Enrichment of heavy nitrogen in Titan's HC3N detected using ALMA

Trace isotopic ratios contain unique information on the chemical histories of atmospheric gases. From the combined efforts of previous ground and space-based sub-mm and infrared observations, Titan's atmospheric HCN has been found to be substantially enriched in 15N compared with the bulk N2 reservoir. This is theorized to occur as a consequence of isotope-selective photodissociation of N2 in the upper atmosphere, giving rise to an enhanced abundance of atomic 15N that subsequently becomes incorporated into HCN. Here, 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 May 2017 (see Figure 1). The HC3N emission is found to be strongly enhanced over the 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 previous (2015) ALMA observations. The H13CCCN/HCCC15N flux ratio is consistent with an HC3N/HCCC15N ratio of 67 ± 14, which is significantly enriched in 15N compared with the main molecular nitrogen reservoir (which has a 14N/15N ratio of 167). This confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan's organic inventory. In this presentation I will discuss the impact of these results on our understanding of the origin of Titan's N2, the evolution of its atmospheric 14N/15N ratio, and on our knowledge of the detailed photochemical processes occurring in nitrogen-rich planetary atmospheres.