Detection of Propadiene on Titan

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.

References in corpus (14)

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• Detection of Propene in Titan's Stratosphere
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• Upper limits for undetected trace species in the stratosphere of Titan
• Propane on Titan
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• Ethane in Titan's Stratosphere from Cassini CIRS Far- and Mid-Infrared Spectra
• Measurement of CH${}_3$D on Titan at Submillimeter Wavelengths

Cited by in corpus (10)

• The Composition and Chemistry of Titan's Atmosphere
• Understanding planetary context to enable life detection on exoplanets and test the Copernican principle
• Data availability and requirements relevant for the Ariel space mission and other exoplanet atmosphere applications
• Material Properties of Organic Liquids, Ices, and Hazes on Titan
• Vertical distribution of cyclopropenylidene and propadiene in the atmosphere of Titan
• Spitzer IRS Observations of Titan as a Precursor to JWST MIRI Observations
• Open Questions and Future Directions in Titan Science
• The D/H ratio in Titan's acetylene from high spectral resolution IRTF/TEXES observations
• Detection of propadiene (CH${}_2$CCH${}_2$), propene (C${}_3$H${}_6$) and non-detection of propane (C${}_3$H${}_8$) in Jupiter's northern polar stratosphere
• Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres