Late accretion to the early Earth and rocky exoplanets

The final stages of Earth's formation were characterised by the violent collisions of planetesimals, asteroids and comets -- by-products of the primary stage of planet formation -- with the nascent planet. It is thought these impacts, delivering vast sums of energy, may have transformed surface environments -- melting Earth's primitive crust, and sterilizing the surface of putative life. In contrast, it is also thought these impacts may have assumed a central role in the delivery of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur -- the major chemical constituents of life on Earth. Specifically, an early period of cometary bombardment has long been speculated to provide a key source of prebiotic feedstock molecules, required for the origins of life. Late accretion may therefore have profoundly controlled the emergence of habitable conditions on Earth, and the prebiotic chemistry that led to the emergence of life. Yet, our understanding of both the amount of material delivered, and the role these impacts played in the origins of life remains far from complete. In this thesis I address the evidence for, and implications of late accretion to Earth. The first strand of my research investigates the excess abundance of highly-siderophile elements (HSEs) in the Earth and Moon's mantles -- commonly assumed to record a `late veneer' of impacts delivering the final ∼0.5% of Earth's mass. I will show that the geodynamics of HSE delivery challenge the foundational assumption that HSE abundances reliably trace total mass accretion, with HSEs from large planetesimals liable to sink directly to Earth's core, leaving no geochemical signature in the mantle. Coupling geodynamical constraints on HSE delivery with isotopic constraints on the composition of late accretion, I will argue that the majority of Earth's HSEs must pre-date Moon formation -- challenging the canonical late veneer hypothesis. This is shown to provide a self-consistent explanation for the HSE records of both the Earth and Moon. The second strand of my research explores the ability of comets to deliver key feedstock molecules to planetary surfaces, reassessing their assumed role in the origins of life on Earth. Solar System comets boast a rich diversity of organic molecules -- many of which thought to be limiting for prebiotic chemistry -- yet, recent work highlights that a significant proportion of these molecules will be destroyed during hypervelocity impact with Earth. Leveraging recent advances in cometary impact simulations, comet-atmosphere interactions, and the dynamical modelling of short-period comets’ origin, I will show that the successful delivery of prebiotic feedstock molecules to Earth was an exceedingly rare event. I finally demonstrate that the prospects for cometary delivery are substantially worse in an exoplanetary context, challenging the long-held hypothesis that cometary impacts are likely to play an important role in the origins of life.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2026

Author dc:creator

• Anslow, Richard

Advisor dc:contributor.advisor

• Bonsor-Matthews, Amy

• Exoplanets
• Planet Formation
• Late Veneer
• Prebiotic Chemistry
• Origins of Life
• Comets
• Impact Processes
• Geochemistry

Licence

• https://www.repository.cam.ac.uk/bitstreams/fbe33a5a-1b10-4785-9ee6-8ccd5e76a4fc/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.131364

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/404972