Biology
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p14a..01z&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P14A-01
Biology
[0325] Atmospheric Composition And Structure / Evolution Of The Atmosphere, [5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution
Scientific paper
Modern theories of planetary accumulation do not build Venus dry and Earth wet save by unlucky chance. If Venus and Earth were built of the same stuff, Venus’s descent into ruin must have been caused by its proximity to the Sun: too much sunlight brought a runaway greenhouse effect, the oceans and seas evaporated, and the hydrogen in the water was irrevocably lost to space. If the story has a moral, Venus would be the earth that lost its water. A complication to the story is that early Venus occupies an ambiguous position with respect to the runaway greenhouse effect. If Venus began as an ocean planet like Earth, both runaway and non-runaway states are plausible at first. In the 50 Myr before the Sun reached the main sequence, the Sun was both bright and faint, with Venus moving in and out of the conventional habitable zone. Once the Sun reached the main sequence it settled to a luminosity 70% of today’s. At this point the critical albedo triggering the runaway greenhouse on Venus was ~0.32, slightly higher than Earth’s today. This means that Earth’s albedo would put an ocean-covered Venus in the runaway greenhouse state, but only just barely, while an albedo of a slightly cloudier planet would have let Venus’s ancient oceans condenses. Early Venus’s indecisive state makes the recovery of liquid water oceans from giant impacts such as Earth’s moon-forming impact questionable. Another interesting plot twist is that dry planets (desert worlds with limited surface water) are expected to have some immunity with respect to the runaway greenhouse effect, because the limited water can be cold trapped at high latitudes. On a hot dry planet the dearth of tropical water vapor has two effects: it stabilizes the greenhouse effect (the tropics can radiate at rates well above the traditional runaway limit because the tropical atmosphere is not saturated) and it creates a dry stratosphere that severely limits hydrogen escape. Young Venus, if dry, would have been well within the dry-planet habitable zone. Here we consider the possibility that an ocean planet can evolve into a dry but habitable planet as the Sun brightens without first passing through an uninhabitable runaway greenhouse phase. This can happen because hydrogen escape in the diffusion limit depends on the wetness of the stratosphere: a warm planet can have a wet stratosphere and fast hydrogen escape while maintaining liquid water at the surface, provided that the atmosphere is not too thick. This requires in particular that most of the CO2 now in Venus’s atmosphere was sequestered as carbonate rock. If these conditions were all met, it is possible that Venus may have been a habitable planet as recently as 1 billion years ago.
Abe Yasuhiro
Abe-Ouchi Ayako
Sleep Norman H.
Zahnle Kevin J.
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