Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?

TL;DR

This study investigates how chemical evolution in protoplanetary disk midplanes influences the composition of ices and gases, revealing that ionisation levels and initial conditions significantly affect volatile abundances and ratios over time.

Contribution

It provides a comprehensive analysis of chemical evolution effects on volatile species in disk midplanes, highlighting the importance of ionisation and initial conditions in planetary material composition.

Findings

Chemical evolution alters volatile abundances and ratios.

Ionisation level and initial abundances significantly influence chemical outcomes.

Chemical processing impacts C/O ratios in gas and ice with radius.

Abstract

[Abridged] Chemical evolution in the protoplanetary disk midplane can modify the composition of ices and gases. We have investigated if and how chemical evolution affects the abundances and distributions of key volatile species in the midplane of a protoplanetary disk in the 0.2-30 AU range. A full chemical network including gas-phase, gas-grain interactions and grain-surface chemistry is used to evolve chemistry in time, for 1 Myr. Great diversity is observed in the relative abundance ratios of the main considered species: H2O, CO, CO2, CH4, O2, NH3 and N2. The choice of ionisation level, the choice of initial abundances, as well as the extent of chemical reaction types included are all factors that affect the chemical evolution. The only exception is the inheritance scenario with a low ionisation level, which results in negligible changes compared with the initial abundances, regardless of whether grain-surface chemistry is included. The chemical processing changes the C/O ratios for gas and ice significantly with radius and with model assumptions. For high ionisation levels, chemical processing becomes significant after a few times $10^{5}$~yrs. It is concluded that chemical evolution in the disk midplane needs to be considered in the determination of the volatile composition of planetesimals.