The team also found that while the crater's surface is bright, its walls are even brighter, which was at first surprising. Any ice couldn't survive on those walls because it would sublimate into space the moment it was exposed to sunlight. But it turns out it may not be ice after all, but brighter lunar soil. Small moonquakes could cause some of the crater walls to slough off darker lunar regolith and expose fresh soil underneath, according to NASA. Zuber said the newer soil and the ice on the floor could both explain the crater's unusual brightness. Other than the occasional wall-shaking, the crater is remarkably pristine, having changed very little since its formation 3 billion years ago.