Depletion of Moderately Volatile Elements by Open-System loss in the Early Solar Nebula

TL;DR

This study proposes a new model where open-system loss of vaporized moderately volatile elements at high nebula altitudes explains their depletion in inner solar system bodies, accounting for observed compositional patterns.

Contribution

It introduces a novel hypothesis involving open-system loss processes at high nebula altitudes, supported by modeling of mass loss and particle dynamics, to explain MVE depletion.

Findings

Open system loss processes can cause significant MVE depletion.

Mass loss rates higher than typical disk winds are needed, indicating early intense outbursts.

Irreversible loss of refractory elements explains their relative enhancement in chondrites.

Abstract

Rocky bodies of the inner solar system display a systematic depletion of the "Moderately Volatile Elements" (MVEs) that correlates with the expected condensation temperature of their likely host materials under protoplanetary nebula conditions. In this paper, we present and test a new hypothesis in which open system loss processes irreversibly remove vaporized MVEs from high nebula altitudes, leaving behind the more refractory solids residing much closer to the midplane. The MVEs irreversibly lost from the nebula through these open system loss processes are then simply unavailable for condensation onto planetesimals forming even much later, after the nebula cooled, overcoming a critical difficulty encountered by previous models of this type. We model open system loss processes operating at high nebula altitudes, such as resulting from disk winds flowing out of the system entirely, or layered accretion directly onto the young sun. We find that mass loss rates higher than found in typical T-Tauri disk winds, lasting short periods of time, are most satisfactory, pointing to multiple intense early outburst stages. Using our global nebula model, incorporating realistic particle growth and inward drift for solids, we constrain how much the MVE depletion signature in the inner region is diluted by the drift of undepleted material from the outer nebula. We also find that a significant irreversible loss of the common rock-forming elements (Fe, Mg, Si) can occur, leading to a new explanation of another longstanding puzzle of the apparent "enhancement" in the relative abundance of highly refractory elements in chondrites.