Measurement of CH<sub>3</sub>D on Titan at Submillimeter Wavelengths

Thelen, Alexander; Nixon, C. A.; Cordiner, Martin; Charnley, Steven B.; Irwin, Patrick; Kisiel, Zbigniew

doi:10.3847/1538-3881/ab19bb

Cited by 15 publications

(18 citation statements)

References 52 publications

(87 reference statements)

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“…Another “hydrologic” cycle of interest is found on Titan, where methane precipitates in a nitrogen atmosphere (Hayes et al., 2018; Mitchell & Lora, 2016; Roe, 2012). There has been some study of methane isotopes (especially CH 3 D) in the atmosphere of Titan (Ádámkovics & Mitchell, 2016; Hörst, 2017; Nixon et al., 2012; Thelen et al., 2019). Information exists on equilibrium fractionation of CH 3 D (Armstrong et al., 1955; Calado et al., 1997), but the relative diffusivities have not been studied.…”

Section: Introductionmentioning

confidence: 99%

First‐Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan

Hellmann

Harvey

2021

JGR Planets

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On Earth, the fractionation of stable water isotopes such as HDO and 18 2 H O is important for understanding the hydrologic cycle (Gat, 1996). In addition to the equilibrium fractionation that occurs between water vapor and ice or liquid water, in some cases a significant role is played by kinetic fractionation that depends on the ratio of the atmospheric diffusivity of the isotopologue of interest to that of H 2 O. Recently, we reported temperature-dependent diffusivity ratios in air calculated from the kinetic theory of molecular gases applied to state-of-the-art intermolecular potentials based on ab initio calculations for the interaction between water and molecular nitrogen and oxygen (Hellmann & Harvey, 2020). We found that, especially for HDO, the often-assumed constant diffusivity ratios from simple hard-sphere kinetic theory were in error, and that the temperature dependence of the diffusivity ratios was not negligible. We supplied recommended diffusivity ratios and their uncertainties from 190 to 500 K, greatly exceeding the range in which the (scattered) experimental data exist.Earth is not the only place in our solar system with a hydrologic cycle. The distribution and seasonal variation of isotopic water species (especially HDO) has been used to study the climate of Mars (Encrenaz Abstract Recent work used the kinetic theory of molecular gases, along with state-of-the-art intermolecular potentials, to calculate from first principles the diffusivity ratios necessary for modeling kinetic fractionation of water isotopes in air. Here, we extend that work to the Martian atmosphere, employing potential-energy surfaces for the interaction of water with carbon dioxide and with nitrogen. We also derive diffusivity ratios for methane isotopes in the atmosphere of Titan by using a highquality potential for the methane-nitrogen pair. The Mars calculations cover 100-400 K, while the Titan calculations cover 50-200 K. Surprisingly, the simple hard-sphere theory that is inaccurate for Earth's atmosphere is in good agreement with the rigorous results for the diffusion of water isotopes in the Martian atmosphere. A modest disagreement with the hard-sphere results is observed for the diffusivity ratio of CH 3 D in the atmosphere of Titan. We present temperature-dependent correlations, as well as estimates of uncertainty, for the diffusivity ratios involving HDO, H 2 17 O, and H 2 18 O in the Martian atmosphere, and for CH 3 D and 13 CH 4 in the atmosphere of Titan, providing for the first time the necessary data to be able to model kinetic isotope fractionation in these environments.Plain Language Summary Different isotopes distribute unevenly between the vapor phase and liquid or solid phases during precipitation and evaporation, and the resulting changes in isotope ratios are used in the study of climate and other geophysical processes. While equilibrium aspects of this fractionation are fairly well understood, in some circumstances there is also a kinetic component that depends on the relative diffusivities of diffe...

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Section: Introductionmentioning

confidence: 99%

First‐Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan

Hellmann

Harvey

2021

JGR Planets

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“…Disk-averaged data were extracted from spectral image cubes using a circular mask that encompassed pixels with ≥ 90% of Titan's continuum flux (Lai et al, 2017;Thelen et al, 2019a;Nixon et al, 2020). Variations in Titan's distance and relative velocity between integrations were accounted for in the previous calibration and imaging steps.…”

Section: Radiative Transfer Modeling and Resultsmentioning

confidence: 99%

“…We employed the Non-linear optimal Estimator for Multi-variatE spectral analySIS (NEMESIS) radiative transfer package (Irwin et al, 2008) to model Titan's atmospheric emission and retrieve vertical temperature and volume mixing ratio profiles, as has been used previously for Cassini Composite Infrared Spectrometer and ALMA observations of Titan (see, for example, Nixon et al, 2010, Teanby et al, 2010. The NEMESIS atmospheric model parameterization we used follows the prescription of previous studies of Titan with ALMA (Thelen et al, 2018;Thelen et al, 2019a;Thelen et al, 2019b). Spectral line parameters from the Cologne Database for Molecular Spectroscopy (CDMS; Müller et al, 2001;Müller et al, 2005;Endres et al, 2016) were used for models of CH 3 C 3 N (Moïses et al, 1982;Bester et al, 1984;Bester et al, 1985; with purely K-dependent line parameters taken from CH 3 CN, Anttila et al, 1993).…”

