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. 2023 Mar 14;120(11):e2209751120.
doi: 10.1073/pnas.2209751120. Epub 2023 Mar 6.

Narrow range of early habitable Venus scenarios permitted by modeling of oxygen loss and radiogenic argon degassing

Alexandra O Warren  1 , Edwin S Kite  1
Affiliations

Narrow range of early habitable Venus scenarios permitted by modeling of oxygen loss and radiogenic argon degassing

Alexandra O Warren et al. Proc Natl Acad Sci U S A. .

Abstract

Whether Venus was ever habitable is a key question driving missions to Earth's sister planet in the next decade. Venus today has a dry, O2-poor atmosphere, but recent work has proposed that early Venus may have had liquid water [J. Krissansen-Totton, J. J. Fortney, F. Nimmo, Planet. Sci. J. 2, 216 (2021)] and reflective clouds that could have sustained habitable conditions until 0.7 Ga [J. Yang, G. BouÃĐ, D. C. Fabrycky, D. S. Abbot, Astrophys. J. 787, L2 (2014), M. J. Way, A. D. Del Genio, J. Geophys. Res.: Planets 125, e2019JE006276 (2020)]. Water present at the end of a habitable era must since have been lost by photodissociation and H escape, causing buildup of atmospheric oxygen [F. Tian, Earth Planet. Sci. Lett. 432, 126-132 (2015)]. We present a time-dependent model of Venus's atmospheric composition starting from the end of a hypothetical habitable era with surface liquid water. We find that O2 loss to space, oxidation of reduced atmospheric species, oxidation of lava, and oxidation of a surface magma layer formed in a runaway greenhouse climate can remove O2 from up to 500 m global equivalent layer (GEL) (30% of an Earth ocean), unless melts on Venus had a much lower oxygen fugacity than Mid Ocean Ridge melts on Earth, which increases the upper limit twofold. Volcanism is required to supply oxidizable fresh basalt and reduced gases to the atmosphere but also contributes 40Ar. Consistency with Venus's modern atmospheric composition occurs in less than 0.4% of runs, in a narrow parameter range where the reducing power introduced by O2 loss processes can balance O2 introduced by H escape. Our models favor hypothetical habitable eras ending before 3 Ga and very reduced melt oxygen fugacities three log units below the fayalite-magnetite-quartz buffer (fO2< FMQ-3), among other constraints.

Keywords: Venus; atmospheric evolution; water.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Schematic illustration of our model for Venus’s atmospheric evolution starting from the end of a hypothetical early habitable era. Prior to the model start, we set the total end-habitable-era water inventory (Table 1). After the end of the habitable era, the following processes occur: 1) Additional volatiles are added to the atmosphere by volcanism. These volatiles are sourced from H and C species dissolved in the melt, and their speciation into H2O, H2, CO2, CO, and CH4 is set by the oxygen fugacity of the erupting melt. 2) Water from volcanism and the early habitable era photodissociates to form H and O2. 3) H escapes to space (purple arrow). 4) O is removed from the atmosphere by escape to space, oxidation of reduced atmospheric species such as H2, CH4, and CO to form H2O and CO2, and oxidation of FeO in volcanic products (teal arrows). In models where the habitable era ends in a runaway greenhouse (RGH), O can also be removed from the atmosphere by oxidation of a molten basalt layer if the surface temperature exceeds the basalt solidus.
Fig. 2.
Fig. 2.
Example model output: Melts with lower oxygen fugacity can enable larger end-habitable-era water inventories to be compatible with modern atmospheric composition; however, melts with low oxygen fugacities also outgas reduced gasses such as CO which can overwhelm the O2 available to oxidize them and produce an atmosphere with far greater CO than observed on modern Venus. Time evolution of column masses of i O2 and ii C species for different melt oxygen fugacities in a. our baseline model, with all early habitable eras ending with 300 m GEL 4.0 Ga ago and b. with 1000m GEL RGH surface melting. Line colors indicate melt oxygen fugacity. In ii, CO2, CO, and CH4 are indicated with solid, dashed, and dotted lines, respectively. Black dashed horizontal lines show upper limits for modern Venus O2 (< 70 ppm) and CO (< 52 ppm) in i and ii, respectively (26). Only models that fall beneath both of these limits are considered successful.
Fig. 3.
Fig. 3.
Example model output: Time evolution of column masses of i H2O (purple) and ii O2 (teal) for (A) our baseline model, with all early habitable eras ending with 100 m GEL 4.0 Ga ago, and (B) with RGH surface melting, showing all habitable eras ending with 500 m GEL at 4.0 Ga. Where red CO2 curve is flat, surface is molten and volcanism does not occur. Runs are shown as translucent lines. Solid lines illustrate cases where atmospheric evolution is compatible with modern atmospheric H2O and O2. Dashed purple lines show upper limits for modern Venus H2O, and dashed teal lines show upper limits for O2. Adding runaway greenhouse (RGH) surface melting to our models enables slightly larger end-habitable-era water inventories to be compatible with modern atmospheric H2O and O2.
Fig. 4.
Fig. 4.
Percentage of model runs that end with atmospheric H2O, O2, and CO concentrations consistent with data for modern Venus (25) for different combinations of parameters (Table 1) for runs without runaway greenhouse (RGH) surface melting. Contour plots show model success rates for combinations of different values of 2 parameters. Dashed and solid white lines indicate 2σ and 1σ levels, respectively. Summary bar charts show model success rates for different values of a single parameter. fext is the extrusive magmatism fraction, fvolc is the fraction of Venus’s modern atmospheric CO2 derived from post-habitable era volcanic degassing, and log fO2ΔFMQ is the oxygen fugacity of erupting melts since the end of the habitable era relative to the fayalite–magnetite–quartz buffer.
Fig. 5.
Fig. 5.
Comparison of fraction of successful runs (runs that have final atmospheric O2, H2O, and CO concentrations less than upper limits for Venus’s present atmosphere) compatible with modern Venus atmosphere for models without runaway greenhouse (RGH) melting (dark blue) and with RGH melting (dark green). Successful runs that also fall within or below the range of Venus’s modern atmospheric 40Ar abundance are also shown for models with (light blue) and without (light green) RGH. All parameter values correspond to Table 1.
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