Lunar Day Eleven
We start our observing evening on the beautiful Moon as we return to landmark crater Gassendi standing at the north edge of Mare Humorum (Figs. 12.1 and 12.2). Closely examine the northwestern rim of the mare for crater Mersenius. It is a typical Nectarian geological formation, spanning approximately 83 km in diameter in all directions. Power up in a telescope to look for fine features such as steep slopes supporting newer impact crater Mersenius P and tiny interior craterlet chains. Can you spot white formations and crevices along its terraced walls? How about Rimae Mersenius? Further south you will spy tiny Liebig helping to support Mersenius D’s older structure, along with its own small set of mountains known as the Rupes Liebig. Continue to follow the edge of Mare Humorum around the wall known as Rimae Doppelmayer until you reach the shallow old crater Doppelmayer. As you can see, the whole floor fractured crater has been filled with lava flow from Mare Humorum’s formation, pointing to an age older than Humorum itself. Look for a shallow mountain peak in its center - there is a very good chance that this peak is actually higher than the crater walls. Did this crater begin to upwell as it filled? Or did it experience some volcanic activity of its own? Take a closer look at the floor if the lighting is right to spy a small lava dome and evidence of dark pyroclastic deposits - it is a testament to what once was!