Interferometric imaging of Titan's HC$_3$N, H$^{13}$CCCN and HCCC$^{15}$N

TL;DR

This study presents the first high-resolution maps of Titan's cyanoacetylene isotopologues and vibrationally excited HC$_3$N, revealing spatial distribution, isotopic ratios, and insights into Titan's atmospheric photochemistry.

Contribution

It provides the first interferometric maps of Titan's HC$_3$N isotopologues and vibrationally excited states, offering new insights into atmospheric composition and nitrogen isotopic ratios.

Findings

HC$_3$N is strongly enhanced over Titan's south pole.

HC$_3$N/HCCC$^{15}$N ratio is approximately 67.

Significant $^{15}$N enrichment compared to Titan's main nitrogen reservoir.

Abstract

We present the first maps of cyanoacetylene isotopologues in Titan's atmosphere, including H$^{13}$CCCN and HCCC$^{15}$N, 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. The first high-resolution map of HC$_3$N in its $v_7=1$ vibrationally excited state is also presented, revealing a unique snapshot of the global HC$_3$N distribution, free from the strong optical depth effects that adversely impact the ground-state ($v=0$) map. The HC$_3$N 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 HC$_3$N from the summer hemisphere combined with production and transport to the winter pole since the April 2015 ALMA observations. The H$^{13}$CCCN/HCCC$^{15}$N flux ratio is derived at the southern HC$_3$N peak, and implies an HC$_3$N/HCCC$^{15}$N ratio of $67\pm14$. This represents a significant enrichment in $^{15}$N compared with Titan's main molecular nitrogen reservoir, which has a $^{14}$N/$^{15}$N ratio of 167, and confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan's organic inventory.