Deciphering the thermal evolution of small planetary bodies.

Change log
Dodds, Kathryn 

The parent bodies of meteorites were the first bodies to form in our solar system and the building blocks of the terrestrial planets, as well as the cores of the gas giants. They also played an important role in the delivery of volatiles to the inner solar system planets, including the Earth. These bodies also hosted a wide range of geological processes, from low temperature aqueous alteration, to explosive volcanism. However, determining certain properties of these asteroid-sized bodies such as their size and structure can be difficult.

In this thesis, I use detailed models of planetesimal thermal evolution to constrain the accretionary histories and structures of a range of different meteorite parent bodies from a variety of observed meteorite properties. This model describes the thermochemical evolution of a planetesimal, from accretion and differentiation, through a period of early magma ocean convection and subsequent diffusive cooling, to solidification of its liquid iron core, for a wide range of accretionary scenarios that result in the proposed parent body structures. I then use this model for three different projects, which investigate: 1) the conditions for thermally-driven dynamo activity in planetesimal cores, 2) the accretionary histories of the magmatic iron parent bodies, and 3) the possible parent body structures of an unusual greenschist chondrite, Almahata Sitta stone AhS 202.

Conditions for thermally-driven dynamo activity: The ability for a planetesimal to generate a thermally-driven field from 4 - 35 Myr after the start of the solar system is found to depend critically on its accretion rate and duration of core formation as this controls the depth and location of any thermal stratification that develops during core formation. This result allows us to constrain the accretion rate of these bodies for the first time as thermal dynamo generation requires accretion durations of > 100kyr. Additionally, the timings of the thermally-driven fields on the fully-differentiated angrite parent body and partially-differentiated, CVOx parent body require that both these bodies were > 420 km with > 200 km radius cores. The CVOx parent body also had a 7 - 12 km thick unmelted, chondritic lid at the surface, from which the CVOx chondrites originate.

Accretionary histories of the magmatic iron parent bodies: The measured 182W anomalies in magmatic iron meteorites, which originate from the cores of their parent bodies, are a product of the timing of core formation and differentiation. I use these anomalies to infer the accretion start times and durations of their parent bodies. I find that these parent bodies may have been either fully or partially differentiated, challenging the canonical assumption that they were fully differentiated. As a result, it is not possible to use the measured 182W anomalies in iron meteorites to uniquely define the relative timings of planetesimal accretion in the inner and outer solar system, as done in many previous studies.

Properties of the AhS 202 parent body: Almahata Sitta stone, AhS 202 is the only known meteorite that has experienced high pressure, greenschist-like metamorphism, requiring its parent body to be 300 - 900 km in radius. However, its association with the CR chondrites, the youngest meteorite group, means that its parent body did not accrete with sufficient 26Al, the dominant planetesimal heat source, to reach the temperatures required for this metamorphism. Instead, the heat for this metamorphism could have been provided by either internal heating by the decay of long-lived radioistopes in a > 550 km chondritic parent body or diffusive heating of a thick chondritic lid by an differentiated interior in a > 380 km partially-differentiated parent body.

Finally, the crystallization of asteroid-sized cores is not well understood, which has made using the timings of compositionally-driven dynamo fields in planetesimal cores to constrain the properties of their parent bodies challenging. I have used thermodynamic calculations to show that due to their low pressures, these cores crystallized inwardly, requiring a different dynamo mechanism to the geodynamo. However, previous studies into dynamo generation in this regime have largely been restricted to numerical models. In this work, I have used novel analogue experiments to identify the key physics involved in inwards crystallization in asteroid cores. These experiments have allowed the identification of a new core crystallisation mechanism in which iron crystals form below the CMB and fall into the interior in crystal-rich downwelling plumes. However, whether this mechanism is capable of driving dynamo fields in cores of meteorite parent bodies is still uncertain due to difficulties in scaling our experimental results to the relevant core conditions. If future work shows that this is possible, this new mode of core crystallization will allow more accurate constraints to be placed on the size and structure of the parent bodies of meteorites that experienced a compositionally-driven field from 65 - 200 Myr after the start of the solar system.

Harrison, Richard
asteroid thermal evolution, dyamo generation, meteorites
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
NERC (2073224)