Upper limits for undetected trace species in the stratosphere of Titan

TL;DR

This study uses Cassini CIRS infrared spectra to set upper limits on undetected trace molecules in Titan's stratosphere, providing constraints for photochemical models and suggesting future observational improvements.

Contribution

First quantitative search for multiple trace molecules in Titan's stratosphere using infrared spectra, establishing upper limits and guiding future detection efforts.

Findings

No detections of NH3, CH3OH, H2CO, or CH3CN in spectra.

Upper limits suggest NH3 and CH3OH are undetectable with current data.

CH3CN and H2CO are near detection thresholds, guiding future observations.

Abstract

In this paper we describe a first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25{\deg}S and 75{\deg}N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.