The first self-consistent dynamical simulations of asteroid dynamos

Many meteorites come from the deep interiors of asteroids that formed very early in the history of our solar system. Over time, some early asteroids collided and merged to form the planetesimals that later grew into the Earth and the other terrestrial planets. Knowing the size, chemical composition, and rate of growth of these planetesimals would thus help us better understand the early evolution of our solar system and the formation of the Earth and other planets. Not only can knowing more about these early asteroids help us to characterise the origins of Earth, it would help us compare our solar system to the protoplanetary systems and exoplanets being observed around other stars. Our tool for studying the early solar system is magnetisation.

When an early asteroid grew large enough, internal heat caused it to partially melt, allowing the iron within to sink to its centre, forming a metallic core overlain by a predominantly rocky mantle. Convection in this liquid metal core could create a magnetic field, similar to what happens in the liquid outer core of Earth. This internally generated magnetic field could be imprinted into minerals within the asteroid's mantle, a magnetisation that could survive the later breakup of the asteroid and the meteorite's journey to Earth. So, meteorites have been found that contain a permanent mineral magnetisation acquired billions of years ago, early in the lifetime of our solar system. The strength of that magnetisation reflects the strength of the magnetic field generated within the asteroid's core, which in turn depends on the size and chemical composition of the asteroid. The group in Oxford has studied meteorite magnetisation and constructed mathematical models of early asteroids to understand how they grow and their interior temperature and chemistry evolve through time. In this work, meteorite magnetisation of different ages has been used to estimate what the parent asteroids were made from, how fast they initially grew and heated up, and then how they cooled. However, making the connection from meteorite magnetisation to asteroid growth and composition requires a good understanding of how magnetic fields are generated in asteroid cores and that understanding is currently lacking.

Although other mechanisms have been proposed (e.g., dynamos powered by rotational precession), the evolution models derived from meteorite magnetisation focus on early asteroid magnetic fields generated by thermochemical convection in their fluid cores. Much of the theory and simulations that have been developed to understand this process has been designed to study the present-day magnetic field of Earth. However, studies of Mercury's oddly weak magnetic field, the failure of the early dynamos of Mars and the Moon, and the magnetic field of Ganymede have all shown that the behaviour of such planetary magnetic fields depends on the details of the core's geometry and how the convection is powered. For example, magnetic fields generated in the cores of early asteroids would behave differently to that of modern Earth because asteroid cores would solidify from the top-down whereas Earth's core is freezing from the bottom-up. The team in Leeds has much experience in theoretical modelling and numerical simulation of the processes by which magnetic fields are generated in the fluid cores of planets and we will produce new simulations and analyses specifically designed to determine how magnetic field generation worked in the cores of asteroids early in the solar system.

The teams will combine our expertise to relate the new theoretical work on magnetic field generation to the existing measurements of meteorite magnetisation. This will let us test current hypotheses for how the meteorite parent asteroids formed, grew, and eventually cooled during the first few hundred million years of our solar system. Thus, we will better determine the early evolution of these key building blocks of the terrestrial planets.