Nitrogen in the stratosphere of Titan from Cassini CIRS infrared spectroscopy

TL;DR

This study uses Cassini CIRS infrared spectroscopy to measure nitrogen-bearing gases in Titan's atmosphere, mapping their distributions, detecting ices, and analyzing isotopic ratios to understand atmospheric composition and seasonal changes.

Contribution

It provides detailed measurements of nitrogen compounds in Titan's atmosphere, including their vertical and latitudinal distributions, ices, and isotopic ratios, advancing understanding of Titan's atmospheric chemistry.

Findings

Detection of HCN, HC3N, and C2N2 in Titan's stratosphere.

Mapping of gas distributions across latitudes and altitudes.

Measurement of 14N/15N ratio in HC(15)N.

Abstract

In this chapter we describe the remote sensing measurement of nitrogen-bearing species in Titan's atmosphere by the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft. This instrument, which detects the thermal infrared spectrum from 10-1500 cm-1 (1000-7 microns) is sensitive to vibrational emissions of gases and condensates in Titan's stratosphere and lower mesosphere, permitting the measurement of ambient temperature and the abundances of gases and particulates. Three N-bearing species are firmly detected: HCN, HC3N and C2N2, and their vertical and latitudinal distributions have been mapped. In addition, ices of HC3N and possibly C4N2 are also seen in the far-infrared spectrum at high latitudes during the northern winter. The HC(15)N isotopologue has been measured, permitting the inference of the 14N/15N ratio in this species, which differs markedly (lower) than in the bulk nitrogen reservoir (N2). We also describe the search in the CIRS spectrum, and inferred upper limits, for NH3 and CH3CN. CIRS is now observing seasonal transition on Titan and the gas abundance distributions are changing accordingly, acting as tracers of the changing atmospheric circulation. The prospects for further CIRS science in the remaining five years of the Cassini mission are discussed.