It is hot and dry – but why does Venus still have significantly less water than previous models suggest? Results from computer simulations now suggest that the extreme drying is due to a process in which hydrogen atoms are very effectively ejected from the atmosphere into space. This is a so-called dissociative recombination, in which HCO+ ions react with electrons, causing the potential building blocks of water to quickly escape. Future missions to our planetary neighbor could also demonstrate the importance of this effect directly in the atmosphere of Venus, say the researchers.
Both are similar in size, consist largely of rocky material and have an atmosphere: Venus and Earth are sisters, but they have developed very differently over the course of their history. Our planet produced life-friendly conditions and water on the surface, while Venus turned into a dry hellworld. It is assumed that the features were very similar at the beginning: billions of years ago, Venus still had about as much water as Earth.
But then the high levels of carbon dioxide in their atmosphere caused the strongest greenhouse effect in the solar system. He ended up giving Venus’ surface about 900 degrees Celsius, turning all of its water into steam. Over time, most of the gaseous water drifted into space, so that today there is only relatively little water vapor in the Venus atmosphere. According to research results, the amount would be enough to cover the planet’s surface with a three centimeter deep layer of water.
Why so extremely dry?
“Venus has about 100,000 times less water than Earth, even though it has roughly the same size and mass,” says lead author Michael Chaffin from the University of Colorado in Boulder. As he and his colleagues explain, this is a surprisingly small amount of residual water. This makes Venus appear significantly more parched than can be explained by the evaporation process based on the heating of water molecules. “Let’s say I pour out the water in a water bottle. There would still be a few droplets left. “However, on Venus, almost all of these remaining drops have also disappeared,” says Chaffin, illustrating the surprising feature. Apparently there were other processes that contributed to the intensive water loss. However, previous explanations have not led to a conclusive model of drying out.
In their study, Chaffin and his colleagues have now developed complex model simulations of the processes in the Venus atmosphere. All known parameters of this “hell air” characterized by heat, high pressure and various substances were included. As the researchers report, a substance finally emerged in the products of the simulated Venus atmosphere that could be linked to a strong loss of water: it is HCO+. According to the simulations, this positively charged ion from one atom each of hydrogen, carbon and oxygen would have to be formed at higher altitudes in the Venusian atmosphere, the team reports. “Surprisingly, it became apparent that HCO+ would be one of the most abundant ions in the Venusian atmosphere,” says Chaffin.
Hydrogen atoms are transported into space
The researchers explain the drying effect of HCO+ as follows: These ions are therefore constantly produced in the upper atmosphere – but do not survive for long. There they react with electrons, which leads to a so-called dissociative recombination. The hydrogen atom is split off from the carbon-oxygen residue: it literally whizzes away and can easily escape into space. The Venus atmosphere is increasingly losing the building blocks for the formation of water (H₂O). This effect could have contributed to the surprisingly severe dryness of Venus, say the researchers.
As they emphasize, the new explanatory approach remains theoretical so far. Because there is still no concrete evidence of HCO+ in the atmosphere of Venus. “Due to design limitations, previous Venus missions were unable to measure HCO+ or escaping hydrogen,” the authors write. But they hope that future probe missions to our planetary neighbor can provide this evidence. Co-author Eryn Cangi of the University of Colorado concludes: “Because water is central to the evolution of life, we should understand the conditions that favor liquid water in the universe and that may have led to the very dry state of Venus.” said the scientist.
Source: University of Colorado at Boulder, specialist article: Nature 10.1038/s41586-024-07261-y