Far-infrared opacity sources in Titan’s troposphere reconsidered

Kok, Remco de; Irwin, Patrick; Teanby, N. A.

doi:10.1016/j.icarus.2010.06.035

Cited by 16 publications

(15 citation statements)

References 34 publications

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“…Titan's atmosphere is photochemically active even at lower altitudes and leads to the formation of complex tholin-like potential prebiotic molecules due to the penetrating longer wavelength photons. This is consistent with observations of Titan's haze stretching between B500 km and B50 km above the surface 9 , containing several regions of haze (0, A, B and C) as recently characterized by the Cassini CIRS instrument 34,35 .…”

supporting

confidence: 92%

Photochemical activity of Titan’s low-altitude condensed haze

et al. 2013

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Titan, the largest moon of Saturn and similar to Earth in many aspects, has unique orangeyellow colour that comes from its atmospheric haze, whose formation and dynamics are far from well understood. Present models assume that Titan's tholin-like haze formation occurs high in atmosphere through gas-phase chemical reactions initiated by high-energy solar radiation. Here we address an important question: Is the lower atmosphere of Titan photochemically active or inert? We demonstrate that indeed tholin-like haze formation could occur on condensed aerosols throughout the atmospheric column of Titan. Detected in Titan's atmosphere, dicyanoacetylene (C 4 N 2 ) is used in our laboratory simulations as a model system for other larger unsaturated condensing compounds. We show that C 4 N 2 ices undergo condensed-phase photopolymerization (tholin formation) at wavelengths as long as 355 nm pertinent to solar radiation reaching a large portion of Titan's atmosphere, almost close to the surface.

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supporting

confidence: 92%

Photochemical activity of Titan’s low-altitude condensed haze

et al. 2013

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“…Following Tomasko et al (2008a) and de Kok et al (2010), we confirm that the N2-CH4 collision-induced absorption predicted by Borysow and Tang (1993) is too weak at temperatures of 70-85 K. We found that a constant multiplicative factor of ~1.52 yields a satisfactory reproduction of the CIRS spectra, in contrast with the conclusions of Anderson and Samuelson (2011) which were essentially based on limb and high emission angle spectra.…”

Section: Discussionsupporting

confidence: 74%

On the H2 abundance and ortho-to-para ratio in Titan's troposphere

Bézard

Vinatier

2020

Icarus

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We have analyzed spectra recorded between 50 and 650 cm -1 by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles to determine simultaneously the H2 mole fraction and ortho-to-para ratio in Titan's troposphere.We used constraints from limb spectra between 50 and 900 cm -1 and from in situ measurements by the Huygens probe to characterize the temperature, haze and gaseous absorber profiles. We confirm that the N2-CH4 collision-induced absorption (CIA) coefficients used up to now need to be increased by about 52% at temperatures of 70-85 K.We find that the N2-N2 CIA coefficients are also too low in the N2 band far wing, beyond 110 cm -1 , in agreement with recent quantum mechanical calculations. We derived a H2 mole fraction equal to (0.88 ± 0.13) × 10 -3 , which pertains to the ~1-34 km altitude range probed by the S0(0) and S0(1) lines. This result agrees with a previous determination based only on the H2-N2 dimer transition in the S0(0) line, and with the in situ measurement by the Gas Chromatograph Mass Spectrometer (GCMS) aboard Huygens. It is 3-4 times smaller than the value measured in situ by the Ion Neutral Mass Spectrometer (INMS) of Cassini at 1000-1100 km. The H2 para fraction is close to equilibrium in the 20-km region. CIRS spectra can be fitted assuming ortho-to-para (o-p) H2 thermodynamical equilibrium at all levels or a constant para fraction in the range 0.49-0.53. We have investigated different mechanisms that may operate in Titan's atmosphere to equilibrate the H2 o-p ratio and we have developed a onedimensional model that solves the continuity equation in presence of such conversion mechanisms. We conclude that exchange with H atoms in the gas phase or magnetic interaction of H2 in a physisorbed state on the surface of aerosols are too slow compared with atmospheric mixing to play a significant role. On the other hand, magnetic interaction of H2 with CH4, and to a lesser extent N2, can operate on a timescale similar to the vertical mixing time in the troposphere. This process is thus likely responsible for the o-p equilibration of H2 in the mid-troposphere implied by CIRS measurements. The model can reproduce the inferred o-p ratio in the 20-km region, assuming low atmospheric mixing in the troposphere down to 15-20 km and conversion rates with CH4 or N2 slightly larger than obtained from an extrapolation of natural ortho-para conversion rate measured in gaseous hydrogen.

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“…The model used a CH 4 -N 2 opacity based on the CH 4 altitude profile of Niemann et al (2010). Absorption coefficients for CH 4 -N 2 were taken from Borysow & Tang (1993) and were increased by 50% following de Kok et al (2010). We adopted an H 2 -N 2 opacity based on a 0.001 mole fraction of H 2 (Courtin et al 1995;Jennings et al 2009;Niemann et al 2010) using H 2 -N 2 absorption coefficients from Courtin (1988) and Dore et al (1986).…”

Section: Discussionmentioning

confidence: 99%

Surface Temperatures on Titan During Northern Winter and Spring

Jennings

Cottini

Nixon

et al. 2016

ApJL

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Meridional brightness temperatures were measured on the surface of Titan during the 2004-2014 portion of the Cassini mission by the Composite Infrared Spectrometer. Temperatures mapped from pole to pole during five twoyear periods show a marked seasonal dependence. The surface temperature near the south pole over this time decreased by 2 K from 91.7±0.3 to 89.7±0.5 K while at the north pole the temperature increased by 1 K from 90.7±0.5 to 91.5±0.2 K. The latitude of maximum temperature moved from 19 S to 16 N, tracking the subsolar latitude. As the latitude changed, the maximum temperature remained constant at 93.65±0.15 K. In 2010 our temperatures repeated the north-south symmetry seen by Voyager one Titan year earlier in 1980. Early in the mission, temperatures at all latitudes had agreed with GCM predictions, but by 2014 temperatures in the north were lower than modeled by 1 K. The temperature rise in the north may be delayed by cooling of sea surfaces and moist ground brought on by seasonal methane precipitation and evaporation.

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Far-infrared opacity sources in Titan’s troposphere reconsidered

Cited by 16 publications

References 34 publications

Photochemical activity of Titan’s low-altitude condensed haze

Photochemical activity of Titan’s low-altitude condensed haze

On the H2 abundance and ortho-to-para ratio in Titan's troposphere

Surface Temperatures on Titan During Northern Winter and Spring

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