Free Astronomy Magazine July-August 2019

16 ASTRONAUTICS event, the land on which the Apen- nine was built was the inner part of the Serenitatis-equivalent of the Fra Mauro Formation. Therefore, there was a layer of Serenitatis ejecta deposited on the crust, later contaminated by the formation of Mare Imbrium, so that the under- lying crustal blocks were exposed by the new event. It is possible that part of this material remained on the peaks, but it is likely that most of it slipped down, accumulating in the valleys. When an impact crater is formed, material dug from deeper underground is left at the edge of the crater itself. When a basin is formed, there is also a pulse of semi-melted material dug into the depths of the surface crust, and this material would have coated the Apennine. Fra Mauro rocks revealed that Imbrium was formed 3.85 billion years ago, but the Fra Mauro Formation only rep- resented the ejecta of the crust. A POLLO 16 − Image taken aboard the LRV as the astro- nauts were heading to the lunar module. [NASA, Project Apollo Archive]