Search for methane isotope fractionation due to Rayleigh distillation on Titan

Ádámkovics, Máté; Mitchell, Jonathan L.

doi:10.1016/j.icarus.2016.04.006

Cited by 2 publications

(1 citation statement)

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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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The near-surface methane humidity on Titan

Lora

Ádámkovics

2017

Icarus

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We retrieve vertical and meridional variations of methane mole fraction in Titan's lower troposphere by re-analyzing near-infrared ground-based observations from 17 July 2014 UT (Ádámkovics et al., 2016). We generate synthetic spectra using atmospheric methane profiles that do not contain supersaturation or discontinuities to fit the observations, and thereby retrieve minimum saturation altitudes and corresponding specific humidities in the boundary layer. We relate these in turn to surface-level relative humidities using independent surface temperature measurements. We also compare our results with general circulation model simulations to interpret and constrain the relationship between humidities and surface liquids. The results show that Titan's lower troposphere is undersaturated at latitudes south of 60• N, consistent with a dry surface there, but increases in humidity toward the north pole indicate appreciable surface liquid coverage. While our observations are consistent with considerably more liquid methane existing at the north pole than is present in observed lakes, a degeneracy between low-level methane and haze leads to substantial uncertainty in determining the extent of the source region.

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Search for methane isotope fractionation due to Rayleigh distillation on Titan

Cited by 2 publications

References 50 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

The near-surface methane humidity on Titan

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