Measurement of CH${}_3$D on Titan at Submillimeter Wavelengths

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 ($\sim 0.644$ 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 = $6.157\times 10^{-6}$ in Titan's stratosphere (at altitudes $>130$ 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 $(1.033\pm 0.081)\times 10^{-4}$. 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.

9 pages, 4 figures

References in corpus (3)

• ALMA measurements of the HNC and HC${}_3$N distributions in Titan's atmosphere
• Mapping Vinyl Cyanide and Other Nitriles in Titan's Atmosphere Using ALMA
• Photochemical enrichment of deuterium in Titan's atmosphere: new insights from Cassini-Huygens