Sky's Up July-September 2017 - Page 52

Quadrant 18: — Images and text provided by Howard Eskildsen Aristarchus Plateau: The fire within If any area of the Moon could be considered unique, this is the place, a strange, crustal block thrust upward by some catastrophic event, possibly the Imbrium impact. Measuring 170 by 200 kilometers, it rises as much as 2,000 meters above the surrounding mare basalts of Oceanus Procellarum. It is bounded by the Aristarchus Crater on the south and by the Montes Agricola to the north and is covered with low hills. It is darker than the surrounding maria in visible light as well as in ultraviolet light, and radar reflection studies show its surface to be smoother than other areas of the Moon, implying that it is nearly devoid of rocks and boulders. Multiple rilles that radiate in various directions once belched lava from reservoirs deep below the surface. Vallis Schröteri (Schröter’s Valley) is the most notable rille. It winds for 160 kilometers from the origin, known as the Cobra Head, to the plains of Procellarum, and is by far the largest rille on the Moon. It is nearly 10 km wide near the Cobra Head and, at its peak activity, may have gushed forth between 10 million and 10 billion kilograms of molten basalt every second. If so, it could have covered the entire Oceanus Procellarum to a depth of 200 feet. Later lava flows from a different source cover significant portions of lava from Schröteri, so its actual extent is not known. These massive flows were accompanied by fire- fountains spewing glass beads that spread over the plateau, coating it with tens of meters of volcanic ash and producing the unusual surface smoothness. It also may account for the unusual color noted at various illuminations. Near full moon it has been described 52 as mustard-colored, ruddy or even olive in color. Other curious sightings in recent times have raised the question of further eruptions or haze emanating from the valley. While any active volcanism has been essentially ruled out by thermal studies, Apollo 15 did detect radon gas in the region, presumably produced from the decay of uranium and thorium, which may have escaped through fissures from beneath the surface. Other rilles show that Schröter’s Valley was not Sky ’ s Up the only source of flows from the plateau. These various smaller rilles are collectively known as Rimae Aristarchus and are marked with Roman numerals on the image. All are much smaller than the valley, but note the apparent fossil flow-channel remnant that is unofficially named Fossa Caruso near the end of rima VIII. This appears to have been an outflow channel that was eventually blocked and partly flooded by subsequent flows. Its source has been obscured, but it makes one wonder what, if any, connection it might have had with Vallis Schröteri. Long after the volcanic fires died out, a huge space rock crashed on the southern margin of the plateau and created the name-sake crater Aristarchus. This 40-kilometer crater is thought to be around 500 million years in age and is the brightest area on the moon. It is readily visible to the naked eye and even notable during lunar eclipses or in the Earthshine on the dark side of the terminator during the crescent phases of the Moon. Ironically it is located next to one of the darkest areas of the moon. Its rays scatter about but are hard to see on the plateau. On its interior, two distinct types of dark basalt are visible as well as bright anorthosite in the central peak. Radial bands can be seen within the crater at high sun angles, and an especially bright ray plume from Aristarchus drapes across the floor of Herodotus. In contrast to Aristarchus, Herodotus looks tired and worn with its central peak buried in basalt that partly fills the crater. Faring even worse, the crater Prinz has much of its rim buried as well. Had volcanic activity continued much longer Prinz might have been totally obliterated. North of the plateau the strange Montes Agricola forms a thin linear ridge unlike most other ranges on the Moon, which are usually arcuate rather than linear. I know of no official explanation for its morphology, but it does remind me of buckling seen on Earth where one plate pivots against another like a hinge. The whole area is a place of wonder with its complex structure, history and appearance. Its startling landforms, and changing appearance as the sun-elevation changes, entices the observer to return again and again to this wonderfully unique plateau. Sky ’ s Up 53