Formation and evolution of small planets and moons

Our research aims to aid understanding of the formation and evolution of smaller terrestrial planetary cores (e.g., Mercury, the Moon, Mars and Ganymede) and the icy moons and smaller bodies of the outer solar system. We shall do this in the following ways: (i) by investigating core crystallisation processes (Project 1), (ii) by determining the physical properties of core-forming materials under the relevant conditions of pressure and temperature (Project 2) and (iii) by investigating the physical properties of icy materials (including their vibrational spectra) at high pressures and very low temperatures (Project 3).

Key observables from recent and future missions are surface morphologies, spectroscopic observations, magnetic fields, gravity data and, for the Moon and for Mars (via the current InSight mission) seismic observations. The generation of magnetic fields relies on energy sources which include the release of gravitational energy and latent heat of crystallisation. Detailed understanding of these energy sources in small planetary cores is currently uncertain, as it is not even known whether they crystallise from the bottom up or from the top down, and this has first order implications for dynamo driving mechanisms; these are the main issues to be addressed by Project 1. Key deliverables of this project will be ab initio predictions over the entire pressure-temperature range relevant to the cores of Mercury, Ganymede, Mars and the Moon of: i) the adiabatic temperature gradients and melting curves of iron-alloys, which will allow us to determine the mode of core crystallisation, ii) the densities of the liquids and the liquid-solid density contrast, which contributes to gravitational energy release, and iii) the latent heat of fusion in iron alloy systems.

Project 2 is predominantly experimental, and addresses major gaps in our knowledge of the fundamental physical properties of iron alloys and related materials at the conditions found within the smaller planetary cores. If we are to understand planetary evolution, geophysical modelling of planetary interiors - the outputs of which can be tested against data from spacecraft and ground-based observations - is essential, but these geophysical models will be reliable only if the physical properties of the constituent core-forming materials (essentially iron and iron alloys) are accurately known in the relevant pressure and temperature ranges. At present, this is often not the case for the conditions expected in the interiors of the Moon, Mercury, Mars and Ganymede. Surprisingly, even for the simplest approximation - a core of pure iron - the data currently available are inadequate. Key deliverables of the project will be the determination of the phase diagrams and physical properties of iron alloys, especially ternary systems such as Fe-Ni-Si and Fe-S-Si, at pressures up to ~40 GPa.

Project 3 addresses a different class of materials - water ice and hydrated sulphate salts - which form the mantles of the icy bodies of the outer solar system. Application of pressure to these readily induces changes in crystal structure and, in the case of the hydrated sulphate salts, can lead to expulsion of water and transformation to a lower hydration state. The consequence of these structural changes is to produce a layered structure within the icy body. In addition, the large volume changes accompanying dehydration may result in global expansion and rifting and/or global contraction and crumpling of the surface. Key deliverables of this project include the phase diagrams, equations of state and thermal and electrical conductivities of these icy materials under the relevant conditions of pressure and temperature. In addition to this we shall determine the vibrational spectra of these various phases which will allow direct comparison with mission data.

Impact on Planetary Science in general: multi-billion dollar investments in solar system exploration are of little use without a broad foundation of materials science upon which to build a framework to interpret the observations. We expect our research in Project 1 to yield crucial new insight into the origins of magnetic fields and core formation in small terrestrial bodies. Magnetic fields provide a precious observational window into the deep interior and long term chemical and thermal evolution of telluric planets and moons. The GRAIL mission, together with re-analyses of Apollo seismic data with modern methods, have revolutionised our understanding of the lunar core, providing new constraints on the size and solidity of the core. Mercury was the target of the recent MESSENGER mission and the BepiColumbo orbiters are en route there; the gravity and magnetic fields measured by MESSENGER revealed an active core dynamo suggestive of a liquid outer core. Our knowledge of the interior of Mars has benefited from Mars Global Surveyor (MGS), and is a primary target of the InSight mission's geophysical lander which has the goal of determining the size and fluidity of the Martian core. The material properties obtained from Project 2 will be central to all geophysical and evolutionary models of the interiors of the smaller planetary bodies such as Mercury, the Moon, Mars and Ganymede. Surprisingly, many of the properties necessary to construct models of the core of, e.g. Mars, are currently poorly known, if known at all, under the relevant conditions of pressure and temperature - this lack of knowledge extends even to the properties required to construct the simplest and most basic models, i.e. those based on pure iron in the appropriate phase. In Project 3 we will focus on the materials expected to be found in the mantles of the icy moons of the outer solar system which exist at high pressure and very low temperature. As part of the project we shall determine the vibrational spectral signatures of various ice phases as a function of pressure, temperature and composition which will allow direct comparison with mission data.

Impact on Materials Science: the materials science aspects of these three projects provide us with plentiful scope for engagement with a wide of spectrum of physicists, chemists and metallurgists. For example, the properties of solid and liquid metal alloys at extremes of temperature and pressure are of great interest to those working in the defence and nuclear industries. Similarly the effects of isotopic substitution in water ice are of great interest to solid-state physicists and chemists.

Training of skilled personnel: the project will result in the training of three PDRAs. The first will be trained in computer simulation methods and the other two will be trained in high-pressure and high/low temperature techniques, X-ray diffraction, electron microscopy, ultrasonics and spectroscopy, making all three suitable for employment in a wide range of positions within the general area of materials science. Career destinations of those who have worked previously with the PI and Co-Is at UCL on planetary materials include employment both in the UK and abroad, split between academic research on planets, academic research in materials science and careers in promoting the public understanding of science.

Educational and Societal Impact: as evidenced by the many recent space missions, the planets and moons of our solar system continue to attract wide public interest. Together with "big physics" (e.g. the LHC at CERN) they provide one of the best routes by which scientists can engage with both young and old. For the old, this can be viewed as enhancing the cultural life of the nation; for the young it is a way of achieving the national goal of increasing the number of trained scientists and technologists.