Loss of Water on the Young Venus: The Effect of a Strong Primitive Solar Wind

Abstract

An enhanced primitive solar wind, such as may have prevailed during the first few 100 million years of the solar system history, is shown to have had the potential to stimulate strong thermal atmospheric escape from the young Venus. Due to heating by solar wind bombardment of an extended dense planetary corona, typically 10 times more extensive than the solid planet, an escape flux of pure atomic hydrogen as large as 3 × 10 14cm −2sec −1is found to be possible, provided the solar wind was ≈10 3–10 4more intense than now. Even if escape was diffusion-limited, an enhanced primitive solar UV flux (a factor of ≈5 above present level), absorbed by ≈0.3 mbar of thermospheric water vapor, was able to supply the flow at the required rate. For these high escape rates, oxygen was massively dragged off along with hydrogen, and water molecules could be lost at a rate of ≈6 × 10 13molecules cm −2sec −1. Because, at this rate, a terrestrial-type ocean was completely lost in ≈10 million years, short compared to typical accretion and outgassing times, water was lost “as soon” as it was outgassed. This mechanism could explain the present lack of oxygen in the Venus atmosphere. Because it is expected to affect all sunlike stars in the early phase of planet formation, abiotic oxygen atmospheres could be rare in the universe.