Scientists suggest that young Venus lost its oceans, filling its atmosphere with steam instead.

The habitable zone is the range of distances from a star where water can exist in liquid form on the surface of terrestrial-type planets, and water is a necessary condition for life as we know it. Following the strictest criteria, today only Earth falls within this zone in our system. More daring estimates suggest that the habitable zone of the Solar System extends from 0.38 to 10 astronomical units (AU) — the average distance from Earth to the Sun — meaning from Mercury to Saturn. Of course, as a star and its system evolve, this zone shifts. The climate history of Venus may help us understand where the inner boundary of our habitable zone lay.

Venus and Earth formed under similar conditions. Theoretically, after formation, our "neighbor" in the system had enough water for a global ocean three kilometers deep. However, it is unlikely that such an ocean existed: there are no clear signs of water erosion on the surface, but there are traces of meteorite impacts, albeit somewhat "erased" by the renewal of the surface due to volcanic activity.

Today, there are no bodies of water on Venus: the average surface temperature (72 AU from the Sun) is around 465 degrees Celsius, and the pressure is 90 times greater than that at sea level on Earth. Nevertheless, scientists remain uncertain that liquid water has never existed there.

Based on computer simulations, two "pictures" of the climate on the young planet have emerged: "temperate and wet Venus" and "dry Venus." In the first scenario, it resembled Earth for billions of years. The planet formed cool, with liquid water on the surface, and clouds covering the daytime side. A dramatic climate shift was likely triggered by volcanoes that saturated the atmosphere with carbon dioxide and sulfur gases.

The "wet Venus" hypothesis is supported by the abundance of deuterium in the atmosphere, but this has already been explained in other ways. The key point is that if there was so much water, the planet's interior should still be rich in hydrogen.

In the "dry Venus" scenario, the initial ocean of magma on the surface cooled slowly, all the water evaporated, its molecules broke down in the atmosphere, and hydrogen escaped into outer space. Estimates suggest that even if Venus had ten times more water than Earth's oceans, if the magma cooled slowly, the planet could have retained less than a tenth of the mass of Earth's oceans. Over the subsequent billions of years, the atmosphere lost even that water. Thus, the interior of the planet today should have little hydrogen left.

The authors of a new study, published in the journal Nature Astronomy, assessed the hydrogen content inside Venus based on how the balance of compounds in its modern atmosphere is maintained through "feeding" from volcanic activity, as the composition of volcanic emissions reflects the abundance of hydrogen in its depths.

Scientists analyzed the dynamics of water (H₂O), carbon dioxide (CO₂), and carbonyl sulfide (OCS) in Venus's current atmosphere: their formation, decay, the influence of solar radiation, and potential interactions with surface rocks. This allowed them to confirm that they accounted for all possible sources and losses of these compounds and elements. Today, there is a balance in Venus's atmosphere, meaning that any deficiencies of elements in the calculated model are compensated by volcanic emissions. It turns out that Venus's magma is rich in sulfur and carbon, but contains very little water.

Thus, this planet formed with a water vapor-saturated atmosphere. While the ocean of magma slowly cooled on the surface, the water in the atmosphere broke down, and hydrogen "escaped." Over several tens of millions of years, Venus "dried up." This indicates that similar exoplanets and planets in the "Venus zone" around their stars are unlikely to have ever cooled enough to retain liquid water on their surfaces. Overall, these are unlikely candidates for the discovery of life as we know it, which only exists in the presence of liquid water.