HYDRODYNAMIC ESCAPE OF OXYGEN FROM PRIMITIVE ATMOSPHERES - APPLICATIONS TO THE CASES OF VENUS AND MARS

Authors
Citation
E. Chassefiere, HYDRODYNAMIC ESCAPE OF OXYGEN FROM PRIMITIVE ATMOSPHERES - APPLICATIONS TO THE CASES OF VENUS AND MARS, Icarus, 124(2), 1996, pp. 537-552
Citations number
25
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Astronomy & Astrophysics
Journal title
IcarusACNP
ISSN journal
0019-1035
Volume
124
Issue
2
Year of publication
1996
Pages
537 - 552
Database
ISI
SICI code
0019-1035(1996)124:2<537:HEOOFP>2.0.ZU;2-8
Abstract
It is shown that oxygen produced by photodissociation of water vapor i n an earlier stage of terrestrial planet evolution may be lost by hydr odynamic escape, although in relatively modest amounts. If hydrodynami c escape of hydrogen contained in an ocean equivalent to a few present terrestrial oceans occurred at a relatively slow rate, over the first gigayear of a planet's life, less than approximate to 10% of oxygen w ould be expected to be lost to space (typically approximate to 10% for Mars, approximate to 2% for Venus, and approximate to 0% for Earth). In particular, this result applies to the case of a continuous supply of water by comets during the period of heavy bombardment (approximate to 1 Gyr): it is shown, from a comparison study of the Earth and Venu s, that no more than 0.3 terrestrial ocean is expected to have been ac creted in this way. On the other side, a short episode of intense esca pe (approximate to 2 x 10(7) years), during which the available solar EUV flux is fully consumed to drive escape, at the early times when vo latiles are supposed to have been outgassed (approximate to 10(8) year s), may yield more substantial oxygen escape. By using amounts of oxyg en that are believed to be involved in crustal iron oxidation (and car bonates) on Venus and Mars, as well as in the massive Venus atmosphere , it is shown that primitive oceans equivalent to respectively 0.45 an d 0.2 present terrestrial ocean (respectively 1300 and 600 m average d epth) could be lost, with respectively 30 and 50% of oxygen initially contained in the ocean released to space by hydrodynamic escape. An im portant corollary is that if Venus had been supplied with more than ap proximate to 0.45 terrestrial ocean, it would have been left with an o xygen-rich atmosphere. If the fraction of available solar energy consu med in hydrodynamic escape is definitely smaller than unity, for examp le, by a factor of 4, the previous initial water endowments of Venus a nd Mars are reduced, and less than 10% of oxygen could be lost to spac e by hydrodynamic escape. The question of whether escape can work at h igh energy-limited rates, from a photochemical-dynamic point of view, is not solved in the present work. Finally, it must be noted that no s ignificant oxygen escape is found for Earth, whatever the model parame ters may be: when comparing the three terrestrial planets, the Earth i s the least favorable one to escape, since Venus receives more solar e nergy, whereas Mars, although more distant from the Sun, has a weaker gravitational field. (C) 1996 Academic Press, Inc.