Atomic and molecular hydrogen budget in Titan’s atmosphere

Abstract

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.

Atmospheric Structure and Composition

“…As these particles fall through the ignorosphere and grow, they become detectable by remote sensing: UVIS at ~1000 km, ISS at ~500 km and eventual become ubiquitous throughout the stratosphere. These haze particles are strong absorbers of solar UV and visible radiation and play a fundamental role in heating Titan's stratosphere and perhaps the ignorosphere (Lavvas et al 2009) . The differential heating with latitude drives wind systems in Titan's middle atmosphere, much as ozone does in the Earth's middle atmosphere.…”

“…This feature was previously detected in stellar occultation measurements, (e. g., Sicardy et al 2006) and is the location of a detached haze layer (Porco et al 2005) . Only a very small optical depth would suffi ce to absorb suffi cient solar radiation to balance the implied substantial heat loss by thermal conduction (Lavvas et al 2009) .…”

“…Above this haze layer, Liang et al (2007b) present evidence that the aerosols extend up to 1000 km. Heating associated with discreet haze layers and the broad distribution of aerosols above the visible disk of Titan were not included in the Yelle (1991) model, but were included in Lavvas et al (2009) , who used the Yelle computer code and found that the net effect was to raise the mesopause by ~50 km without changing its temperature, in farther disagreement with the HASI inferred mesopause at 494 km.…”

Sputtering and heating of Titan's upper atmosphere

“…Loss of hydrogen was known to occur by thermal escape (Lebonnois et al 2003). Heating effects in the exobase region induced by the ambient plasma ions, pick-up ions, ionospheric outflow and energetic re-impacting neutrals have been estimated by a number of groups.…”

State-Resolved Dynamics of the CN(B2Σ+) and CH(A2Δ) Excited Products Resulting from the VUV Photodissociation of CH3CN

“…18 Experiment and theory show that this scheme results in the production of higher molecular weight nitriles and, after polymerization processes, the aerosol particles that form a stratospheric haze layer around this celestial body. [19][20][21] Recent studies suggest that further investigation into the product identities and abundances resulting from processes involving nitrile species is necessary to improve the accuracy of models detailing Titan's atmospheric chemistry. 18,22 Furthermore, CN provides a mechanism for incorporating nitrogen into the ring structure of polycyclic aromatic hydrocarbons to form polycyclic aromatic nitrogen heterocycles (PANHs).…”