The photochemical production of aromatics in the atmosphere of Titan

Loison, Jean‐Christophe; Dobrijévic, M.; Hickson, Kevin M.

doi:10.1016/j.icarus.2019.03.024

Cited by 49 publications

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“…On Titan, the CN radical is mainly generated from the photolysis of HCN, which is formed through reactions of N( 4 S) or through ion chemistry. 11 Benzonitrile has not been detected on Titan and is predicted to be formed in low quantities, largely due to CN being sequestered by reaction with CH 4 . Even with the larger rate constants measured in this work, this is likely also the case for the products of the reaction between CN and toluene, though implementation of these results into Titan models may still be beneficial to determine if they have any influence in the atmosphere.…”

Section: Discussionmentioning

confidence: 99%

Rate Constants of the CN + Toluene Reaction from 15 to 294 K and Interstellar Implications

et al. 2020

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CN is known for its fast reactions with hydrocarbons at low temperatures, but relatively few studies have focused on the reactions between CN and aromatic molecules. The recent detection of benzonitrile in the interstellar medium, believed to be produced by the reaction of CN and benzene, has ignited interest in studying these reactions. Here, we report rate constants of the CN + toluene (C 7 H 8) reaction between 15 and 294 K using a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme; reaction kinetics in uniform supersonic flow) apparatus coupled with the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. We also present the stationary points on the potential energy surface of this reaction to study the available reaction pathways. We find the rate constant does not change over this temperature range, with an average value of (4.1 ± 0.2) × 10-10 cm 3 s-1 , which is notably faster than the only previous measurement at 105 K. While the reason for this disagreement is unknown, we discuss the possibility that it is related to enhanced multiphoton effects in the previous work.

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Section: Discussionmentioning

confidence: 99%

Rate Constants of the CN + Toluene Reaction from 15 to 294 K and Interstellar Implications

et al. 2020

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“…Indeed, 1D neutral photochemical models of hydrocarbons do not predict C 6 H 6 to be abundant in Neptune's atmosphere (Moses et al, 2005). However, photochemical models of Titan (Vuitton et al, 2018;Loison et al, 2019) show that the ion chemistry favors the production of C 6 H 6 , suggesting that C 6 H 6 might also be efficiently produced in the atmosphere of Neptune. This motivates the need to develop a coupled ion-neutral photochemical model for this planet.…”

Section: Introductionmentioning

confidence: 99%

1D photochemical model of the ionosphere and the stratosphere of Neptune

Dobrijévic

Loison

Hue

et al. 2020

Icarus

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Neptune remains a mysterious world that deserves further exploration and is a high-priority objective for a future planetary mission in order to better understand the formation and evolution of ice giant planets. We have developed a coupled ion-neutral 1D photochemical model of Neptune's atmosphere to study the origin and evolution of the hydrocarbons and the oxygen species. The up-to-date chemical scheme is derived from one used for Titan's atmosphere, which led to good agreements with the Cassini-CIRS observations for oxygen species and the main hydrocarbons. The main results we obtain are the following: The ion-neutral chemistry coupling produces aromatics (and benzene in particular) in the atmosphere of Neptune with relatively high abundances. Our model results are in good agreement with observations (taking model uncertainties into account). Two ionospheric peaks are present in the atmosphere located above the pressure level of 10 −5 mbar and around 10 −3 mbar. The influx of oxygen species in the upper atmosphere of Neptune has an effect on the concentration of many ions. We show that in situ exploration of Neptune's atmosphere would provide very interesting constraints for photochemical models concerning in particular the origin of oxygen species and the contribution of ion chemistry. A precise description of upper atmospheric chemistry is crucial for a better understanding of the internal composition and the formation processes of this planet.

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“…Unfortunately, this is an imprecise methodology since model results have strong uncertainties (see, for instance, Dobrijevic et al 2010 for Neptune andDobrijevic et al 2011 for Saturn), which can be much larger than uncertainties on observational data. In a recent photochemical model of Titan, Loison et al (2019) used H 2 O and HCN to constrain K zz in the lower stratosphere. One of the reasons for this is that the chemical processes that drive their abundances are expected to be simpler than those for hydrocarbons and the model uncertainties caused by chemical rates are, therefore, more limited.…”

Section: Discussionmentioning

confidence: 99%

Monitoring of the evolution of H₂O vapor in the stratosphere of Jupiter over an 18-yr period with the Odin space telescope

Benmahi¹,

Cavalié

Dobrijévic

et al. 2020

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Context. The comet Shoemaker-Levy 9 impacted Jupiter in July 1994, leaving its stratosphere with several new species, with water vapor (H2O) among them. Aims. With the aid of a photochemical model, H2O can be used as a dynamical tracer in the Jovian stratosphere. In this paper, we aim to constrain the vertical eddy diffusion (Kzz) at levels where H2O is present. Methods. We monitored the H2O disk-averaged emission at 556.936 GHz with the space telescope between 2002 and 2019, covering nearly two decades. We analyzed the data with a combination of 1D photochemical and radiative transfer models to constrain the vertical eddy diffusion in the stratosphere of Jupiter. Results. Odin observations show us that the emission of H2O has an almost linear decrease of about 40% between 2002 and 2019. We can only reproduce our time series if we increase the magnitude of Kzz in the pressure range where H2O diffuses downward from 2002 to 2019, that is, from ~0.2 mbar to ~5 mbar. However, this modified Kzz is incompatible with hydrocarbon observations. We find that even if an allowance is made for the initially large abundances of H2O and CO at the impact latitudes, the photochemical conversion of H2O to CO2 is not sufficient to explain the progressive decline of the H2O line emission, which is suggestive of additional loss mechanisms. Conclusions. The Kzz we derived from the Odin observations of H2O can only be viewed as an upper limit in the ~0.2 mbar to ~5 mbar pressure range. The incompatibility between the interpretations made from H2O and hydrocarbon observations probably results from 1D modeling limitations. Meridional variability of H2O, most probably at auroral latitudes, would need to be assessed and compared with that of hydrocarbons to quantify the role of auroral chemistry in the temporal evolution of the H2O abundance since the SL9 impacts. Modeling the temporal evolution of SL9 species with a 2D model would naturally be the next step in this area of study.

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The photochemical production of aromatics in the atmosphere of Titan

Cited by 49 publications

References 100 publications

Rate Constants of the CN + Toluene Reaction from 15 to 294 K and Interstellar Implications

Rate Constants of the CN + Toluene Reaction from 15 to 294 K and Interstellar Implications

1D photochemical model of the ionosphere and the stratosphere of Neptune

Monitoring of the evolution of H₂O vapor in the stratosphere of Jupiter over an 18-yr period with the Odin space telescope

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The photochemical production of aromatics in the atmosphere of Titan

Cited by 49 publications

References 100 publications

Rate Constants of the CN + Toluene Reaction from 15 to 294 K and Interstellar Implications

Rate Constants of the CN + Toluene Reaction from 15 to 294 K and Interstellar Implications

1D photochemical model of the ionosphere and the stratosphere of Neptune

Monitoring of the evolution of H2O vapor in the stratosphere of Jupiter over an 18-yr period with the Odin space telescope

Contact Info

Product

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Monitoring of the evolution of H₂O vapor in the stratosphere of Jupiter over an 18-yr period with the Odin space telescope