Institute for Advanced Study/Princeton University Joint Astrophysics Colloquium

NASA’s GRAIL mission to the Moon was completed in 2012 when both spacecraft were intentionally “de-orbited” (i.e. crashed) into a nearside mountainside. This orbital mission measured tiny variations in the Moon’s gravitational field by continuously monitoring the distance between two co-orbiting spacecraft (dubbed Ebb and Flow) to a precision of less than 0.1 micron. The resulting enormous improvement in the gravity field reveals buried structures otherwise hidden from view, from the underpinnings of large impact scars and the nearside lava flows down to a detection of the Moon’s core and perhaps an inner core. We have finally achieved a clear understanding of the previously mysterious mascons that posed a navigational hazard to the Apollo spacecraft and can now document a complete catalog of all of the ancient and, in many cases, otherwise invisible scars of large impacts that have breached the Moon’s crust. The Moon’s gravity field turns out to be qualitatively different from that of the other terrestrial-type planets in our solar system, but is now yielding insights into the nature of the earliest crusts of planetary bodies.