Yale University, USA
We propose a new phenomenon affecting the settling of dust grains at the terrestrial region in early protoplanetary disks, which is one of the most fundamental processes in planetary formation.
Dust grains are subjected to evaporation in the hot inner region of an early disk stage, and the effects of condensation and evaporation on the vertical dust settling are crucial, although these effects of chemistry have not been considered in previous studies. We will here present a simple thermodynamically-consistent model to quantify the effects of chemistry on vertical dust settling. Our approach includes at least two following notable points. First, dust properties are calculated through Gibbs free energy minimization rather than using condensation temperature. Second, the disk thermal structure is calculated using the opacity consistent with the stable phases calculated from Gibbs free energy minimization. The existence of dust will change the opacity by orders of magnitude, thus affecting the whole disk temperature structure. Such temperature change will feed directly back to the chemistry, possibly altering the stable phases, and further modifying the opacity. The purpose of this study is to demonstrate quantitatively this feedback between physics and chemistry during the dust settling process. Our study shows that dust grains evaporate as they sink towards the hotter mid-plane and form a “condensation front” above which dust-composing elements, Mg, Si, and Fe, are concentrated creating a large temperature gap. Modeling results show that repeating evaporation at the front inhibits grain growth, resulting in slow settling with a time scale comparable to that of radial evolution. The predicted time scale at 1 AU could be longer than 105 years, while an order of 103 years is suggested by the standard theory of planetary formation.
The new model might also explain some refractory element abundances recorded in chondrites, whereas the existing explanation of incomplete condensation fails to explain. Our results show different settling behaviors between refractor elements and Mg, Si, and Fe. Mg, Si, Fe-bearing grains would form a condensation front and settle in a long timescale, but Al and Ca would not form the front and are likely to settle quickly towards the mid-plane. Al and Ca could end up in a large grain size and experience radial drifting. This could potentially explain the high Al/Si ratio in the terrestrial mantle and the low Al/Si ratio in ordinary chondrites. Our study shows an importance of building a complete model that takes into account both the physical and chemical evolution of protoplanetary disks.
Keywords: Protoplanetary disk evolution, Cosmochemistry, Terrestrial planets formation