L26 Mare Frigoris
The “Sea of Cold,” an arc-shaped mare sweeping across the Moon’s northern nearside in a broad, dark band — unlike the circular impact basins, this elongated sea provides a stark, low-albedo contrast against the bright, rugged terrain of the northern highlands.
L26 Mare Frigoris
Far Northern Near Side📉 Vital Statistics
🔭 Field Notes
Mare Frigoris is the odd one out among the named maria — a long, narrow ribbon of basalt arcing east-west across the Moon’s far north for roughly 1,600 km, yet rarely more than 200 km wide. Unlike Crisium or Imbrium, it isn’t ponded inside a single circular impact basin. Its origin is debated: some researchers connect it to an outer ring of the Imbrium basin, while others tie it to the much larger, still-hypothetical Procellarum basin — a proposed (not confirmed) ancient impact structure underlying much of the Moon’s near side. Whatever the cause, at the eyepiece it behaves more like a dark seam running along the edge of Mare Imbrium and Mare Serenitatis than a “sea” in its own right.
- ▶ Two-Age Floor: LROC and geologic mapping show the basin material framing the mare is Lower Imbrian in age, the eastern mare fill is Upper Imbrian, and the western fill near Plato is younger Eratosthenian basalt — a visible difference in albedo and crater density along the mare’s length.
- ▶ Bürg and Lacus Mortis: At the mare’s eastern end it merges into Lacus Mortis (“Lake of Death”), home to the floor-fractured crater Bürg and the rille system Rimae Bürg — a worthwhile side trip once you’ve traced Frigoris that far east.
- ▶ Wrinkle Ridges: The narrow mare floor is crossed by low compressional wrinkle ridges, products of the basalt cooling and contracting — most visible near the terminator rather than at full illumination, since the mare’s low overall contrast already makes it one of the less conspicuous maria to track down.
📍 Nearby L100 Targets
- L23 Mons Pico: An isolated 2.4 km peak on the floor of Mare Imbrium just south of Plato — a surviving fragment of the Imbrium basin’s buried inner mountain ring, throwing one of the longest mountain shadows on the near side near the terminator.
- L19 Alpine Valley (Vallis Alpes): A 166 km straight-walled graben slicing through the Montes Alpes, directly connecting Mare Imbrium to Mare Frigoris’s southwestern shore — the most obvious “doorway” between the two seas in a small telescope.
- L83 Plato Craterlets: The 101 km dark, flat-floored crater Plato sits right on Mare Frigoris’s southwestern shore, where the mare, the Alpine Valley, and the Montes Alpes all meet. Its handful of sub-3 km floor pits are a classic seeing/aperture resolution test, with experienced imagers under exceptional conditions resolving dozens.
🚀 Mission Log
Target Acquisition
Use Plato and the Alpine Valley as your gateway north
Don’t hunt for Mare Frigoris directly — approach it through Plato, the dark, flat-floored 101 km crater sitting right on its southwestern shore. Plato is unmistakable at low power against the bright Montes Alpes. From there, follow the straight-walled gash of the Alpine Valley (Vallis Alpes) northeast; it cuts through the Montes Alpes and opens out into the northern lowlands where Frigoris begins, making an obvious “doorway” out of the Imbrium region. Once through it, you’re on Frigoris itself — a long, narrow, comparatively low-contrast ribbon of mare running east-west along the Moon’s far north, easy to lose track of if you haven’t anchored on Plato first.
Wait for the terminator — Frigoris is a low-contrast target at high Sun
Mare Frigoris is already one of the less conspicuous maria under full illumination, and its wrinkle ridges — low compressional folds from the cooling basalt — are essentially invisible without shadow to define them. The best time to trace the mare’s full length and pick up the ridges is when the terminator crosses the mare directly, whichever end you’re working. Because Frigoris stretches roughly 1,600 km east-west, the terminator won’t cross the whole thing at once — plan to work one end per session rather than trying to catch it all in a single sitting.
