by Matt Tarlach
Directly E of Crisium the basin of Mare Marginus is easy to detect under low lighting one day after Full Moon. The scarp defining the basin rim appears nearly continuous, though lower and less dramatic than corresponding scarps and mountains around Nectaris, for example. The rim of Marginus appears soft and eroded in comparison, and the surface inside the rim is so well marked with craters that its texture is nearly as rough as the surrounding highlands. Clearly this is a very old basin.
North of Crisium and Marginus, Mare Humboldtianum is a well-defined two-ring basin. With the terminator near 80E, 1 day after Full Moon and with a good libration, both rings appear nearly continuous and more sharply defined than the Marginus basin to the South. The smooth mare material of Humboldtianum is contained almost fully within the inner ring, except for some splashes between the two rings and NW of the basin center. The remainder of the annulus between the rings is pocked with small and medium-sized craters but not as broken up as the interior of Marginus.
On the southeast limb, directly E of Petavius, lies the huge (207km) crater Humboldt. With the terminator falling directly across the crater, its fractured floor and volcanic features were undetectable but the floor is clearly convex, as dark flooding lava near the center of the crater catches sunlight while all the outer reaches of the floor remain in shadow. A chain of peaks or possibly craters running N-S across the center was also visible right on the terminator. Humboldt is quite a dramatic sight 1 night after Full Moon and under a good libration. If it were better located for viewing from Earth, it would be one of the showpiece objects of the Moon!
Unlike many lunar objects which are striking near the terminator but disappear under a high Sun, Hyginus retains interest under all conditions. 1 night after Full Moon, the rim or inner wall of the crater appears continuous and bright white, as do the branches of the Hyginus rille leading off in either direction. The floor of the crater has an albedo similar to the surrounding plain. Around the rim of the crater is an irregular patch of dark material. My knowledge of geology is very limited but considering the views under both high and low Suns, Hyginus really looks quite different from other craters of similar size, and more like a volcanic than an impact feature.
Atlas is an interesting object under high Sun, near Full Moon. Two splotches of dark material on the floor are striking even at moderate power, and there is a small bright ring at the center of the crater floor which does not really match up with the central peaks that are visible when near the terminator.
Observations with 150mm Mak-Cass at 215x, seeing Ant III-IV.
Posted on shallow-sky Oct 28, 2004 23:59:25 PT
Converted by report.pm 1.2 Jan 16, 2005 08:20:54 PT