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Impacts by icy bodies likely played a key role in shaping the composition of solar systems objects, including the Earth's habitability. Hence, it is likely that they play a similar role in exoplanetary systems. Here I discuss how an impact from a comet affects the atmospheric chemistry, climate, and composition of two Earth-like terrestrial exoplanets with differing orbital configurations: a short (6 days), tidally-locked, orbit and an Earth-analogue orbit with a diurnal cycle.
To investigate this, I coupled a cometary impact model, which includes thermal ablation and pressure-drive breakup, with the 3D Earth-System-Model WACCM6/CESM2, quantifying the impact of a 2.5 km radius pure water ice comet. This revealed how both the impact-delivered water and thermal energy together affect the planetary atmosphere, including changing i) the cloud greenhouse effect, ii) the planetary albedo, iii) and the overall atmospheric composition. For the latter, we generally find an increase in the abundance of oxygen-bearing molecules with one key exception: ozone, the abundance of which is highly sensitive to products of the photodissociation of water, e.g OH.
My models also revealed how the response of the planetary atmosphere to the impact is shaped by the orbital configuration, and hence circulation regime of the planet. I find that the global atmospheric circulation may play a key role in setting the potential observability of individual massive impacts in future observations of exo-Earths. On the other hand, longer term changes to atmospheric composition appear less sensitive to orbital configuration, suggesting that sustained bombardment, or multiple large impacts, have the potential to measurably change the composition, and hence the habitability, of terrestrial exo-Earths.
In summary we find that cometary impacts may play an important role in shaping terrestrial exoplanetary atmospheres, and that to fully understand their impact we must also understand the underlying atmosphere they interact with.
Type Ia supernova are known to be thermonuclear explosions in a carbon-oxygen white dwarf that has acquired a helium envelope by accretion from a close companion star. The consensus model is that a flame starts at the centre of such a star and then becomes a detonation. This only works if the star is close to its maximum possible mass, the Chandrakhar mass (~ 1.4 solar masses).
I will discuss an alternative model in which a detonation in the helium envelope triggers a detonation the carbon-oxygen core. Recent work has shown that such a model could explain all "normal" type Ia supernovae, but there are disparities in length scales that make direct numerical simulations extremely difficult. The talk will consider some alternative techniques, such as front tracking using level set methods.
The term 'submesoscale' refers to ocean features with a horizontal lengthscale of 100m-10km and a timescale of hours-days. This regime consists of a range of eddies, fronts and waves where the typical non-dimensional parameters of the system are all order 1. I will explain the importance and challenges of modelling submesoscale motion and present some of my work on modelling submesoscale flow features, such as ocean fronts and frontal instabilities.
The source of angular momentum transport in stellar radiative zones remains an open, fundamental problem in stellar physics. One candidate mechanism is turbulence driven by the Tayler instability of toroidal magnetic fields. I will discuss a recent systematic revision of the linear stability analysis, followed by its implications for the efficiency of angular momentum transport out of evolved stellar cores. In particular, the Tayler instability is suppressed in the compositionally stratified regions of evolved low mass stars, suggesting that the Tayler instability cannot explain observations of core-envelope coupling on its own.
Changes in the Earth's rotation originate mostly from torques by the atmosphere and oceans, with a relatively small contribution from the Earth's core. The angular momentum exchange between core and mantle is however a long-standing problem. In this talk I will describe some of the mechanisms potentially at play and how they can be used to infer key properties of the core-mantle boundary. I will focus in particular on torsional Alfvén eigenmodes. The energy in these modes is equal parts magnetic and kinetic, and their motion is mostly columnar. The latter property has been used in the past to build approximate inviscid 1-D models. We have built a 3-D numerical model including viscosity, an electrically conductive inner core, and a thin, electrically conductive layer at the bottom of the mantle allowing the outer core to exchange angular momentum with the solid inner core and the mantle. I will present a systematic study the most relevant properties of these modes, particularly their columnarity, their torques, and their lifetimes as functions of the magnetic diffusivity and viscosity of the outer core, as well as functions of the electrical conductance of the bottom of the mantle.
