Distribution and escape of molecular hydrogen in Titan's thermosphere and exosphere

Abstract

[1] We present an in-depth study of the distribution and escape of molecular hydrogen (H 2 ) on Titan, based on the global average H 2 distribution at altitudes between 1000 and 6000 km, extracted from a large sample of Cassini/Ion and Neutral Mass Spectrometer (INMS) measurements. Below Titan's exobase, the observed H 2 distribution can be described by an isothermal diffusion model, with a most probable flux of (1.37 ± 0.01) Â 10 10 cm À2 s À1 , referred to the surface. This is a factor of $3 higher than the Jeans flux of 4.5 Â 10 9 cm À2 s À1 , corresponding to a temperature of 152.5 ± 1.7 K, derived from the background N 2 distribution. The H 2 distribution in Titan's exosphere is modeled with a collisionless approach, with a most probable exobase temperature of 151.2 ± 2.2 K. Kinetic model calculations in the 13-moment approximation indicate a modest temperature decrement of several kelvin for H 2 , as a consequence of the local energy balance between heating/cooling through thermal conduction, viscosity, neutral collision, and adiabatic outflow. The variation of the total energy flux defines an exobase level of $1600 km, where the perturbation of the Maxwellian velocity distribution function, driven primarily by the heat flow, becomes strong enough to raise the H 2 escape flux considerably higher than the Jeans value. Nonthermal processes may not be required to interpret the H 2 escape on Titan. In a more general context, we suggest that the widely used Jeans formula may significantly underestimate the actual thermal escape flux and that a gas kinetic model in the 13-moment approximation provides a better description of thermal escape in planetary atmospheres.Citation: Cui, J., R. V. Yelle, and K. Volk (2008), Distribution and escape of molecular hydrogen in Titan's thermosphere and exosphere,

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

“…The H 2 fluxes increase until they reach values very close to the H 2 limiting flux profile (green). After rising to meet the limiting flux curve between 900 and 1000 km altitude, the H 2 fluxes then become asymptotic, suggesting that the limiting flux is setting the eventual escape flux of H 2 out of the atmosphere-consistent with Strobel [2010Strobel [ , 2012 and Cui et al [2008].…”

“…There are currently two peer-reviewed methods for analyzing the INMS raw data: (1) the methods developed by Magee et al [2009] and (2) the methods developed by Cui et al [2008] and Cui et al [2012]. Both methods produce similar results for the major species densities but differ in their minor species composition.…”

“…Cui et al [2008] and Strobel [2009] demonstrated that H 2 densities and mixing ratios measured by INMS could be simulated using escape rates consistent with a limiting flux mechanism [Hunten, 1974], where upwelling H 2 from the lower atmosphere is limited through the homopause region. While most studies agree on the H 2 escape rates, Strobel [2010,2012] found that 1-D simulations could not simultaneously match both the INMS H 2 measurements and those of CIRS and GCMS.…”

“…Previously, we accounted for both diurnal and sun-Saturn orbital distance variations over the course of our simulations, which are not accounted for in the collective works of Cui et al [2008], Strobel [2008Strobel [ , 2009Strobel [ , 2010Strobel [ , 2012, and Yelle et al [2008]. To address these differences, we now focus on a series of simulations that approximate a global mean state during the TA-T40 timeframe, by specifying a fixed solar zenith angle of 60 • , an orbital distance of 9.25 AU, and an F 10.7−cm flux that we divide by 2.0 (see Table 1).…”

Cassini INMS observations of neutral molecules in Saturn's E‐ring

“…In the IM, the primary dissociation products of H 2 O are H and OH, and the density of ambient H 2 is much lower, below the measurement threshold of INMS. Nor would the approximately 10 28 molecules/s of H 2 that enter the magnetosphere from Titan near 20 R S provide H 2 measureable near 4 R S [Cui et al, 2008]. The H 2 molecules observed by INMS are created from H 2 O in the INMS antechamber due to the molecules' high velocity and the presence of unoxidized titanium.…”