Whole planet coupling between climate, mantle, and core: Implications for the evolution of rocky planets (1711.06801v1)

Abstract: Earth's climate, mantle, and core interact over geologic timescales. Climate influences whether plate tectonics can take place on a planet, with cool climates being favorable for plate tectonics because they enhance stresses in the lithosphere, suppress plate boundary annealing, and promote hydration and weakening of the lithosphere. Plate tectonics plays a vital role in the long-term carbon cycle, which helps to maintain a temperate climate. Plate tectonics provides long-term cooling of the core, which is vital for generating a magnetic field, and the magnetic field is capable of shielding atmospheric volatiles from the solar wind. Coupling between climate, mantle, and core can potentially explain the divergent evolution of Earth and Venus. As Venus lies too close to the sun for liquid water to exist, there is no long-term carbon cycle and thus an extremely hot climate. Therefore plate tectonics cannot operate and a long-lived core dynamo cannot be sustained due to insufficient core cooling. On planets within the habitable zone where liquid water is possible, a wide range of evolutionary scenarios can take place depending on initial atmospheric composition, bulk volatile content, or the timing of when plate tectonics initiates, among other factors. Many of these evolutionary trajectories would render the planet uninhabitable. However, there is still significant uncertainty over the nature of the coupling between climate, mantle, and core. Future work is needed to constrain potential evolutionary scenarios and the likelihood of an Earth-like evolution.

Whole Planet Coupling Between Climate, Mantle, and Core

The research paper by Bradford J. Foley and Peter E. Driscoll presents an in-depth analysis of the interactions between Earth's climate, mantle, and core over geological timescales and explores their collective influence on the evolutionary pathways of rocky planets. These interactions, termed "whole planet coupling," are pivotal for understanding why Earth and Venus, despite their similarities, have evolved so differently.

Summary of Key Findings

The authors investigate the coupling mechanisms among Earth's climate, mantle dynamics, and core processes, highlighting the implications for plate tectonics, carbon cycling, and magnetic field generation. They propose that:

1. Climate and Tectonics: Earth's cooler climate facilitates plate tectonics by increasing lithospheric stresses and promoting hydrous weakening. This, in turn, supports the long-term carbon cycle by exposing fresh rock for silicate weathering, which regulates atmospheric CO$_2$ and maintains a temperate climate.
2. Tectonics and Core: Plate tectonics aids in the cooling of Earth's core by allowing efficient heat transfer through subduction processes. This cooling is essential for the dynamo action that generates Earth's magnetic field, providing atmospheric shielding from solar wind.
3. Divergent Evolution of Earth and Venus: The lack of liquid water on Venus prevented silicate weathering and the long-term carbon cycle, leading to its current hot, dry climate, stagnant lid tectonics, and absence of a sustained magnetic field. In contrast, Earth's position within the habitable zone enabled the conditions necessary for its current geodynamic and magnetic evolution.

Implications and Future Considerations

The paper's findings suggest a framework for predicting the evolutionary trajectories of rocky exoplanets based on initial conditions such as atmospheric composition, volatile content, and orbital distance. The implications for future planetary research and astrobiology include:

• Habitability Criteria: Understanding whole planet coupling is essential for identifying exoplanets capable of developing Earth-like habitable conditions. The presence of liquid water, plate tectonics, and a magnetic field are interdependent factors crucial for sustaining life.
• Exoplanetary Evolution: The paper speculates on various evolutionary scenarios, emphasizing that even within a star's habitable zone, planets may fail to become habitable due to initial atmospheric conditions or delayed initiation of tectonics.
• Magnetic Field and Atmospheric Retention: The paper underscores the role of magnetic fields in protecting atmospheres, particularly during early thermal adjustment periods, which may be vital for preserving a planet's water inventory.

Conclusion

Foley and Driscoll's work provides a detailed model of the interplay between climatic, tectonic, and magnetic processes on rocky planets, offering insights into the geophysical conditions necessary for Earth-like evolution. This comprehensive understanding is a critical step towards assessing the potential habitability of exoplanets and interpreting the geological histories of terrestrial bodies within our own solar system and beyond. Future research will need to resolve uncertainties in mantle dynamics, the deep carbon cycle, and the exact conditions required for sustaining plate tectonics and magnetic fields over geological timescales.