The resultant cessation in silicate weathering then leads to the buildup of the

# The resultant cessation in silicate weathering then

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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.
Geosciences 2018 , 8 , 280 8 of 48 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 ~2.6 × 10 24 kg. The degree of magma ocean drawdown is obtained by calculating the FeO that can be oxidized to FeO 1.5 [ 55 ]. I assume Fe 3+ /(Fe 2+ + Fe 3+ ) ratios and Fe 2+ (by weight) values consistent with the Earth (0.025 and 8%, respectively; ibid), which yields ~0.21% Fe 3+ or ~5.5 × 10 21 kg. For FeO 1.5 , a maximum of (24/56) × 5.5 × 10 21 = 2.3 × 10 21 kg of O can be oxidized by Fe 3+ . Given that the mass of Earth’s atmosphere (1 bar) is 5.1 × 10 18 kg, nearly 500 bars of O can be taken up by this magma ocean. Although the specifics depend on the temporal evolution and other magma ocean characteristics, including size and temperature, large amounts of atmospheric oxygen can theoretically be removed in this manner, possibly explaining the lack of remnant oxygen in the Venusian atmosphere. Moreover, the empirical HZ would shrink if Venus had lost its surface ocean by the end of accretion. As shown in Ramirez [ 22 ], this ‘early Venus limit’ would be computed at ~4.56 Ga, when solar luminosity was ~70% of that of today [ 48 ]. The effective solar flux ( S EFF ) at Venus’ orbit at that time was 0.7 × 1.92 = 1.35, which corresponds to an orbital distance today of d = 1/0.35 = 0.86 AU (Equation (3)). Thus, our solar system’s empirical HZ would shrink in size by ~0.11 AU (~10%) if Venus had lost its water early. 3.2. The Fate of Mars Venus lost whatever carbonate–silicate it may have had and on Earth, this cycle is sustained by plate tectonics. Mars had recently exhibited hot spot volcanism, similar to Hawaii [ 56 ], but hot spot volcanism is unlikely, by itself, to regulate surface temperatures over geologic timescales. This is consistent with the absence of standing bodies of water on present Mars. The differences in bulk sizes between Earth and Mars may also be key to explaining the divergent evolutionary histories between the two planets. Mars has less volume available for its surface area, causing it to lose heat more rapidly than our planet. As time proceeds, the Martian dynamo weakens and the effects of solar

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