Detection of Cyclopropenylidene on Titan with ALMA

Abstract

We report the first detection on Titan of the small cyclic molecule cyclopropenylidene (c-C3H2) from high sensitivity spectroscopic observations made with the Atacama Large Millimeter/sub-millimeter Array (ALMA). Multiple lines of cyclopropenylidene were detected in two separate datasets: ~251 GHz in 2016 (Band 6) and ~352 GHz in 2017 (Band 7). Modeling of these emissions indicates abundances of 0.50 +/- 0.14 ppb (2016) and 0.28 +/- 0.08 (2017) for a 350 km step model, which may either signify a decrease in abundance, or a mean value of 0.33 +/- 0.07 ppb. Inferred column abundances are (3-5)E12 cm-2 in 2016 and (1-2)E12 cm-2 in 2017, similar to photochemical model predictions. Previously the C3H3+ ion has been measured in Titan's ionosphere by Cassini's Ion and Neutral Mass Spectrometer (INMS), but the neutral (unprotonated) species has not been detected until now, and aromatic versus aliphatic structure could not be determined by the INMS. Our work therefore represents the first unambiguous detection of cyclopropenylidene, the second known cyclic molecule in Titan's atmosphere along with benzene (C6H6) and the first time this molecule has been detected in a planetary atmosphere. We also searched for the N-heterocycle molecules pyridine and pyrimidine finding non-detections in both cases, and determining 2-σ upper limits of 1.15 ppb (c-C5H5N) and 0.85 ppb (c-C4H4N2) for uniform abundances above 300 km. These new results on cyclic molecules provide fresh constraints on photochemical pathways in Titan's atmosphere, and will require new modeling and experimental work to fully understand the implications for complex molecule formation.

arXiv comment34 pages, 13 figures. 8 tables

References in corpus (23)

• Detection of the Aromatic Molecule Benzonitrile ($c$-C${}_6$H${}_5$CN) in the Interstellar Medium
• Titan's Atmosphere and Climate
• TEXES: A Sensitive High-Resolution Grating Spectrograph for the Mid-Infrared
• Molecular Carbon Chains and Rings in TMC-1
• Detection of Propene in Titan's Stratosphere
• Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA
• A Search for Interstellar Pyrimidine
• A new astrobiological model of the atmosphere of Titan
• Rotational spectroscopy of isotopic vinyl cyanide, H${}_2$C=CH$-$C$\equiv$N, in the laboratory and in space
• ALMA Observations of HCN and its Isotopologues on Titan
• Isotopic Ratios of Carbon and Oxygen in Titan's CO using ALMA
• Abundance Measurements of Titan's Stratospheric HCN, HC${}_3$N, C${}_3$H${}_4$, and CH${}_3$CN from ALMA Observations
• Upper limits for undetected trace species in the stratosphere of Titan
• Deep K-band observations of TMC-1 with the Green Bank Telescope: Detection of HC7O, non-detection of HC11N, and a search for new organic molecules
• Spatial and Seasonal Variations in C3Hx Hydrocarbon Abundance in Titan's Stratosphere from Cassini CIRS Observations
• Spatial Variations in Titan's Atmospheric Temperature: ALMA and Cassini Comparisons from 2012 to 2015
• ALMA Spectral Imaging of Titan Contemporaneous with Cassini's Grand Finale
• Interferometric imaging of Titan's HC${}_3$N, H${}^{13}$CCCN and HCCC${}^{15}$N
• Mapping Vinyl Cyanide and Other Nitriles in Titan's Atmosphere Using ALMA
• Optimization by Smoothed Bandpass Calibration in Radio Spectroscopy
• ${}^{14}$N/${}^{15}$N isotopic ratio in CH${}_3$CN of Titan's atmosphere measured with ALMA
• Upper limits for PH3 and H2S in Titan's Atmosphere from Cassini CIRS
• Measurement of CH${}_3$D on Titan at Submillimeter Wavelengths

Cited by in corpus (6)

• The Composition and Chemistry of Titan's Atmosphere
• Detection of CH${}_3$C${}_3$N in Titan's Atmosphere
• Material Properties of Organic Liquids, Ices, and Hazes on Titan
• Experimental Characterization of the Pyridine:Acetylene Co-crystal and Implications for Titan's Surface
• The D/H ratio in Titan's acetylene from high spectral resolution IRTF/TEXES observations
• Open Questions and Future Directions in Titan Science