Identification (or Otherwise) of Martian Carbon in Martian Meteorites

Introduction: One of the goals of current martian exploration is to search for evidence of extinct (or even extant) life. In recent years, this goal has been interpreted as a search for evidence of water on Mars’ surface. The success of instrumentation (high resolution cameras and infrared spectrometers) on orbiting spacecraft, coupled with in situ data from the MER rovers have revolutionized our understanding of the surface of Mars, the type of rocks and minerals present, and their stratigraphy [1 and subsequent papers]. It is clear that there has been extensive and intensive aqueous action at different periods in Mars’ history, and evidence for water is, to all intents and purposes, irrefutable. It is now time to define the next phase in the search for life as a search for regions of habitability on Mars [2]. A search for life could also be seen as a search for carbon, as one of the major assumptions that has been made in the search for life on Mars is that any life present is likely to be carbon-based. To date, there have been no unambiguous measurements of carbon on Mars’ surface. Data from the Viking labelled-release experiments were not clear [3]. Although Mars’ thin atmosphere is approximately 95% carbon dioxide, and carbonate grains have been identified in dust in the atmosphere [4], predictions of vast carbonate reservoirs [5] have not been verified, and it is now thought that fluid on the surface might have been too acidic for the survival of carbonates [6]. In contrast to observations of the martian surface, measurements on martian meteorites have shown the presence of several different carbon-bearing components, including carbonates [7, 8]. We use data from martian meteorites to estimate the abundance and δC of organic carbon, primary magmatic carbon and secondary carbonates that might be present on Mars. Much is known about the behaviour of carbon on Earth as it cycles through the atmosphere, hydrosphere and lithosphere. The biosphere is a fourth carbon reservoir, and its presence influences the fixing and release of carbon in these reservoirs over different timescales. The overall carbon balance is kept in equilibrium at the surface by a combination of tectonic processes (which bury carbon), volcanism (which releases it) and biology (which mediates it). In contrast, to Earth, Mars currently has no active tectonic system; neither does it possess a significant biosphere. However, these observations might not necessarily have held in the past. By constructing a carbon cycle for Mars based on the carbon chemistry of martian meteorites, we investigate whether or not there is evidence for a martian biosphere Analytical techniques: Three separate techniques were used to determine the abundance and isotopic composition of different carbon-bearing components in martian meteorites. Component identification by stepped combustion. Carbon-bearing phases are oxidized by incremental heating in an atmosphere of oxygen (stepped combustion). Following purification of the resulting CO2, the isotopic composition of the gas is measured by mass spectrometry [9]. An example of the type of data acquired is shown in Figure 1, with the temperature ranges shown over which different carbon-bearing components combust or decrepitate.