Thus Venus could have been in a moist or runaway greenhouse state for a

Thus venus could have been in a moist or runaway

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Thus, Venus could have been in a moist or runaway greenhouse state for a comparable (if not longer) amount of time, which would have depleted any hypothetical surface ocean. According to this idea, as the planet desiccates, weathering reactions slow down and the water incorporated within subducting plates decreases. This causes the plates to become too brittle for subduction and plate tectonics ceases. The resultant cessation in silicate weathering then leads to the buildup of the currently observed ~90-bar CO 2 atmosphere. Alternatively, perhaps Venus never had a surface ocean, as the water was lost to a runaway greenhouse during the accretion process itself [50,51]. The high atmospheric escape rates in both scenarios are consistent with the very high measured atmospheric D/H ratio (>100× greater than that for the Earth; [52]). However, the disappearance of the resultant atmospheric oxygen is difficult to explain if Venus had managed to acquire a surface ocean. A leading idea is that a magma ocean had formed during accretion, removing the remnant atmospheric oxygen via drawdown [50]. I examine the efficiency of this oxygen drawdown with the following calculation. Magma oceans can be of many different sizes and depths, including global bodies that are thousands of kms deep [53]. If I assume a ~2000 km deep global ocean on Venus (radius = 6052 km), its volume would be ~6.5 × 10 11 km 3 , after subtracting out the core and solid mantle volumes. Assuming a typical mantle density (4000 kg/m 3 from the literature [54]), the magma ocean mass is Figure 3. The classic CO 2 –H 2 O habitable zone with stellar effective temperature ( T EFF = 2600–7200 K) as a function of the effective stellar flux ( S EFF ). From left to right, the “recent Venus” and “early Mars” limits (solid blue curves) are the empirical (optimistic) classical HZ limits, whereas the “Leconte et al.” and “Maximum Greenhouse” limits compose the conservative (pessimistic) classical HZ limits. Some solar system planets and confirmed exoplanets are shown. Based on work from Kopparapu et al. [ 26 , 49 ]. 3. Habitability case studies: Venus and Mars The subsequent sections delve into the specific cases of Mars and Venus, which are examples of planets that have significantly diverged from our own, providing the rationale for the classical HZ as defined [ 1 ]. 3.1. The Fate of Venus In conjunction with HZ theory, the carbonate–silicate cycle could potentially explain the state of the current Venusian atmosphere. The inner edge of the HZ has likely been past Venus’ orbital distance (~0.72 AU) for at least the past several hundred million years, possibly 1 billion years [ 1 ]. Thus, Venus could have been in a moist or runaway greenhouse state for a comparable (if not longer) amount of time, which would have depleted any hypothetical surface ocean. According to this idea, as the planet desiccates, weathering reactions slow down and the water incorporated within subducting plates decreases. This causes the plates to become too brittle for subduction and plate tectonics ceases.
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