Dynamic escape of H from Titan as consequence of sputtering induced heating

doi:10.1016/S0032-0633(98)00050-6

Planetary and Space Science

Volume 46, Issues 9–10, October 1998, Pages 1207-1213

https://doi.org/10.1016/S0032-0633(98)00050-6 Get rights and content

Abstract

In previous investigations on nonthermal escape processes on Saturns large moon Titan, the authors (Lammer and Bauer) showed that dissociation of ⁺₂ ions and impact dissociation of N₂ by magnetospheric electron precipitation lead to nitrogen escape rates much lower than atmospheric sputtering processes via energetic magnetospheric N⁺ ions and protons or solar wind protons. Energetic processes like Titans plasma interaction with Saturns magnetosphere, however, can affect Titans upper atmosphere. It is shown that the bulk of the energetic N⁺ ions is deposited below the exobase, a region where no temperature determinations by occultation methods exist. This present study takes into account all sources of energy fluxes, heat conduction and other thermal transport processes and estimated horizontal wind velocities of about 100–300 m/s. We found that the energy deposition of magnetospheric N⁺ ions is responsible for heating effects in Titans atmosphere, corresponding to a rise in the temperature up to 30 K above the presently assumed. Thus, sputtering-induced heating would lead to higher Jeans escape rates for neutral hydrogen atoms and may represent an additional source for populating and heating Saturns neutral hydrogen torus. Sputtering by magnetospheric and solar wind protons will yield, however, only negligible small heating effects of Titans thermosphere.

Introduction

Analysis of the occultation measurements by the ultraviolet spectrometer (UVS) on board of Voyager 1 has yielded the temperature and density near the exobase (approx. 1400 km altitude). The temperature at 1265 km altitude has been inferred to be 176±20 K at the evening terminator and 196±20 K at the morning terminator, while the molecular nitrogen density at the same altitude was found to be about 2.7±0.2×10⁸ cm⁻³ at each terminator (Smith et al., 1982). Friedson and Yung, 1984had calculated the diurnal variation of the vertical structure of Titans thermosphere by using solar heating, low-energy magnetospheric electron precipitation and infrared cooling. Lellouch et al., 1990modified the model of Friedson and Yung, 1984, since they discovered a numerical error in the calculation of the heating profile. Yelle, 1991has pointed out the importance of HCN cooling. Lara et al. (1996)have recently reinvestigated the vertical structure of Titans atmosphere, especially the CH₄ and HCN profiles. However, in neither of these studies the heating effects of incident magnetospheric ions, which cause sputtering (Lammer and Bauer, 1993), have been investigated. Our present observational knowledge of Titans interaction with the surrounding plasma flow is based almost exclusively on the data from the single encounter of the Voyager 1 spacecraft. Titans orbital radius of 20.2 Saturn radii is such that the satellite may be located in the solar wind, in the magnetosheath of Saturn or in Saturns magnetosphere. During the Voyager 1 spacecraft encounter, Titan was inside the magnetosphere of Saturn. In this case, one has two different incident particle populations which act as sputtering agents. Protons with energies of about 210 eV and a number density of about 0.1 cm⁻³, and N⁺ ions with energies of about 2.9 keV and a number density of about 0.2 cm⁻³ (Neubauer et al., 1984). The corresponding average N⁺ ion flux φ_N+ at the exobase level is about 2.4×10⁶ cm⁻² s⁻¹, and for the protons, φ_H+ is about 1.2×10⁶ cm⁻² s⁻¹. When Titan is outside Saturns magnetosphere we have 1 keV solar wind protons as incident particles (Lammer and Bauer, 1993). In previous papers, it was shown that atmospheric mass loss by sputtering from Titan and Triton is the main nonthermal escape mechanism of their nitrogen atmospheres (Lammer and Bauer, 1993; Lammer, 1995). The sputtering process discussed in these papers can act to change the character of the upper atmosphere. Whereas solids are good heat conductors, atmospheres are not. Therefore, the bulk of the energy deposited by incoming energetic particles below the exobase provides a heat source which rises the temperature and expands the upper atmosphere. If the energy is deposited very close to the exobase, where the collision frequency is small, one can get higher thermal escape rates. Energy deposition has been investigated for other planetary bodies. Plasma ion heating was worked out for Mars and Venus (e.g. Luhmann and Kozyra, 1991) for Earth (e.g. Ishimoto et al., 1992) and for Io by Pospieszalska and Johnson (1992). We shall investigate here the sputtering induced heating rates and correlated effects, corresponding to the bulk of the energetic N⁺ ions, which is deposited below the exobase.

Section snippets

Titans thermosphere

In previous works, various authors investigated two principal energy sources for Titans thermospheric heating (e.g. Strobel and Shemansky, 1982; Friedson and Yung, 1984; Lellouch et al., 1990; Yelle, 1991). These sources are solar energy and magnetospheric electrons. Solar radiation is absorbed in the thermosphere from the extreme ultraviolet (EUV) to about 2000 . The most important part of heating occurs through absorption of Lyman α radiation by methane. Heating by nitrogen is globally weak

Sputtering induced heating : a source for Titans neutral hydrogen torus

According to the concept of Jeans, the existence of particles having sufficient velocity for hyperbolic orbits leads to the so-called Jeans flux that depends on the number density of the escaping constituent at the exobase and an effusion velocity. The escape efficiency depends on the escape parameter X(r). $X(r) = GMm_{j} r_{exo} k_{B} T_{exo} ≡ v^{2}_{esc} u^{2}_{0}$ n(10)with G being the gravitational constant of 6.672×10⁻¹¹ N m² kg⁻², M the mass of Titan of about 1.35×10²³ kg, m_j the mass of the escaping particle in kg, r

Conclusion

Our calculations show that sputtering induced heating of Titans thermosphere by magnetospheric N⁺ ions causes a temperature rise of approximately 30 K. The heating effects depend on the penetrating ion fluxes, diffusion cross-sections and solar wind activity. We also conclude that magnetospheric N⁺ ion heating will increase the escape rates and thus provide an additional mechanism for populating the observed neutral hydrogen torus in the Saturn system during periods of high exospheric

Acknowledgements

. The authors wish to thank R. E. Johnson (Department of Nuclear Engineering and Engineering Physics, University of Virginia, U.S.A.), E. Lellouch (Observatoire de Paris-Meudon, France) and two anonymous referees for enlightening discussions relating to this work.

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The deposition of energy, escape of atomic and molecular nitrogen and heating of the upper atmosphere of Titan are studied using a Direct Simulation Monte Carlo method. It is found that the globally averaged flux of deflected magnetospheric atomic nitrogen ions and molecular pickup ions deposit more energy in Titan's upper atmosphere than solar radiation. The energy deposition in this region determines the atmospheric loss and the production of the nitrogen neutral torus. The temperature structure near the exobase is also calculated. It is found that, due to the inclusion of the molecular pickup ions more energy is deposited closer to the exobase than assumed in earlier plasma ion heating calculations. Although the temperature at the exobase is only a few degrees larger than it is at depth, the density above the exobase is enhanced by the incident plasma.

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^fn1: Also at Institute for Meteorology and Geophysics, University of Graz, Halbärthgasse 1, A-8010 Graz, Austria.

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Dynamic escape of H from Titan as consequence of sputtering induced heating

Abstract

Introduction

Section snippets

Titans thermosphere

Sputtering induced heating : a source for Titans neutral hydrogen torus

Conclusion

Acknowledgements

Planet. Space Sci.

Science

Icarus

Science

Science

Academic Press

J. Geophys. Res.

J. Geophys. Res.

Astrophysical Journal

J. Geophys. Res.