# Measurement of CH$_3$D on Titan at Submillimeter Wavelengths

## TL;DR

This study reports the first detection of monodeuterated methane (CH$_3$D) in Titan's atmosphere using ALMA, providing new insights into Titan's methane D/H ratio and demonstrating the potential for spatially resolved observations.

## Contribution

First radio/submillimeter detection of CH$_3$D in Titan's atmosphere, enabling measurement of D/H ratio and opening avenues for spatially resolved studies with ALMA.

## Key findings

- Detected CH$_3$D at 4.6σ and 5.7σ significance levels.
- Measured CH$_3$D volume mixing ratio as 6.157×10⁻⁶ in Titan's stratosphere.
- Derived D/H ratio in CH$_4$ as (1.033±0.081)×10⁻⁴, consistent with previous measurements.

## Abstract

We present the first radio/submillimeter detection of monodeuterated methane (CH$_3$D) in Titan's atmosphere, using archival data from of the Atacama Large Millimeter/submillimeter Array (ALMA). The $J_K=2_1-1_1$ and $J_K=2_0-1_0$ transitions at 465.235 and 465.250 GHz ($\sim0.644$ mm) were measured at significance levels of $4.6\sigma$ and $5.7\sigma$, respectively. These two lines were modeled using the Non-linear optimal Estimator for MultivariatE spectral analySIS (NEMESIS) radiative transfer code to determine the disk-averaged CH$_3$D volume mixing ratio = $6.157\times10^{-6}$ in Titan's stratosphere (at altitudes $\gt130$ km). By comparison with the CH$_4$ vertical abundance profile measured by Cassini-Huygens mass spectrometry, the resulting value for D/H in CH$_4$ is $(1.033\pm0.081)\times10^{-4}$. This is consistent with previous ground-based and in-situ measurements from the Cassini-Huygens mission, though slightly lower than the average of the previous values. Additional CH$_3$D observations at higher spatial resolution will be required to determine a value truly comparable with the Cassini-Huygens CH$_4$ measurements, by measuring CH$_3$D with ALMA close to Titan's equator. In the post-Cassini era, spatially resolved observations of CH$_3$D with ALMA will enable the latitudinal distribution of methane to be determined, making this an important molecule for further studies.