To test these ideas, we computed detailed models of the thermal state of the Moon during accretion. We pay special attention to the structure of the silicate atmosphere and its loss rate by calculating the chemical species at equilibrium. Because of the large heats of vaporization and ionization, the adiabatic atmosphere is nearly isothermal and massive escape is expected. The pressure of the atmosphere is determined by the balance between vaporization of accreting material and escape. If the accretion time is one month, a 0.3 bar atmosphere is expected.
Elemental fractionation depends strongly on the temperature of the accreting material.
The initial temperature of the material can be estimated from the condition of gravitational instability in the proto-lunar disk. As shown by Ida et al., accretion starts when gravitational instability spreads the disk beyond the Roche limit. If the Moon is formed from the accretion of such material, volatile elements such as Na and K are retained in the Moon only atearly in accretion. At later times, K and Na are lost, and a fraction of the Mg, Si and Fe are lost. However, refractory elements such as Ca and Al are retained.