PX282 - J7c - earth and venus

why are venus' and earth's atmospheres so different?

CO $_{2}$ dissolves in water, forming carbonic acid
this causes acid rain, which erodes silicate rocks
the calcium carbonate is deposited as ocean sediments (limestones), which comprises of $99.6 %$ of earth's carbon
this negative feedback in carbon-silicate cycle stabilizes the climate

increase in CO $_{2}$ $\to$ increase in the surface temperature $\to$ increase in ocean evaporation $\to$ erosion $\to$ deposition of calcium carbonate $\to$ decrease in CO $_{2}$
CO $_{2}$ is also returned to the atmosphere by volcanoes when ocean plates are subducted
on venus, there is a competing positive feedback loop, which leads to runaway greenhouse
increase in CO $_{2} \to$ increase in the surface temperature $\to$ increase in ocean evaporation $\to$ increase in water vapour $\to$ increase in surface temperature
this happens until all the water is evaporated into the atmosphere: $T_{s} ≃ 1000$ 's K
this forms magma ocean, and the carbon-silicate cycle shuts off
CO $_{2}$ from volcanoes accumulates in atmosphere
earth is not heated enough for runaway greenhouse, even with 100 bar CO $_{2}$ , unlike venus
early earth probably had a thicker CO $_{2}$ atmosphere, avoiding freezing when the sun was fainter
this could be the answer to the faint-young sun paradox
the total quantity of CO $_{2}$ and N $_{2}$ is actually very similar for earth and venus, with CO $_{2}$ as ocean sediments on earth
the water on venus is believed to be split by UV photosynthesis into H and O
H evaporated into space
supported by the large excess of deuterium on venus, which is less vulnerable to evaporate than H