Atmospheric and water loss from early Venus

Abstract

Previous interpretations of the Pioneer Venus mass spectrometer data of the deuterium to hydrogen ( D / H ) ratio of 1.9 × 10 - 2 or 120 ± 40 times the terrestrial value indicate that Venus may have had at least an H 2 O content of the order of about 0.3% of a terrestrial ocean (TO), and even much more during and shortly after the accretion period of ⩽ 300 Myr , depending on the unknown ratio of a continuous supply of H 2 O by comets to a hydrogen blow-off loss and impact erosion of the early atmosphere. In view of the low H 2 O abundance in the present atmosphere, several studies suggest that the planet should have lost most of its H 2 O during the early high X-ray, EUV and solar wind period of the active young Sun. Because oxygen did not accumulate in Venus’ atmosphere it is commonly believed that a part of the oxygen from dissociated H 2 O vapor was dragged off to space along with the escaping hydrogen during a blow-off period, or could have oxidized the surface minerals to produce FeO and Fe 2 O 3 to the depths of a few kilometers to tens of kilometers depending on the initial amount of H 2 O . We use in the present study, for the first time, multi-wavelength X-ray and EUV (XUV) observations by the ASCA, ROSAT, EUVE, FUSE and IUE satellites and stellar winds inferred from mass loss observations by the Hubble Space Telescope of solar proxies with ages < 4.6 Gyr for the investigation of how efficiently the radiation and particle environment of the young Sun could have influenced the evolution of the early Venusian atmosphere and its H 2 O inventory due to the removal of oxygen picked up by the solar wind. For modelling the Venusian thermosphere over the planetary history we apply a diffusive-gravitational equilibrium and thermal balance model and investigate the heating of the early thermosphere by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by CO 2 IR emission in the 15 μ m band. Our model simulations result in expanded thermospheres with exobase levels between about 200 km at present and about 2200 km 4.5 Gyr ago. Moreover, our results yield high exospheric temperatures during the active phase of the young Sun even if we assume a “dry” CO 2 atmosphere with similar composition that is observed on present Venus of more than 8000 K after the Sun arrived at the zero-age-main-sequence (ZAMS). Exospheric temperatures above about 4000 K lead to diffusion-limited escape and high loss rates for atomic hydrogen. The duration of this blow-off phase for atomic hydrogen essentially depends on the mixing ratios of CO 2 , N 2 and H 2 O in the early Venusian atmosphere and could last between about 150 to several hundred Myr, which could result in a large thermal loss of hydrogen from Venus. For studying how much of the H 2 O -related oxygen could have been lost to space by the ion pick up process due to the stronger solar wind and higher XUV fluxes of the young Sun we used our modelled atmospheric density profiles and studied the loss of O + ion pick up from the upper atmosphere of Venus over the planet's history by applying a numerical test particle model. Depending on the used solar wind parameters, our model simulations show that ion pick up by a strong early solar wind on a non-magnetized Venus could erode during 4.6 Gyr more than about 250 bar of O + ions, that corresponds to an equivalent amount of one terrestrial ocean. Finally, we discuss the implications of our findings for the formation of the Venusian atmosphere and discuss our results in the frame of previous studies.