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Harrison Nicholls (he/him)

PhD student

Research theme

  • Astronomy and astrophysics

Sub department

  • Atmospheric, Oceanic and Planetary Physics

Research groups

  • Planetary Climate Dynamics
harrison.nicholls@physics.ox.ac.uk
Atmospheric Physics Clarendon Laboratory, room 113
www.h-nicholls.space
  • About
  • Publications

Convective shutdown in the atmospheres of lava worlds

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 536:3 (2024) 2957-2971

Authors:

Harrison Nicholls, Raymond T Pierrehumbert, Tim Lichtenberg, Laurent Soucasse, Stef Smeets
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Self-limited tidal heating and prolonged magma oceans in the L 98-59 system

(2025)

Authors:

Harrison Nicholls, Claire Marie Guimond, Hamish CFC Hay, Richard D Chatterjee, Tim Lichtenberg, Raymond T Pierrehumbert
Details from ArXiV

Magma Ocean Evolution at Arbitrary Redox State.

Journal of geophysical research. Planets 129:12 (2024) e2024JE008576

Authors:

Harrison Nicholls, Tim Lichtenberg, Dan J Bower, Raymond Pierrehumbert

Abstract:

Interactions between magma oceans and overlying atmospheres on young rocky planets leads to an evolving feedback of outgassing, greenhouse forcing, and mantle melt fraction. Previous studies have predominantly focused on the solidification of oxidized Earth-similar planets, but the diversity in mean density and irradiation observed in the low-mass exoplanet census motivate exploration of strongly varying geochemical scenarios. We aim to explore how variable redox properties alter the duration of magma ocean solidification, the equilibrium thermodynamic state, melt fraction of the mantle, and atmospheric composition. We develop a 1D coupled interior-atmosphere model that can simulate the time-evolution of lava planets. This is applied across a grid of fixed redox states, orbital separations, hydrogen endowments, and C/H ratios around a Sun-like star. The composition of these atmospheres is highly variable before and during solidification. The evolutionary path of an Earth-like planet at 1 AU ranges between permanent magma ocean states and solidification within 1 Myr. Recently solidified planets typically host H 2 O - or H 2 -dominated atmospheres in the absence of escape. Orbital separation is the primary factor determining magma ocean evolution, followed by the total hydrogen endowment, mantle oxygen fugacity, and finally the planet's C/H ratio. Collisional absorption by H 2 induces a greenhouse effect which can prevent or stall magma ocean solidification. Through this effect, as well as the outgassing of other volatiles, geochemical properties exert significant control over the fate of magma oceans on rocky planets.
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AGNI: A radiative-convective model for lava planet atmospheres

Journal of Open Source Software The Open Journal 10:109 (2025) 7726-7726

Authors:

Harrison Nicholls, Raymond Pierrehumbert, Tim Lichtenberg
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Temperature–chemistry coupling in the evolution of gas giant atmospheres driven by stellar flares

Monthly Notices of the Royal Astronomical Society Oxford University Press 523:4 (2023) 5681-5702

Authors:

Harrison Nicholls, Olivia Venot

Abstract:

The effect of enhanced UV irradiation associated with stellar flares on the atmospheric composition and temperature of gas giant exoplanets was investigated. This was done using a 1D radiative-convective-chemical model with self-consistent feedback between the temperature and the non-equilibrium chemistry. It was found that flare-driven changes to chemical composition and temperature give rise to prolonged trends in evolution across a broad range of pressure levels and species. Allowing feedback between chemistry and temperature plays an important role in establishing the quiescent structure of these atmospheres, and determines their evolution due to flares. It was found that cooler planets are more susceptible to flares than warmer ones, seeing larger changes in composition and temperature, and that temperature–chemistry feedback modifies their evolution. Long-term exposure to flares changes the transmission spectra of gas giant atmospheres; these changes differed when the temperature structure was allowed to evolve self-consistently with the chemistry. Changes in spectral features due to the effects of flares on these atmospheres can be associated with changes in composition. The effects of flares on the atmospheres of sufficiently cool planets will impact observations made with JWST. It is necessary to use self-consistent models of temperature and chemistry in order to accurately capture the effects of flares on features in the transmission spectra of cooler gas giants, but this depends heavily on the radiation environment of the planet.
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