Scientists have observed something unexpected in the atmosphere of Venus – an increase in the level of deuterium relative to hydrogen. Okay, sure, this doesn’t sound like the most exciting statement. However, the consequences of this discovery could turn our current understanding of the amber world upside down.
As it turns out, this would throw into question our assumption that Venus is a barren, forever uninhabitable planet. Here’s how it happens.
“Venus is often called Earth’s twin because of its similar size,” Hiroki Karyu, a researcher at Tohoku University and one of the scientists involved in the study, said in a statement. “Despite the similarities between the two planets, they evolved differently. Unlike Earth, Venus has harsh surface conditions.”
Liquid water cannot exist in sufficient quantities due to the extreme temperatures and pressures beneath Venus’ thick cloud layers. “To put this in perspective, these altitudes contain about 150,000 times less water than similar altitudes on Earth,” the scientists wrote in their study.
But that doesn’t mean this was always the case.
Deuterium and hydrogen are isotopes of each other, meaning they are different forms of the same element, containing the same number of protons but different numbers of neutrons in their nuclei. This results in their different atomic masses, but their chemical properties remain relatively the same.
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A lot of information can be gleaned from isotope ratios. Take carbon dating, for example, a powerful tool scientists use to determine the age of organic matter using relative ratios of carbon-12 and carbon-14 isotopes. The ratio of these isotopes in a material changes over time as carbon-14 radioactively decays and is not replaced.
The HDO/H2O ratios are thought to have been similar on Venus and Earth once, as the two planets formed in a hot region of the early solar system where water could not condense. Later, water is thought to have been transported to the worlds by water-rich asteroids, likely from the outer asteroid belt, which should have resulted in similar deuterium-to-hydrogen (D/H) ratios on both planets. This hypothesis is also supported by similar levels of other volatile elements, such as carbon and nitrogen, on Venus and Earth.
But after looking at data from the Solar Occultation Infrared (SOIR) instrument on the Venus Express spacecraft (which operated from 2006 to 2014), scientists have put a twist on this story. They found that HDO is now 120 times higher than H2O in the Venusian atmosphere. “This enrichment is primarily due to the decay of water isotopes in the upper atmosphere by solar radiation, producing hydrogen (H) and deuterium (D) atoms,” the ESA scientists wrote. “Because hydrogen atoms escape to space more easily due to their lower mass, the HDO/H2O ratio gradually increases.”
They also found that the concentration of water molecules, both H2O and HDO, increases with altitude, specifically between 70 and 110 kilometers (43 and 68 miles) above the Venusian surface. Moreover, they found that the ratio of HDO to H2O becomes very high at these altitudes, more than 1,500 times higher than in Earth’s oceans. This suggests that Venus’ atmosphere contains much more deuterium-rich water than Earth’s, suggesting significant differences in the atmospheric processes of the two planets.
The team believes that these processes may be controlled by climate mechanisms involving sulfuric acid aerosols (H2SO4), which make up the majority of Venus’ clouds.
“These aerosols form just above the clouds, where temperatures drop below the dew point of sulfurized water, creating deuterium-enriched aerosols,” the scientists explained. “These particles rise to higher altitudes, where the higher temperatures cause them to evaporate, releasing a larger fraction of HDO than H2O. The vapor then travels downward, restarting the cycle.”
As for how the results will impact our understanding of the planet, the team first hopes that future studies will examine how the deuterium-to-hydrogen (D/H) ratio changes with altitude when calculating the total amounts of these gases in Venus’ atmosphere. Second, the way deuterium to hydrogen changes with altitude affects how quickly hydrogen and deuterium escape into space. For example, much more deuterium is released into the high atmosphere than expected, which could affect the overall deuterium-to-hydrogen ratio if some of that deuterium escapes.
This means that in order to accurately understand how Venus’s atmosphere has evolved and how much water it may have lost over time, scientists need to use detailed models that take into account these changes in altitude.
