Moon Atlas

There are many large basaltic features once attributed to bodies of water on the Moon's near-side: seas (Mare), bays (Sinus), lakes (Lacus), marshes (Palus) and a single ocean (Oceanus Procellarum). Most formed between 3 and 3.5 billion years ago when basaltic lavas filled the mare basins created by an earlier series of massive impacts dating back to 4 billion years ago. Mare Imbrium is probably one of the largest impact basins in the solar system with a diameter of 1,145 km. At ~4 milliom km² in area, Oceanus Procellarum is the largest mare feature, though unlike most others, it is not confined to a well defined impact basin.

(A): Sinus Iridum; (B): Mare Cognitum; (C): Sinus Aestuum; (D): Mare Vaporum; (E): Sinus Asperitatis; (F): Mare Nectaris; (G): Mare Australe; (H): Mare Crisium; (I): Mare Fecunditatis.

The following are only visible during favourable libration. (J): Mare Humboldtianum; (K): Mare Marginis; (L): Mare Smythii; (M): Lacus Veris and Mare Orientale.

Crater chains are unusual features composed of linear rows of small craters. One of the best examples is Catena Davy, "Davy Chain" (left) C|G9, which extends some 50 km and contains over 20 craterlets, most with diameters under 2 km. It was probably created by near simultaneous multiple impacts, similar to the Shoemaker-Levy-9 comet impact on Jupiter in 1994. Another nice example is Catena Abulfeda C|J10 which extends for 210 km. Double craters created by near simultaneous double impact are a related feature. Hesiodus A S|F12 on the South edge of Mare Nubium is a good example. Other crater chains appear to have been caused by secondary impacts of ejecta thrown out by large impacts. Vallis Capella E|L9 cuts through crater Capella. There are a number of crater chain features associated with the Copernicus impact.

Prominent ray systems are associated with a number of the younger craters. Over great expanses of time, ray systems are obliterated by more recent impacts, continual micro-impacts and space weathering by solar radiation. Tycho S|G13 has the most prominent ray system, with rays extending at least 1,500 km from the impact. The ray system associated with Copernicus W|E7 is also substantial, extending some 800 km from the crater. Kepler's rays W|C7(right), extend obliquely for 300 km across the surrounding basaltic plains. Proclus E|M6 has a very asymmetric 600 km ray system, indicative of a low angle of impact. An unusual linear 100 km ray system is associated with Messier A E|M8 on Mare Fecunditatis, resulting from an almost glancing impact. Whilst most ray systems are "bright" (highly reflective), Dionysius C|J7 is notable for its dark ray system that extends for ~130 km. Other ray craters include Anaxagoras N|H1, Aristarchus NW|C5, Aristillus N|H4, Byrgius A SW|B11 and Manilius C|I6.

Unlike the picture scanned from my 1963 "How and Why Wonder Book of the Moon", lunar mountains have relatively gentle slopes. They are extremely old, most being formed billions of years ago by mechanisms very different to those on Earth. Portions of the outer rim of the massive Mare Imbrium impact basin form the most significant visible mountain ranges including the Alps N|H2+3, the Apennines N|H5 with peaks rising almost 5 km above Mare Imbrium, and the Caucasus N|I4+5 with peaks approaching 6 km in height. Lesser ranges are associated with other impact basins. Isolated mountains Mons Pico C|G3, Mons Piton C|H3, Montes Teneriffe C|G3 and Montes Spitzbergen C|G4 were part of a now obliterated Imbrium basin inner ring.

Volcanic domes are amongst my favourite lunar features. Most are located on Oceanus Procellarum. With a diameter of 70 km, the largest is Mons Rumker NW|C3 which is actually a cluster of domes. The domes that lie between Copernicus and Kepler, the Hortensius domes W|E7 and Milichus domes W|D6, are more typical, with diameters of ~10 km, heights of 200 - 400 m and ~1 km caldera pits. Due to their gentle slopes, they are only visible at local sunrise and sunset, and quickly vanish. They represent a nice imaging challenge. Dozens of small domes cluster East of crater Marius in the largest lunar dome field, the Marius Hills W|B6. The impressive 20 km dome Mons Gruithuisen Gamma NW|D4 with its 1 km pit sits at the Northern end of Oceanus Procellarum. A number of small domes are associated with Vallis Schroteri NW|D5, itself a dramatic volcanic feature. Several domes are located on other maria. Mare Tranquillitatis harbours domes associated with rilles and faults near crater Cauchy E|L7, and Birt rille originates from a shallow dome in Mare Nubium S|G11.

Lunar faults give rise to several types of lunar features: Wrinkle Ridges that occur on basaltic plains, a type of lunar valley or rille called Graben, and isolated faults. Rupes Recta (left) on Mare Nubium is an example of a "normal" isolated fault. It is 110 km in length and has a width of 2.5 km and maximum height of ~300 m - it is not a steep slope as suggested by its image. Other examples include Rupes Cauchy and Rupes Altai. Isolated faults are relatively rare. Lunar faults did not arise due to plate tectonics as in the case of many terrestrial faults. Rather they resulted from stresses in the lunar crust often associated with the cooling, shrinkage and subsidance of basaltic impact basins.

Wrinkle ridges are a common feature on basaltic mare deposits, such is Mare Imbrium and Mare Tranquilitatis, and on a few basalt flooded craters such as Wargentin. They formed as the freshly filled basalt basins subsided under their own weight. Compression buckled the surface, resulting in long raised folds and ridges that often extend for long distances. Wrinkle ridges are thrust faults. Due to their low profile - they are usually only 100-200 m in height, they can only be seen at local lunar sunrise and sunset. However the basaltic plain can be at quite different elevations on either side of the ridge as a consequence of the buckling process. Sometimes Wrinkle ridges follow the outline of ancient craters buried under basaltic plains, for example Lamont.

Superficial resemblance is the only thing the ancient lunar valleys have in common with terrestial valleys. A handful of larger depressions are called valleys ("vallis"), whilst the more plentiful smaller examples are called rilles or rimae. There are several different types. Sinuous rilles resulted from lava flows and/or collapsed lava tubes. The largest sinuous rille is Vallis Schroteri NW|C5 (right), which is 160 km in length, up to 1 km in depth and 10 km across at its widest point. By any measure it is a large valley. For comparison, the Grand Canyon is 446 km length, up to 29 km across and up to 1.8 km in depth. Vallis Schroteri's flat floor hosts a narrow sinuous rille that looks like a river - I've not been able to see this directly. Other sinuous rilles include Rima Birt and Rima Hadley

Straight rilles are graben, flat valleys formed when the crust between two adjacent faults subsided (right). Graben often occur at the edges of marie, where they have resulted from subsidance of the basalt causing faults to open up in long tracks. They can also occur in older cratered terrain. The prominent 180 km Vallis Alpes N|H2 is probably a flooded graben. A further example is Rimae Hippalus SW|E11, a group of concentric graben that follow the edge of Mare Humorium.

Vallis Alpes (Alpine Valley) (right) mentioned above is an example shaped by several processes. The large structure was formed between two parallel faults when the crust was pulled apart. The flat floor and narrow sinuous rille running down its length was formed by subsequent lava flooding from Mare Imbrium and Mare Frigoris. The sinuous rille is good observing challenge.

A very different formation process is exemplified by Vallis Rheita SE|L13, which probably resulted from a destructive rain of secondary impacts associated with the formation of the Mare Nubium impact. It is essentially a highly eroded crater chain. Vallis Capella mentioned in Crater Chains above is a less highly eroded example.