Research

Entrapment of CO in CO₂ Ice

Planet atmosphere and hydrosphere compositions are fundamentally set by accretion of volatiles, and therefore by the division of volatiles between gas and solids in planet-forming disks. For hyper-volatiles such as CO, this division is regulated by volatile sublimation energies, and by the ability of other ice components to entrap. Water ice is known for its ability to trap CO and other volatile species. In this study we explore whether another common interstellar and cometary ice component, CO₂, is able to trap CO as well. We measure entrapment of CO molecules in CO₂ ice through temperature-programmed desorption experiments on CO₂:CO ice mixtures. We find that CO2₂ice traps CO with a typical efficiency of 40%–60% of the initially deposited CO molecules for a range of ice thicknesses between 7 and 50 monolayers, and ice mixture ratios between 1:1 and 9:1. The entrapment efficiency increases with ice thickness and CO dilution. We also run analogous H₂O:CO experiments and find that under comparable experimental conditions, CO₂ ice entraps CO more efficiently than H₂O ice up to the onset of CO₂ desorption at ∼70 K. We speculate that this may be due to different ice restructuring dynamics in H₂O and CO₂ ices around the CO desorption temperature. Importantly, in planet-forming disks, the ability of CO₂ to entrap CO may change the expected division between gas and solids for CO and other hyper-volatiles exterior to the CO₂ snowline. Read more.

Entrapment of Hypervolatiles in Interstellar and Cometary H₂O and CO₂ Ice Analogs

Planets and planetesimals acquire their volatiles through ice and gas accretion in protoplanetary disks. In these disks, the division of volatile molecules between the condensed and gaseous phases determines the quantity of volatiles accreted by planets in different regions of the disk. This division can be strongly affected by entrapment of volatiles into less volatile ice matrices, resulting in different radial profiles of common volatiles and elemental ratios than would otherwise be expected. In this study we use laboratory experiments to explore the ability of abundant interstellar and cometary ice matrices, i.e., H₂O and CO₂, to trap the hypervolatiles ¹³CO, ¹²CH₄, ¹⁵N₂, and Ar. We measure entrapment efficiencies through temperature programmed desorption for two ice thicknesses (10 and 50 monolayers) and two mixing ratios (3:1 and 10:1) for each matrix:volatile combination. We find that ice entrapment efficiencies increase with ice thickness and ice mixing ratio to a maximum of ∼65% for all hypervolatiles. Entrapment efficiencies are comparable for all hypervolatiles, and for the two ice matrices. We further find that the entrapment efficiency is relatively insensitive to the ice deposition temperature between 10 and 30 K with the possible exception of CH₄ in CO₂ ice. Together these results suggest that hypervolatile entrapment at low temperatures (<30 K) is a remarkably robust and species-independent process. Read more.

Characterizing H-D exchange in UV irradiated water:methane ice mixtures (in prep.)

Present-day D/H ratios in cometary water and organics provide clues about the formation environment of the Solar System’s planets and planetesimals, and by extension to the chemistry of planet formation more generally. Decoding these clues is complicated, however, possible by a range of H-D exchange reactions in ice. In particular, H-D exchanges can occur thermally, or through irradiation or atom bombardment. Quantifying the relative importance of these processes for the H/D record requires laboratory experiments. I will present the outcome of one such experiment, focusing on the UV-induced H-D scrambling between water and organics in ice mixtures maintained at different temperatures, ranging from 10 to 70 K. In these experiments D₂O:CH₄ or H₂O:CD₄ ices are deposited at ~10 K and subsequently heated to a desired temperature. This is followed by prolonged exposure to UV photons from a well-characterized UV lamp. We use this data to constrain the D-H exchange rate at different ice temperatures, any energy barriers, as well as the steady-state D/H ratio in water and methane under different ice conditions. To gain a deeper understanding of the D-H exchange, we have explored multiple experimental conditions, including different ratios, thicknesses, and the choice between co-deposition and pre-mixed components. We then use these results to discuss the interpretation of observations of D/H in water and small organics within comets.

Formation of Deuterated Methanol from Oxygen Insertion (in prep.)

This project focuses on the formation of deuterated methanol ices from different O-bearing species. We focus on CH₃D and O-bearing (O₂, CO₂, or oxygen bombardment) under VUV irradiation. The resulting d-methanol could be CH₂DOH or CH₃OD, giving us insights into deuterium selection.

Database for Pure Deuterated Organics & Water:Organic Ice Mixtures (in prep.)

This project focuses on creating a spectroscopy and temperature-programmed desorption (TPD) database for a large variety of pure deuterated species: ND₃, CD₄, DCOOD, D₂O, and all D-methanol species. Mixed organics in water will also be studied.