Lunar Maria

Mare Nubium occupies a broad, irregularly shaped depression in the southern near side, filling the ancient Nubium basin — one of the oldest confirmed impact structures on the Moon, predating even the Nectarian period. Unlike the more geometrically pristine circular maria to the north, Nubium’s basin was so thoroughly overprinted by subsequent impacts that its original ring structure is almost entirely obliterated. What remains is a wide, gently undulating plain of solidified basalt roughly 254,000 km² in extent.

The mare’s southern boundary blurs into the heavily cratered southern highlands without a sharp transition — a consequence of the basin’s antiquity and the sheer density of later impact gardening across that latitude. The prominent crater Bullialdus, sitting near the mare’s centre, is a striking exception: a well-preserved Nectarian-age complex crater whose crisp terraced walls and central peaks stand in sharp relief against the dark basalt floor.

In 1964, Ranger 7 became the first American spacecraft to successfully image the Moon at close range, impacting in the northwestern part of Mare Nubium — a region subsequently named Mare Cognitum (“Sea That Has Become Known”) in its honour. It returned over 4,000 photographs in the final minutes of its descent and transformed understanding of lunar surface texture. The images revealed a surface far more heavily cratered at small scales than most scientists had predicted.

Mare Humorum is one of the more geometrically complete circular mare basins on the near side — a roughly 424 km diameter disc of dark basalt that preserves its basin rim more clearly than the sprawling, overprinted Nubium to the northeast. The Humorum basin formed during the Nectarian period, approximately 3.9 billion years ago, and was subsequently flooded with basaltic lavas over a timespan stretching to at least 3.3 Ga.

One of Humorum’s most visually striking features is the system of arcuate rilles — concentric fractures that curve around the basin interior parallel to its rim. These formed as the dense basalt infill subsided under its own weight, stretching the surrounding crust into a series of graben. The rilles are visible in even modest amateur telescopes and serve as a textbook example of mare tectonics driven by volcanic loading.

In June 1966, Surveyor 1 landed in the mare and became the first American spacecraft to achieve a soft landing on the Moon. It returned over 11,000 photographs and demonstrated that the lunar surface could support the weight of a crewed lander — an essential data point ahead of Apollo. The bearing strength measurements Surveyor 1 made here directly informed the design of the Apollo landing gear.

Mare Nectaris holds an outsized place in lunar chronology. The Nectaris basin is so ancient — and its formation so clearly recorded in the surrounding highland stratigraphy — that geologists use it to define an entire period of lunar history: the Nectarian (~3.92–3.85 Ga). Any crater or surface that postdates Nectaris but predates Imbrium is classified as Nectarian in age. In this sense, Nectaris is not merely a sea — it is a timestamp embedded in the Moon’s geological timescale.

The basin’s three-ring structure is partially preserved and visible from Earth: the outermost ring is expressed as the Altai Scarp (Rupes Altai), a magnificent curved escarpment that rises up to 3 km above the surrounding terrain and arcs for over 500 km. From a telescope, Rupes Altai is one of the most visually arresting features on the near side, particularly when lit at a low angle near the terminator. Nectaris itself is a relatively small, compact sea — roughly 350 km across — and is noticeably lighter in colour than Tranquillitatis or Serenitatis, reflecting its lower titanium content.

The mare was a candidate target for the Apollo programme’s later missions. Its proximity to a diverse array of highland terrains — offering access to both ancient crustal material and younger mare basalts within a short traverse distance — made it scientifically attractive. It remains under active consideration for future sample-return planning.

Mare Fecunditatis is one of the larger mare basins on the near side, covering 326,000 km², yet it receives comparatively little scientific attention relative to its extent. Its basin is ancient — Pre-Nectarian — and like Nubium, has been so heavily overprinted that only fragments of the original ring structure survive. The mare’s irregular, elongated shape reflects the overlap of multiple ancient basins that have been collectively flooded by younger lavas.

The basalt flows within Fecunditatis span an unusually wide age range — from roughly 3.6 to 3.1 billion years ago — indicating that volcanic activity here was episodic and prolonged. Remote sensing data show compositionally distinct patches of lava across the mare floor, some richer in titanium and some markedly poorer, suggesting eruptions from different mantle source regions at different times.

The twin craters Messier and Messier A, sitting in the western reaches of Fecunditatis, are among the most studied impact features on the Moon. They formed when an impactor struck at an extremely oblique angle, producing a pair of overlapping craters and two parallel rays that extend westward across the mare for over 100 km with no corresponding rays to the east — a geometric signature unique among all lunar craters and a valuable natural experiment in low-angle impact mechanics.

Mare Marginis sits at the very eastern edge of the Moon’s near side, so close to the limb that it is only visible from Earth under favourable libration conditions — the gentle monthly wobble in the Moon’s apparent orientation that allows observers to peek around the edges. For most of history it was essentially unstudied, glimpsed only in foreshortened views that revealed little of its true character.

Spacecraft imagery transformed understanding of Marginis. It is a relatively small, irregular mare without a clear single-basin origin — more a patchwork of lava infill in ancient lowlands than a single coherent flood. The basalt here is moderately dark and compositionally distinct from the high-Ti flows of Tranquillitatis to its south, suggesting lava from different mantle source depths reached this peripheral region during the Moon’s mid-Imbrian volcanic peak.

