Deuterium Isotope Inversion at 72km
Analysis of the unexpected deuterium enrichment in Venus's mesosphere from Venus Express data and its implications for atmospheric evolution and ISRU.
The Mesospheric Isotope Trap
Venus functions as a planetary-scale fractional distillation column for hydrogen isotopes. Over billions of years, hydrodynamic escape and solar wind stripping have preferentially removed lighter hydrogen ($^1H$), enriching the atmosphere in deuterium ($^2H$ or $D$).
Data from the Venus Express mission’s SOIR instrument reveal an unexpected further enrichment in the mesosphere: the Deuterium Inversion Layer.
The Cold Trap Mechanism
Enrichment is driven by the sulfuric acid hydration cycle in the upper haze:
- Condensation: HDO condenses preferentially into H₂SO₄ aerosols due to higher boiling point and mass.
- Photolysis: Solar UV breaks down vapor, producing H and D atoms; lighter H escapes more easily.
- Accumulation and Transport: Deuterium-enriched aerosols evaporate at higher altitudes, releasing disproportionate HDO; downward transport restarts the cycle.
This vapor-pump mechanism amplifies fractionation beyond simple escape models.
Primary Source: 2024 PNAS Paper and Press Release
The key discovery comes from reanalysis of Venus Express SOIR solar occultation data:
- Paper: Mahieux et al. (2024) report HDO/H₂O rising dramatically above 70 km, challenging prior models of uniform D/H.
- Popular Summary: Tohoku University press release (August 2024) highlights the “heavy water factory,” with contributions from H. Nakagawa and others.
Additional Supporting Studies
Earlier Venus Express work established baseline mesospheric distributions:
General water loss and deuterium enrichment from Venus Express:
- ESA Summary: Progressive deuterium enrichment in upper regions due to easier H escape.
Implications for ISRU and Future Missions
This extreme mesospheric enrichment has major implications for in-situ resource utilization. Missions targeting atmospheric propellants (e.g., methalox or hydrolox from CO₂ and H₂SO₄ processing) could repurpose buoyant platforms for deuterium harvesting at ~70-90 km, accessing high-purity heavy water precursors for fusion fuel or advanced propulsion.
Source Data:
- Venus Express SOIR solar occultation spectra
- VIRA (Venus International Reference Atmosphere) models
- Historical D/H measurements (Pioneer Venus, ground-based)