Spectacular increase in the D/H ratio in Venus’ atmosphere

2023-2024
Thanks to observations made by the Solar Occultation in the Infrared (SOIR) instrument on board ESA’s Venus Express space probe, BIRA-IASB researchers have revealed an unanticipated increase in the abundances of two variants of water molecules – H2O and HDO – along with their ratio 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 for deuterium enrichment in the upper mesosphere.

Venus, Earth’s twin, evolved differently than our planet

Venus, though Earth-sized, has extreme surface conditions: pressures nearly 100 times higher and temperatures around 460°C.

Thick clouds of sulfuric acid and water span 45–65 km, and its atmosphere is over 100,000 times drier than Earth’s.

Studying Venus’ water isotopologues (H2O and HDO) reveals its water history. 

Venus and Earth likely had similar HDO/H2O ratios, but Venus’ bulk atmosphere now shows 120 times higher values, indicating deuterium enrichment. This results from solar photolysis breaking water molecules in the upper atmosphere, releasing hydrogen (H) and deuterium (D), with lighter H escaping more easily. Over time, this process increases Venus’ HDO/H2O ratio.

Large increase of H2O and HDO in the upper mesosphere measured by the Belgian instrument SOIR

To determine H and D escape rates, measuring water isotopologues above ~70 km (above the clouds) - where sunlight breaks them down - is essential. Our study is the first to map the H2O and HDO distribution in Venus’ mesosphere up to 110 km. We analysed data from the SOIR spectrometer, built and operated at the Royal Belgian Institute for Space Aeronomy. This instrument, aboard ESA’s Venus Express, orbited the planet from 2006 to 2014.

Our research revealed two surprises: 

  1. H2O and HDO concentrations increase with altitude from 70 to 110 km, contrary to expectations.
  2. the HDO/H2O ratio rises tenfold in this range, reaching over 1,500 times the level in Earth’s oceans today.

To explain these findings, we propose a new mechanism based on hydrated sulfuric acid (H₂SO₄) aerosol behaviour, aligning with SOIR observations (see the figure for the referenced numbers).

  1. Aerosols form just above the clouds as temperatures drop below the sulfurated water dew point, preferentially condensing deuterated water. 
  2. These deuterium-rich aerosols rise above 100 km, where 
  3. warming by ~80°C causes evaporation, releasing more HDO than H₂O. 
  4. The vapour then descends, completing the cycle. 

This process also explains the observed increase in sulfur species like SO₂ above 90 km, previously unaccounted for.

The discovery impacts the possible evolution of the planet

Our study underscores two key points. 

  • First, understanding altitude variations is crucial for identifying deuterium and hydrogen reservoirs, shedding light on Venus’ water history. 
  • Second, the rising HDO/H₂O ratio affects H and D escape rates, as photolysis at higher altitudes increases deuterium release, altering the long-term D/H ratio. 

These findings highlight the need for altitude-dependent models to refine predictions on D/H evolution and reassess whether ancient Venus was wetter or drier, impacting our knowledge about its past habitability.

Reference

  • Mahieux A. et al., 2024. Unexpected increase of the deuterium to hydrogen ratio in the Venus mesosphere. Proceedings of the National Academy of Sciences. 121, e2401638121, doi:10.1073/pnas.2401638121.
Surface Venus
Radar image of the surface of Venus by Magellan. Credits: Magellan Project/NASA/JPL.
Illustration increase H2O and HDO in upper mesosphere Venus
Large increase of H2O and HDO in the upper mesosphere. Credits BIRA-IASB.