Venusian Habitable Climate Scenarios: Modeling Venus through time and applications to slowly rotating Venus-Like Exoplanets (2003.05704v1)

Abstract: One popular view of Venus' climate history describes a world that has spent much of its life with surface liquid water, plate tectonics, and a stable temperate climate. Part of the basis for this optimistic scenario is the high deuterium to hydrogen ratio from the Pioneer Venus mission that was interpreted to imply Venus had a shallow ocean's worth of water throughout much of its history. Another view is that Venus had a long lived (approximately 100 million year) primordial magma ocean with a CO2 and steam atmosphere. Venus' long lived steam atmosphere would sufficient time to dissociate most of the water vapor, allow significant hydrogen escape and oxidize the magma ocean. A third scenario is that Venus had surface water and habitable conditions early in its history for a short period of time (< 1 Gyr), but that a moist/runaway greenhouse took effect because of a gradually warming sun, leaving the planet desiccated ever since. Using a general circulation model we demonstrate the viability of the first scenario using the few observational constraints available. We further speculate that Large Igneous Provinces and the global resurfacing 100s of millions of years ago played key roles in ending the clement period in its history and presenting the Venus we see today. The results have implications for what astronomers term "the habitable zone," and if Venus-like exoplanets exist with clement conditions akin to modern Earth we propose to place them in what we term the "optimistic Venus zone."

Venusian Habitable Climate Scenarios: Modeling Through Time and Applications to Exoplanets

The paper explores the climatic evolution of Venus, proposing scenarios where Venus could have supported habitable conditions in its past and extrapolating these findings to Venus-like exoplanets. The authors employ a general circulation model (GCM) to explore three primary scenarios for Venus' climate history, each with implications for the planet's ability to sustain liquid water on its surface.

Key Findings and Methodological Approaches

1. Habitable Periods on Venus:
   • The paper suggests that Venus could have maintained habitable conditions with surface liquid water for nearly 3 billion years. The presence of a carbonate-silicate cycle, which stabilizes CO2 levels, plays a critical role in these scenarios, potentially negating the impact of increasing solar insolation over geological timescales.
2. Scenarios for Climate Evolution:
   • Scenario 1 posits long-term stability with conditions conducive to surface liquid water, aligning with data from isotopic ratios suggesting past water presence.
   • Scenario 2 involves a primordial magma ocean and sustained steam atmosphere, leading to extensive dissociation and hydrogen escape, suggesting a drier history.
   • Scenario 3 outlines a short-lived habitable period with a subsequent runaway greenhouse effect due to solar warming, leading to the dry state observed today.
3. Modeling Considerations:
   • The paper uses different atmospheric compositions, surface characteristics, and solar insolation levels across simulations to test the validity of these scenarios. The model accounts for cloud feedback mechanisms, which are crucial in maintaining cooler surface temperatures under greater solar intensities.
4. Exoplanetary Implications:
   • By identifying 'optimistic Venus zones,' the paper suggests that exoplanets with similar slow rotation and cloud feedback mechanisms might reside within the inner habitable zone, challenging traditional boundaries of habitability.

Implications and Theoretical Extensions

• Understanding Habitability:

The research carries implications for defining habitable zones, suggesting that even planets closer to their stars than previously thought possible might sustain life-supporting conditions, given the right atmospheric processes and surface properties.

• Mission Planning:

The findings call for re-evaluating target selection in exoplanet explorations, highlighting the need for missions that can assess atmospheric compositions indicative of long-term habitability akin to early Venus.

• Geophysical and Atmospheric Dynamics:

The scenarios also raise questions about the geological and atmospheric evolution mechanisms that can maintain or disrupt habitable conditions, applicable to both solar system bodies and exoplanetary systems.

Future Directions in Planetary Science

• Venus Exploration:

Further missions to Venus could aim to validate these models by uncovering geological and atmospheric evidence of its climatic past, providing insights into the conditions necessary for habitable climates.

• Exoplanet Studies:

Observations and modeling of exoplanets in inner habitable zones could leverage this framework to evaluate potential biosignatures or signs of atmospheric stability akin to Earth's.

This paper offers a comprehensive framework for understanding Venus' potential past habitability and extends these insights to a broader understanding of planet habitability across the galaxy. As such, it enhances our understanding of planetary atmospheres and the delicate balances that may allow life to persist under varying cosmic circumstances.