Detection of Propadiene on Titan

TL;DR

This paper reports the first confirmed detection of propadiene in Titan's atmosphere using infrared spectroscopy, providing new data to understand Titan's organic chemistry and atmospheric composition.

Contribution

The study presents the first unambiguous detection of propadiene in an astronomical object, with spectroscopic analysis and abundance measurement on Titan.

Findings

Detected propadiene at 12 μm in Titan's atmosphere.

Measured a volume mixing ratio of (6.9 ± 0.8) × 10^{-10}.

Found an abundance ratio of propyne to propadiene of 8.2 ± 1.1.

Abstract

The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH$_{2}$CCH$_{2}$, an isomer of the well-measured propyne (also called methylacetylene) CH$_{3}$CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule - which has recently been remedied with an updated spectral line list. Here we present the first unambiguous detection of the molecule in any astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility (IRTF) in July 2017. We model its emission line near 12 $\mu$m and measure a volume mixing ratio (VMR) of (6.9 $\pm$ 0.8) $\times$10$^{-10}$ at 175 km, assuming a vertically increasing abundance profile as predicted in photochemical models. Cassini measurements of propyne made during April 2017 indicate that the abundance ratio of propyne to propadiene is 8.2$\pm$1.1 at the same altitude. This initial measurement of the molecule in Titan's stratosphere paves the way towards constraining the amount of atomic hydrogen available on Titan, as well as future mapping of propadiene on Titan from 8 meter and larger ground based observatories, and future detection on other planetary bodies.