Atmospheric Impacts of a Close Cometary Encounter

This thesis comprises both experimental and modelling investigations relating to the atmospheric impacts of a close cometary encounter. Two laboratory techniques were used to study the optical properties of iron oxide meteoric smoke particle (MSP) analogues: the Photochemical Aerosol Flow System (PAFS) and the Molecular flow Ice Cell in the Trapped Reactive Atmospheric Particle Spectrometer (MICE/TRAPS). Results from the two experiments were combined using an iterative photochemical model to derive complex refractive indices for the nanoparticles.

A number of sensitivity simulations were performed using the global climate model, WACCM (the Whole Atmosphere Community Climate Model) to simulate the effects of a cometary dust-loading following a close encounter with Halley’s comet at a distance of 100,000 km. The effects of a chemical perturbation (a metal injection) and a dynamical perturbation (a heating tendency) on the mesosphere-lower thermosphere (MLT) were investigated. The increase in metal density following the chemical perturbation caused the destruction of up to 100 % of the ozone (O3), atomic hydrogen (H) and water (H2O) in the MLT, and up to around 30 % of the atomic oxygen (O). This affected the chemical heating rates, leading to changes in temperature and changes to global transport and mixing. The dynamical perturbation produced up to a ~200 K temperature increase that led to small changes to the densities of O and O3. When the two perturbations were combined, the dynamical perturbation essentially amplified the effects of the chemical perturbation.

The effects of an injection of meteoric sulfur (S) and MSPs on the middle atmosphere were also investigated. Notable perturbations to both gas-phase sulfur chemistry and aerosol size distributions were observed. The injection also produced large increases in the surface area density of MSPs and mixed sulfate aerosol. No significant changes to the total aerosol deposition pattern were observed. However, a significant increase in the S deposited via this aerosol was seen compared to observations in a Greenlandic ice-core.