R. E. Johnson

Mars solar wind interaction: Formation of the Martian corona and atmospheric loss to space

escaping fluxes of pickup ions are derived from a 3-D hybrid model describing the interaction of the solar wind with our computed Martian oxygen exosphere. In this work it is shown that the role of the sputtering crucially depends on an accurate description of the Martian corona as well as of its interaction with the solar wind. The sputtering contribution to the total oxygen escape is smaller by one order of magnitude than the contribution due to the dissociative recombination. The neutral escape is dominant at both solar activities (1 � 10 25 s � 1 for low solar activity and 4 � 10 25 s � 1 for high solar activity), and the ion escape flux is estimated to be equal to 2 � 10 23 s � 1 at low solar activity and to 3.4 � 10 24 s � 1 at high solar activity. This work illustrates one more time the strong dependency of these loss rates on solar conditions. It underlines the difficulty of extrapolating the present measured loss rates to the past solar conditions without a better theoretical and observational knowledge of this dependency.

Sputtering of water ice surfaces and the production of extended neutral atmospheres

Plasma and UV photon bombardment of an icy object in the outer solar system can lead to ejection of atoms and molecules from the surface which can, in turn, produce an extended neutral atmosphere. We present new laboratory studies of the sputtering of water ice by keV ions (H+ through Ne+) made using a sensitive microbalance technique that allows measurements at very low ion fluences. These results for the sputtering yield of ice by keV O+ ions, the dominant sputtering agents in the Saturnian magnetosphere, are much larger than those used previously to model the neutral cloud associated with the icy satellites. The data presented are used to recalculate previously published sputtering rates for the icy satellites of Jupiter and Saturn, and for the E-ring grains at Saturn. The new results can account, in part, for the discrepancy between the predicted and observed OH cloud near Tethys in Saturns inner magnetosphere. We compare the yields induced by the incident ions to the recently measured UV photosputtering yield, and discuss possible synergism between UV photon and plasma ion induced erosion.

Magnetospheric ion bombardment profiles of satellites - Europa and Dione

Abstract The spatial dependence across a satellite surface of the ion bombardment/implantation rate is calculated for satellites imbedded in planetary magnetospheric plasmas. These bombardment profiles are created by tracking ions in the plasma onto the surface of the object using an appropriate description of the ion motion. A parameter study is made indicating the general dependence on ion gyroradius and pitch angle. It is shown that access to the leading hemisphere depends strongly on the pitch angle distribution and that the gyromotion can cause differences in the bombardment of the inner and outer hemisphere. Profiles are then calculated for sulfur ions incident on Europa and oxygen ions incident on Dione using reasonable speed and pitch angle distributions. The results indicate that the longitudinal dependence of the UV absorption seen by Voyager closely follows the sulfur ion implantation profile at Europa if the hot plasma measured by LECP is not dominated by sulfur. This would appear to confirm that the feature is produced by ion bombardment. It is also found that for the assumed pitch angle distribution the dominant sputtering component at Dione bombards that surface nearly isotopically. This affects the analysis of the surface reflectance properties and the calculation of the heavy ion plasma source distribution.

Sputtering processes: Erosion and chemical change

Abstract We review laboratory data and models on sputter-induced erosion and chemical alterations of ice films and apply the results to icy grains and satellites exposed to magnetospheric ion bombardment. We show that the source of the plasma in the inner magnetosphere of Saturn is likely to be the sputter erosion of the icy objects in this region and consider the sputter erosion and possible stabilization of the E-ring. Ion-induced polymerization is discussed as a source of the darkened rings of Uranus.

Primordial comet mantle: Irradiation production of a stable organic crust

Abstract Laboratory data and corrected estimates of cosmic ray dose are used to predict the thickness and survivability of the cosmic ray-produced, primordial comet mantle (“crust”). These results support the hypothesis that the refractory mantle produced by cosmic ray irradiation of a new comet may be able to survive a comets entry into the inner solar system for many revolutions. Because this mantle may extend to several meters in depth, the proposed CRAF and Rosetta probes would have to be extended to reach unprocessed cometary material.

Charging of ice grains by low‐energy plasmas: Application to Saturn's E ring

The charging of ice grains in planetary plasmas is studied, including the effects of secondary electron emission and backscattering of the incident electrons. It is shown that existing charging models can not be simply extrapolated to the low-energy electron regime (below 30 eV) common in planetary magnetospheric plasmas. We derive expressions for the electrical potential of a grain immersed in a low-energy plasma which more carefully account for electron reflection and the threshold for secondary electron emission. Using plasma parameters from Voyager PLS experiment, we calculate the potential of Saturns E ring grains to vary from −5.5 V at 4 Rs to 5 V at 10 Rs.

Sputtering of solid SO2

Abstract Solid SO2 has been bombarded by MeV and keV ions, and the transformation of the deposited energy studied by investigating sputtering from the surface. Measurements have been made of the total yield, and the mass and energy distributions of ejected atoms and molecules. These are examined in terms of existing sputtering models.

Micrometeorite Erosion of the Main Rings as a Source of Plasma in the Inner Saturnian Plasma Torus

Abstract Micrometeorite bombardment is thought to erode the main rings of Saturn and to produce a plasma locally, accounting for transient ring phenomena and the supply of water molecules to Saturns atmosphere. This process is shown here to be also a significant source of water molecules and molecular ions in the region between the outer edge of the main rings and Enceladus, adding to those neutrals and plasma produced by sputtering of the icy satellites. However, the magnitude and distribution of the ring source depends on the micrometeorite flux and the energy distribution of the neutral ejecta, both of which are uncertain. Limiting cases are examined, their implications in the Cassini division are calculated, and their relevance to the neutral and plasma clouds in this region is discussed.

Sputtering of molecular gas solids by keV ions

Abstract The sputtering of D2O and CO solids by keV ions has been studied by measuring the absolute sputtering yield and the masses of ejected particles. The yield results for several ions on D2O indicate that for the case where the energy deposited in nuclear motion is comparable to that deposited electronically, the sputtering yield is still determined to a large extent by the electronic energy. The mass spectra ejected from D2O by 30 keV Kr+ ions have a behavior very much like those produced by MeV ions. The bombardment of CO by kr+ results in the rapid production of a dark residue, stable at room temperature, and the ejection of CO, CO2, O2, O, C2 and (CO)2.

MOLECULAR DYNAMICS STUDY OF NON-EQUILIBRIUM ENERGY TRANSPORT FROM A CYLINDRICAL TRACK : I. TEST OF SPIKE MODELS

Molecular Dynamics (MD) calculations were carried out to describe the kinetic energy transport in a low temperature, condensed-gas solid following electronic excitation of a narrow cylindrical region by fast ions. Solid Ar is used as a model material. The atoms in a cylindrical region of an Ar sample are each given a kinetic energy, which they might obtain by non-radiative radiative relaxation processes, and the evolution of the sample is followed in time. Whereas a uniformly energized sample is found to equilibrate in a mean collision time, equilibration competes with radial transport of energy from the cylindrically excited region. The radial evolution of the deposited energy is compared to that predicted by thermal spike models because of their extensive use. Both analytic and finite diAerence calculations are examined. Although the numerical model, using a realistic conductivity, gives the best comparison, none of the thermal spike models very accurately describe the radial energy profile with time. Initially, energy is collisionaly transported along nearest neighbor directions. Very rapidly a melt front forms, which has nearly constant radial extent, and a pressure wave moves outward in the region not melted, carrying oA part of the total energy deposited. In paper II these results are applied to electronic sputtering of condensed-gas solids. ” 1998 Elsevier Science B.V. All rights reserved.