In the giant impact theory for lunar origin, the Moon forms from material ejected by the impact into an Earth-orbiting disk. Here we report the initial results from a silicate melt-vapor equilibrium chemistry model for such impact-generated planetary
debris disks. In order to simulate the chemical behavior of a two-phase (melt+vapor) disk, we calculate the temperature-dependent pressure and chemical composition of vapor in equilibrium with molten silicate from 2000 to 4000 K. We consider the elements O, Na, K, Fe, Si, Mg, Ca, Al, Ti, and Zn for a range of bulk silicate compositions (Earth, Moon, Mars, eucrite parent body, angrites, and ureilites). In general, the disk atmosphere is dominated by Na, Zn, and O2 at lower temperatures (< 3000 K) and SiO, O2, and O at higher temperatures. The high-temperature chemistry is consistent for any silicate melt composition, and we thus expect abundant SiO, O2, and O to be a common feature of hot, impact-generated debris disks. In addition, the saturated silicate vapor is highly oxidizing, with oxygen fugacity (fO2) values (and hence H2O/H2 and CO2/CO ratios) several orders of magnitude higher than those in a solar-composition gas. High fO2 values in the disk atmosphere are found for any silicate composition because oxygen is the most abundant element in rock. We thus expect high oxygen fugacity to be a ubiquitous feature of any silicate melt-vapor disk produced via collisions between rocky planets.
Currently, there are about 3 dozen known super-Earth (M < 10 MEarth), of which 8 are transiting planets suitable for atmospheric follow-up observations. Some of the planets are exposed to extreme temperatures as they orbit close to their host stars,
e.g., CoRot-7b, and all of these planets have equilibrium temperatures significantly hotter than the Earth. Such planets can develop atmospheres through (partial) vaporization of their crustal and/or mantle silicates. We investigated the chemical equilibrium composition of such heated systems from 500 - 4000 K and total pressures from 10-6 to 10+2 bars. The major gases are H2O and CO2 over broad temperature and pressure ranges, and Na, K, O2, SiO, and O at high temperatures and low pressures. We discuss the differences in atmospheric composition arising from vaporization of SiO2-rich (i.e., felsic) silicates (like Earths continental crust) and MgO-, FeO-rich (i.e., mafic) silicates like the bulk silicate Earth. The computational results will be useful in planning spectroscopic studies of the atmospheres of Earth-like exoplanets.
