Scientists from the Department of Physics at the University of Oxford have monitored the variations of water vapour (H2O) and semi-heavy water (HDO) in the atmosphere of Mars using observations from the ESA-Roscosmos ExoMars Trace Gas Orbiter. A new article published today in Nature Astronomy provides further insights into the different behaviour of these water isotopes in the Martian atmosphere and what these can tell us about the escape of water through time.
Understanding the climate of early Mars
The slightly different properties of hydrogen and deuterium atoms make H2O and HDO respond differently to several atmospheric processes that affect them. In particular, the higher mass of the deuterium atoms makes these more strongly bound by the gravitational field of Mars, while the hydrogen atoms escape more easily to space. Therefore, as water is lost from the atmosphere of Mars, the ratio of deuterium-to-hydrogen atoms increases over time.
‘The ratio of heavy-to-normal water provides important constraints on the evolution of the water reservoir on Mars,' explains Dr Juan Alday from the Department of Physics at the University of Oxford and lead author of the article. ‘The enrichment in the heavy isotopes of water vapour with respect to Earth tells us that Mars had much more water in the past, which could be indicative of a previously more habitable planet where liquid water was able to flow on the surface.'
Seasonal changes in the escape of water vapour
‘We have monitored the isotopic composition of water in the atmosphere of Mars for more than a full Martian year and have observed drastic variations driven by the proximity of Mars to the Sun,' says Professor Patrick Irwin, a co-author of the paper. ‘During perihelion, when Mars is closest to the Sun, we observe water vapour rising to the upper layers of the atmosphere where the escape mechanisms take place.'
One process changing the D/H ratio is condensation: when water vapour partially condenses in the cold Martian upper atmosphere, this changes the D/H of the remaining water vapour. A second process affecting the D/H ratio of water vapour is when solar energetic photons break the water vapour molecules into lighter species: this is called photolysis.
‘We have calculated the effect that condensation and photolysis have in both normal and semi-heavy water, which also affects the rates at which deuterium and hydrogen atoms escape to space,' explains Dr Colin Wilson, another co-author of the paper. ‘We observe that condensation produces large variations of the D/H ratio in water – as had been expected from previous work. However, the new work reveals for the first time that photolysis is more important for shaping the isotopic composition of the escaping gases.'
Dr Alday is excited by the new results: ‘This work really provides new insights about the mechanisms that affect the escape of the water isotopes; it’s great how planetary spacecraft can give such a detailed understanding of atmospheric processes on worlds so far away.’
Isotopic fractionation of water and its photolytic products in the atmosphere of Mars, Alday et al, Nature Astronomy, 24 June 2021