VERTICAL DISTRIBUTION OF C ₃ -HYDROCARBONS IN THE STRATOSPHERE OF TITAN

Abstract

Motivated by the recent detection of propene (C 3 H 6) in the atmosphere of Titan, we use a one-dimensional Titan photochemical model with an updated eddy diffusion profile to systematically study the vertical profiles of the stable species in the C 3-hydrocarbon family. We find that the stratospheric volume mixing ratio of propene (C 3 H 6) peaks at 150 km with a value of 5 × 10 −9 , which is in good agreement with recent observations by the Composite Infrared Spectrometer on the Cassini spacecraft. Another important species that is currently missing from the hydrocarbon family in Titan's stratosphere is allene (CH 2 CCH 2), an isomer of methylacetylene (CH 3 C 2 H). We predict that its mixing ratio in the stratosphere is about 10 −9 , which is on the margin of the detection limit. CH 2 CCH 2 and CH 3 C 2 H equilibrate at a constant ratio in the stratosphere by hydrogen-exchanging reactions. Thus, by precisely measuring the ratio of CH 2 CCH 2 to CH 3 C 2 H, the abundance of atomic hydrogen in the atmosphere can be inferred. No direct yield for the production of cyclopropane (c-C 3 H 6) is available. From the discharge experiments of Navarro-González & Ramírez, the abundance of cyclopropane is estimated to be 100 times less than that of C 3 H 6 .

Titan's atmosphere and climate

“…Photochemical models are now able to reproduce the observed abundances of most of the small (less than 100 Da) molecules in Titan's atmosphere [see, e.g., Vuitton et al , ; Hörst et al , ; Lavvas et al , , ; Vuitton et al , ; Yelle et al , ; Krasnopolsky , ; Vuitton et al , ; Westlake et al , ; Hébrard et al , ; Dobrijevic et al , ; Li et al , ; Dobrijevic et al , ] but the formation of heavier molecules is not well understood. Figure shows a summary of the major production and loss pathways for 10 of the most abundant photochemically produced molecules in Titan's atmosphere (see extensive discussion in Vuitton et al []).…”

“…Photochemical models are now able to reproduce the observed abundances of most of the small (less than 100 Da) molecules in Titan's atmosphere [see, e.g., Vuitton et al, 2007;Hörst et al, 2008;Lavvas et al, 2008aLavvas et al, , 2008bVuitton et al, 2008;Yelle et al, 2010;Krasnopolsky, 2012;Vuitton et al, 2012;Westlake et al, 2012;Hébrard et al, 2013;Dobrijevic et al, 2014;Li et al, 2015;Dobrijevic et al, 2016] but the formation of heavier molecules is HÖRST Figure 3. Shown here are the 10 most abundant photochemically generated molecules in Titan's equatorial stratosphere in approximately decreasing order of abundance with their major atmospheric production and loss pathways listed [see, e.g., Vuitton et al, 2014].…”

Atmospheric Circulation, Chemistry, and Infrared Spectra of Titan-like Exoplanets around Different Stellar Types

“…KINETICS solves the 1-D continuity equation for each chemical species, taking into account chemical production and loss rates, and transport in a 1-D column due to molecular and eddy diffusion. KINETICS has been used to model Titan photochemistry since its inception (Yung et al 1984), and more recently it has been validated using Cassini observations (Li et al 2014(Li et al , 2015 that constrained Titan's atmospheric eddy diffusivity and the chemical pathways controlling the abundances of C 2 and C 3 hydrocarbons.…”

Solid‐state photochemistry as a formation mechanism for Titan's stratospheric C4N2 ice clouds

“…Although quantitative investigations of these and other questions lie outside the scope of the present study, a few advances are already under way. The photochemical model of Titan developed by Yung et al [] has recently been updated in light of Cassini Ultraviolet Imaging Spectrograph and CIRS measurements [ Liang et al , ; Li et al , , ]. Reactions (R1)–(R6) have been incorporated into the model, and preliminary microphysical modeling using the Community Aerosol and Radiative Model for Atmospheres [ Bardeen et al , ] provides a highly provisional initial estimate that the equivalent of a vertical column of 1 × 10 17 molecules cm −2 of C 4 N 2 may be sequestered in Titan's atmospheric particles.…”