Editing Openai/6946e998-79f0-8007-9a65-19828450838d (section)

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Models show that early Venus may have received lower solar flux (~30% lower Sun luminosity in its first 0.5–1 Gyr).
During that time:
* Venus may have had surface water, possibly shallow oceans.
* Climate models support multi-hundred-million-year periods where liquid water was stable.

But Venus received ~2× Earth’s solar flux even early on.
This placed it very close to the inner edge of the habitable zone from the start.

==== As the Sun brightened: ====
# Venus crossed the moist greenhouse threshold: - More water vapor enters the stratosphere. - UV radiation photolyzes H₂O → H + O. - Hydrogen escapes to space.
# After enough water was lost aloft, Venus entered a runaway greenhouse: - The planet retains more heat than it can radiate away. - Surface temperature rises until the oceans rapidly boil off (geologically rapid, 10⁵–10⁶ years).

==== Once water is in vapor form: ====
* Hydrogen escapes into space extremely efficiently.
* Oxygen reacts with surface rocks or is lost to space as well.

Over 10⁸–10⁹ years, Venus becomes essentially dry.

With no water:
* No carbon-silicate cycle → CO₂ removed from the atmosphere stops.
* Volcanic outgassing accumulates CO₂ indefinitely.

==== Key properties: ====
* Atmosphere: ~96% CO₂, ~92 bar surface pressure
* Temperature: ~735 K (462°C)
* No oceans, no plate tectonics
* Runaway greenhouse fully stabilized

The physics summary:

Venus crossed a critical flux threshold where water vapor itself becomes a greenhouse amplifier.

Without water, feedbacks that stabilize climate on Earth collapse.

The planet irreversibly locked into a high-temperature, high-CO₂ state.

This is why Venus is inhospitable for complex life to evolve:
* No stable liquid water
* No clement temperatures
* No long-term geochemical cycling
* No stable atmosphere for biochemistry on the surface

Even microbial life would struggle to persist after oceans were lost.