MoonWatch
December 1998
Lichtenberg crater and wrinkle ridges in Oceanus Procellarum
11 March 1998 Peter Grego
December begins with a waxing gibbous Moon that climbs high into the sky by midnight. On December 1 the morning terminator will be sweeping slowly (at an average speed of just 15 km/hour) across the vast dark plains of Oceanus Procellarum, the Ocean of Storms. Scanning the terminator along Oceanus Procellarum at high powers you will be able to study the topography in detail. Examine the ridges south of the bright feature Reiner Gamma in western Procellarum and notice how they twist into odd curves. These are examples of wrinkle ridges, or (more scientifically) dorsa.
Oceanus Procellarum occupies much of the western hemisphere of the Moon's near side. It is the largest expanse of lava on the Moon, having a surface area of more than 2.1 million square kilometres, making it a little smaller than the Earth's Mediterranean Sea. Oceanus Procellarum consists of a patchwork of lava flows that erupted around 3.9 billion years ago, burying more ancient structures beneath several kilometres of basaltic exudate. Unlike the other lunar seas, Procellarum does not conform to one single massive impact basin - the idea of an ancient 3,200 km diameter Procellarum Basin has now been discredited. The complicated structure of Procellarum was dramatically revealed in images taken at different wavelengths by the Clementine probe during its lunar survey in 1994. There is no single mascon (concentration of mass) at the centre of Procellarum - Clementine showed a clumpy gravity field that corresponds with denser portions of mare lava, and its successor Lunar Prospector continues to refine our knowledge yet further.
Although the old marine nomenclature of the lunar seas remains, even Galileo did not believe that they were expanses of water. When illuminated by a low Sun, the lunar maria are shown by the smallest of telescopes to be somewhat uneven, with craters, hills and ridges here and there. All of the Moon's seas contain wrinkle ridges. The circular seas contain dorsa that generally appear parallel to the shoreline. Some dorsa contort themselves into features that look like melted craters, notably the Lamont formation in Mare Tranquillitatis. Oceanus Procellarum contains extensive systems of dorsa, some of which wind their way across the surface for hundreds of kilometres.
How were the dorsa formed? Undoubtedly, many are remnants of ancient lava flows or crumpled lava fronts. Some dorsa, like those that lie radial to Aristarchus in Procellarum, may be piles of ejecta or redistributed surface material shaped by the blast of the Aristarchus impact. Low ridges may appear to conform to deep-seated structures such as craters smothered by lava flows. Other dorsa may be extrusions of viscous lava along the line of crustal faults. Dorsa running around the coastline of the maria, like the prominent ridges of mare Serenitatis, may have resulted from surface shrinkage and collapse following the cessation of vulcanism. To be sure, no one process has caused all of the Moon's wrinkle ridges.
