Thanks to observations by the Solar Occultation in the Infrared (SOIR) instrument on the Venus Express space probe of the European Space Agency (ESA), researchers have discovered an unexpected increase in the abundances of two water molecule variants -- H2O and HDO -- and their ratio HDO/H2O in Venus' mesosphere. This phenomenon challenges our understanding of Venus' water history and the potential that it was once habitable in the past.
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