Probing planetary cores with global magnetic fields

Summary

The global magnetic fields produced by terrestrial planets exhibit remarkable variability in both strength and structure. Earth’s field is strong and long-lived, while Mercury’s current field is weak and Mars’s field died a long time ago. These fields are all generated by turbulent motions in liquid iron cores and thus provide a unique probe into the dynamics and evolution of planetary interiors. So why are the fields so different if they are all produced by the same basic process? This fundamental question in planetary science is still under debate. We hypothesise that a key factor is the way solids freeze from liquid iron inside cores during planetary cooling. Freezing releases heat and light material that provide crucial power for global magnetic fields, but many different regimes are possible: depending on the size and thermal/chemical properties of the body, freezing can begin at the top, middle or bottom of the core and produce solid particles that “snow” into the deeper core or “float” to the core surface. While bottom-up freezing of heavy solid is well-studied, little is known about the other regimes and so fundamental questions remain: under what conditions can snow and floatation generate global magnetic fields? What are the characteristics of the generated fields? Do these characteristics differ between the different crystallization regimes? Are the fields compatible with observations?

We have recently developed a new model for studying iron snow, including its role in magnetic field generation. In this project, you will extend this model, including developing the first model of the floatation regime, producing a unique tool for investigating the origin of global terrestrial magnetic fields. Through a systematic analysis you will establish whether differences in the magnetic fields of bodies including Earth, Mercury, Mars, Ganymede and our moon reflect different crystallization regimes in their cores. The results will make new predictions regarding the interior structure and evolution of these bodies, constrained by and informing existing and forthcoming observational data from space missions.

You will join a vibrant team within the School of Earth and Environment at Leeds that is currently leading large NERC- and NSF-funded projects on solid-liquid interactions in planetary cores in collaboration with University College London and Scripps Institution of Oceanography. The School hosts one of the largest deep Earth research groups in the world and your training will also benefit from the strong links to the astrophysical fluid dynamics research group in the School of Mathematics. Within this environment you will be trained in the skills that will enable you to develop and lead the next generation of core crystallization models and innovative planetary science investigations.

Planetary magnetic fields and core crystallization

The interiors of planets and moons can be probed via both the history and the present state of their global magnetic fields. Orbital and flyby missions can measure the current strength and geometry of planetary magnetic fields; whereas maps of crustal magnetism, combined with cratering histories, and paleomagnetic measurements of returned samples or meteorites can provide constraints on ancient field variations. Global fields are generated in the liquid cores of terrestrial bodies by a dynamo process that converts the kinetic energy of turbulent fluid motions into magnetic energy. Dynamo action in the Earth’s core has generated a strong dipole-dominated magnetic field for billions of years, whereas Mars and the Moon possessed core dynamos that shut off early in their history, and Venus may have never had an internally generated magnetic field (Schubert and Soderlund, 2011). Mercury’s dynamo currently generates a weak (relative to Earth) dipolar field, as do icy moons such as Ganymede. New data on these magnetic fields will be acquired by the BepiColumbo and JUICE missions, respectively. The striking variety of field strengths, geometries, and histories (Figure 1) reflects differences in interior structure, composition, and thermodynamic evolution, which control the thermal, chemical, and dynamical conditions of the dynamo regions.

The start and end of core crystallization are among the most significant events in the history of a planet. Earth’s liquid iron core is slowly freezing from the bottom upwards as the planet cools because the melting curve of the iron alloy is steeper than the temperature profile of the core fluid (Nimmo 2015; Figure 2). Freezing releases heat and light elements (e.g. sulphur) into the liquid; these effects are crucial, and currently provide the main power sources for generating the geomagnetic field (Nimmo 2015). The inner core is a young feature of the planet, less than a billion years old, and the conditions that generated the geomagnetic field prior to its nucleation are still debated (Davies et al 2015; O’Rourke and Stevenson 2016,). The presence of the inner core may also determine the strength and geometry of the geomagnetic field (Driscoll 2016).

Figure 1: Radial magnetic field at the surface of Earth, Mercury, Ganymede and Uranus. Note the weak field of Mercury and the non-dipolar field of Uranus. Figure from Schubert and Soderlund (2011).

In stark contrast to Earth, the cores of small planetary bodies can freeze from the top downwards if the melting curve is shallower than the core temperature profile at the relevant pressure-temperature-composition conditions (e.g. Stewart et al, 2007). If the frozen solid is heavier than the liquid it will fall, snowing into the deeper core where it may melt. This “iron snow” regime has been proposed to exist in Mercury (Dumberry and Rivoldini, 2015), Mars (Davies and Pommier, 2018) and Ganymede (Rückriemen et al, 2015). Melting and freezing together with movement of solid and liquid can provide power for magnetic field generation. Therefore, analogy with Earth suggests that formation of an iron snow layer will profoundly affect the dynamics and evolution of a planetary core. Simple models suggest this is the case (Rückriemen et al, 2015) while also predicting that the iron snow regime is very different to the bottom-up crystallization of Earth.

More exotic crystallization regimes are possible (Figure 2). Depending on planetary structure and thermal/chemical conditions, freezing can begin from the middle of the core as has been proposed for Ganymede (Hauck et al, 2006; Breuer et al, 2016). At very high light element concentrations the frozen material can even be lighter than the liquid – in this “floatation” regime the solid rises while the dense liquid sinks to greater depths. The fluxes of heat, light elements and solid material in these different crystallization regimes will generate latent heat and chemical energy and fundamentally control the evolution of the core and the properties of its magnetic field. However, whether any of these regimes produce the magnetic fields we observe is presently unknown.

