Photoreactivity of condensed acetylene on Titan aerosols analogues

Abstract

Volatile organic molecules formed by photochemistry in the upper atmosphere of Titan can undergo condensation as pure ices in the stratosphere and the troposphere as well as condense as ice layers onto the organic aerosols that are visible as the haze layers of Titan. As solar photons penetrate through Titan's atmosphere, shorter-wavelength photons are attenuated and longerwavelength photons make it into the lower altitudes, where aerosols become abundant. We conducted an experimental study to evaluate the long wavelength (λ > 300 nm) photo-reactivity of these ices accreted on the Titan aerosol-analogs (also known as tholins) made in the laboratory. We have focused on acetylene, the third most abundant hydrocarbon in Titan's atmosphere (after CH 4 and C 2 H 6 ). Further, acetylene is the most abundant unsaturated hydrocarbon in Titan's atmosphere. Our results indicate that the aerosols can act as activation centers to drive the photoreactivity of acetylene with the aerosols at the accretion interface at wavelengths where acetylene-ice alone does not show photoreactivity. We found that along with photochemistry, photodesorption plays an important role. We observed that about 15% of the initial acetylene is photodesorbed, with a photodesorption rate of (2.1 ± 0.2) × 10 -6 molecules·photon -1 at 355 nm. This photodesorption is wavelength-dependent, confirming that it is mediated by the UV absorption of the aerosol analogues, similar to photochemistry. We conclude that the UV-Vis properties of aerosols would determine how they evolve further in Titan's atmosphere and on the surface through photochemical alterations involving longerwavelength photons. Stronger extinction coefficients in the longer wavelength UV-Vis regions (>300 nm) of the aerosol give higher efficiencies of photodesorption of accreted volatiles as well as photochemical incorporation of unsaturated condensates (such as acetylene) into the aerosols.

Infrared spectra and optical constants of astronomical ices: III. Propane, propylene, and propyne

“…Studies of the chemistry of worlds on which solid hydrocarbons exist, such as Pluto, Titan, and trans-Neptunian objects (TNOs), can benefit from our results in the quantification of ice composition (e.g., Brown et al, 2015;Sasaki et al, 2005). Similarly, laboratory studies of the reaction chemistry of small molecules of planetary and interstellar interest find our work relevant for measuring molecular abundances and reaction yields (e.g., Dartois et al, 2017;Fleury et al, 2019;Behmard et al, 2019).…”

Detection of Aerosols at Microbar Pressures in an Exoplanet Atmosphere

“…These observations lead to another question: if photochemical haze produces the Rayleigh scattering observed in WASP-69b, why is it probing more scale heights in this planet than in other observations of more strongly irradiated planets? Also, laboratory studies of haze formation in hot Jupiter atmospheres show that photochemical haze formation occurs when the C/O > 1 and is less efficient for C/O < 1 (Fleury et al 2019(Fleury et al , 2020. Although the C/O retrieved here for WASP-69b does not have tight constraints, there is some preference for C/O lower than sub-solar values.…”

Adsorption isotherms and nucleation of methane and ethane on an analog of Titan’s photochemical aerosols

“…Following Dimitrov & Bar-Nun (2002), these chemical changes should correspond to a hardening of the material and an increase of the surface tension by a factor 10-100. A151, page 9 of 12 A&A 631, A151 2019In a series of papers, Gudipati et al (2013), Couturier-Tamburelli et al (2014, 2015, and Fleury et al (2019) investigated the transformation under irradiation of C 4 N 2 , HC 5 N, HC 3 N, HCN, and C 2 H 2 ices into photochemical material. They used wavelengths larger than 230 nm or 300 nm, relevant to process that may occur in the low atmosphere.…”