arXiv subjects

Hannah R. Sanderson

Publications and source records attributed to Hannah R. Sanderson.

4 recordsLinked to original sources

arXiv2026

Long-lived thermal dynamo generation on differentiated, impact-disrupted planetesimals

Meteorites containing both metal and silicates indicate that some planetesimals were partially differentiated and/or processes mixed planetesimal cores and mantles post-differentiation. Time-resolved paleomagnetic records in these meteorite groups can shed light on the differentiation and mixing histories of such bodies. Previous studies measured paleomagnetic remanences in two such meteorite groups, the IIE irons and the Main Group pallasites, and used thermal evolution and dynamo generation modelling to recover parent body properties. However, these studies assumed that these meteorites only recorded magnetic fields generated by core solidification; an assumption recently shown to be invalid. We use a refined planetesimal thermal evolution and dynamo generation model that considers thermal and compositional drivers of dynamo generation simultaneously to re-evaluate the parent body properties of the IIE irons and Main Group pallasites and constrain the formation mechanisms of these meteorites. We find that none of the remanences require core solidification, but later formed remanences are more likely to record dynamos driven by core solidification. The Main Group pallasites and IIE iron parent bodies likely had radii of $\sim$400km with core radius fractions of $\sim$0.5 and $\sim$0.7, respectively. Impacts shaped both parent bodies: the Main Group pallasites formed too far from the core-mantle-boundary to result from ferromagmatism and instead formed by impacts, and the IIE iron's core radius fraction suggests the parent body experienced mantle-stripping collisions. Overall, combining meteorite paleomagnetism with thermal evolution and dynamo generation models provides insights into the long-term evolution of differentiated planetesimals, their interior structures, and metal-silicate mixing on these bodies.

astro-ph.EP↗

arXiv2026

Dynamo generation reveals redox conditions during formation of differentiated planetesimals

In the early Solar System, an isotopic dichotomy existed between non-carbonaceous (NC) and carbonaceous (CC) planetesimals. Depending on the formation location of these planetesimals relative to condensation lines in the protoplanetary disk, NC and CC differentiated planetesimals could have had distinct redox states and water contents. However, the extent of these differences and the resulting accretion environments of NC and CC planetesimals are debated. Here, we use thermal evolution and dynamo generation models to explore the effect of planetesimal core size, a proxy for redox state, and mantle water content on planetesimal dynamo generation. We find that combinations of core size and water content consistent with different formation scenarios produce planetesimals with stark contrasts in both magnetic field strength and duration. By comparing our models to existing paleomagnetic data for NC planetesimals, we suggest these bodies formed with a small amount of water-ice and degassed efficiently during differentiation. Future paleomagnetic measurements could determine whether CC planetesimals degassed as efficiently as NC planetesimals and the number of planetesimal formation regions in the NC reservoir. Overall, we demonstrate that meteorite paleomagnetism combined with dynamo generation models provides novel insight into the accretion environments of planetesimals and the evolution of their water contents.

astro-ph.EP↗

arXiv2024

Early and elongated epochs of planetesimal dynamo generation

Accreting in the first few million years (Ma) of the Solar System, planetesimals record conditions in the protoplanetary disc and are the remnants of planetary formation processes. The meteorite paleomagnetic record carries key insights into the thermal history of planetesimals and their extent of differentiation. The current paradigm splits the meteorite paleomagnetic record into three magnetic field generation epochs: an early nebula field ($\lesssim$5 Ma after CAI formation), followed by thermal dynamos ($\sim$5-34 Ma after CAI formation), then a gap in dynamo generation, before the onset of core solidification and compositional dynamos. These epochs have been defined using current thermal evolution and dynamo generation models of planetesimals. Here, we demonstrate these epochs are not as distinct as previously thought based on refined thermal evolution models that include more realistic parametrisations for mantle convection, non-eutectic core solidification, and radiogenic $^{60}Fe$ in the core. We find thermal dynamos can start earlier and last longer. Inclusion of appreciable $^{60}Fe$ in the core brings forward the onset of dynamo generation to $\sim$1-2 Ma after CAI formation, which overlaps with the existence of the nebula field. The second epoch of dynamo generation begins prior to the onset of core solidification, suggesting this epoch is not purely compositionally driven. Planetesimal radius is the dominant control on the strength and duration of dynamo generation, and the choice of reference viscosity can widen the gap between epochs of dynamo generation from 0-200 Ma. Overall, variations in planetesimal properties lead to more variable timings of different planetesimal magnetic field generation mechanisms than previously thought. This alters the information we can glean from the meteorite paleomagnetic record about the early Solar System.

astro-ph.EP↗

arXiv2024

Unlocking planetesimal magnetic field histories: a refined, versatile model for thermal evolution and dynamo generation

The thermal and magnetic histories of planetesimals provide unique insights into the formation and evolution of Earth's building blocks. These histories can be gleaned from meteorites by using numerical models to translate measured properties into planetesimal behaviour. In this paper, we present a new 1D planetesimal thermal evolution and dynamo generation model. This magnetic field generation model is the first of a differentiated, mantled planetesimal that includes both mantle convection and non-eutectic core solidification. We have improved fundamental aspects of mantle heat transport by including a more detailed viscosity model and stagnant lid convection parametrisations consistent with internal heating. We have also added radiogenic heating from $^{60}Fe$ in the metallic Fe-FeS core. Additionally, we implement a combined thermal and compositional buoyancy flux, as well as the latest magnetic field scaling laws to predict magnetic field strengths during the planetesimal's thermal evolution until core solidification is complete. We illustrate the consequences of our model changes with an example run for a 500 km radius planetesimal. These effects include more rapid erosion of core thermal stratification and longer duration of mantle convection compared to previous studies. The additional buoyancy from core solidification has a marginal effect on dynamo strength, but for some initial core sulfur contents it can prevent cessation of the dynamo when mantle convection ends. Our model can be used to investigate the effects of individual parameters on dynamo generation and constrain properties of specific meteorite parent bodies. Combined, these updates mean this model can predict the most reliable and complete magnetic field history for a planetesimal to date, so is a valuable tool for deciphering planetesimal behaviour from meteorite properties.

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