Their limb positions make these seas challenging targets for surface missions but geologically valuable ones, positioned near the boundary between near and far side crustal structure. Any future mission here would be traversing terrain where the thin near-side crust transitions to the thicker far-side crust — a boundary that may be recorded in the composition and thickness of the lavas themselves.

Mare Smythii straddles the lunar equator at the very edge of the near side, centred almost precisely on the 90°E meridian — the boundary between near and far hemisphere. This limb position means it is perpetually foreshortened from Earth and was essentially unmappable before the space age. Its name honours the nineteenth-century English astronomer William Henry Smyth, one of the most diligent visual observers of the pre-photographic era.

The Smythii basin is one of the oldest confirmed impact structures on the Moon, formed in the Pre-Nectarian period over four billion years ago. Despite its antiquity, the basin is reasonably well-preserved in circular outline — approximately 840 km across — and was subsequently flooded with basaltic lavas that are moderately enriched in titanium compared to the older, low-Ti lavas of Nubium and Humorum.

Like Marginis and Spumans to its north, Smythii represents the eastern frontier of near-side volcanism — lavas that reached this far east were at the outermost extent of the mantle upwelling system that fed the great western seas. The composition gradient visible in remote sensing data, with titanium content rising toward the eastern limb, may reflect the changing depth and temperature of mantle source regions across that hemisphere.

Named after Alexander von Humboldt, the Prussian polymath and pioneering naturalist, Mare Humboldtianum occupies a foreshortened position near the northeastern limb that kept it out of reach of systematic study until orbital missions could photograph it from overhead. Even then, its limb position meant it received a fraction of the analytical attention lavished on more centrally placed maria.

The Humboldtianum basin is a multi-ring impact structure approximately 600 km across at its inner ring, with evidence of an outer ring extending to roughly 650 km. The mare basalt occupies only the innermost depression — a comparatively small dark patch relative to the total basin extent — surrounded by a broad annular zone of highland material that constitutes the basin’s ejecta-modified terrain. This geometry, in which the mare fill covers only the central basin floor while the ring structures remain exposed as highland ridges, is characteristic of basins where lava supply was limited.

Like Smythii and Marginis, Humboldtianum represents the outer boundary of near-side volcanic activity. Its small mare fill suggests the magma supply that reached this latitude was already depleted relative to the great western basins. Nonetheless, its well-preserved multi-ring structure makes it one of the better natural laboratories for studying the relationship between basin geometry and subsequent lava flooding — questions that inform our understanding of similar structures on Mars, Mercury, and the large icy moons.

Mare Frigoris is an anomaly among the lunar maria — not a circular basin-fill but an elongated, irregular band of dark basalt stretching approximately 1,600 km from west to east across the northern near side, at latitudes around 56°N. No single large impact basin underlies it. Instead, Frigoris appears to have formed by lava flooding a series of connected lowlands associated with the peripheral structures of the adjacent Imbrium and Serenitatis basins, exploiting the pre-existing topography rather than occupying a single impact scar.

The basalt of Frigoris is notably low in titanium — among the lowest of any near-side mare — suggesting relatively shallow mantle source depths. Its lavas are also younger than many southern maria, ranging from roughly 3.5 to 3.2 billion years ago, which aligns with the prolonged volcanic activity of the KREEP-enriched Imbrium region to its south. In effect, Frigoris may represent the final northward expression of the same volcanic system that fed Mare Imbrium, with lava flows spilling over the Imbrium basin rim into the lower terrain beyond.

The mare’s high northern latitude places it within reach of the permanently shadowed polar regions that may harbour water ice — making the Frigoris region an area of growing interest for future resource prospecting, as any mission to a northern polar ice deposit would traverse or pass near this sea.

Mare Vaporum occupies the central near side between the great basins of Imbrium and Serenitatis to the north and Tranquillitatis to the southeast. It lacks the confident circular geometry of a classic basin-fill mare — its underlying impact structure, if any, is deeply buried and ambiguous in the geological record. What is visible is a broad plain of moderately dark basalt that merges to the south with Sinus Medii (the Bay of the Centre) — the small, near-perfectly centred dark patch that sits almost exactly at the geometric centre of the lunar near side.

Sinus Medii’s central location made it the default choice for early landing site surveys. In November 1967, Surveyor 6 landed here and performed a feat no spacecraft had attempted before: after completing its initial science objectives, it briefly fired its vernier engines and hopped 2.5 metres to a new position, allowing stereoscopic photography of its own landing footpads — demonstrating the plume effects of a rocket engine firing on the lunar surface and providing crucial data for designing the Apollo descent engine firing procedures.

Vaporum’s central position also made it a zero-point reference for early lunar coordinate systems. The Mösting A crater, a small, fresh impact crater in the region, served as the primary geodetic reference point for the Moon’s coordinate system from the 18th century until modern laser-ranging and spacecraft imagery established a more precise framework. In a literal sense, Mare Vaporum was once the place from which all lunar positions were measured.