Interferometric Imaging of Titan's HC₃N, H¹³CCCN, and HCCC¹⁵N

We present the first maps of cyanoacetylene isotopologues in Titan's atmosphere, including H¹³CCCN and HCCC¹⁵N, detected in the 0.9 mm band using the Atacama Large Millimeter/submillimeter array (ALMA) around the time of Titan's (southern winter) solstice in 2017 May. The first high-resolution map of HC₃N in its v ₇ = 1 vibrationally excited state is also presented, revealing a unique snapshot of the global HC₃N distribution, free from the strong optical depth effects that adversely impact the ground-state (v = 0) map. The HC₃N emission is found to be strongly enhanced over Titan's south pole (by a factor of 5.7 compared to the north pole), consistent with rapid photochemical loss of HC₃N from the summer hemisphere combined with production and transport to the winter pole since the 2015 April ALMA observations. The H¹³CCCN/HCCC¹⁵N flux ratio is derived at the southern HC₃N peak, and implies an HC₃N/HCCC¹⁵N ratio of 67 ± 14. This represents a significant enrichment in ¹⁵N compared with Titan's main molecular nitrogen reservoir, which has a ¹⁴N/¹⁵N ratio of 167, and confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan's organic inventory.