The study of top-down and middle-out crystallization has only recently begun and so fundamental questions remain:

  • Under what conditions can snow and floatation generate global magnetic fields?
  • What are the characteristics of the generated fields?
  • Do these characteristics differ between the different crystallization regimes?
  • Are the fields compatible with observations?

We have recently developed a new model (Davies and Pommier, 2018) for studying the long-term evolution of two-phase regions such as snow and floatation zones. You will further develop this model to answer Questions (1)-(4) above.

The project

You will simulate the interior dynamics and past evolution of a number of terrestrial bodies, including Mercury, Earth, Mars, the Moon, and Ganymede, in order to elucidate the origin of their observed magnetic fields. To do this you will extend the functionality of our existing snow model, including developing the first model of the floatation regime. The project will initially focus on modelling the core behaviour, but there is flexibility to also analyse the mantle behaviour in detail. By matching (or not) the observed magnetic fields the research will make new predictions regarding the interior structure and evolution of these bodies, constrained by and informing existing and forthcoming observational data.

Figure 2: Cartoon of bottom-up crystallization (a), iron snow (b) and floatation (c) regimes (Breuer et al, 2016). Time advances from left to right.

You will extend the model to study the floatation regime and to allow crystallization to proceed from the top, middle and bottom of the core (possibly simultaneously). It is expected that the basic physics of the floatation regime is not fundamentally different to the snow regime and so the coding effort can be tailored to suit your interests. This work will give you a general framework for investigating crystallisation in terrestrial planets.

The next phase of the project is to explore how core-mantle evolution and magnetic field generation are affected by different crystallization regimes: snow and floatation, beginning from the top, middle and bottom of the core. The crystallization regime is not imposed – it emerges naturally from the model and can change as the core evolves. A reasonable strategy is therefore to consider an arbitrary planetary body and vary the model inputs (initial structure, thermal and chemical state of the core and mantle 4.5 billion years ago) systematically until the different crystallization regimes emerge. Using a large suite of model results you will be able to map out the physical conditions that lead to different crystallization scenarios, which is required in order to answer Questions 1-3 above.

The final phase of the project is to use the model to study the evolution of different terrestrial bodies. The small terrestrial bodies exist at much lower pressure and temperature than Earth’s core, and there is a large literature covering their structural, thermal and chemical properties. You will exploit this wealth of data and our collaboration with Prof. Anne Pommier, an international expert in experimental petrology at UC San Diego, to create a range of evolutionary models for each body that demonstrate the range of plausible behaviour. This work will show which planets can enter the snow/floatation regime and where these bodies tend to crystallize. Preferred evolutionary scenarios will be obtained by comparing model outputs (present thermal and chemical state of the core, snow/floatation zone properties, etc) to present-day thermal, chemical and structural data and to available magnetic field observations. Comparing preferred scenarios between different planets will enable you to establish whether different crystallization regimes can explain the variety of magnetic field behaviour observed in terrestrial bodies.

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 international team you will have the opportunity to engage with and learn about other aspects of the project including petrological and ab initio determinations of material properties at extreme pressure-temperature conditions and non-equilibrium processes that govern the formation of solid particles at the atomic scale. To allow you to complete the project you will learn a range of computational and mathematical methods as well as developing an understanding of the properties of planetary interiors. You will also learn 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 (Figure 3). 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 PANORAMA NERC doctoral training partnership.

Figure 3: University of Leeds hosted high performance computing facility (https://arc.leeds.ac.uk/), one of the supercomputers that will be used in this project.

Student profile

This project offers plenty of flexibility depending on the interests and experience of the candidate. There is ample scope for code development, while those interested in the more fundamental fluid dynamical aspects of the problem may choose to consider the effects of rotation and magnetic fields in the 2D models, which are basically unexplored. 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. Davies, C., Pozzo, M., Gubbins, D. and Alfè, D., 2015. Constraints from material properties on the dynamics and evolution of Earth’s core. Nature Geoscience8(9), pp.678-685.
  2. Davies, C.J. and Pommier, A., 2018. Iron snow in the Martian core? Earth and Planetary Science Letters481, pp.189-200.
  3. Driscoll, P.E., 2016. Simulating 2 Ga of geodynamo history. Geophysical Research Letters43(11), pp.5680-5687.
  4. Dumberry, M., Rivoldini, A., 2015. Mercury’s inner core size and core-crystallization regime. Icarus 248, 254–268.
  5. Hauck, S.A., Aurnou, J.M. and Dombard, A.J., 2006. Sulfur’s impact on core evolution and magnetic field generation on Ganymede. Journal of Geophysical Research: Planets111(E9).
  6. Nimmo, F., 2015. Energetics of the core. In: Schubert, G. (Ed.), Treatise on Geophysics, vol. 8, 2nd ed. Elsevier, Amsterdam, pp. 27–55.
  7. O’Rourke, J.G. and Stevenson, D.J., 2016. Powering Earth’s dynamo with magnesium precipitation from the core. Nature529(7586), pp.387-389.
  8. Rückriemen, T., Breuer, D., Spohn, T., 2015. The Fe snow regime in Ganymede’s core: a deep-seated dynamo below a stable snow zone. J. Geophys. Res., Planets 120 (6), 1095–1118.
  9. Schubert, G. and Soderlund, K.M., 2011. Planetary magnetic fields: Observations and models. Physics of the Earth and Planetary Interiors187(3), pp.92-108.
  10. Stewart, A.J., et al., 2007. Mars: a new core-crystallization regime. Science 316, 1323–1325.