主讲人简介：

Professor of Engineering Physics, Department of Materials Science and Engineering, University of Virginia, USA

He studies ion, electron, and photon interactions with surfaces of low-temperature condensed-gas solids and biomolecular solids and with atmospheric gases. The principal processes studied are the ejection of molecules from the surface, called desorption or sputtering, and atomic collision cross sections. The applications are to surfaces and atmospheres of objects in the outer solar system or the interstellar medium, in order to understand the ambient gas or plasma observed, and samples of biomolecules, in order to produce a source of gas-phase biomolecules for studies in molecular biology.

内容简介：

Our understanding of the evolution of planetary atmospheres is being enormously enhanced by in situ spacecraft data on objects in the outer solar system and by telescopic observation of exoplanets. Cassini is orbiting in Saturn’s system, New Horizon is on its way to the Pluto-Charon system, and the MAVEN mission to study escape from Mars will be launched next year. Surprisingly, the large amount of Cassini data on the thick atmosphere of Titan, instead of re-enforcing our understanding of escape, led to rates that differed by orders of magnitude. This disagreement was due to a lack of a detailed description of how escape changes in character from evaporation on a molecule by molecule basis (Jeans-like escape) to an organized flow (hydrodynamic escape). The latter is a process of considerable interest for the early evolution of terrestrial atmospheres and for atmospheres on exoplanets orbiting close to their parent star. Therefore, we carried out extensive molecular kinetic simulations of escape and found that the transition from Jeans-like to hydrodynamic escape occurs over a surprisingly narrow range of the Jeans parameter, the ratio of the gravitational energy to the thermal energy of the molecules. Application of a molecular-kinetic model to escape from Titan and a fluid/ kinetic hybrid model to escape from Pluto will be described. Using the fluid/ kinetic hybrid model we also evaluated the applicability of the ‘energy-limited escape’ expression which is used frequently for modeling exoplanet atmospheres.