  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-{\sigma} 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.
