Orenthal J. Tucker

The Enceladus and OH Tori at Saturn

The remarkable observation that Enceladus, a small icy satellite of Saturn, is actively venting has led to the suggestion that ejected water molecules are the source of the toroidal atmosphere observed at Saturn for over a decade using the Hubble Space Telescope (HST). Here we show that the venting leads directly to a new feature, a narrow Enceladus neutral torus. The larger torus, observed using HST, is populated by charge exchange, the process that limits the lifetime of the neutrals in the Enceladus torus.

THERMALLY DRIVEN ATMOSPHERIC ESCAPE: TRANSITION FROM HYDRODYNAMIC TO JEANS ESCAPE

Thermally driven escape from planetary atmospheres changes in nature from an organized outflow (hydrodynamic escape) to escape on a molecule-by-molecule basis (Jeans escape) with increasing Jeans parameter, λ, the ratio of the gravitational to thermal energy of the atmospheric molecules. This change is described here for the first time using the direct simulation Monte Carlo method. When heating is predominantly below the lower boundary of the simulation region, R 0, and well below the exobase of a single-component atmosphere, the nature of the escape process changes over a surprisingly narrow range of Jeans parameters, λ0, evaluated at R 0. For an atomic gas, the transition occurs over λ0 ~ 2-3, where the lower bound, λ0 ~ 2.1, corresponds to the upper limit for isentropic, supersonic outflow. For λ0 > 3 escape occurs on a molecule-by-molecule basis and we show that, contrary to earlier suggestions, for λ0 > ~6 the escape rate does not deviate significantly from the familiar Jeans rate. In a gas composed of diatomic molecules, the transition shifts to λ0 ~ 2.4-3.6 and at λ0 > ~4 the escape rate increases a few tens of percent over that for the monatomic gas. Scaling by the Jeans parameter and the Knudsen number, these results can be applied to thermally induced escape of the major species from solar and extrasolar planets.

Thermally driven escape from Pluto's atmosphere: A combined fluid/kinetic model

A formula is derived for the rate of thermal atmospheric escape, valid, and asymptotically exact, at low Knudsen number. 1. The thermal escape problem Consider a planetary atmosphere without stirring and radiative heating or cooling above the nominal surface. The atmosphere cannot be static since a static atmosphere reaches thermodynamic equilibrium. The Boltzmann distribution would then give a finite density at infinite distances, which is unphysical. The atmosphere must be in a state of permanent escape. We want to calculate the rate of escape and the resulting temperature and density profiles. The thermal escape problem has a long history (Jeans 1904, Parker 1958, Hunten 1982). But in recent applications to planetary bodies, there has been disagreement on which model – hydrodynamic or free molecular flow – should be applied (Johnson 2010). We believe that all issues have been finally settled by a direct molecular dynamics simulations of Volkov et. al. (2010). Here we just show how the Volkov et. al. (2010) results can be obtained analytically. It would seem that the escape rate calculation is only possible via molecular dynamics or Boltzmann numerical simulation, because the exobase region, where the transition from hydrodynamic to free molecular flow occurs, cannot be treated analytically. But we show, for low Knudsen number at the surface, that the exobase boundary conditions are either irrelevant or can be deduced due to large overlap between hydrodynamic and free molecular flow regions. This allows one to calculate the atmospheric escape using hydrodynamics and to derive a formula for the escape rate. For clarity, we approximate the molecular in0 10 20 30 40 -15 -10 -5 0 Fig. 1.— Escape rate for Kn = 10, cp = 5/2, α = 1. Je < 2.5 is the Parker regime, 2.5 < Je < 24 is the Fourier regime, Je > 24 is the Jeans regime.

Mass Loss Processes in Titan's Upper Atmosphere

Although Titans atmospheric column density is about ten times that of the Earths, its measured 15N/14N ratio suggests that considerable escape has occurred or that Titans original material had a ratio closer to that of cometary materials. A number of active escape processes have been proposed: thermal escape, chemical-induced escape, slow hydrodynamic escape, pick-up ion loss, ionospheric outflow and plasma-ion-induced atmospheric sputtering. These loss processes and relevant simulations are reviewed in light of recent Cassini data.

Kinetic simulations of thermal escape from a single component atmosphere

The one-dimensional steady-state expansion of a monatomic gas from a spherical source in a gravity field is studied by the direct simulation Monte Carlo method. Collisions between molecules are described by the hard sphere model, the distribution of gas molecules leaving the source surface is assumed to be Maxwellian, and no heat is directly deposited in the simulation region. The flow structure and the escape rate (number flux of molecules escaping the atmosphere) are analyzed for the source Jeans parameter λ0 (ratio of the gravitational energy to thermal energy of the molecules) and Knudsen number Kn0 (ratio of the mean free path to the source radius) ranging from 0 to 15 and from 0.0001 to ∞, respectively. In the collisionless regime, flows are analyzed for λ0=0-100 and analytical equations are obtained for asymptotic values of gas parameters that are found to be non-monotonic functions of λ0. For collisional flows, simulations predict the transition in the nature of atmospheric loss from escape on a m...

