Guest Benjamin Greenhagen, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory in Maryland, outlines the lunar surface’s remaining mysteries --- everything from permanently shaded regions at the North and South poles to near and far side impact craters.  Greenhagen is the deputy principal investigator of the Diviner Lunar Radiometer instrument onboard NASA’s Lunar Reconnaissance Orbiter (LRO) and is an expert on the geology of the Moon’s surface.