Hydrocarbons as Recorders of Cosmic Environments

Abstract

Hydrocarbons are ubiquitous in the Cosmos. Carbon stars jettison large amounts of hydrocarbons into interstellar space and these are incorporated into forming planetary systems alongside newly synthesized hydrocarbon material. The structure of a hydrocarbon reveals its origin with non-biological, biological, low temperature, high temperature, reduced, oxidised and aqueously altered hydrocarbons all having structural features that imply their provenance. These features are explored throughout this work, with a focus on the insoluble macromolecular organic carbon of meteorites and comparative terrestrial samples. Analytical pyrolysis of macromolecular material in meteorites is a well established technique. By subjecting samples to multiple heating steps, rather than the more usual single step, new insights into the structure and composition of the macromolecular material have been obtained. In addition, simple typing of chondrites and a reconstruction of the conditions experienced on their asteroid parent bodies is possible using the products of pyrolysis. It is the carbonaceous chondrites that have received the most attention for their organic content but some ordinary chondrites also contain appreciable quantities of organic materials. The organic inventory of both carbonaceous and ordinary chondrites has been explored in this study. Carbonaceous chondrites contain authentic abiotic organic material and are in great demand for scientific analysis and experimentation. Yet these objects are extremely rare and valuable and there is a need for analogue materials that are available in larger quantities and on which specific experiments can be carried out. Uses of effective meteorite organic analogues include the training of personnel, testing of analytical methods, contamination studies, and optimisation of space mission instruments. Most of the carbon in carbonaceous chondrites is a non-biological aromatic and intractable macromolecular material and previously unsatisfactory analogues have included coals and other so-called type III kerogens. Following a comparison of a number of candidate materials a new analogue has been identified in reworked fossil soils from the Jurassic of southern England. This type IV kerogen displays great similarities to the macromolecular material in meteorites and can be employed to lessen the burden on our curated collections of rare carbonaceous meteorites. The thermal and chemical stability of hydrocarbons ensures that they exhibit excellent preservation potential and can often be found when other molecular information carriers have long since perished. This feature is important when studying planetary environments for indicators of biogenicity. Yet there is a multitude of information to process and the organic signals can often be confusing owing to diagenesis, catagenesis, oxidation and weathering. In this study a wide range of terrestrial and extraterrestrial materials have been examined using statistical techniques to develop a method for the discrimination of abiotic from biotic macromolecular materials, based only upon the distributions of simple aromatic hydrocarbons and related compounds. This has important implications for life-detection missions destined for Mars, which are currently under development.