The Dry Fate of a Twin World

The long-held belief was that the sun simply baked Venus's water away over time.
However, this theory hits a mathematical wall: the process would leave behind a massive, suffocating residue of oxygen, which should have stalled the evaporation process long before all the water was gone.

By simulating the terminal stage of planetary accretion, researchers found that a barrage of space rocks acted as a planetary-scale sponge.

• The total impactor mass was between 1 x 10^22 kg and 5 x 10^22 kg.
• The cumulative mass of excavated rock reached approximately 1 wt% of the host planet, equivalent to 10,000 times the mass of Earth’s current atmosphere.
• If the total mass of the impactors exceeded 2 x 10^22 kg, the process could remove more than 0.6 Terrestrial Oceans (TO) of water.

Act 1: Solar Radiation Splits Water

Solar radiation broke atmospheric water vapor into hydrogen and oxygen. The light hydrogen escaped into space, while the heavier oxygen remained trapped in the atmosphere.

Act 2: Hypervelocity Impacts Create Dust

Hypervelocity impacts struck the surface with shock pressures exceeding the 5 GPa Hugoniot Elastic Limit. This pulverized the Venusian crust into a fine-grained, iron-rich dust with particles measuring 45–100 μm in diameter.

Act 3: Iron Reacts with Oxygen

The dust was blasted into a high-temperature steam atmosphere of 300 atm. In this pressurized cauldron, the iron in the dust hungrily reacted with the leftover oxygen.

• For particles at temperatures of 1,023–1,173 K, this oxidation was likely complete within 30 minutes.
• This reaction permanently locked the oxygen into the rock, removing it from the atmosphere and allowing more water to be split apart.

This process was thorough and planet-wide:

• Large impactors greater than 500 km in diameter excavated the planet down to depths of 100–1,000 km, ensuring a continuous supply of fresh, unoxidized minerals.
• With a size-frequency distribution exponent of 2.5, Venus would have undergone more than 3 global surface turnovers, effectively "inhaling" its own atmosphere’s oxygen over and over again.

Model Efficiency

While the model offers a robust solution to the "oxygen bottleneck," it relies on a maximum stoichiometric efficiency of 10.6 g of water removed per kg of ejecta. Real-world mixing dynamics might lower this yield.

Evidence on the Surface

The modern presence of ferrous iron on Venus's surface suggests that more recent volcanic resurfacing, occurring roughly 300 to 600 million years ago, may have buried the ancient, rusted layers created by this violent, dry beginning.