Sky's Up - Fall 2015 - Page 12

Quadrant 37: — Images and text provided by Howard Eskildsen Copernicus to Lansberg Copernicus stands out as an eye-catching crater on the upper right corner of the quadrant. The 96 km diameter crater typifies the structure of large craters on the moon. A prominent group of peaks rises near the center of the crater, surrounded by a flat inner floor. The inner rim rises from the floor in a series of steps or terraces to the roughly circular outer crater rim. Outside the rim, rough rubble extends in an ever thinning ring that extends nearly a full crater diameter beyond the outer ring. This material, known as ejecta, was violently ejected during the formation of the crater by an explosive impact of an asteroid or comet. Intermittent fingers of ejecta extend even farther than the continuous apron surrounding the crater. Additionally, small irregular craters appear in the surrounding area that are secondary craters from blocks of material ejected during the crater formation. Reinhold and Lansberg, 49 and 41 km diameters respectively, show similar structural features to Copernicus on a smaller scale, but appear softened in appearance as if there has been some weathering compared to Copernicus. They are older than Copernicus and have endured eons of erosion caused by strikes from small meteors, known as gardening. Both also endured a massive moonquake by the formation of Copernicus as well as scouring from material ejected from it. The lower left half of the quadrant consists of a mostly smooth surface interrupted by scattered craters. Some of those craters appear fresh and deep, while others appear worn and shallow; indeed there is a nearly completely buried crater between Lansberg and Hortensius with only its outer rim rising above the plains. What could have buried and nearly completely filled the crater? The plains consist of a type of lava known as basalt and are known as mare (singular) or maria (plural), such as Mare Insularum on the image. It arose from swarms of fractures in the lunar surface and, layer by layer, laid down the smooth plains. Either before 12 or during the emplacement of the basalt, the flooded crater formed by meteor impact, and then was filled inside and out by rising lava until only the uppermost portion of its outer rim remained. Tests run on the basalt recovered from the moon’s lava plains by the Apollo missions show that it was about as thin and runny as warm motor oil or pancake syrup, and flowed freely into flat layers. Later, when that phase of volcanism had ended, some last gasps of volcanic activity left domes of lava that was much thicker when it erupted. The rounded domes the lava formed feature central pits or caldera. Several of these are visible on this image including the Hortensius Domes and Dome π near Milichius. In this image it is possible to see several processes that have shaped the moon over the eons. There are craters caused by impacts with meteors, asteroids or comets. Erosion of the craters shows up as wear from impacts of small meteors