Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere

Lebonnois, Sébastien; Bakes, E. L. O.; McKay, Christopher P.

doi:10.1006/icar.2002.6943

Cited by 98 publications

(72 citation statements)

References 41 publications

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“…The introduction of the scheme used for aerosol growth in Lebonnois et al (2002) also has an impact on the hydrogen budget. Since it adds a loss of gas phase molecules (e.g., C 2 H 2 , HCN) to the haze, it slightly reduces the mole fraction of acetylene in the stratosphere.…”

Section: Discussionmentioning

confidence: 99%

“…In this model, which we also used for our recent study of the photochemical production of aerosols in Titan's atmosphere (Lebonnois et al, 2002), the number of hydrocarbon and nitrile species and the number of photochemical reactions have been enhanced, especially to improve the nitrile schemes and to introduce benzene. Oxygen species are not included.…”

Section: Photochemical Modelmentioning

confidence: 99%

“…L96: Lara et al (1996). c See text, and also Lebonnois et al (2002). d All compounds produced and not destroyed in their Table 5.…”

Section: Hydrogen Fluxesmentioning

confidence: 99%

“…The loss of gas compounds into aerosol production (e.g., Lebonnois et al, 2002) has not been included in these initial simulations. However, we have done some calculations that include this process in order to test the sensitivity of our conclusions (see Section 2.3).…”

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confidence: 99%

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Atomic and molecular hydrogen budget in Titan’s atmosphere

Lebonnois

Bakes

McKay

2003

Icarus

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Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan's atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 ϫ 10 8 cm 2 s Ϫ1 and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300 -500] km, where it recombines into H 2 . Production of both H and H 2 also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 ϫ 10 9 cm Ϫ2 s Ϫ1 and the latest retrieved value of the H 2 mole fraction in the stratosphere: (1.1 Ϯ 0.1) ϫ 10 Ϫ3 . Our results for both of these values are at least 50 -100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H 2 is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C 4 species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C 3 H 4 , C 4 H 2 and C 6 H 6 . Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons.

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Section: Discussionmentioning

confidence: 99%

Section: Photochemical Modelmentioning

confidence: 99%

“…L96: Lara et al (1996). c See text, and also Lebonnois et al (2002). d All compounds produced and not destroyed in their Table 5.…”

Section: Hydrogen Fluxesmentioning

confidence: 99%

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confidence: 99%

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Atomic and molecular hydrogen budget in Titan’s atmosphere

Lebonnois

Bakes

McKay

2003

Icarus

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“…Far ultraviolet solar radiation dissociates the major neutral species (nitrogen and methane) in the upper atmosphere and produces primary species such as HCN, C 2 H 2 and C 2 H 4 (Yung et al 1984;Wilson & Atreya 2004). The first photochemical models investigating aerosol formation found a maximum production at approximately 200 km through neutral chemistry involving complex hydrocarbons and nitriles (Lebonnois et al 2002;. However, this result was in disagreement with microphysical models that required a production above 400 km in order to fit the geometric albedo (McKay et al 2001).…”

Section: Introductionmentioning

confidence: 99%

Composition and chemistry of Titan's thermosphere and ionosphere

Vuitton

Yelle

Lavvas

2008

Phil. Trans. R. Soc. A.

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Titan has long been known to harbour the richest atmospheric chemistry in the Solar System. Until recently, it had been believed that complex hydrocarbons and nitriles were produced through neutral chemistry that would eventually lead to the formation of micrometre sized organic aerosols. However, recent measurements by the Cassini spacecraft are drastically changing our understanding of Titan's chemistry. The Ion and Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) revealed an extraordinary complex ionospheric composition. INMS detected roughly 50 positive ions with m/z!100 and a density higher than 0.1 cm K3 . CAPS provided evidence for heavy (up to 350 amu) positively and negatively charged (up to 4000 amu) ions. These observations all indicate that Titan's ionospheric chemistry is incredibly complex and that molecular growth starts in the upper atmosphere rather than at lower altitude. Here, we review the recent progress made on ionospheric chemistry. The presence of heavy neutrals in the upper atmosphere has been inferred as a direct consequence of the presence of complex positive ions. Benzene (C 6 H 6 ) is created by ion chemistry at high altitudes and its main photolysis product, the phenyl radical (C 6 H 5 ), is at the origin of the formation of aromatic species at lower altitude.

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Titan's atmosphere and climate

2017

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Titan is the only moon with a substantial atmosphere, the only other thick N2 atmosphere besides Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and atmosphere is also unique in our solar system; atmospheric chemistry produces materials that are deposited on the surface and subsequently altered by surface‐atmosphere interactions such as aeolian and fluvial processes resulting in the formation of extensive dune fields and expansive lakes and seas. Titan's atmosphere is favorable for organic haze formation, which combined with the presence of some oxygen‐bearing molecules indicates that Titan's atmosphere may produce molecules of prebiotic interest. The combination of organics and liquid, in the form of water in a subsurface ocean and methane/ethane in the surface lakes and seas, means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the universe. The Cassini‐Huygens mission to the Saturn system has provided a wealth of new information allowing for study of Titan as a complex system. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth‐based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini‐Huygens comes to an end.

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Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere

Cited by 98 publications

References 41 publications

Atomic and molecular hydrogen budget in Titan’s atmosphere

Atomic and molecular hydrogen budget in Titan’s atmosphere

Composition and chemistry of Titan's thermosphere and ionosphere

Titan's atmosphere and climate

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