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Communication Dans Un Congrès Année : 2009

The History of Water on Venus: a Scenario Accounting for Present Neon and Hydrogen Isotopic Ratios

Résumé

In order to study the evolution of the primitive atmosphere of Venus, we developped a time dependent model of hydrogen hydrodynamic escape powered by solar EUV (Extreme UV) flux and solar wind, and accounting for oxygen frictional escape We study specifically the isotopic fractionation of noble gases resulting from hydrodynamic escape. The fractionation’s primary cause is the effect of diffusive/gravitational separation between the homopause and the base of the escape. Heavy noble gases such as Kr and Xe are not fractionated. Ar is only marginally fractionated whereas Ne is moderately fractionated. We also take into account oxygen dragged off along with hydrogen by hydrodynamic process. In that case, most of the available energy is consumed by oxygen and the amount of energy available for the escape of hydrogen is reduced by one order of magnitude. We find nonetheless scenarios that are compatible with present-day Ne and Ar fractionation in Venus atmosphere. Our model suggests that during the first 100 Myr of the planetary accretion of Venus, no more than the content of five terrestrial oceans (5 TO) of water have been lost to space. Our preferred scenario shows that around 60% of the oxygen contained in this water was left behind in the atmosphere. During the end of the accretion, the atmospheric water vapor pressure could have been maintained at the value required to maintain the surface temperature above the liquidus. We argue that hydrodynamic escape could have controlled the solidification rate of the magma ocean during the end of the accretion period by pumping the water out of the magma, through the atmosphere, remaining at a pressure of around 300 bar. After most of the water in the magma has been extracted, the atmosphere progressively dried up, and the magma ocean crystallized, leading to a final collapse of the hydrodynamic escape. The end of the hydrodynamic escape phase and the crystallization of the primitive magma ocean would thus roughly coincide and have occurred around 100 Myr. Since hydrodynamic escape would leave large amounts of oxygen that are not found in the present day atmosphere, an efficient mechanism has removed it. We propose that this mechanism is the dissolution of oxygen in the magma ocean and the ultimate oxidation of ferrous iron into ferric iron. From 100 Myr to 500 Myr, the hydrogen of the water delivered by a late veneer of comets could have been removed by continued thermal escape of hydrogen only. The energy available then would have been insufficient to allow oxygen to be dragged off along with hydrogen. At 500 Myr, on Venus would be left a water global equivalent layer (GEL) of a few meters depth and a dense molecular oxygen atmosphere of around 15 bar would have been left at the surface of Venus. At later times non-thermal mechanisms (pick-up ion escape) may have removed most of the remaining water and led to the present D/H ratio. The 15 bar of oxygen may have been lost to crustal oxidation if the resurfacing and oxidation rates have been high enough during the last past 4 billion years.
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Dates et versions

hal-04113507 , version 1 (01-06-2023)

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C. Gillmann, Eric Chassefiere, P. Lognonne. The History of Water on Venus: a Scenario Accounting for Present Neon and Hydrogen Isotopic Ratios. American Geophysical Union fall Meeting 2009, Dec 2009, San Francisco, United States. pp.abstract id.P31D-05. ⟨hal-04113507⟩
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