The Imbrium Basin is the largest impact basin on the Moon’s near side, with a diameter of around 1,160 kilometres. The South Pole-Aitken Basin on the far side is twice as large. The massive impact event that formed Imbrium, one of the most violent in the history of the Solar System that occurred 3.85 billion years ago, left a giant crater which was subsequently infilled by basaltic lava.
Mare Imbrium (Sea of Rains), the huge lava plain that we see today in the Moon’s north-western quadrant, is the most obvious legacy of that ancient, cataclysmic event. Second only in size to neighbouring Oceanus Procellarum (Ocean of Storms), Mare Imbrium is obvious to the naked eye on a 10 day-old gibbous Moon; indeed, Imbrium forms the left eye of the famous ‘Man in the Moon’ feature. Raise a pair of binoculars or train a small telescope on Imbrium and it shouldn’t take long to realise that Mare Imbrium is bordered by a number of very impressive mountain ranges.
Montes Apenninus
The most striking range is Montes Apenninus (the lunar Apennines), which majestically guard the south-eastern shore of Mare Imbrium. They sweep in a 600-kilometres arc from Promontorium Fresnel in the north to the peaks east of crater Eratosthenes. Montes Apenninus’ highest peaks include the impressive Mons Huygens (5,500 metres), the highest peak on the Moon, and Mons Hadley (4,600 metres), lying close to its eastern extremities. A 150–200m (six- to eight-inch) telescope, operating at a power of around 150× to 200×, zooms in nicely on Mons Huygens and just to its west Mons Ampère (3,000 metres).
Montes Caucasus, Carpatus and Alpes
Montes Caucasus, to the east of Mare Imbrium, form a continuation of Montes Apenninus to the north-east (as far as crater Eudoxus). A third major range is Montes Carpatus (Carpathian Mountains), found just north of the mighty Copernicus impact crater, that mark the southern border of Imbrium. Together, Apenninus, Caucasus and Carpatus form the outermost of Imbrium’s three concentric rings of mountains, part of what is left of the rim of the basin following the lava flooding.
Montes Alpes (the Alpes Mountains), in the northeastern portion of the Imbrium Basin, is another famous feature that’s easily located as a rugged 250-kilometre-long south-east arc sweeping from the dark-floored crater Plato to crater Cassini. Look out for the striking Vallis Alpes, a rift valley that cuts right through the Alpine range
Through binoculars it’s easy to see that at its southern extremities Montes Alpes lies just inside the western flanks of Montes Caucasus. This is because Montes Alpes was part of middle ring of the Imbrium basin.
The Straight Range: part of an inner ring
You’re no doubt familiar with the Straight Wall (Rupes Recta), the 110-kilometre-long linear fault in the south-eastern part of Mare Nubian. How about Montes Recti, the Straight Range? It is an east-west orientated rectangular formation of peaks, around 90 kilometres in length and just 20 kilometres wide.
Individual peaks and groups of peaks, including Montes Recti, are common close to the north shore of Mare Imbrium. Lying just to the east of Montes Recti is the better known range Montes Teneriffe and to the south of Plato is the isolated peak Mons Pico, which towers 2,400 metres or so above the plain. Close by to the south-east is Mons Piton (2,300 metres), another stand-alone massif. However, they only seem to be individual peaks as they are easily-observable traces of an inner ring some 790 kilometres in diameter, parts of the inner terracing of the basin that were high enough not to be drowned by lava that formed the mare surface.
Further inspection southwards reveal more evidence of the inner-ring; Montes Spitzbergen (Spitzbergen Mountains) is located about 80 kilometres north of impact crater Archimedes.
The west to north-western section of the Imbrium Basin lack anything like as substantial a mountain range, but the vast semicircular scarp of Montes Jura, bordering Sinus Iridum (Bay of Rainbows) indented in the north-western edge of Mare Imbrium, is a magnificent sight.
Imbrium impact: craters
There are a handful of outstanding craters seen in the encircling mountains and standing in splendid isolation on the Imbrium plain.
The flooded crater Archimedes (81km) is the best and most prominent impact crater seen on the floor of the Mare Imbrium, at its eastern edge. Together with close companions Aristillus (55km) and Autolycus (39km), lying east and north-east, respectively, the trio provide a great sight. Looking through a small telescope, Archimedes has a smooth, Plato-like dark floor, which contrasts nicely with the marvellous central peaks of Aristillus.
Cassini is a curious crater lying to the north-east of Aristillus. Like Archimedes it’s a flooded crater, but it’s floor contains the interior craters Cassini A and Cassini B, the former having an unusual floor. Archimedes and companions and Cassini are all on show on the morning of 17 October.
Crater Eratosthenes (60km)lies in the foothills of south-western Montes Apenninus (Apennines). It hasasharprimwithwideinternallyterracedwallsandahillyfloor,abovewhichrisesagroupof mountains. Many observers liken it to a mini-Copernicus. Before you finish observing Imbrium, be sure to take a look at dark-floored Plato, lying at the western end of Montes Alpes.
Ghosts and crater chains: Take on challenges in Imbrium
Crater Lambert (30km) lies in glorious isolation on the Imbrium plane, around 350 kilometres west of Archimedes. Lambert is an easy target for any telescope but can you spot larger Lambert R (Ruin; 56km) lying just to the south? It is one of the Moon’s many ‘ghost’ craters. Astronomers believe it is an impact crater that was subsequently flooded by massive lava flows, left behind its rim as evidence of its former existence.
Now head to Imbrium’s far south-eastern quadrant, around 100 kilometres north-east of crater Eratosthenes, to track down a little ghost crater called Wallace (26km). Both craters are much easier to spot when they are illuminated by a low Sun.
Multiple impacts
Who can forget when in 1994 over 20 fragments of Comet Shoemaker–Levy 9 – dubbed ‘string of pearls’ – slammed into Jupiter’s cloud-tops, producing a series of dark scars on the planet’s southern hemisphere, the largest of which persisted for months. The comet was torn apart by Jupiter’s overwhelming tidal forces Astronomers believe similar impacts have occurred on the Moon and, unlike Jupiter’s long-dispersed scaring, we can observe the results. Perhaps the most famous of such features on the Moon is Cantena Davy, lying between crater Davy and majestic Ptolemaeus. However, there are a couple of them worthy of attention in the Imbrium Basin.
Cantena Beer
The small impact crater Beer (10km) lies around 115 kilometres south-west of the large crater Archimedes (80km). Astronomers think multiple impacts from a single, disrupted body, a comet or an asteroid, formed the chain of tiny craters (Cantena Beer; the largest crater is about 1.5 kilometres in diameter) seen arcing eastwards from Beer, eventually turning into a straight rille. You’ll need a telescope on the 250mm (10-inch) class to spot them, though Beer itself is an easy capture.
Crater Timocharis (34km) lies about 90 kilometres south-east of Beer. Lying just south-west of Timocharis are two much smaller craters, Heinrich (6km), the larger of the pair, with Timoocharis-C due east. Running north-north-eastwards from Timocharis-C is Cantena Timocharis, a 20-kilometre-long string of diminutive craters. This feature is probably best left to high-resolution imagers, though a large Dobsonian on a steady night could be successful. Try for Cantenae Beer and Timocharis on the morning