THE IMPLANTATION AND INTERACTIONS OF O+ IN TITAN'S ATMOSPHERE: LABORATORY MEASUREMENTS OF COLLISION-INDUCED DISSOCIATION OF N2 AND MODELING OF POSITIVE ION FORMATION

Energetic oxygen ions are an important component of the plasma incident onto Titans atmosphere. Therefore, we report measurements of electron capture and ionization collisions of N2 with incident O+ over the energy range 10-100 keV. Using time of flight coincidence counting techniques we also measured the collision-induced dissociation of N2 following ionization and electron capture. The electron capture and ionization cross sections were found to have comparable magnitudes. Capture collisions are dominated by non-dissociative processes with the dissociative processes providing contributions that are only slightly smaller. In contrast, ionization is entirely dominated by the dissociative processes. The energy distributions of the N+ and N atom fragments ejected by 20, 50, and 100 keV incident O+ projectiles have also been determined. These fragments carry considerable amounts of energy and if produce in the exobase region can readily escape. The cross sections measured here have been used with Cassini energetic ion and atmospheric density data to determine the ionization by and neutralization of energetic O+ penetrating Titans N2 rich atmosphere. Neutralization by charge exchange is found not to occur efficiently above Titans exobase, so energetic particles with large gyroradii penetrate the atmosphere primarily as ions. When the energetic O+ flux is large, we also show it is an important source of ionization and heating at depth into Titans atmosphere and the fragments contribute to the net atmospheric loss rate.

Hybrid fluid/kinetic modeling of Pluto’s escaping atmosphere

Abstract Predicting the rate of escape and thermal structure of Pluto’s upper atmosphere in preparation for the New Horizons Spacecraft encounter in 2015 is important for planning and interpreting the expected measurements. Having a moderate Jeans parameter Pluto’s atmosphere does not fit the classic definition of Jeans escape for light species escaping from the terrestrial planets, nor does it fit the hydrodynamic outflow from comets and certain exoplanets. It has been proposed for some time that Pluto lies in the region of slow hydrodynamic escape. Using a hybrid fluid/molecular-kinetic model, we previously demonstrated the typical implementation of this model fails to correctly describe the appropriate temperature structure for the upper atmosphere for solar minimum conditions. Here we use a time-dependent solver to allow us to extend those simulations to higher heating rates and we examine fluid models in which Jeans-like escape expressions are used for the upper boundary conditions. We compare these to hybrid simulations of the atmosphere under heating conditions roughly representative of solar minimum and mean conditions as these bracket conditions expected during the New Horizon encounter. Although we find escape rates comparable to those previously estimated by the slow hydrodynamic escape model, and roughly consistent with energy limited escape, our model produces a much more extended atmosphere with higher temperatures roughly consistent with recent observations of CO. Such an extended atmosphere will be affected by Charon and will affect Pluto’s interaction with the solar wind at the New Horizon encounter. For the parameter space covered, we also find an inverse relationship between exobase temperature and altitude and the Jeans escape rate that is consistent with the energy limited escape rate. Since we have previously shown that such models can be scaled, these results have implications for modeling exoplanet atmospheres for which the energy limited escape approximation is often used.

Expansion of monatomic and diatomic gases from a spherical source into a vacuum in a gravitational field

Steady monatomic and diatomic gas flows from a spherical source into a vacuum in a gravitational field are studied using direct statistical simulation. The qualitative gravitation effect on the flow is shown to be independent of the intermolecular collision model. Three characteristic Jeans parameter ranges can always be distinguished, namely, the subcritical range, on which the flow in a weak gravitational field is similar with the outflow in the absence of gravitation, the supercritical range, on which the outflow velocity remains small even at large distances from the source, and a narrow transitional range between the two former ranges. The presence of internal degrees of freedom of gas molecules displaces the transitional range toward the greater values of the Jeans parameter and leads to an increase in the outflow velocity and the gas temperature; however, in the initial region the latter effect is expressed only slightly. The normalized escape flow is a nonmonotonic function of both the Jeans parameter and the Knudsen number and is different for monatomic and diatomic gases within 50% on the parameter range considered.

Evolution of an early Titan atmosphere

Abstract Rapid escape from a proposed early CH 4 /NH 3 atmosphere on Titan could, in principle, limit the amount of NH 3 that is converted by photolysis into the present N 2 atmosphere. Assuming that this conversion occurred, a recent estimate of escape driven by the surface temperature and pressure was used to constrain Titans accretion temperature. Here we show that for the range of temperatures of interest, heating of the surface is not the primary driver for escape. Atmospheric loss from a thick Titan atmosphere is predominantly driven by heating of the upper atmosphere; therefore, the loss rate cannot be used to easily constrain the accretion temperature. We give an estimate of the solar driven escape rate from an early atmosphere on Titan, and then briefly discuss its relevance to the cooling rate, isotope ratios, and the time period suggested to convert NH 3 to the present N 2 atmosphere.

Fluid/Kinetic Hybrid Simulation of Atmospheric Escape: Pluto

A hybrid fluid/molecular kinetic model was developed to describe the escape of molecules from the gravitational well of a planet’s atmosphere. This model was applied to a one dimensional, radial description of molecular escape from the atmosphere of Pluto and compared to purely fluid dynamic simulations of escape for two solar heating cases. The hybrid simulations show that the atmospheric temperature vs. altitude and the escape rates can differ significantly from those obtained using only a fluid description of the atmosphere.