Figure. Diagram of the proposed Venus mesospheric water cycle mechanism to sustain the observed HDO and H2O VMR and HDO/H2O ratio increase with altitude. ©Mathieux et al. Thanks to observations by the Venus Express space probe of the European Space Agency (ESA), an international research group led by Arnaud Mahiuex, a researcher at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Hiroki Karyu, a PhD student, and Assoc. Prof. Hiromu Nakagawa, at this laboratory, has discovered an unexpected increase in the abundances of two water molecule variants – H2O and HDO – and their ratio HDO/H2O in Venus’ mesosphere. The...