Lava Filled Craters and the Thermal History of the Lunar Surface

THE concept of a “cooled crust” is useful in studies of the thermal history of a planetary surface originating from a differentiated liquid layer of considerable thickness. The cooled crust is defined as the solid outer layer of a planetary surface increasing in temperature with depth, the lower boundary of which, at a depth Z 0 , is at a temperature T 0 corresponding to the zero-pressure melting point of the planetary material. The physical state of the material below the boundary Z 0 will depend on the thermal gradient d T /d Z and the elevation of the melting point with increasing depth due to lithostatic pressure which we indicate by d M /d Z . I assume material of basaltic composition under lunar conditions 1 .