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 $3.03\pm0.29$ ppb, with a C$_2$H$_5$CN/C$_2$H$_3$CN abundance ratio of $2.43\pm0.26$. Our HC$_3$N and CH$_3$CN spectra can be accurately modeled using abundance gradients above the tropopause, with fractional scale-heights of $2.05\pm0.16$ and $1.63\pm0.02$, respectively.