Mapping Vinyl Cyanide and Other Nitriles in Titan’s Atmosphere Using ALMA

Abstract

Vinyl cyanide (C 2 H 3 CN) is theorized to form in Titan's atmosphere via high-altitude photochemistry and is of interest regarding the astrobiology of cold planetary surfaces due to its predicted ability to form cell membrane-like structures (azotosomes) in liquid methane. In this work, we follow up on the initial spectroscopic detection of C 2 H 3 CN on Titan by Palmer et al. (2017) with the detection of three new C 2 H 3 CN rotational emission lines at submillimeter frequencies. These new, high-resolution detections have allowed for the first spatial distribution mapping of C 2 H 3 CN on Titan. We present simultaneous observations of C 2 H 5 CN, HC 3 N, and CH 3 CN emission, and obtain the first (tentative) detection of C 3 H 8 (propane) at radio wavelengths. We present disk-averaged vertical abundance profiles, two-dimensional spatial maps, and latitudinal flux profiles for the observed nitriles. Similarly to HC 3 N and C 2 H 5 CN, which are theorized to be short-lived in Titan's atmosphere, C 2 H 3 CN is most abundant over the southern (winter) pole, whereas the longer-lived CH 3 CN is more concentrated in the north. This abundance pattern is consistent with the combined effects of high-altitude photochemical production, poleward advection, and the subsequent reversal of Titan's atmospheric circulation system following the recent transition from northern to southern winter. We confirm that C 2 H 3 CN and C 2 H 5 CN are most abundant at altitudes above 200 km. Using a 300 km step model, the average abundance of C 2 H 3 CN is found to be 3.03 ± 0.29 ppb, with a C 2 H 5 CN/C 2 H 3 CN abundance ratio of 2.43 ± 0.26. Our HC 3 N and CH 3 CN spectra can be accurately modeled using abundance gradients above the tropopause, with fractional scale-heights of 2.05 ± 0.16 and 1.63 ± 0.02, respectively.

Abundance measurements of Titan’s stratospheric HCN, HC3N, C3H4, and CH3CN from ALMA observations

“…For both molecules, we assumed a Lorentzian broadening HWHM (Γ) value = 0.1 cm −1 bar −1 and temperature dependence (α) = 0.75 as in previous studies (Vinatier et al, 2007;Cordiner et al, 2014;Lai et al, 2017) and recommended by HITRAN. The strongest 4-5 C 3 H 4 transitions were modeled each year, as weaker lines did not significantly contribute to the retrieved vertical abundance profiles.…”

Measurement of CH3D on Titan at Submillimeter Wavelengths

“…We extracted a disk-averaged spectrum of Titan by defining a mask of pixels containing at least 90% of Titan's continuum flux, as in Lai et al (2017). Using the method in previous studies (e.g.…”

“…We thus include N 2 and CH 4 vertical abundance profiles from Niemann et al (2010) and Teanby et al (2013); CO with a constant volume mixing ratio of 50 ppm (see Serigano et al, 2016, and references therein); HC 3 N, using the 2015 disk-averaged abundance profile found in Thelen et al (2019). Though nearby emission lines of C 2 H 3 CN and C 2 H 5 CN do not strongly affect the line shape of the CH 3 D lines and are undetected in the modeled spectral range of these data, these species were included for completeness using abundance profiles from Lai et al (2017). The vertical temperature and abundance profiles included in our model are shown in Fig.…”

Vinyl cyanide (CH2CHCN) in interstellar space: potential spectral lines for its detection

“…Vinyl cyanide (CH 2 CHCN), also known as the acrylonitrile or propenenitrile, may be the best candidate for the formation of cell-like membranes in Titan's hydrocarbon-rich lakes and seas [1] . Lai et al [2] have performed the first spatial distribution mapping of CH 2 CHCN in the atmosphere of Titan with the help of ALMA. Along with C 2 H 5 CN, HC 3 N and CH 3 CN, the new maps of CH 2 CHCN has been spatially resolved on Titan and their abundances are derived using the radiative transfer modeling.…”