Sky's Up July-September 2017 - Page 50

Quadrant 31: — Images and text provided by Howard Eskildsen Mare Serenitatis: The serene sea Mare Serenitatis, the sea of serenity, lies peacefully on the face of the Moon as it has for billions of years, truly serene with nearly nothing happening since last layers of lava covered its floor. See the paltry few craters on its interior? That is nearly all that visibly happened to it in more than 3 billion years; quite serene, almost to the point of boredom. Perhaps that is why so many minuscule craters in this area have names, whereas they would have been ignored in the Southern Highlands where craters abound. But it wasn’t always that way; once it likely was as rugged as the Moon’s Southern Highlands, but a powerful impact around four billion years ago annihilated all pre-existing craters and left the huge basin now known as Mare Serenitatis. Originally, it probably had multiple rings that survived for a time but were then erased by subsequent basin-forming impacts or covered with lava flows. Its only clear ring remnant is the Montes Haemus, which likely rivaled the Apennine Mountains in the distant past, but now sits scoured and partly buried in Apennine tailings. A close look at the Montes Haemus shows rounded, rutted mounds northwest of Menelaus, with less evidence of devastation to the east of Menelaus since that part was farther from the Imbrium impact. On Earth, such degradation and wearing down of massive ranges took eons of glaciation and erosion, but on the Moon the violence of the moment took but minutes to hours as blasted material seethed over the terrain and then later settled while the Moon continued shaking with seismic tremors for days afterwards. The impacts that created the basins also left deep cracks that traveled downward and provided conduits for lava to rise to the surface and spread across the basin interior. Gradually the solidified basalt accumulated to the point where its weight caused the center to sink relative to the surrounding areas. This subsidence created cracks around the margins of the mare, known as arcuate rilles. A few of these rilles are visible on the image; Rimae Sulpicius Gallus, Rimae Menelaus and Rimae Plinius are examples. More lava arose over eons, adding further basalt layers over the floor of Serenitatis, covering most but not all of the earlier layers. Note the dark area around Promontorium Archerusia and Rimae Plinius; these are remnants of the oldest visible basalts in the basin and are high in titanium. 50 They are thought to be 3.7 to 3.8 billion years old. Subsequent flows settled to the sunken center of the basin and as it solidified, the basin sank further causing compressive stress on the interior of the mare. Small ridges, known as dorsa or wrinkle ridges, are overthrust faults that formed to relieve the compression stresses. The youngest lava layer is believed to be about three billion years old, which means the volcanic activity persisted for 700-800 million years. That is longer than it took complex life to evolve on Earth from simple life forms! Since that time very litt le else has happened that can be seen through a telescope. A few craters such as Menelaus and Bessel appeared in addition to a handful of otherwise insignificant craters. Bessel is notable primarily for the ray named after it even though it had nothing to do with the origination of the ray. Some have surmised that it is a streak ejected from Tycho more than 2,100 kilometers to the southwest, while others believe it may have originated from some fresh crater to the northeast. Another one-time source of confusion was the crater Linné, a diminutive dot on the upper portion of the image. It was once thought to have disappeared and been replaced by a brilliant white patch. While it caused quite a Sky ’ s Up stir, as any such discovery today would, it turned out to be a result of varying illumination of a tiny, young crater at the limit of Earth-bound telescopic visibility. So, Mare Serenitatis serenely awaits repeated discovery by Sky ’ s Up telescopic tourists. It’s peaceful now, but it was not always so serene. Its face has gazed sedately towards Earth for eons before our kind came into existence on Earth, and will continue, perhaps, after we are all long gone. 51