Mersenius is located west of Mare Humorum not far from the moon’s western limb. It is of average size but still rich in detail. The crater walls are broad and terraced; the floor has been flooded with lava and bulges markedly upwards in the middle.
My observation was done under excellent conditions with steady, transparent skies and favorable illumination. The terraces were prominent, especially on the western side (right i sketch). The convex shape of the floor was indicated by a soft shadow tracing the walls in the northern half of the crater. Two lighter rays were also clearly visible on the floor. (After checking up on the observation it seems that the northern and thinner ray, is a rille rather than a ray.) Several secondary craters were also seen, most notably Mersenius H on the southern part of the rim, and Mersenius L adjacent to the crater on the northern side.
The Procellarum Basin draws it’s name from Oceanus Procellarum (Ocean of Storms) and is the unofficial name for a hypothetical impact crater covering a large part of the moon’s nearside. There seems to be at least two alternative explanations to the features associated with the basin, and I will get back to them below.
Working through the Lunar 100 the Procellarum Basin is something of an enigma. What exactly are we supposed to observe? If it is the circular shape that can be traced in the nearside maria, why is it inserted at the difficult end of the Lunar 100 list, and not right after L3: the Mare/Highland dichotomy? So far I haven’t managed to find any information on the web that might shed light on these questions.
Some days ago I had my rarely used 120 mm refractor set up at the kitchen table, doing some maintenance. When done I looked out through an open window and noticed the moon rising over a nearby cliff. Still seated I swung the telescope over and was treated with a really nice view. The indoor/outdoor temperature difference didn’t seem to affect the image, at least not at low power. Comfortable seated I decided to sketch the (almost) full moon, and to try to outline the Procellarum Basin as well as some of the more prominent ejecta ray systems. As the moon would eventually wander out of sight, I had to work quite swiftly, and the sketch was done in about 45 minutes.
Anyone who have tried to sketch the full face of the moon, know that it is quite challenging. The amount of detail is overwhelming, and it is easy getting stuck somewhere in the highlands, sketching away at the many craters. I decided to go for a rather crude sketch, just outlining the maria and sketching only a few of the craters. I didn’t even bother to trace the terminator — the moon was about a day from full — I just pretended that the moon was a perfect circle.
In the image to the right I have tried to mark what I believe is recognized as the outline of the Procellarum Basin [Edit: see new image at the end]. The maria seems organised in a circular pattern, most notably to the north and southwest, and in accordance with one interpretation it might be traces of an ancient and enormous impact crater. If this is correct the energy of the impact might also have been enough to create the highlands on the far-side of the moon.
A later and opposing view do away with the impactor, instead claiming that the Procellarum region is the result of internal processes active 3 to 4 billion years ago. Data from the Gravity Recovery and Interior Laboratory (GRAIL) mission show evidence of remnants of lava-filled rifts forming a squarish pattern (red in the image to the right, original image here). It is suggested that as the original lava oceans that covered the young moon solidified, they also contracted. Molten lava then rose at the edges creating the rifts.
If the latter interpretation is true there is no Procellarum basin, which of course again raises the question of what we are suppose to observe. Hopefully I have done enough to tick off L 95 from my list. Come to think of it, and since the sketch also depicts the mare/highland dichotomy, I will tick off L 3 as well.
Edit: After a short exchange with Jef De Wit at Cloudy Nights’ sketching forum, I realized that my depiction of the basin needed to be adjusted. In the paper that suggested the Procellarum Basin (linked in text above), E. A, Whitaker found evidence of three concentric rings associated with the hypothetical impact. “The rings have surface diameters of 1700, 2400 and 3200 km, and are centered at about 23º N, 15º W, near the crater Timocharis.” In the image inserted below I have drawn these circles onto my sketch.
The original Lunar 100 list only mentions the 3200 km ring. And since only a small part of that ring can be clearly seen, and since the impact as such is contested, it might explain why the Procellarum basin ended up on the difficult end of the list.