Edith C. Fayolle

Hydrogen sulfide (H₂S) is the fifth most abundant molecule observed in the coma of comet 67P/Churyumov–Gerasimenko (67P). Prior to its incorporation into cometary materials, H₂S likely underwent ultraviolet (UV) radiation exposure, which is thought to initiate a complex sulfur chemistry. We present an investigation into the UV photochemistry of H₂S ices using infrared, Raman, and mass spectrometry techniques. Our study reveals the production of complex sulfur allotropes ranging from S₂ to S₆, alongside polysulfanes (H₂S _n , n = 2–3). Temperature-programmed desorption measurements postirradiation of H₂S exhibit two peaks for S₂ molecules: a broad peak between 80 and 140 K and a distinct peak at ∼245 K. Notably, larger allotropes S₃–S₅ exclusively display the 245 K peak. Furthermore, ROSINA measurements of the S₂/H₂S ratio during dust impact events and previously reported S₂/H₂S ratios in the undisturbed coma are compared to our laboratory-determined S₂/H₂S values. This analysis identifies two distinct sources of S₂: a volatile S₂ potentially sublimated directly from the comet’s surface and a secondary source likely resulting from fragmentation of larger sulfur chains during dust impacts. We determined the ratio of produced S₂ to the initial H₂S for both the volatile component and the refractory component at 245 K with both measurements conducted at an irradiation incident fluence of 2.25 × 10¹⁷ photons cm⁻². These laboratory-derived S₂/H₂S ratios exhibit concordance with ROSINA measurements. When extrapolated to incident fluences anticipated in molecular clouds, this photoprocessing mechanism offers a plausible explanation for the measured S₂/H₂S ratio in comet 67P.

H₂CO is a small organic molecule widely detected in protoplanetary disks. As a precursor to grain-surface formation of CH₃OH, H₂CO is considered an important precursor of O-bearing organic molecules that are locked in ices. Still, since gas-phase reactions can also form H₂CO, there remains an open question on the channels by which organics form in disks, and how much the grain versus the gas pathways impact the overall organic reservoir. We present spectrally and spatially resolved Atacama Large Millimeter/submillimeter Array observations of several ortho- and para-H₂CO transitions toward the bright protoplanetary disk around the Herbig Ae star HD 163296. We derive column density, excitation temperature, and ortho-to-para ratio (OPR) radial profiles for H₂CO, as well as disk-averaged values of N _T ∼ 4 × 10¹² cm⁻², T _ex ∼ 20 K, and OPR ∼ 2.7, respectively. We empirically determine the vertical structure of the emission, finding vertical heights of z/r ∼ 0.1. From the profiles, we find a relatively constant OPR ∼ 2.7 with radius, but still consistent with 3.0 among the uncertainties, a secondary increase of N _T in the outer disk, and low T _ex values that decrease with disk radius. Our resulting radial, vertical, and OPR constraints suggest an increased UV penetration beyond the dust millimeter edge, consistent with an icy origin but also with cold gas-phase chemistry. This Herbig disk contrasts previous results for the T Tauri disk, TW Hya, which had a larger contribution from cold gas-phase chemistry. More observations of other sources are needed to disentangle the dominant formation pathway of H₂CO in protoplanetary disks.

Enceladus is unique as an astrobiology target in that it hosts an active plume sourced directly from its habitable subsurface ocean. Ice particles from the plume contain geochemical constituents that are diagnostic of the ocean conditions, and may hold traces of life and/or biosignatures, if they exist. Up to 93% of the plume particles fall back onto the surface of Enceladus. The low radiation environment and present-day activity are favorable to the preservation of any complex organics and putative biosignatures contained within these particles. Laboratory experiments and modeling suggest that plume deposits would likely be weakly consolidated and relatively easy to sample. Sampling systems like a dual rasp, under development to achieve technology readiness level (TRL) 5 in 2021, would enable a landed mission on Enceladus’ surface to acquire large amounts of surface materials, a requirement for analysis of trace constituents. A landed mission on Enceladus could greatly enhance our understanding of the chemical makeup of plume particles and the subsurface ocean, and seek traces of life and/or biosignatures.

