Evolution of Earth's Noble Gases: Consequences of Assuming Hydrodynamic Loss Driven by Giant Impact

✍ Scribed by Robert O. Pepin

• Book ID: 102569212
• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 278 KB
• Volume: 126
• Category: Article
• ISSN: 0019-1035
• DOI: 10.1006/icar.1996.5639

✦ Synopsis

In this evolutionary model a planet acquires two isotopically primordial (or ''solar'')

A simple analytic model of hydrodynamic escape is applied to atmospheric loss from Earth under conditions that are un-volatile reservoirs during accretion, one occluded from the proven but could plausibly have existed following deposition nebula into planetary embryo materials and the other coacof thermal energy by a giant Moon-forming impact. Primordial creted as a primary atmosphere degassed from impacting xenon in the primary (pre-impact) atmosphere is readily fracplanetesimals during later planetary growth. Subsequent tionated to its contemporary nonradiogenic isotopic composihydrodynamic losses of primary and outgassed volatiles tion by appropriate selection of parameters in the equations are driven by intense EUV radiation from the young evolvgoverning the escape process. Subsequent mixing of the fracing sun. Hydrogen outflow fluxes strong enough to enable tionated residuals of lighter primordial noble gases surviving Xe escape from Earth and fractionation of Xe to its present in the post-escape atmosphere with solar-composition gases isotopic composition require large but not unrealistic early outgassed from the deep planetary interior yields close matches solar EUV enhancements (by up to ȁ450ϫ present levels)

to the present-day abundances and isotopic compositions of atmospheric krypton and argon. Replication of present-day and atmospheric H 2 inventories (equivalent to water comneon composition requires an additional later episode of hydroposing Յ a few wt% of the planet's mass).

dynamic H 2 escape, now powered by extreme-ultraviolet (EUV)

Energy sources other than solar EUV absorption may solar radiation just intense enough for entrainment and loss of have powered atmospheric escape. Benz and Cameron Ne but not of heavier species. Requirements for EUV flux (1990) suggest that hydrodynamic loss driven by thermal intensity and planetary water inventory are substantially reenergy deposited in a giant Moon-forming impact could duced compared to an earlier model of EUV-driven Xe loss have generated the well-known fractionation signature in from Earth. A noteworthy result of this approach is the close terrestrial Xe. Their model of the event calls for rapid agreement of the noble gas elemental composition characterizinvasion of the pre-existing primary atmosphere by exing the pre-impact terrestrial atmosphere with that derived tremely hot (ȁ16,000 K) dissociated rock and iron vapor, for Venus's primary atmosphere from a parallel evolutionary model involving only solar EUV radiation as an energy source. emplacement of an orbiting rock-vapor disk with an inner No claim is made that the modeling parameters used here edge at an altitude comparable to the atmospheric scale adequately describe the complex and rapidly evolving physical height at this temperature, and longer-term heating of the nature of the post-impact terrestrial atmosphere, or that these top of the atmosphere by reaccretion of dissipating disk solutions are unique. But they do suggest a basic unity in material. Detailed studies of atmospheric structure and primordial noble gas distributions on the two planets, and point dynamics during and following such an impact have not to separate mechanisms that could account for divergent evoluyet been carried out and clearly will be a challenging task.

tion to their presently radically different compositional

A central issue is whether any remnant of the original states. © 1997 Academic Press atmosphere could have survived the event. Ahrens (1990Ahrens ( , 1993)), for example, argues that virtually complete expulsion might have occurred by direct ejection from the im-

1. Introduction

pacted hemisphere and by shock-induced outward ramming of the antipodal planetary surface. However, a Hydrodynamic escape of hydrogen-rich primary atmoplausible alternative is that losses were driven in part by spheres and outgassed volatiles from the terrestrial planets, thermal rather than mechanical processes, and if so it may operating in an astrophysical environment for the early prove difficult to blow off the entire atmosphere, in particusolar system inferred from observation of young star-forming regions in the galaxy, can account for most of the lar its heavier constituents. In this paper I will assume that Benz and Cameron's (1990) suggested post-impact known details of noble gas distributions in their present-148