Free Astronomy Magazine July-August 2019

19 ASTRONAUTICS In 1965, D.E. Wilhelms made a dis- tinction between Fra Mauro’s corru- gated area and the gentle clear plains of the adjacent highland, producing a new map in which the latter was labeled as Cayley Forma- tion. While the maria were effusive eruptions of a dark basalt rich in mafic silicates that produced low- viscosity lava capable of forming smooth plains, it was thought that ‘highland basalt’ was sufficiently enriched with silica to render it semi-viscous, with the result that, after oozing from fissures, it settled as isolated spots in low areas. When R.E. Eggleton mapped the clear hills near Descartes Crater, in the central highlands, he reported them as an atypical patch of the Fra Mauro For- mation. Lacking evidence to the contrary, it was natural to consider these domic hills as extrusions of sil- ica-rich rhyolite which, being vis- cous, had accumulated to become hills. Originally called ‘Material of the Descartes Mountains’, this hilly A POLLO 16 − Videos showing Thomas Mattingly and John Young having fun aboard the LRV. [NASA, Project Apollo Archive]