The formation of Venus (and Earth) as told by isotopes

Jennika Greer, Planetary Seminar Fall 2015

Similarities: size, mass, distance from Sun- early history Differences: satellites, magnetic fields, tectonics, rotation, atmosphere

Image: ESA

Noble Gases

• Inert • Depleted in

planets; enriched in radiogenic component

Composition of Interior

• Venus has 25% less 40Ar than Earth – 25% less 40K (?) – Less degassed

Source of Volatiles

• 20Ne (primordial) • 21Ne (produced by 24Mg; cosmic rays) • 22Ne (produced by 25Mg) • Same δ21Ne and δ22Ne: same source of noble gas • Different δ21Ne and δ22Ne: different source of noble gas

Source of Volatiles

• Venus enriched in Ar and Ne relative to Earth (5:1) – Implanted by

solar wind – Impacted by

large comet – Accreted, but

held on to

Source of Volatiles

• Venus 20Ne/36Ar = 0.5 ± 0.3 • Earth 20Ne/36Ar = 0.57 • Solar Wind 20Ne/36Ar = 50 • Has solar wind flux changed? May not be

source of Ar enrichment

Source of Volatiles

• Kr/Xe and Ar/Kr • Low Kr on Venus: solar wind, Jupiter’s

atmosphere • High Kr on Venus: meteorites, Earth, Mars

Source of Volatiles

• High D/H ratio in atmosphere – 120x Earth • If Venus’ water is in a steady state, source

water must be 10x that on Earth – Massive escape

early in history – Continuous

outgassing

Modified from Hartogh, 2011

Venus

Fractionation Processes

• Xe isotopic signature not inherited from solar nebula

• Xe loss => loss of all lighter gases • Earth’s Xe loss: Escape during accretion and

after giant impact

Fractionation Processes

Fractionation Processes Atmosphere Blowoff Erosion due to accretion

Exogenous Source Ionized Escape

Fractionation Processes

• Atmospheric blowoff different Xe on Earth and Venus • Erosion due to accretion (can’t account for

mass fractionation) same Xe on Earth and Venus • Exogenous source of Xe (comets) same Xe on Earth and Venus • Ionized Xe escape different Xe on Earth and Venus

U-Xe: primordial Xe

• Approximate solar wind composition, depleted in heavy isotopes- not found

• Venus- only place where U-Xe is still present? If 136Xe/130Xe 8% lower than solar wind

Mixing History

Variations in O isotopic composition

Mixing History

• Ground-based IR observation – 2645-2651 cm-1: 16O12C17O and 16O12C18O – 4577-4587 cm-1: 16O12C18O and 16O12C16O

• δ17O = 92 ± 158 ‰ • δ18O = -42 ± 85 ‰ (-3 ± 50 ‰) • Source of error

– Day to day variations – Uncertainty in temperature

Mixing History

Agrees with Earth/Moon/Mars fractionation line within error- terrestrial planets from well-mixed protosolar nebula

Wish List – in situ measurements

• 22Ne/20Ne and 21Ne/20Ne source of Ne, other volatiles • Xe isotopic measurements (136Xe/130Xe) source of Xe- solar or planetesimal; early

solar wind processes • Kr/Xe and Ar/Kr (bulk) distribution of materials in solar system

Conclusions

• Isotopic measurements act as tracers for bulk composition of planet during formation

• Source of volatiles: – Solar wind – Comets, meteorites

• Distinguish between processes that affect Venus and Earth

References • Baines, K.H., Atreya, S.K., Carlson, R.W., Crisp, D., Grinspoon, D., Russell, C.T.,

Schubert, G., Zahnle, K., (2007). Experiencing Venus: Clues to the Origin, Evolution, and Chemistry of Terrestrial Planets via In-Situ Exploration of our Sister World. Geophysical Monograph Series

• Grinspoon, D., (1993). Implications of the high D/H ratio for the sources of water in Venus’ atmosphere. Nature 363, 428-431

• Hartogh, P., Lis, D.C., Bockelee-Morvan, D., de Val-Boro, M., Biver, N., Kuppers, M., Emprechtinger, M., Bergin, E.A., Crovisier, J., Rengel, M., Moreno, R., Szutowicz, S., Blake, G.A., (2011). Ocean-like water in the Jupiter-family comet 103P/Hartley 2. Nature 478, 218-220

• Hoffman, J.H., Hodges, R. R., McElroy, M. B., Donahue, T. M., Kolpin, M., (1979). Composition and Structure of the Venus Atmosphere: Results from Pioneer Venus. Science 205, 49-52

• Iwagami N., Hashimoto, G.L., Ohtsuki, S., Takagi, S., Robert, S., (2015). Ground-based IR observation of oxygen isotope ratios in Venus’s atmosphere. Planetary and Space Science 113-114, 292-297

• Pepin, R. O., (1998). Isotopic evidence for a solar argon component in the Earth’s mantle. Nature 394, 664-667

• Periodic Table Images: Theodore Gray • Turcotte, D. L., Schubert G., (1988). Tectonic Implications of radiogenic noble gases

in planetary atmospheres Icarus 74, 36-46