Swirling flows induced by the combination of rotation and shear in orthogonal directions are ubiquitous in various natural phenomena, such as tornadoes and tropical cyclones, meandering rivers, vortex rings with swirl, and geophysical and astrophysical flows. These flows also occur in trailing vortices of aircraft wingtips and in branching junctions of everyday piping systems and physiological flows, where identifying instabilities that lead to vortex breakdown is of paramount importance. Swirling flows are present in industrial processes, such as filtration or purification of wastewater, isotope separation through centrifugation, and oil-drilling systems. They are also characteristic of convective flows with rotation, associated with cooling or lubrication of rotating machinery, crystal growth, and solidification of metals.
From a hydrodynamic perspective, the base state of a swirling flow has azimuthal and axial velocity components in either open or confined geometries. The open flow configuration is typical of swirling jets in natural phenomena, while the confined one is more common in engineering. A convenient setup to study swirling flows, both theoretically and experimentally, confines the fluid in a cylindrical annulus with differentially rotating cylinders, creating the classical circular Couette-Taylor flow. The axial component in this setup can be induced by an external pressure gradient, as in Spiral Poiseuille flow (SPF), by sliding inner cylinder, as in Spiral Couette flow (SCF), or by a radial temperature gradient, as in baroclinic Couette flow (BCF).
In this talk I present a universal theory of instabilities in swirling flows, occurring in both natural settings and industrial applications. The theory encompasses a wide range of open and confined flows, including spiral isothermal flows and baroclinic flows driven by radial temperature gradients and natural gravity in rotating fluids. By employing short-wavelength local analysis, the theory generalizes previous findings from numerical simulations and linear stability analyses of specific swirling flows, such as spiral Couette flow, spiral Poiseuille flow, and baroclinic Couette flow. A general criterion, extending and unifying existing criteria for instability to both centrifugal and shear-driven perturbations in swirling flows is derived, taking into account viscosity and thermal diffusion and guiding experimental and numerical investigations in the otherwise inaccessible parameter regimes.
The icy ocean worlds (e.g., Enceladus, Europa) are promising astrobiological targets since they potentially have regions with active water-rock interaction and hydrothermal activity at present-day. However, these habitable environments are typically overlain by a thick (>10 km) ocean and ice shell. Thus, to interpret surface observations, we need to understand the efficiency and the timescale over which fluids and particles get transported from the ocean-- core to the surface. In this study, we use high-resolution (~ 40-80m grid resolution) fluid dynamical simulations to analyze hydrothermal plume dynamics in an icy ocean world context. Our results significantly expand upon previous work by Goodman et al. (2004, 2012) by considering a larger range of: (i) hydrothermal heat fluxes (in particular lower heat fluxes < 100 W/m2 - consistent with estimates from tidal dissipation models), (ii) planetary rotation rates, and (iii) plume latitudes (polar to equatorial). We also consider the possibility of rapid vertical transport by bubble rich plumes.
We find that, in contrast to typical terrestrial hydrothermal plumes, baroclinic eddies play a critical role in the rotational plume dynamics in a deep icy ocean worlds in presence of minimal ocean stratification. The eddies efficiently transport heat laterally away from the vent location on a timescale faster than plume rise timescale. Consequently, a buoyant rotating plume rises much more slowly compared to a non-rotating plume. Using scaling results calibrated with the simulations, we find that the transit time across Enceladus's ocean for highest 1% of hydrothermal plume particles is at least ~ 100 yrs, if not significantly longer. This timescale significantly exceeds the months-to-a-few-years estimate based on a core hydrothermal activity model for silica nanoparticles observed in Enceladus’s plume. Thus, alternative models for silica nanoparticle formation need to be considered given the physical implausibility of fast transit times. Although bubbles can provide additional plume buoyancy, we find that unrealistically large bubbles are needed for rapid across-ocean transit. Our results also have significant implications for interpreting the measured geyser fluid compositions (e.g., methane, hydrogen, CO2) in the context of seafloor habitability.
