — often the first thing visible before the peak itself resolves at 50x–75x.
- Mons Pico β and Wrinkle Ridges: Same terminator window. The secondary peak, roughly 25 km to the south, becomes distinguishable as a separate summit at moderate magnification. The compressional wrinkle ridges crossing the mare between the peaks and extending southward become extremely difficult or impossible to detect visually under high Sun — catch them in the same terminator session.
What to Look For
At 50x–75x near the terminator, Mons Pico announces itself before you have finished looking — the shadow it throws across the mare floor is disproportionately long for a feature of its basal footprint, projecting dramatically away from the rising Sun before the peak itself is easily resolved as a bright compact massif. Watching the shadow shorten across a single evening session as the Sun climbs is one of the more tangible real-time observations available at the lunar telescope. The rate of change is measurable over intervals of 30–45 minutes, and sketching or photographing the shadow length at two or three points in the session makes the change concrete.
Roughly 25 km south of the main peak, Mons Pico β rises to approximately 1,900 m above the mare floor — lower than Pico’s ~2,400 m, but tall enough to cast its own distinct shadow near the terminator and to resolve as a separate summit rather than a shoulder of the main massif. Confirming both peaks as distinct features in the same field at 50x–100x is one of the cleaner double-peak observations available on the northern mare. Under high Sun both peaks flatten to undistinguished bright spots and the separation between them becomes difficult to appreciate.
Under low raking terminator light at 100x, the flat lava plain surrounding both peaks is revealed to be anything but flat. Compressional wrinkle ridges cross the mare between and around the two peaks — low, sinuous swells caused by the cooling and lateral contraction of the lava fill after the basin flooded. They are among the subtler targets in this part of Mare Imbrium but are accessible on a steady terminator night, winding southward toward the Apennine foothills. Under high Sun they become extremely difficult or impossible to detect visually, leaving no trace of the relief that was so apparent in raking light.
Step back to low power — 30x–50x — and take in the full setting. Mons Pico and Mons Pico β sit in open northern Mare Imbrium between Plato to the northwest and the Montes Spitzbergen group to the southeast, with no significant topographic feature nearby at the same scale. That isolation is not accidental: Pico is a fragment of one of the Imbrium basin’s inner mountainous rings, one of the few places where ancient highland crust survived above the level of the lava plain that buried the rest of the ring. Most of that ring is invisible beneath the mare. Pico is what sticking up looks like.
The Science: A Mountain That Should Not Be There
Mons Pico is not a volcano. It is not a crater rim. It is a piece of the original highland crust of the Moon that survived the flooding of Mare Imbrium by being tall enough to stick out above the lava surface — a geological accident of altitude. Understanding why it is where it is requires understanding what Mare Imbrium actually is.
The Imbrium Impact and Its Ring Structure
Mare Imbrium occupies the interior of one of the Moon’s largest and best-preserved impact basins, formed roughly 3.85 billion years ago when an impactor of extraordinary scale struck the near side. The impact excavated a multi-ring basin structure — concentric rings of uplifted crust at different radii from the impact centre, analogous to the ripples spreading outward from a stone dropped in water, but frozen in rock. These rings are now expressed as the mountain ranges that form the spectacular border of Mare Imbrium: Montes Apenninus, Montes Caucasus, Montes Alpes. But a smaller, inner ring also formed, closer to the basin centre, and that ring is now almost entirely buried beneath the mare lavas that flooded the basin in the billion years following the impact. Mons Pico is one of the few surviving remnants of that inner ring — ancient highland crust that was tall enough to project above the lava surface while the rest of the ring drowned.
Why the Rest of the Ring Is Gone
The inner ring of Imbrium did not disappear all at once. Over the billion-plus years following the impact, successive eruptions of low-viscosity basaltic lava poured onto the basin floor from vents in the crust, gradually raising the level of the lava plain. Each eruption buried a little more of the ring. Ground-penetrating radar aboard China’s Yutu rover, which landed in northern Mare Imbrium in 2013, detected at least nine distinct subsurface lava layers beneath the local surface — evidence of multiple discrete flooding episodes rather than a single catastrophic eruption. The ring that Mons Pico belongs to is estimated at roughly 650 km in diameter based on the positions of surviving exposures; between those exposures, it lies buried under several kilometres of basalt. Montes Teneriffe, Montes Recti, and Montes Spitzbergen are the other principal surviving fragments.
What the LRO Found on Pico’s Flanks
Lunar Reconnaissance Orbiter Narrow Angle Camera imagery resolved the upper slopes of Mons Pico at sub-metre resolution, revealing boulder fields and downslope mass-movement features on the flanks — material that has slid or rolled partway down the steep sides of the massif over geological time. Similar features are documented on the nearby Mons Piton, where a boulder more than 30 m across left a visible track down the mountainside. These features indicate that the flanks of Mons Pico are geologically active in a slow, gravity-driven sense even today, as material loosened by micrometeorite impacts and thermal cycling gradually migrates downslope toward the mare floor.
What Mons Pico offers the observer is rare on the Moon: a single isolated object with a well-understood geological identity, a dramatic and time-varying shadow, and a clear relationship to the larger basin structure visible in the same low-power field. Most lunar targets reward patience and aperture. Pico rewards showing up at the right phase — and then watching what happens next.