The evolution of terrestrial-type planets

The present internal state of the earth and the age of various events such as core formation and continental formation can be used to place fairly strong constraints on the early history of the earth and, presumably, the other terrestrial planets. The present internal configuration of the earth shows that it is an extremely differentiated body, and the ages of surface rocks indicate that this differentiation took place early in the history of the earth. The gravitational energy associated with the formation of the earth and the thermal energy of core formation are the primary sources of energy for the melting and subsequent gravitational separation of the earth into a core, a crust, a chemically inhomogeneous mantle, and an atmosphere and hydrosphere. The larger the planet the more likely it is to be extensively differentiaed and outgassed. The earth probably accreted in 10(5) years or less. This causes the initial temperatures to be high and results in core formation to be essentially simultaneous with accretion. Mars can be accreted without differentiation and with minor outgassing.