Evolution/Alteration of Venus

Isotope Ratios in the Venusian Atmosphere (A Wish List…)

• The terrestrial planets likely formed in similar conditions, yet today they 
look very different.

• The evolution of Venus must have been very different than that of Earth: - No global magnetic field (but likely in the past) - No plate tectonics (also likely in the past) - Enormous surface pressure and temperature (94 bar, 700K) - Atmosphere almost entirely CO2 - Incredibly dry atmosphere (water content 1000x below Earth)

• One way to understand this evolution is through studying isotopes..

The Many Morphings of Venus

Isotopes as a Tracer of Evolution

McKeegan et al. (2011)

Isotopes as a Tracer of Evolution

H21

8 O(‰

)δ

Lee et al. (2005)

Cias et al. (1995)

D/H - Did Venus Have an Ocean?• D/H ratio is a tracer of planetary water loss.

• Venus’ atmospheric D/H ratio is very high (~250x Earth)

H2O HDOH

O

D

O

—Out to Space—

—Down into Crust(?)—

- At one point, Venus was wet. But how wet?

Owen, Tobias, et al. (2001)

• To find out how much water was present, you need:

- Current D/H Ratio

- Primordial D/H Ratio

- Fractionation factor

- Current Absolute Abundance

Owen, Tobias, et al. (2001)

D/H - Did Venus Have an Ocean?

• Same as Earth? Same as Comets?

• Relative efficiency of H and D loss

• H2O and H2 are fairly well measured

Current calculations give a global
ocean depth of 5-500 meters.

• To find out how much water was present, you need:

- Current D/H Ratio

- Primordial D/H Ratio

- Fractionation factor

- Current Absolute Abundance

Owen, Tobias, et al. (2001)

D/H - Did Venus Have an Ocean?

• Same as Earth? Same as Comets?

• Relative efficiency of H and D loss

• H2O and H2 are fairly well measured

Current calculations give a global
ocean depth of 5-500 meters.

Atmospheric Escape - Nitrogen

Owen, Tobias, et al. (2001)

Atmospheric Escape - Nitrogen

Owen, Tobias, et al. (2001)

Stronger escape on 
Mars due to low gravity

Atmospheric Escape - Nitrogen

Owen, Tobias, et al. (2001)

Stronger escape on 
Mars due to low gravity

Jupiter system formed 
from a different source

Atmospheric Escape - Nitrogen

Owen, Tobias, et al. (2001)

Stronger escape on 
Mars due to low gravity

Jupiter system formed 
from a different source

What about Venus?

Atmospheric Replenishment - Ar and Ne• The strength of degassing processes is a clue to geologic activity.

• Argon - 40Ar is produced radiogenically from 40K, while 36Ar is primordial.

- Low 40Ar/36Ar indicates low level of current outgassing. Less active geology.

• Helium - 4He produced radiogenically, 3He primordial. - Currently no accurate measurements of He isotopes.

Turcotte and Schubert (1988)

Current Geological Processes• 33S/32S and 34S/32S are good tracers of volcanic processes.

• Isotope ratios are largely determined
in planetary interiors and volcanism.

• They therefore are a clue to the 
current state of geologic activity.

• Deviations from the average level 
inform you of both surface chemistry 
and the atmospheric sulfur cycle 



34S(‰)δ

Thode, H.G. (1991)

Measurements at ~1% accuracy are 
needed to provide useful constraints 
on atmospheric chemistry

Questions?

References • Baines, Kevin H., et al. "Experiencing Venus: Clues to the Origin, Evolution, and Chemistry of Terrestrial Planets

Via In‐Situ Exploration of Our Sister World." Exploring Venus as a Terrestrial Planet (2007): 171-189.

• Baines, Kevin H., et al. "The Atmospheres of the Terrestrial Planets: Clues to the Origins and Early Evolution of Venus, Earth, and Mars." Comparative Climatology of Terrestrial Planets 137 (2014).

• Ciais, P., et al. "A large northern hemisphere terrestrial CO2 sink indicated by the 13C/12C ratio of atmospheric CO2." SCIENCE-NEW YORK THEN WASHINGTON- (1995): 1098-1098.

• Donahue, T. M., and C. T. Russell. "The Venus atmosphere and ionosphere and their interaction with the solar wind: An overview." Venus II (1997): 3-31.

• Farquhar, James, and Boswell A. Wing. "Multiple sulfur isotopes and the evolution of the atmosphere." Earth and Planetary Science Letters 213.1 (2003): 1-13.

• Fedorova, A., et al. "HDO and H2O vertical distributions and isotopic ratio in the Venus mesosphere by Solar Occultation at Infrared spectrometer on board Venus Express." Journal of Geophysical Research: Planets (1991–2012) 113.E5 (2008).

• Owen, Tobias, et al. "Protosolar nitrogen." The Astrophysical Journal Letters 553.1 (2001): L77.

• Thode, H. G. Sulphur isotopes in nature and the environment: an overview. Wiley, 1991.

• Turcotte, D. L., and G. Schubert. "Tectonic implications of radiogenic noble gases in planetary atmospheres." Icarus 74.1 (1988): 36-46.