Timothy A. Cassidy

MASSIVE SATELLITES OF CLOSE-IN GAS GIANT EXOPLANETS

We study the orbits, tidal heating and mass loss from satellites around close-in gas giant exoplanets. The focus is on large satellites which are potentially observable by their transit signature. We argue that even Earth-size satellites around hot Jupiters can be immune to destruction by orbital decay; detection of such a massive satellite would strongly constrain theories of tidal dissipation in gas giants, in a manner complementary to orbital circularization. The stars gravity induces significant periodic eccentricity in the satellites orbit. The resulting tidal heating rates, per unit mass, are far in excess of Ios and dominate radioactive heating out to planet orbital periods of months for reasonable satellite tidal Q. Inside planet orbital periods of about a week, tidal heating can completely melt the satellite. Lastly, we compute an upper limit to the satellite mass loss rate due to thermal evaporation from the surface, valid if the satellites atmosphere is thin and vapor pressure is negligible. Using this upper limit, we find that although rocky satellites around hot Jupiters with orbital periods less than a few days can be significantly evaporated in their lifetimes, detectable satellites suffer negligible mass loss at longer orbital periods.

Sputtering from a Porous Material by Penetrating Ions

Porous materials are ubiquitous in the universe and weathering of porous surfaces plays an important role in the evolution of planetary and interstellar materials. Sputtering of porous solids in particular can influence atmosphere formation, surface reflectivity, and the production of the ambient gas around materials in space. Several previous studies and models have shown a large reduction in the sputtering of a porous solid compared to the sputtering of the non-porous solid. Using molecular dynamics simulations we study the sputtering of a nanoporous solid with 55% of the solid density. We calculate the electronic sputtering induced by a fast, penetrating ion, using a thermal spike representation of the deposited energy. We find that sputtering for this porous solid is, surprisingly, the same as that for a full-density solid, even though the sticking coefficient is high.

Sputtering of Ices

Data obtained from the exploration of the outer solar system has led to a new area of physics: electronically induced sputtering of low-temperature, condensed-gas solids, here referred to as ices. Icy bodies in the outer solar system are bombarded by relatively intense fluxes of ions and electrons, as well as the background solar UV flux, causing changes in their optical reflectance and ejection (sputtering/desorption) of molecules from their surfaces. The low cohesive energies of ices lead to relatively large sputtering rates by both momentum transfer (‘knock-on’ collisions) and the electronic excitations produced by the incident particles. Such sputtering produces an ambient gas about an icy body, often the source of the local plasma. This chapter focuses on the ejection of material by ionizing radiation from a surface that is predominantly a molecular condensed gas solid. The incident radiation types considered are photons, electrons and ions with the emphasis on the ejection processes. This radiation also produces the chemical effects described in the chapters of sections II and III. The induced-chemistry can produce both more refractory and more volatile products and so affect the molecular ejection rate. The emphasis in this chapter is on the production of gas-phase species from icy surfaces in space. We describe the physics and chemistry leading to the ejection of atoms and molecules, give semi-empirical expressions based on these processes, and describe some applications.

Collisional Evolution of the Enceladus Neutral Cloud

Water vapor ejected from Saturn’s small moon Enceladus easily escapes its meager gravity to form a Saturn‐encircling cloud with a low collision rate. Observations show that the cloud is quite broad in the radial direction, and we show here that collisions, though quite rare, may be largely responsible for this radial spreading. We modeled this cloud using the Direct Simulation Monte Carlo method, as fluid methods would be inappropriate for such a tenuous gas.

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.