Water and heavy water in Venus’s atmosphere

This plot shows water (H2O) and heavy water (HDO) measured by the Stellar and Solar Occultation in the InfraRed (SOIR) instrument on board
ESA’s Venus Express in the upper atmosphere of Venus.

The data shows measured abundances of water and heavy water at altitudes from 70 to 110 km above the surface of the planet. The figure on the right
shows the ratio of HDO to H2O as a function of altitude. The measured ratio is found to be more than twice the value previously measured in the
lower atmosphere.