Explore Mare Imbrium

Mare Imbrium, the vast lava plain in the Moon’s north-western quadrant, is the most evident legacy of an ancient, cataclysmic impact event that formed the Imbrium Basin. This immense crater, subsequently flooded by basaltic lava, is encircled by a series of impressive mountain ranges that serve as a testament to the violence of its formation.

The most striking of these ranges is Montes Apenninus (the lunar Apennines), a majestic 600-kilometer arc sweeping from Promontorium Fresnel in the north to the peaks east of crater Eratosthenes. Its highest summits, including the towering Mons Huygens (5,500 meters) and Mons Hadley (4,600 meters), form a formidable barrier guarding the south-eastern shore of Mare Imbrium.

To the east, Montes Caucasus extends the arc of Montes Apenninus towards the north-east, while Montes Carpatus (the Carpathian Mountains) marks the southern border of Imbrium, just north of the mighty Copernicus crater. Together, these three ranges form the outermost concentric ring of mountains, remnants of the basin’s original rim.

In the northeastern portion of the basin lies Montes Alpes (the Alpes Mountains), a rugged 250-kilometer-long arc sweeping from the dark-floored crater Plato to crater Cassini. The striking Vallis Alpes, a rift valley cutting through the Alpine range, is a notable feature in this region.

Montes Alpes is part of the middle ring of the Imbrium Basin, lying just inside the western flanks of Montes Caucasus. This positioning highlights the concentric nature of the mountain ranges that encircle the basin.

Closer to the northern shore of Mare Imbrium, traces of an inner ring can be observed. Montes Recti, the Straight Range, is an east-west oriented rectangular formation of peaks, approximately 90 kilometers in length and just 20 kilometers wide. Nearby lies the better-known Montes Teneriffe range, as well as the isolated peaks Mons Pico (2,400 meters) and Mons Piton (2,300 meters), towering above the lava plain.

These individual peaks and groups of peaks, including Montes Recti, are remnants of an inner ring some 790 kilometers in diameter, parts of the inner terracing of the basin that stood high enough to avoid being drowned by the lava flows that formed the mare surface.

Further evidence of this inner ring can be found in the Montes Spitzbergen (Spitzbergen Mountains), located about 80 kilometers north of the impact crater Archimedes. While the west to north-western section of the Imbrium Basin lacks a substantial mountain range, the vast semicircular scarp of Montes Jura, bordering Sinus Iridum (Bay of Rainbows) indented in the north-western edge of Mare Imbrium, presents a magnificent sight.

Within the encircling mountain ranges of Mare Imbrium, a handful of outstanding craters stand in splendid isolation on the vast lava plain. The flooded crater Archimedes (81km), located along the eastern edge of Mare Imbrium, is the most prominent impact crater on the mare’s floor. Together with its close companions Aristillus (55km) and Autolycus (39km), lying east and north-east respectively, this trio provides a stunning sight.

Through a small telescope, Archimedes reveals a smooth, Plato-like dark floor that contrasts beautifully with the marvellous central peaks of Aristillus. Nearby, Cassini is a curious flooded crater with an unusual floor featuring the interior craters Cassini A and Cassini B, the former boasting an intriguing floor of its own.

In the foothills of the south-western Montes Apenninus (Apennines) lies the crater Eratosthenes (60km), a sharp-rimmed formation with wide, internally terraced walls and a hilly floor. Above this floor rises a group of mountains, lending Eratosthenes an appearance reminiscent of a mini-Copernicus.

Before concluding observations of Imbrium, be sure to take in the dark-floored Plato, situated at the western end of Montes Alpes. This crater offers a striking contrast to the surrounding terrain.

For those seeking additional challenges within Imbrium, think hunting for the elusive “ghost” craters. Crater Lambert (30km), a prominent feature on the Imbrium plain, is easily spotted by any telescope. However, the larger Lambert R (Ruin; 56km) lying just to the south is one of the Moon’s many ghost craters – an impact crater subsequently flooded by lava flows, leaving only its rim as evidence of its former existence.

In the far south-eastern quadrant of Imbrium, around 100 kilometers north-east of crater Eratosthenes, lies another ghost crater, the smaller Wallace (26km). Both Lambert R and Wallace are more easily discernible when illuminated by a low Sun.

Multiple impacts from a single, disrupted body, such as a comet or an asteroid, have left their mark on the lunar surface. One such feature within Imbrium is Cantena Beer, a chain of tiny craters arcing eastwards from the small crater Beer (10km), located about 115 kilometers south-west of Archimedes. The largest crater in this chain measures around 1.5 kilometers in diameter, requiring a telescope of at least 250mm (10-inch) aperture to spot.

Another example of a crater chain can be found near the crater Timocharis (34km), lying about 90 kilometers south-east of Beer. Just south-west of Timocharis are two smaller craters, Heinrich (6km) and Timoocharis-C. Running north-north-eastwards from Timoocharis-C is Cantena Timocharis, a 20-kilometer-long string of diminutive craters, best left to high-resolution imagers or large Dobsonian telescopes on steady nights.