Trace the mare’s two-age floor from west to east
At 50x–75x, work along the mare’s length. Geologic mapping shows younger, Eratosthenian-age basalt units in the west near Plato, giving way to older Upper Imbrian fill toward the east — a real distinction, though not something you should expect to visually confirm at the eyepiece the way you can with sharper features; it’s more useful as context for what you’re looking at than as an observing target in itself. Follow the mare east to where it widens and merges into Lacus Mortis, home to the floor-fractured crater Bürg and its rille system, Rimae Bürg — a natural stopping point and a worthwhile side trip once you’ve traced Frigoris that far.
Work the western anchor point in detail
Back at the western end, spend time on the cluster around Plato. Mons Pico (L23), an isolated 2.4 km peak just south of Plato on the Imbrium floor, throws a remarkably long and conspicuous shadow near the terminator — a striking companion target while you’re already in the area. Plato itself (L83) is worth a return visit at higher power: its handful of sub-3 km floor pits are a classic resolution test — most visual observers do well to spot four, while experienced imagers can resolve far more craterlets than visual observers typically see. Between Plato, the Alpine Valley, and Frigoris meeting at one point, this corner of the Moon rewards slow, repeated visits more than a single pass.
📝 Observation Log — L26 Mare Frigoris
0/4 CompleteIs Mare Frigoris visible tonight?
Mare Frigoris can be seen at any phase, but its full extent — a narrow dark arc stretching over 1,400 km across the northern near side — is most legible near Full Moon when the entire mare is illuminated and its full dark arc can be traced against the brighter surrounding highlands. The southern shore features, including Plato and Montes Alpes, are best near the terminator around First Quarter (Day 7–8) or Last Quarter (Day 21–22).
Check Moon Phase TodayReturn to the List
Ready to find the next target? Go back to the full map to see what else is visible tonight.
← Back to Lunar 100 Map
When to Observe Mare Frigoris
Mare Frigoris is one of the few Lunar 100 targets that is genuinely visible at almost any phase — it is simply too large to miss once the Moon is well past crescent. But what you see depends entirely on the illumination. Near Full Moon, with the entire mare in flat light, its full dark arc can be traced across the northern near side from west to east — one of the few major maria whose overall shape is easier to appreciate under high-Sun illumination than near the terminator. Near the terminator, the southern shore’s craters and mountain ranges catch the low raking light and dominate; Frigoris itself fades into a dark background.
- Full Extent of the Mare: Best near Full Moon (Day 14–15), when the entire dark arc from Oceanus Procellarum in the west to the region north of Mare Serenitatis in the east is illuminated simultaneously. This is the only phase at which Frigoris’s anomalous elongated shape and its relationship to the surrounding mare system can be properly appreciated.
- Southern Shore Features: Best around Day 7–8 (First Quarter) or Day 21–22 (Last Quarter), when the terminator crosses Plato, the Alpine Valley, and Montes Alpes along the southern margin. These features overshadow Frigoris itself at this phase — catch the mare’s extent first at Full Moon, then return to the southern rim detail at the terminator.
What to Look For
Near Full Moon at low power — 20x–40x — sweep the northern near side from west to east along Frigoris’s length. The mare runs from Oceanus Procellarum in the west, curves through its central section just north of Plato, then continues east to north of Mare Serenitatis. Most major maria are roughly circular; this one is not — it is a narrow, distinctly elongated dark band, typically described as roughly 1,400 km or more in length but averaging only around 250 km in width. Tracing it continuously from one end to the other in a single sweep establishes its scale and its unusual geometry in a way that no map quite conveys.
Near First or Last Quarter at 50x–75x, the southern boundary of Frigoris becomes the most feature-rich zone on the northern near side. Plato’s dark lava-flooded floor sits at the southwestern corner, flanked by Montes Alpes to the east and, farther to the southwest, the Jura Mountains associated with Sinus Iridum. The Alpine Valley cuts directly through the Montes Alpes as a linear graben, connecting Frigoris to Mare Imbrium below. Under low Sun this entire border region is compressed into a dramatic arc of mountains, craters, and dark mare — one of the most rewarding single sweeps available at the terminator on a moderately magnified view.
