The Effect of Accretion Rate and Composition on the Structure of Ice-rich Super-Earths

TL;DR

This study models the formation and evolution of ice-rich Super-Earths, showing how accretion rate and composition influence their internal structure and surface properties over billions of years.

Contribution

It introduces a thermal evolution model that incorporates accretional heating, radioactive decay, and ice-rock separation to predict planetary structure based on formation parameters.

Findings

Final planetary structure depends on accretion rate and ice-rock ratio.

High accretion rates and low ice content lead to rocky cores.

Significant ice evaporation occurs during accretion, affecting surface composition.

Abstract

It is reasonable to assume that the structure of a planet and the interior distribution of its components are determined by its formation history. We thus follow the growth of a planet from a small embryo through its subsequent evolution. We estimate the accretion rate range based on a protoplanetary disk model at a large enough distance from the central star, for water ice to be a major component. We assume the accreted material to be a mixture of silicate rock and ice, with no H-He envelope, as the accretion timescale is much longer than the time required for the nebular gas to dissipate. We adopt a thermal evolution model that includes accretional heating, radioactive energy release, and separation of ice and rock. Taking the Safronov parameter and the ice-to-rock ratio as free parameters, we compute growth and evolutionary sequences for different parameter combinations, for 4.6 Gyr. We find the final structure to depend significantly on both parameters. Low initial ice to rock ratios and high accretion rates, each resulting in increased heating rate, lead to the formation of extended rocky cores, while the opposite conditions leave the composition almost unchanged and result in relatively low internal temperatures. When rocky cores form, the ice-rich outer mantles still contain rock mixed with the ice. We find that a considerable fraction of the ice evaporates upon accretion, depending on parameters, and assume it is lost, thus the final surface composition and bulk density of the planet do not necessarily reflect the protoplanetary disk composition.