Adsorption of volatiles on dust grains in protoplanetary disks

TL;DR

This study uses advanced quantum calculations to analyze how volatile molecules adsorb onto dust grains in protoplanetary disks, revealing different mechanisms on carbonaceous versus silicate surfaces and implications for planet formation.

Contribution

It provides detailed adsorption energies and mechanisms for H2, H2O, and CO on dust grain analogs, highlighting the contrasting chemisorption and physisorption processes.

Findings

Weak physisorption on carbonaceous surfaces with low adsorption energies.

Strong chemisorption on silicate surfaces with higher adsorption energies.

Cationic molecules like CO have increased desorption temperatures due to co-adsorption effects.

Abstract

The adsorption of volatile molecules onto dust grain surfaces fundamentally influences dust-related processes, including condensation of gas-phase molecules, dust coagulation, and planet formation in protoplanetary disks. Using advanced ab-initio density functional theory with r$^2$SCAN+rVV10 van der Waals functionals, we calculate adsorption energies of H$_2$, H$_2$O, and CO on carbonaceous (graphene, amorphous carbon) and silicate (MgSiO$_3$) surfaces. Results reveal fundamentally different adsorption mechanisms: weak physisorption on carbonaceous surfaces ($|\Delta\epsilon_{\rm ad}|\sim 0.1-0.2~{\rm eV}$) versus strong chemisorption on silicates ($|\Delta\epsilon_{\rm ad}|\sim 0.5-1.5~{\rm eV}$) via coordination bonds. Kinetic Monte Carlo simulations incorporating these energies demonstrate divergent surface evolution: carbonaceous grains exhibit distinct condensation radius compared to silicates, while the cocrystal of H$_2$O and CO significantly increases the desorption temperature of CO. The actual radii of gas-phase molecule depletion could thus be a comprehensive result of temperatures, chemical compositions, and even evolution tracks. Meanwhile, silicates maintain chemisorbed molecular coatings throughout most disk regions. Such dichotomy in surface coverage could also provide a natural mechanism for carbon depletion in inner planetary systems.