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.