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Workshop on Early (1997) 3001.pdf

THERMAL LOSS OF WATER ON YOUNG : THE EFFECT OF A STRONG PRIMITIVE SOLAR WIND. E. Chassefière, Laboratoire de Météorologie Dynamique, Université P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France, (Tel. : 33 (1) 44 27 47 67; Fax : 33 (1) 44 27 62 72; Telex : UPMCSIX 200145F; Electronic mail : [email protected])

This work deals with the problem of thermal escape of heating efficiencies are first considered. The model hydrogen (and, in some cases, both H and O, i.e. water) takes into account the transition to the collisionless state on young planets, in earlier stages of planetary at the exobase through a modified Jeans approach. For evolution. Although it is more specifically focused on the fluid inner planetary corona, the conservation the problem of (and the present lack of oxygen in equations are solved from the base of the expanding Venus ), it may be also applied to Mars. flow (z»200 km) up to the exobase, generally located at an altitude of »1 planetary radius. Solutions are found, It is first assumed that escape is stimulated only by the for which the in the collisional region is equal to solar EUV radiation, which is known to habe been more the Jeans escape flux at the exobase. It is shown that intense in the past (Zahnle and Walker, 1982). From a »2/3 of the escape energy is supplied by energetic simple energetic budget (Chassefière, 1996a), it is neutrals (ENs) formed by charge exchange between shown that, in this case, oxygen produced by escaping H atoms and solar protons in the heliosphere, a photodissociation of water vapor in an earlier stage of fraction of which intercepts the exobase and heats by terrestrial planets evolution may be lost by collision the upper layers of the fluid planetary corona. hydrodynamic escape (see Hunten et al, 1987, for the This mechanism somewhat differs of sputtering (see e.g. theory), although in relatively modest amounts. If Luhman and Kozyra, 1991), although being basically of hydrodynamic escape of hydrogen contained in an ocean the same nature. Any planetary magnetic field pushing equivalent to a few present terrestrial oceans occurred at away the obstacle up to an altitude larger than »3 a relatively slow rate, over the first Gyr of a ’s life, planetary radii would inhibit this effect. The ratio of the less than »10% of oxygen is expected to be lost to space EUV flux to the solar wind strength is shown to be of (typically »10% for Mars, »2% for Venus and »0% for prime importance, since the solar wind regulates the ). On the other side, a short episode of intense escape flux from the outer, through the action of escape (»2 107 yr), during which the available solar energetic neutrals, and the EUV flux acts from the EUV flux is fully consumed to drive escape, at the early inner, by supplying atoms with the energy required to times when volatiles are supposed to have been lift them up to the exobase. outgassed (»108 yr), may yield more substantial O escape. For Venus and Mars, it is shown that primitive The previous model is now being tested by using oceans of respectively 1300 m and 600 m average depth primitive enhanced values of the solar EUV flux. The could be lost by EUV-stimulated hydrodynamic escape, most interesting result at the present stage is that, in with respectively 30% and 50% of oxygen initially early conditions, the radius of the fluid corona may contained in the ocean released to space by reach »4 planetary radii, and a semi-collisional region, hydrodynamic escape, the rest being involved in crustal inside which the mean free path and the scale height are oxidation. An important corollary is that if Venus had close together, is established between 4 and »10 been supplied with more than »0.45 terrestrial ocean, it planetary radii (Chassefière, 1996c). Because the solar would have been left with an O2-rich atmosphere, in wind is an important source of escape energy, as good qualitative agreement with the previous suggestion previously shown, and due to the possibly large cross- of Kasting (1995). sectional area of the venusian corona at early times, this result suggests the potentially important role of the solar In a second step, the question of whether escape can wind. work at high energy-limited rates is assessed more specifically by using a devoted one-dimensional model From a third specific model, an enhanced primitive of hydrodynamic escape (Chassefière, 1996b), as solar wind, such as may have prevailed during the first previously made by Kasting and Pollack (1983). This few 100 million years of the solar system history model is used to study the loss of hydrogen from a hot, (Henney and Ulrich, 1995), is shown to have had the water-rich, atmosphere of the Venus type. A range of potential to stimulate strong thermal EUV heating rates corresponding to the present solar from the young Venus (Chassefière, 1997). Due to cycle fluctuations of the EUV flux and different possible heating by solar wind bombardment of an extended Workshop on Early Mars (1997) 3001.pdf

THERMAL LOSS OF WATER ON YOUNG PLANETS: E. Chassefière

dense planetary corona, an escape flux of pure atomic References: [1] Chassefière, E. 1996a. Hydrodynamic hydrogen as large as 3 1014 cm–2 s–1 is found to be escape of oxygen from primitive : possible, provided the solar wind was »103 – 104 more applications to the cases of Venus and Mars, Icarus 124, intense than now. Even if escape was diffusion-limited, 537-552. [2] Chassefière, E. 1996b. Hydrodynamic an enhanced primitive solar UV flux (a factor of »5 escape of hydrogen from a hot water-rich atmosphere: above present level), absorbed by »0.3 mbar of the case of Venus, J. Geophys. Res. 101, 26039-26056. thermospheric water vapor, was able to supply the flow Chassefière, E. 1996c. The Venus hydrogen corona in at the required rate. For these high escape rates, oxygen primitive solar EUV conditions, Geophys. Res. Lett., was massively dragged off along with hydrogen, and submitted. [3] Chassefière, E. 1997. Loss of water on the water molecules could be lost at a rate of »6 1013 young Venus: the effect of a strong primitive solar wind, molecules cm–2 s–1. Because, at this rate, a terrestrial- Icarus, in press. [4] Henney, C.J. and R.K.Ulrich. 1995. type ocean was completely lost in »10 million years, The effects of heavy-element diffusion and mass loss on short compared to typical and outgassing solar evolution. in Proc. of fourth SOHO workshop, pp times, water was lost “as soon” as it was outgassed. This 3-7. [5] Hunten, D.M., R.O. Pepin, and J.C.G. Walker. mechanism could explain the present lack of oxygen in 1987. Mass fractionation in hydrodynamic escape. the Venus atmosphere. Because it is expected to affect Icarus 69, 532-549. [6] Kasting, J.F., and J.B. Pollack. all sun-like stars in the early phase of planet formation, 1983. Loss of water from Venus. I. Hydrodynamic abiotic oxygen atmospheres could be rare in the escape of hydrogen. Icarus 53, 479-508. [7] Kasting, universe. This result must be considered in relation with J.F. 1995. O2 concentrations in dense primitive the search for life (or at least clues to life) by space- atmospheres: commentary. Planet. Space Sci. 43, 11-13. based IR interferometry (Léger et al, 1996). Although [8] Léger, A., J.M. Mariotti, B. Mennesson, M. Ollivier, Mars and the Earth are not thought to have given rise to J.L. Puget, D. Rouan, and J. Schneider. Icarus 123, 249- a runaway greenhouse effect of the Venus type, strong 255. [9] Luhman, J.G., and J.U. Kozyra. 1991. Dayside water escape could perhaps occur by this way during an pickup oxygen ion precipitation at Venus and Mars : early accretional greenhouse phase (Matsui and Abe, Spatial distributions, energy deposition and 1986), in addition to impact erosion. consequences. J. Geophys. Res. 96, 5457-5467. [10] Matsui, T., and Y. Abe. 1986. Impact-induced atmospheres and oceans on Earth and Venus. Nature 322, 526-528. [11] Zahnle, K.J., and J.C.G. Walker. 1982. The evolution of solar ultraviolet luminosity. Rev. Geophys. Space Phys. 20, 280-292.