There’s a reason we’ve only ever seen one side of the moon. It’s tidally locked, which means only one side of it faces Earth. This is not uncommon for moons in our solar system, but it’s still unclear exactly when this occurs, what conditions encourage it, and how it happens. In Petro’s mind, this mystery of tidal locking harkens back to the question of the origin of the moon, and what was happening in early lunar history. It’s another piece of the 4-billion-year puzzle.