Hydrodynamic escape

These problems are proving to be more tractable in the context of a related thermal loss mechanism, hydrodynamic escape (Zahnle and Kasting 1986 Hunten et al. 1987, 1988, 1989) Sasaki and Nakazawa 1988, 1990 Zahnle et al. 1990a Pepin 1991, 1994, 1997, 2000). Here atmospheric loss is assumed to occur from much larger bodies, partially or fully accreted planets. Their hydrogen-rich primordial atmospheres are heated 

Constituent 1 is usually assumed to be molecular hydrogen. Constituent 2 is taken to be a multicomponent noble gas mixture—excluding He, which is only weakly bound in terrestrial planet atmospheres and escapes readily by other processes. The diffusion parameter b in Equation (3), and thus the crossover mass me, differs for different elements S in constituent 2. If mc(S) is known (or assumed) for one element, say Xe, then Equation (3) for both Xe and S yields 

The energy required for escape of a particle with mass mi from its local gravitational field, at radial distance r rs from a body of mass M and radius rs, is GmiM/r ergs per particle. If the global mean solar EUV input at heliocentric distance R and time t is ( )(R,t) the energy-limited escape flux is 

In the case where energy deposited in an atmosphere by EUV radiation or from some other source declines exponentially with time, the flux Fi of constituent 1 is given by 

Fractionating effects of the escape process can now be calculated analytically if specific assumptions are made about the time dependence of the major (constituent 1) inventory Ni— that it is either replenished as fast as it escapes (constant inventory model), or is lost without replenishment along with the minor atmospheric species (Rayleigh fractionation model). In both cases the inventories N2 of minor species, here the noble gases, are assumed to be in the atmosphere at to and are lost without replenishment during the escape episode. For Rayleigh fractionation, adopted for this discussion. Equations (2), (3), and (7) and the definitions Fi = -dNi/dt and F2 = -dN2/dt may be combined and integrated, in the limit of Xi = 1, mc m2 mi, and mc m2, to yield 

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Support for loss of light gases from the atmosphere via hydrodynamic escape can be found in other atmophile isotopic systems. The Ne/ Ne ratio in the mantle is elevated... 

Therefore, there exist several of lines of evidence to support the view that impact erosion may have had a significant effect on Earth s composition. However, in most cases the evidence is suggestive rather than strongly compelling. Furthermore, we have a very poor idea of how this is possible without fractionating potassium isotopes (Humayun and Clayton, 1995), unless the entire inventory of potassium is vaporized (O Neill, 1991a,b Halliday et al, 1996 Halliday and Porcelli, 2001). We also do not understand how to lose heavy elements except via hydrodynamic escape of a large protoatmosphere (Hunten et al, 1987 Walker, 1986). Some of the loss may have been from the proto-planets that built the Earth. 

Figure 4 Relationships of (a) unfractionated SW-Xe and U-Xe to terrestrial atmospheric Xe, plotted as per mil differences from the air Xe composition (Table 1). (b) SW-Xe and U-Xe, after hydrodynamic escape fractionation to the extent required to match the Xe/ Xe...

Hydrodynamic escape required to modify the isotopic composition of the noble gases will also fractionate nitrogen and carbon in the atmosphere, as discussed in detail by Pepin (1991). The inventories in the remaining atmosphere will be isotopicaUy heavier, and so chondrites supplying volatUes must have had lower and o... 

Venus. Venus is characterized only by the immensely valuable but still incomplete and relatively imprecise reconnaissance data from the Pioneer Venus and Venera spacecraft missions of the late 1970s. Additional in situ measurements, at precisions within the capabilities of current spacecraft instrumentation, are now necessary to refine atmospheric evolution models. Unfortunately, the possibilities of documenting the volatile inventories of the interior of the planet are more remote. A significant question that must be addressed is whether nonradiogenic xenon on Venus is compositionally closer to SW-Xe (as seen on Mars) or to the U-Xe that is seen on the Earth and so is expected to have been present within the inner solar system. Also, the extent of xenon fractionation will be an important parameter for hydrodynamic escape models if intense solar EUV radiation drove hydrodynamic escape on the Earth, it would also impact Venus, while losses from the Earth driven by a giant impact would not be recorded there. 

Much of present understanding of the earliest evolution of Earth s atmosphere can trace descent from Walker (1979) and references therein. The prebiological atmosphere (before the origin of life) was controlled principally by the composition of gases emitted from volcanoes. Emission of H2 in volcanic gases has contributed to net oxidation of the planet through time. This is achieved through several mechanisms. Simplest is hydrodynamic escape... 

Identifying Martian paleoatmosphere is fascinating in its own right, but also could put tight constraints on Martian atmospheric evolution. For example, based on Xe isotope systematics in ALH84001, Mathew and Marti have argued that not only had the Martian atmosphere not acquired its full complement of Xe by 4 Ga ago, the isotopic fractionation that is required (see Fig. 3 and discussion above) had not yet occurred then either (Mathew and Marti 2001). Such a late fractionation would be very difficult to explain by atmospheric loss through hydrodynamic escape (Hunten et al. 1987 Pepin 1994 Pepin and Porcelli 2002, this volume). 

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