Presentation #102.382 in the session Poster Session.
One of Kepler’s key findings is that the most abundant planets in our galaxy, observed to date, are larger than Earth but smaller than Neptune. In order to understand the distribution of elements in sub-Neptunes and their potential evolution to form super-Earths, we are investigating the equilibrium chemistry between metal cores, silicate magma oceans, and initially hydrogen-rich atmospheres for sub-Neptunes. Towards this end, we have developed a reaction network composed of 18 independent reactions among 25 phase components representing sub-Neptune-like exoplanets. Both reactive metal and unreactive metal sequestered in an isolated core, are modeled. Redox chemistry dominates these systems, with oxidation of the hydrogen-rich envelopes and reduction of the magma oceans. We find hydrogen and oxygen should comprise significant fractions of metal cores where the cores, silicate melts, and atmospheres are in chemical equilibrium. Light elements in metal cores leads to density deficits that offer a possible alternative explanation for the densities of the Trappist-1 planets. In addition, hydrogen occurs at < 1% by mass in silicate mantles, setting a maximum limit to the hydrogen-budget for out-gassing by future super-Earths. With a few exceptions, the total hydrogen-budget of most sub-Neptunes can be, to first order, well estimated from their atmospheres alone, as more than 90% of all H resides in the envelopes. However, if silicate and metal equilibrate at T > ~ 5500 K with modest relative masses fractions of H for the planet, roughly comparable amounts of H can reside in the metal core and in the atmosphere. Reactions between magma oceans and hydrogen envelopes produces significant amounts of SiO and H2O in the envelopes directly above the magma ocean, increasing their mean molecular weights by more than a factor of two. Isolation of metal cores from reaction with silicate and hydrogen envelopes removes metal as a sink for light elements, leading to greater concentrations of H2O in magma oceans and in atmospheres.