Recent years have shown that there is still much to be learned about Venus, both present and past. As worlds go, Venus is hell. A surface temperature high enough to melt lead and a surface pressure like a kilometer deep don’t make it particularly comfortable, and the clouds that cover the entire planet are rich in various acids – yet the picture is more complicated. Venus may once have had the same amount of water as Earth.

Evidence of active volcanoes is mounting and fascinating molecules such as phosphine and possibly ammonia have been discovered. Now researchers at Tohoku University have investigated the distribution of water in Venus’ atmosphere and discovered further peculiarities.

“Because of their similar size, Venus is often referred to as Earth’s twin,” said Hiroki Karyu, co-author of the study and researcher at Tohoku University, in a statement. “Despite the similarities between the two planets, they have evolved differently. Unlike Earth, Venus has extreme surface conditions.”

The planet and its clouds are now dry, but there is still water floating around. The researchers used data from the Solar Occultation in the Infrared (SOIR) instrument, part of ESA’s Venus Express spacecraft, to measure the abundance of two different types of water molecules: normal and semi-heavy.

Normal water is just your standard H₂O. We love it because it is critical to the survival of all life on Earth. You may have heard of semi-heavy water in connection with nuclear power plants. It is water in which one of the two hydrogen atoms has been replaced by an atom of deuterium, an isotope of hydrogen. Normal hydrogen consists of one electron orbiting a proton, while deuterium consists of one electron orbiting a proton and one neutron.

The ratio between the two types of water is likely to be the same on Venus and Earth, but Venus’ atmosphere contains 120 times more deuterium than Earth’s water. The enrichment on Venus is caused by sunlight. It splits all the water molecules, but because hydrogen is lighter than deuterium, the normal version is lost in space faster than the heavier version.

But that’s not all: The researchers found that in both variants there is more water higher up in the atmosphere. That is between 70 and 110 kilometers (43.5 and 68 miles), much higher than in the clouds. The ratio between the two variants increases dramatically. At the highest point there is 1,500 times more semi-heavy water than normal.

The dramatic distribution is thought to be caused by a water and sulfuric acid cycle in the atmosphere. Hydrated sulfuric acid forms just above the clouds and rises, where it releases more semi-heavy water. The vapor tends to rise back down, where it condenses again and starts the cycle again. The team believes it is very important to include the altitude-dependent changes in the models.

We’ll soon learn more about Venus. The Rocket Lab Probe, part of the Morning Star Missions, is scheduled to launch in January 2025 and will be the first private mission to another planet. It will penetrate Venus’ atmosphere and provide insights from the site. Then there’s NASA’s DAVINCI+, one of two NASA missions heading to Venus in the next decade. And then there’s EnVision, the European Space Agency’s mission designed to understand the relationship between the planet’s special atmosphere and geological activity.

The article was published in Proceedings of the National Academy of Sciences.