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In this thesis we develop models of the chemical kinetics of prestellar cores, so we can identify molecules which will be able to trace various physical processes, and therefore give us more understanding of how stars form from these objects. First, we investigate non-thermal desorption mechanisms which can operate inside prestellar cores, and make estimates of the desorption efficiencies based on observational data, which before now were poorly constrained by theory. We then use our chemical models to predict molecular abundances during the collapse phase of a prestellar core. In particular we investigate the effect of different initial conditions and collapse models, so we can identify molecules which could be used to distinguish between slow and rapid star formation models. We also generate line profiles for selected molecules using a radiative transfer code, using the abundances predicted from our chemical models. We find that for CS, a double peaked line profile with a blue asymmetry can only be produced for models of core collapse with extended inward motions. This contradicts many previous interpretations of line profiles, which have explained the infall signature by collapse models with a static envelope and temperature gradient. We also explore deuterium chemistry in prestellar cores, to investigate whether core collapse by ambipolar diffusion can leave a signature on deuterium fractionation. We find that the fractionation is very sensitive to the density profile of the core, thus we propose that observations of the spatial distribution of deuterated molecules could be used to distinguish between different collapse models. Finally, we investigate the chemistry in C-shock regions associated with outflows of young stellar objects, where we also include the analysis of some new observations of outflow LI448mm, which show very strong evidence for the presence of a magnetic precursor.

Any microbial life extant in the top meters of the martian subsurface is likely to be held dormant for long periods of time by the current permafrost conditions. In this potential habitable zone, a major environmental hazard is the ionising radiation field generated by the flux of exogenous energetic particles: solar energetic protons and galactic cosmic rays. The research reported here constitutes the first multidisciplinary approach to assessing the astrobiological impact of this radiation on Mars. A sophisticated computer model has been constructed de novo to characterise this complex subsurface ionising radiation field and explore the influence of variation in crucial parameters such as atmospheric density, surface composition, and primary radiation spectra. Microbiological work has been conducted to isolate novel cold-tolerant bacterial strains from the Dry Valleys environment of Antarctica, an analogue site to the martian surface, and determine their phylogenetic diversity and survival under high-dose gamma-ray exposure frozen at -79!C, a temperature characteristic of the martian mid-latitude permafrost. Original results are presented pertinent to microbial survival time, persistence of organic biomarkers, and calibration of the optically stimulated luminescence dating technique, as a function of depth. The model predicts a population of radiation resistant cells to survive in martian permafrost soil for 450,000 years at 2 m depth, the proposed drill length of the ExoMars rover. The Antarctic culturing studies identified representatives of four bacterial genera. The novel isolate Brevundimonas sp. MV.7 is found to show 99% 16S sequence similarity to cells discovered in NASA spacecraft assembly clean rooms, with the experimental irradiation determining this strain to suffer 10-6 population inactivation after a radiation dose of 7.5 kGy in martian permafrost conditions. Integrating the modelling and experimental irradiation, this research finds a contaminant population of such cells deposited just beneath the martian surface would survive the ambient cosmic radiation field for 117,000 years.