What Lies Beneath: Making the Link Between the coma and Surface Composition of Comets

Much is made about the relative abundances and isotope ratios of species detected in the coma of comets and active asteroids via spectroscopic (both remote, telescope, and in situ, orbiter) measurements as if they can be directly assigned to the composition and physical state of the nucleus itself. These assumed compositions have been used to infer constraints to models of formation of these primitive bodies, and by extension the solar system. However, little is known about the kinetics of the sublimation/degassing processes during the state change from the icy solid to the (detectable) volatile phase. Some correction must be necessary to back-calculate the original composition of the body due to fractionation processes - some species will partition preferentially into the vapor or solid phases. Simulation experiments on synthetic analogues with known compositions must be carried out, as actual samples of comet return are rare, so that the volatile phases degassing, under various interplanetary conditions, can be transcribed to the actual primary composition of the nucleus. To this end we will use the new Imperial College Impacts and Astromaterials Research Group's planetary environmental chamber to simulate the exposure of various mixtures of water-ice, inorganic mineral phases (phyllosilicates/carbonates) and astrobiologically relevant organic molecules (aliphatic and aromatic hydrocarbons, amino acids, etc) to ultraviolet irradiation and vacuum. Headspace gases generated will be drawn off and analysed via gas chromatography - mass spectrometry (GC-MS) and isotope ratio - mass spectrometry (IR-MS) to analyse gas phase mixtures, volatile organic compound production and carbon and oxygen isotopes. The composition of water-soluble or insoluble organic residues left behind after the experiment will be analysed by GC-MS and pyrolysis-GC-MS respectively, while the, potentially altered inorganic mineral phases will be analysed by pyrolysis - Fourier transform infrared spectroscopy. The results from these analogue experiments will be directly comparable to observations of comets and active asteroids. This will allow for more informed speculation as to the surface composition, formation and alteration history of these primitive bodies, and, by extension, improve models of solar system formation.