MARE CRISIUM 2004.08.31 19:07 UT Age 15 days. Loíigitudmal libration 6°. 10-ín 66 +■ ToUcam at prme foous
Mare Crisium (Sea of Crises) is a dominating feature near the east limb of the Moon. It is a dark, lava-filled basin surrounded by mountain walls. The mountains average to 3000 m high. The mare measures approximately 560 km east-west and 420 km north-south, but to Earth the north-south diameter always appears longer due to foreshortening. Mare Crisium is the site of a gravity anomaly known as a "mascon," or mass concentration that represents thick accumulation of dense lava, or the fragments of an impactor that created the basin buried beneath the mare surface. Mascons exist in other mare regions and few flat-floored craters as well, e.g. Mare Imbrium, Mare Humorum, Mare Orientale, craters Ptolemaeus and Grimaldi; details in the maps of [Moon Data page. Their high-gravity causes lunar satellites orbiting at low altitudes to either impact the Moon or to be flung out into interplanetary space after a few years.
Under oblique illumination, concentric wrinkle ridges are visible along the inner circumference of Mare Crisium. The most apparent wrinkle ridges are Dorsa Harker and Dorsa Tetyaev on the eastern edge (Image T002); they measure 180 ~ 200 km in length. Crater Eimmart C happens to locate on the northern tip of Dorsa Tetyaev, and the combined formation resembles a string looped at one end. The neighbor of Eimmart is an irregular patch of lava named Mare Anguis (Serpent Sea); it seems to be a leak piece from Mare Crisium. The cape on the southern edge is Promontorium Agarum; it rises about 5000 m above the mare and looks almost disconnected from the rest of the highlands. In 1976, the former USSR probe, Luna 24, returned soil samples from the vicinity of Promontorium Agarum.
The end pages of this map give the closer views of Crisium region.
Proclus is a prominent impact crater on the western highlands of Mare Crisium. It has sharp rim and rough floor, 28 km in diameter, 2400 m deep. Its ejecta begins to shine as asymmetrical pattern of rays around Moon age of 5 ~ 6 days, gradually glowing up to be one of the brightest features on the entire Moon. The longest rays slide through the dark floor of Mare Crisium. The asymmetrical ray pattern is an evidence of a grazing impact, in which the space impactor intruded from southwest (top right corner of T005) and hit the Moon surface at very low angle, likely a
few degrees from ground. A low-angled impact is characterized by ejecta in confined directions. The lower the impact angle, the more unidirectional the ejecta is. This explains Palus Somni (Marsh of Sleep), a diamond-shaped area at immediate west of Proclus, is lack of Proclus' ejecta rays. Palus Somni measures 150 km east-west and looks neither a complete highland nor dark mare. Its northern floor is rough but the southern floor is relatively flat. The peculiar gray tone of Palus Somni has been a study subject by astrogeologists.
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