Section: Radiative Transfer Modeling and Resultsmentioning

confidence: 99%

“…The advent of the Atacama Large Millimeter/submillimeter Array (ALMA) in the past decade has enabled the exploration of Titan's atmospheric composition and dynamics to an unprecedented degree from the ground, allowing for follow-up studies on the distribution of trace molecular species by the Voyager-1 and Cassini-Huygens missions. Comprised of many 12 m antennas spatially separated by up to 16 km and access to frequencies ranging from ∼84-950 GHz (∼3.5-0.3 mm), ALMA has enabled the detection of new molecular species (Cordiner et al, 2015;Palmer et al, 2017;Nixon et al, 2020), isotopes (Serigano et al, 2016;Molter et al, 2016;Thelen et al, 2019a;Iino et al, 2020), and vibrationally excited transitions (Cordiner et al, 2018;Kisiel et al, 2020) in Titan's atmosphere. The spectral and spatial resolution capabilities of ALMA have also provided the means by which to map the distribution and dynamics of many nitrogen-bearing molecules (Cordiner et al, 2014;Lai et al, 2017;Thelen et al, 2019b;Cordiner et al, 2019;Lellouch et al, 2019), allowing for the study of atmospheric variations throughout Titan's long (29.5 yr) seasonal cycle after the end of the Cassini mission in 2017.…”

Section: Introductionmentioning

confidence: 99%

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Detection of CH$_3$C$_3$N in Titan's Atmosphere

Thelen,

Cordiner,

Nixon

et al. 2020

Preprint

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Titan harbors a dense, organic-rich atmosphere primarily composed of N 2 and CH 4 , with lesser amounts of hydrocarbons and nitrogen-bearing species. As a result of high sensitivity observations by the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6 (∼230-272 GHz), we obtained the first spectroscopic detection of CH 3 C 3 N (methylcyanoacetylene or cyanopropyne) in Titan's atmosphere through the observation of seven transitions in the J = 64 → 63 and J = 62 → 61 rotational bands. The presence of CH 3 C 3 N on Titan was suggested by the Cassini Ion and Neutral Mass Spectrometer detection of its protonated form: C 4 H 3 NH + , but the atmospheric abundance of the associated (deprotonated) neutral product is not well constrained due to the lack of appropriate laboratory reaction data. Here, we derive the column density of CH 3 C 3 N to be (3.8-5.7)×10 12 cm −2 based on radiative transfer models sensitive to altitudes above 400 km Titan's middle atmosphere. When compared with laboratory and photochemical model results, the detection of methylcyanoacetylene provides important constraints for the determination of the associated production pathways (such as those involving CN, CCN, and hydrocarbons), and reaction rate coefficients. These results also further demonstrate the importance of ALMA and (sub)millimeter spectroscopy for future investigations of Titan's organic inventory and atmospheric chemistry, as CH 3 C 3 N marks the heaviest polar molecule detected spectroscopically in Titan's atmosphere to date.

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“…With the successful end to the Cassini mission in 2017, ground-based observations have become the only vehicle by which continuing studies of Titan's atmosphere can be made. Recent studies of Titan using the capabilities of the Atacama Large sub-Millimeter Array (ALMA) have resulted in detections of new species, including C 2 H 5 CN (Cordiner et al 2015) and C 2 H 3 CN (Palmer et al 2017), new isotopologues (Serigano et al 2016;Molter et al 2016;Thelen et al 2019a) and the ability to continue studying the spatial and temporal variations of Titan's atmosphere (Lai et al 2017;Cordiner et al 2018Cordiner et al , 2019Teanby et al 2018;Thelen et al 2019b). Many molecular species may be studied using sub-mm observations, which are sensitive to the rotational transitions of small polar molecules.…”

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confidence: 99%

Detection of Propadiene on Titan

Lombardo

Nixon

Greathouse

et al. 2019

ApJL

Self Cite

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The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH 2 CCH 2 , an isomer of the well-measured propyne (also called methylacetylene) CH 3 CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule -which has recently been remedied with an updated spectral line list. Here we present the first unambiguous detection of the molecule in any astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility (IRTF) in July 2017. We model its emission line near 12 µm and measure a volume mixing ratio (VMR) of (6.9 ± 0.8) ×10 −10 at 175 km, assuming a vertically increasing abundance profile as predicted in photochemical models. Cassini measurements of propyne made during April 2017 indicate that the abundance ratio of propyne to propadiene is 8.2±1.1 at the same altitude. This initial measurement of the molecule in Titan's stratosphere paves the way towards constraining the amount of atomic hydrogen available on Titan, as well as future mapping of propadiene on Titan from 8 meter and larger ground based observatories, and future detection on other planetary bodies.

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Measurement of CH₃D on Titan at Submillimeter Wavelengths

Cited by 15 publications

References 52 publications

First‐Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan

First‐Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan

Detection of CH$_3$C$_3$N in Titan's Atmosphere

Detection of Propadiene on Titan

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Measurement of CH3D on Titan at Submillimeter Wavelengths

Cited by 15 publications

References 52 publications

First‐Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan

First‐Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan

Detection of CH$_3$C$_3$N in Titan's Atmosphere

Detection of Propadiene on Titan

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Measurement of CH₃D on Titan at Submillimeter Wavelengths