摘要：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 465.250GHz (∼0.644 mm) were measured at significance levels of 4.6σ and 5.7σ, 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=6.157×10 −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± 0.081)×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.
关键词：planets and satellites: atmospheres;planets and satellites: composition;radiative transfer;submillimeter: planetary systems
