A boulder perched on the rim of a Copernican crater in Mare Serenitatis. NAC frame number M168041107L, image width is 250 meters [NASA/GSFC/Arizona State University].
Posidonius Y is a 2 km diameter Copernican crater located at 30.0°N, 24.9°E, in the upper northeast section of Mare Serenitatis. The crater is remarkably bouldery, which is a indication of a relatively young age. The boulder seen on the rim in today's Feature Image is about 25 meters long from stem to stern (about half the length of an Olympic sized swimming pool). Since Posidonius Y is superposed on the wrinkle ridge system known as Dorsa Smirnov, the crater is also relatively younger than these tectonic features. The WAC context image below shows the beautiful combination of mare basalt volcanism, tectonism, and impact cratering that shape the lunar surface.
LROC WAC 100 m/px mosaic showing Posidonius Y and part of the Dorsa Smirnov wrinkle ridge system. The red box marks the area covered in the NAC frame [NASA/GSFC/Arizona State University].