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Preventing the Forward Contamination of Mars
Mars is known to be well endowed with most of the elemental building blocks of life, although the martian inventory of nitrogen, in particular, remains poorly understood (Mancinelli, 1996). Mars’s inventory of these elements is being continuously supplemented through the influx of cometary and meteoritic material.3 A significant fraction (1 to 10 percent) by weight of infalling material is in the form of organic material synthesized by abiotic processes. The annual delivery of reduced organic compounds to the martian surface, most of it due to interplanetary dust particles and micrometeorites, is estimated to be 2.4 × 108 g/yr (Flynn and McKay, 1990; Benner et al., 2000).
Present data on martian organics come from two sources. The first are the Viking pyrolysis gas chromatographmass spectrometer (GCMS) analyses of martian soils, which detected the solvents used to clean the instrument on Earth but found no traces of martian organics to the limits of the instrument’s sensitivity (Biemann et al., 1977). The second source of data is that fraction of the organics contained in martian meteorites that is not due to terrestrial contamination (Jull et al., 1998; Becker et al., 1999). The organics identified in martian meteorites include polycyclic aromatic hydrocarbons (PAHs) and kerogens, some of which exhibit 13C/12C ratios similar to those observed in organics in primitive meteorites, suggesting that Mars may contain a record of extraterrestrial organic carbon compounds delivered by meteorites (e.g., Jull et al., 2000). The failure of the Viking GCMS to detect martian organics has been explained by (1) the absence of organics in the strongly oxidizing near-surface soil environment (Biemann et al., 1977), (2) insufficient instrument sensitivity (Glavin et al., 2001), (3) inability to achieve the high oven temperatures required to volatilize complex kerogen-like components (Becker, 2002), and (4) inability to detect the presence of the nonvolatile products of oxidative degradation such as the salts of organic acids (Benner et al., 2000).
Oxidants (such as hydrogen peroxide, H2O2) derive from photochemical processes in the martian atmosphere and from the interaction of solar ultraviolet radiation with surface minerals to form superoxides (Yen et al., 2000). However, Mancinelli (1989) has shown that certain Earth soil bacteria can survive much higher concentrations of H2O2 than are implied by the Viking measurements. Moreover, geologic processes have managed to preserve organics in terrestrial sedimentary rocks for billions of years (e.g., Foriel et al., 2004), despite oxidizing surface conditions. Therefore, it is unlikely that the presence of oxidants in the martian atmospheric and near-surface environments in itself represents an insurmountable challenge for martian habitability.
Earth’s biota derive useful energy either from light or from chemical reactions. Sunlight is abundant at the surface of Mars, but in the absence of evidence for the absorption of sunlight by martian biota, most considerations of biologically useful energy on Mars have focused on chemical sources. Utilizable chemical energy becomes available when disequilibrium redox conditions are created in the environment by processes such as volcanism, chemical weathering, and atmospheric photochemistry. There is evidence that all three processes either currently occur, or have occurred recently, on Mars. Crater counts in the calderas of martian volcanoes suggest that that eruptions have occurred as recently as 2 million years ago (Neukum et al., 2004). The weathering of iron minerals has been considered as a potential energy source for an early martian biosphere (Jakosky and Shock, 1998), and like the crust of the Earth, martian meteorites show evidence of incomplete chemical weathering (Treiman et al., 1993). Photochemical processes in the martian atmosphere result in the ongoing production of reactive species, such as H2, O2, and CO (Nair et al., 1994) that could be an energy source for martian biota (Weiss et al., 2000; Summers et al., 2002). The relatively good agreement between the observed concentrations of these species and chemical models that do not include surface sinks (biotic or abiotic) has been used to place upper limits on the overall metabolic activity of hypothetical biota. Also, it is concluded that Mars’s present-day biotic carbon flux
Sufficiently large and fast impactors may eject much of their material to space and erode the martian atmosphere; see Melosh and Vickery (1989) and Chyba (1990).