Is a Global Magnetic Field Required for Planetary Habitability?

Supervisors: Prof Chris Davies (SEE), Prof Jon Mound (SEE), Prof Benjamin Mills (SEE)

Contact email: c.davies@leeds.ac.uk

Life on Earth has persisted for billions of years and it is the only planet where we know life exists. Planets such as Mars and Venus may have had conditions amenable to life in the past but cannot currently maintain an abundant biosphere. Thousands of exoplanets have been discovered, some in the `habitable zone’ of their host star; however, both Earth and Mars are in our habitable zone so it is not clear whether detected exoplanets are truly habitable. The long-term (billions of years) habitability of a planet depends on the maintenance and interaction of a large number of global scale systems linking the atmosphere, hydrosphere, biosphere, and geosphere. One such interaction is the shielding that the global magnetic field, generated in Earth’s core, provides against the solar wind, which acts to strip planets of their atmospheres, a process that is believed to have played a key role in transforming Mars from a wet world to its current arid state. Traditionally, the evolution of the deep Earth has been considered separately from that of the surface environment and thus key questions remain. How important has magnetic shielding been for protecting the habitability of Earth? How has the slow evolution of the mantle and core impacted the strength of the geomagnetic field and hence the protection it provides from the solar wind? Can those changes in “magnetic shielding” meaningfully impact on the bulk composition of the atmosphere?

This project will model the evolution of the Earth’s interior and atmosphere over billions of years making use of VPLanet – the Virtual Planet Simulator (Barnes et al., 2020), a software package that allows flexible investigation of planetary system evolution and planetary habitability. Within VPLanet different computational modules account for different physical processes, which can be turned on or off and supplemented with relative ease. We have developed new models of the thermal evolution of Earth’s core (Greenwood et al., 2020) and how the power available to drive the dynamo relates to the strength of the large-scale magnetic field (Davies et al., 2020) that you will incorporate into the VPLanet framework. Atmospheric evolution is also influenced by surface geochemical cycles (Alcott et al., 2019; Rushby et al., 2018), which need to be included within VPLanet. Once these new capabilities are incorporated into the modelling framework, you will explore a range of possible planetary evolutionary pathways to understand how changes in the deep interior of Earth (or other planets) alter planetary magnetic fields over geologically long time scales and the implications for habitability.

For the Earth, paleomagnetism provides an observational check on the magnetic history of the planet. However, data uncertainties and gaps in the rock record (particularly for the young Earth) allow a range of plausible paleointensity evolutions. By modelling the thermal evolution of the core and mantle you will generate a suite of possible magnetic histories for the Earth. These histories will be used to determine the potential impact of magnetic variations on Earth’s atmosphere. For example, to what extent would prolonged periods of weak magnetic field alter atmospheric composition due to enhanced stripping by the solar wind? Are such impacts compatible with geochemical records of atmospheric evolution? The thermal evolution of the Earth’s interior also plays a critical role in atmospheric redox and greenhouse gas evolution, which may also be compared to geochemical records. On other planets, a broader range of scenarios are possible and the evolution of habitability on Mars or exoplanets may also be explored.

Training environment

You will receive training in skills tailored to the project but also useful to help secure a future career as a research scientist in academia or elsewhere. Moreover, since you will be working as part of a large interdisciplinary team you will have the opportunity to engage with and learn about other aspects of the project including the structure, dynamics, and material properties of the deep Earth, as well as global tectonic and biological processes, and how the can be tested with geochemical data. To allow you to complete the project you will learn a range of computational and mathematical methods as well as how to develop software for the analysis of results and to use large-scale high performance computing resources, including those at the University of Leeds (https://arc.leeds.ac.uk/). Alongside the transferable skills in communication and management this can open a wide range of career pathways. These skills will be developed by a mixture of hands on experience, attending external training courses, and by participating in the YES NERC Doctoral Training Network.

Student profile

This project offers plenty of flexibility depending on the interests and experience of the candidate, with a main focus on code development and the interactions between global-scale systems. Whatever the candidate’s background, strong mathematical skills, curiosity and a desire to learn will form an important part of the project.

References and further reading

1. Barnes, R., et al.(2020), VPLanet: The Virtual Planet Simulator, Publications of the Astronomical Society of the Pacific, 132(1008), 024502. doi: 10.1088/1538-3873/ab3ce8
2. Davies, C.J., Bono, R.K., Meduri, D.G., Greenwood, S. and Biggin, A.J. (2020) Dynamo constraints on the long-term evolution of Earth’s magnetic field strength, Under consideration in GJI.
3. Greenwood, S., Davies, C.J., and Mound, J.E. (2020) On the evolution of thermally stable layers at the top of Earth’s core, Under consideration in Physics of the Earth and Planetary Interiors, preprint:https://eartharxiv.org/bh43s/
4. Alcott, L. J., Mills, B. J. W. & Poulton, S. W. Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling. Science 366, 1333-1337 (2019).
5. Rushby, A. J., Johnson, M., Mills, B. J. W., Watson, A. J. & Claire, M. W. Long-Term Planetary Habitability and the Carbonate-Silicate Cycle. Astrobiology 18, 469-480, doi:10.1089/ast.2017.1693 (2018).