Modelling the ultraviolet environment at the surface of Mars and design of the Beagle 2 UV sensor

Abstract

This thesis describes a multi-layer radiative transfer UV model which was used to aid in the design of the UV sensor on Beagle 2, which will soon provide the first ever in situ measurement of UV flux at the martian surface. The model uses the delta-Eddington approximation for diffuse flux and new low temperature gas absorption cross-sections and aerosol optical properties. Dust, H2O clouds and morning fogs are found to modify the martian surface UV spectrum. Dust storms have been shown to attenuate the surface UV flux by more than an order of magnitude, though some UV persists even at extremely high optical depths. The seasonal variation of surface UV irradiance was found to produce maximum exposure areas highly dependent upon dust activity over the martian year. Dust activity is also shown to distort the annual latitudinal dose, with high dust loading in the southern hemisphere resulting in a higher annual dose than in the north. The introduction of O3 abundances of 1.64 x 1017cm-2 into the model resulted in only partial protection for micro-organisms, since wavelengths shorter than 230 nm still penetrate to the surface. DNA-weighting of a martian UV spectrum shows the surface UV environment to be 103 times more damaging than on Earth. UV detection signatures of atmospheric phenomena and anticipated events for the case of Beagle 2 are presented: a dust devil encounter creates a double minimum profile in the UV flux, and solar eclipses by Phobos produce a single minimum. Clouds increase the diffuse/direct UV ratio, and fogs create a distinct dip in the morning profile when normalised to clear days.