This work investigated microorganism survival under temperature and ultraviolet (UV) radiation conditions found at the surface of ice-covered ocean worlds. These studies were motivated by a desire to understand the ability of resilient forms of life to survive under such conditions as a proxy for potential endogenic life and to inform planetary protection protocols for future missions. To accomplish this, we irradiated Bacillus subtilis spores with solar-like UV photons at temperatures ranging from room temperature down to 11 K and reported survival fractions with respect to fluence. We observed an increase in survival at low temperatures and found that the inactivation rate follows an Arrhenius-type behavior above 60 K. For solar-photon fluxes and surface temperatures at Europa and Enceladus, we found that Bacillus subtilis spores would be inactivated in less than an hour when in direct sunlight.

Small hydrocarbons are an important organic reservoir in protostellar and protoplanetary environments. Constraints on desorption temperatures and binding energies of such hydrocarbons are needed for accurate predictions of where these molecules exist in the ice versus gas phase during the different stages of star and planet formation. Through a series of temperature programmed desorption experiments, we constrain the binding energies of 2- and 3-carbon hydrocarbons (C₂H₂—acetylene, C₂H₄—ethylene, C₂H₆—ethane, C₃H₄—propyne, C₃H₆—propene, and C₃H₈—propane) to 2200–4200 K in the case of pure amorphous ices, to 2400–4400 K on compact amorphous H₂O, and to 2800–4700 K on porous amorphous H₂O. The 3-carbon hydrocarbon binding energies are always larger than the 2-carbon hydrocarbon binding energies. Within the 2- and 3-carbon hydrocarbon families, the alkynes (i.e., least-saturated) hydrocarbons exhibit the largest binding energies, while the alkane and alkene binding energies are comparable. Binding energies are ∼5%–20% higher on water ice substrates compared to pure ices, which is a small increase compared to what has been measured for other volatile molecules such as CO and N₂. Thus in the case of hydrocarbons, H₂O has a less pronounced effect on sublimation front locations (i.e., snowlines) in protoplanetary disks.

Context. Methanol (CH₃OH) is at the root of organic ice chemistry in protoplanetary disks. Its connection to prebiotic chemistry and its role in the chemical environment of the disk midplane make it an important target for disk chemistry studies. However, its weak emission has made detections difficult. To date, gas-phase CH₃OH is detected in only one Class II disk, TW Hya. Aims. We aim to constrain the methanol content of the HD 163296 protoplanetary disk. Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to search for a total of four CH₃OH emission lines in bands six and seven toward the disk around the young Herbig Ae star HD 163296. The disk-averaged column density of methanol and its related species formaldehyde (H₂CO) were estimated assuming optically thin emission in local thermodynamic equilibrium. We compared these results to the gas-phase column densities of the TW Hya disk. Results. No targeted methanol lines were detected with Keplerian masking in the image plane nor with matched filter analysis in the uv plane individually nor after line stacking. The 3σ disk-integrated intensity upper limits are −1 for the band six lines and −1 for the band seven lines. The band seven lines provide the strictest 3σ upper limit on disk-averaged column density with N_avg < 5.0 × 10¹¹ cm⁻². The methanol-to-formaldehyde ratio is CH₃OH∕H₂COConclusions. The HD 163296 protoplanetary disk is less abundant in methanol with respect to formaldehyde compared to the disk around TW Hya. Differences in the stellar irradiation in this Herbig Ae disk as compared to that of a disk around a T Tauri star likely influence the gaseous methanol and formaldehyde content. Possible reasons for the lower HD 163296 methanol-to-formaldehyde ratio include: a higher than expected gas-phase formation of H₂CO in the HD 163296 disk, uncertainties in the grain surface formation efficiency of CH₃OH and H₂CO, and differences in the disk structure and/or CH₃OH and H₂CO desorption processes that drive the release of the molecules from ice mantles back into the gas phase. These results provide observational evidence that the gas-phase chemical complexity found in disks may be strongly influenced by the spectral type of the host star.