Thanks to observations made by the Solar Occultation in the Infrared (SOIR) instrument onboard the Venus Express space probe of the European Space Agency (ESA), researchers have revealed an unanticipated increase in the abundances of two variants of water molecules – H2O and HDO – along with their ratio HDO/H2O, in the Venus mesosphere. This unexpected phenomenon questions our current knowledge of the history of water on Venus and the factors that may or may not have favoured planetary habitability in the past. This breakthrough is based on the identification of a possible mechanism.
Venus, Earth’s twin, evolved differently than our planet
Although called the Earth’s twin due to their similar size, Venus today exhibits surface conditions significantly different from those on our planet, with a surface pressure nearly 100 times higher and temperatures around 460°C. Venus is entirely covered by thick clouds of sulphuric acid and water droplets spanning altitudes from 45 km to 65 km. Moreover, its atmosphere is more than 100,000 times drier than Earth’s, with most water found below and within these cloud layers.
Investigating Venus’ abundances of both water variants – called isotopologues – H2O and its deuterated counterpart HDO sheds light on the planet’s water history. It is commonly accepted that Venus and Earth initially shared a similar HDO/H2O ratio. Previous studies observed this ratio as 120 times higher in Venus’ bulk atmosphere (below 70 km), indicating a significant deuterium enrichment over time. The primary mechanism responsible for this deuterium enrichment involves the destruction by the solar radiation (photolysis) of water isotopologues in the upper atmosphere, leading to the production of hydrogen (H) and deuterium (D) atoms. These atoms can further escape to space, with H doing so more readily due to its half mass compared to D. This differential escape results in a gradual increase in the abundance of D relative to H, thereby raising the HDO/H2O ratio over time.
Large increase of water and deuterated water in the upper mesosphere measured by the Belgian instrument SOIR
To figure out how much H and D are escaping into space, it is crucial to measure the water isotopologue amounts at heights where sunlight can break them down, which occurs above the clouds at altitudes larger than ~70 km. Our study aims to uncover, for the first time, how H2O and HDO are distributed in the mesosphere of Venus up to an altitude of 110 km. For this purpose, we analysed measurements taken from orbit by the Belgian Solar Occultation in the InfraRed (SOIR) instrument, a spectrometer designed, built, and operated at the Royal Belgian Institute for Space Aeronomy in Brussels. This instrument flew on the Venus Express orbiter, a spacecraft of the European Space Agency that visited Venus from 2006 to 2014.
Our research uncovered two surprising findings. First, contrary to what was anticipated, the concentrations of H2O and HDO increase with altitude between 70 and 110 km. Second, over this same altitude range, the HDO/H2O ratio rises significantly by an order of magnitude – a factor of 10, reaching a level over 1500 times higher than the one found nowadays in the Earth's oceans.
To account for these unexpected findings, we propose a novel mechanism – see the figure – that hinges on the behaviour of hydrated sulphuric acid (H2SO4) aerosols through condensation and evaporation processes, aligning more closely with the rapid changes observed in the data acquired by SOIR. This mechanism suggests that ① aerosols form at altitudes just above the clouds where temperatures fall below the sulfurated water dew point. Accounting for the fact that deuterated water condenses more readily than its hydrogenated counterpart, the condensation process leads to the formation of deuterium-enriched aerosols. These particles then ② rise to higher altitudes, surpassing 100 km, where the mean temperature increases by nearly 80°C, causing them to ③ evaporate and release a more significant fraction of HDO compared to H2O. Subsequently, the vapour is ④ transported downwards, completing the cycle. This process would also account for the increase in sulphur-based species, such as sulphur dioxide (SO2) above 90 km altitude. This phenomenon was reported by various studies, including by our team, but until now, it remained without explanation.
The discovery impacts the possible evolution of the planet
In conclusion, our study highlights two important key points. First, a thorough understanding of altitude variations is essential for locating deuterium and hydrogen reservoirs in Venus’s atmosphere, providing a better understanding of the history of water on the planet. Second, increased HDO/H2O isotope ratio impacts hydrogen and deuterium escape rates. Photolysis of H2O and HDO at higher altitudes leads to an increased deuterium release, thus influencing the long-term evolution of the D/H ratio. As a result, these findings encourage incorporating altitude-dependent processes into models to make accurate predictions about D/H evolution and to reassess whether ancient Venus was wetter or dryer than previously thought, thus influencing its past habitability.
Unexpected increase of the deuterium to hydrogen ratio in the Venus mesosphere | PNAS (PNAS)
by Arnaud Mahieux, S. Viscardy, R. V. Yelle, H. Karyu, S. Chamberlain, S. Robert, A. Piccialli, L. Trompet, J. T. Erwin, S. Ubukata, H. Nakagawa, S. Koyama, R. Maggiolo, N. Pereira, G. Cessateur, Y. Willame, and A. C. Vandaele
Press contacts
Scientific contact:
- Sébastien Viscardy (+32 476 51 05 83)
Research group “Planetary aeronomy”, Royal Belgian Institute for Space Aeronomy
- Arnaud Mahieux (+32 495 42 44 60 or +34 699 769 314)
Research group “Planetary aeronomy”, Royal Belgian Institute for Space Aeronomy
Communication cell:
- Stéphanie Fratta (+32 498 90 59 00)
Communication and documentation, Royal Belgian Institute for Space Aeronomy
