Our knowledge of the water history of Venus and its previously considered habitability is being put to the test by new observations.
Research using the Solar Occultation in the Infrared (SOIR) instrument aboard the Venus Express spacecraft from the European Space Agency (ESA) has revealed an unexpected rise in the levels of two forms of water molecules—H2O and HDO—and their ratio (HDO/H2O) in Venus’ mesosphere. This discovery raises questions about what we know regarding Venus’ water history and its possible past habitability.
Today, Venus is known to be a hot and arid world. The atmospheric pressures on Venus are nearly 100 times those on Earth, with temperatures reaching close to 460°C. Its atmosphere, shrouded in thick clouds filled with sulfuric acid and water droplets, is extremely dry. The bulk of the planet’s water is located beneath and within these cloud formations. Nonetheless, it is thought that Venus may have once possessed an amount of water comparable to that of Earth.
“Often referred to as Earth’s twin due to their similar sizes,” notes Hiroki Karyu, a researcher at Tohoku University, “Venus has developed quite differently despite these resemblances. Unlike Earth, Venus showcases dramatic surface conditions.”
By exploring the levels of H2O and its deuterated variant HDO (isotopologues), researchers can gain valuable insights into Venus’ water history. It is commonly believed that both Venus and Earth began with a similar HDO/H2O ratio. However, the ratio found in the dense atmosphere of Venus (below 70 km) is 120 times greater, suggesting a considerable accumulation of deuterium over time. This increase is mainly due to solar radiation breaking down water isotopologues in the upper atmosphere, resulting in the production of hydrogen (H) and deuterium (D) atoms. Since hydrogen atoms, being lighter, escape into space more easily, the HDO/H2O ratio gradually grows.
Understanding how much hydrogen (H) and deuterium (D) escape into space necessitates measuring the amounts of water isotopologues at altitudes where sunlight can decompose them, which happens above the clouds at altitudes greater than approximately 70 km. The study presented two unexpected findings: the concentrations of H2O and HDO grow with altitude in the range of 70 to 110 km, and the HDO/H2O ratio significantly increases, reaching levels more than 1500 times higher than those found in Earth’s oceans.
A potential explanation for these observations involves the behavior of hydrated sulfuric acid (H2SO4) aerosols. These aerosols form just above the cloud layer, where temperatures fall below the dew point of sulfurated water, resulting in the creation of deuterium-rich aerosols. These particles then ascend to higher altitudes, where the rise in temperature causes them to evaporate, releasing a larger proportion of HDO relative to H2O. The vapor subsequently descends, restarting the cycle.
The study highlights two important aspects. First, changes in altitude are vital for identifying the locations of D and H reservoirs. Second, the rise in the HDO/H2O ratio ultimately enhances the release of deuterium, which affects the long-term changes in the D/H ratio. These results advocate for the integration of altitude-dependent dynamics in modeling efforts to accurately foresee shifts in D/H ratios. By better understanding Venus’ habitability and water history, we can gain insights into the factors that could render a planet inhospitable, allowing us to take steps to prevent Earth from following a similar path as its twin.
