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Project Progress

We have made progress on a number of fronts toward establishing boundary conditions for planetary atmospheres in the VPL. The link between planetary thermal evolution and the generation of magnetic fields has been better elucidated through a series of publications by Nimmo and colleagues. Such fields shield atmospheres and biospheres from erosion by lethal doses of stellar radiation and as such are of great interest in astrobiology in general and in the VPL in particular. Also of great interest in astrobiology and the VPL is the treatment of hydrodynamic loss processes of chemical constituents from the upper planetary boundary layer. The details of the preliminary modeling techniques and model validation are given by Parkinson et al., to be submitted for publication July 2003, and are summarised below.

General geochemical (Yang, Holland and Rye 2002) and Fe isotopic studies of an older paleosol (2.75 Ga) has provided further understanding of elemental cycling on ancient Earth. These studies provide empirical constraints for validation runs of the VPL against Archean Earth. A reactive transport modeling study of a 2.2-2.4 billion year old (Ga) paleosol has yielded insights into the composition of Earth’s atmosphere at that time. The paleosol modeling study is the first of a series of process driven kinetic models that will serve to characterize the interactions between atmospheres and planetary surfaces, and the tools developed in this highly specific model are highly transferable to more generalized models that are yet to come.

Existing hydrodynamic escape models produce solutions by integrating the coupled, time independent mass, momentum, and energy equations for the escaping gas from the homopause out to infinity. However, solving the one-dimensional, steady state approximation becomes problematic at the distance where the outflow becomes supersonic. We have developed a new technique for the treatment of hydrodynamic loss processes from planetary atmospheres (Parkinson et al. in prep) utilising the Godunov method. A detailed description of a first order Godunov scheme is given by Godunov (1959), Gombosi (1984), and Leveque (2002). This method overcomes the instabilities inherent in modeling transonic conditions by solving the coupled, time dependent mass, momentum, and energy equations, instead of integrating time independent equations. We validate a preliminary model of hydrodynamic escape against simple, idealised cases (viz., steady state and isothermal conditions) showing that a robust solution obtains and then compares to existing cases in the literature as cited above. The general tools developed here will be applied to various problems such as the early Earth and Venus, and close-in extrasolar gas giant planets.