The eastern half of Mare Frigoris, east of roughly 15°E longitude, contains a more complex tectonic record than the relatively featureless western section. LRO mapping has documented a polygonal pattern of wrinkle ridges near the centre of the eastern mare, with larger arcuate graben running concentric to the ridge cluster along the northern and eastern edges. These features are subtle at the eyepiece — the wrinkle ridges in particular require low terminator light and a steady night — but they are more accessible than the western mare’s comparatively smooth floor. Patience at 100x near the Day 7–8 or Day 21–22 terminator gives the best chance of detecting the ridge pattern.
Near Full Moon at low power, pay deliberate attention to the northern edge of Mare Frigoris — the boundary between the dark mare and the ancient highland terrain to the north. Unlike the relatively straight southern border defined by the Imbrium and Serenitatis basin rims, the northern boundary has a gentle inward curve, concave to the south. This curvature is at the centre of one of the more speculative debates in lunar geology — whether Frigoris occupies part of the rim of an enormous, ancient, and largely obliterated impact structure. Tracing the curve and assessing whether it looks genuinely arc-like is one of the more unusual observational exercises the mare offers.
The Science: Why Mare Frigoris Has No Agreed Origin
Almost every other major mare on the near side sits in an obvious impact basin — a round depression whose rim is still visible, at least in part, in the surrounding terrain. Mare Frigoris does not. It is long, narrow, and arcuate, and after more than a century of telescopic observation and decades of spacecraft data, its origin remains genuinely uncertain and details of its formation are still debated. This is its most scientifically interesting characteristic.
The Procellarum Basin Hypothesis — and Why GRAIL Complicated It
An older and historically prominent explanation holds that Mare Frigoris occupies part of the rim or interior of an ancient, enormous impact basin — sometimes called the Procellarum Basin or Gargantuan Basin — so thoroughly degraded that almost no topographic evidence of its rim survives. Under this hypothesis, Frigoris’s curvature reflects the geometry of this ancient structure. However, NASA’s GRAIL mission, which mapped the Moon’s gravity field in unprecedented detail, found that the structures bordering Oceanus Procellarum form a quasi-rectangular pattern with angular intersections — directly contradicting what would be expected from a giant circular impact basin. GRAIL’s principal investigators interpreted the rectangular pattern as frozen remnants of ancient lava-filled rifts, not an impact rim. The giant impact hypothesis for Procellarum — and by extension the Gargantuan Basin model for Frigoris — has therefore been significantly weakened by modern spacecraft data, though not all researchers accept the GRAIL interpretation as final.
The Crustal Block Separation Hypothesis
An alternative interpretation, proposed by Whitford-Stark in 1975, suggests that Mare Frigoris formed by the lateral separation of blocks of highland crust — essentially a tectonic rift — rather than by the flooding of an impact basin. Under this model, the mare occupies the gap opened between crustal blocks that moved apart, with lava subsequently filling the space. The linear, non-circular geometry of Frigoris is cited as more consistent with rifting than with basin flooding. The hypothesis has not been widely adopted, but it has also not been definitively ruled out by subsequent data, and it remains part of the range of proposed origins for the feature.
What LRO and GRAIL Confirmed — and Did Not
Williams et al. (2019, JPL/NASA) used LROC imagery to map the tectonic landforms of Mare Frigoris in detail, confirming that the eastern and western sections have distinctly different tectonic signatures. The eastern mare shows polygonal wrinkle ridges and concentric graben consistent with mascon-driven subsidence — behaviour expected in a lava-filled impact basin. The western mare lacks both a clear circular topographic rim and a large mascon, behaving more like Oceanus Procellarum than like Imbrium or Crisium. Young lobate scarps, small graben, and recorded shallow moonquakes in and around Frigoris also indicate that the mare is not tectonically dead — some deformation continues into the geologically recent past. What LROC and GRAIL have not provided is a definitive answer to the origin question. The data constrain it without resolving it.
What makes Mare Frigoris unusual as a telescopic target is that the open scientific question is directly readable in what you can see. The non-circular shape, the different character of the eastern and western sections, the ambiguous northern boundary — these are not abstract geological problems. They are visible properties of a dark band you can trace with a pair of binoculars. Most lunar features reward knowing what you are looking at. Frigoris rewards wondering why it is what it is.