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Preventing the Forward Contamination of Mars
TYPES OF CONTAMINANTS
All Earth materials, from atoms to molecules to organisms to the actual spacecraft and landers themselves, could be considered contaminants on Mars in specific contexts. Thus it is necessary to define the nature and the threshold levels of materials of concern for planetary protection—specifically with reference to the science objectives of a particular mission to Mars and to the analytical sensitivities of present-generation instruments, as well as in anticipation of not compromising subsequent missions. At least six categories of materials, biological and nonbiological, as well as organic2 and nonorganic materials should be considered explicitly for their potential to contaminate Mars. They are considered briefly below. The OCSSG report (Mahaffy et al., 2003) addresses organic contaminants in more detail than can be presented here.
Compounds Derived from Biological Sources
Substances derived from biological sources could potentially compromise current or future efforts to detect life on Mars, since many of the techniques used for that purpose work at the molecular level to determine the presence of biomolecules and biological activity (see Chapter 6). Such biological compounds include DNA and individual nucleotides, proteins and individual amino acids, complex lipids, complex carbohydrates, and energy carriers such as adenosine triphosphate (ATP). All such compounds, if present as contaminants, could result in a false-positive result interpreted as signaling the presence of living cells in the present-day Mars environment—and could thus compromise the ability of researchers to evaluate the outcomes of scientific experiments concerned with detection of life on Mars.
Materials That Could Serve as Substrates for Microbial Metabolism
Contamination of Mars by Earth microbes could be aided by the introduction to Mars of sources of carbon and energy-yielding substrates. The transfer to Mars of such materials could change the local Mars environment and hence its habitability for microorganisms. Microbes on Earth display an astonishing diversity of metabolic capability (see Chapter 5); environments have been observed and species isolated in which microbial growth occurs anaerobically by the oxidation of unusual substrates such as benzene, trinitro-toluene (TNT), trichloroethylene, and numerous other solvents, plastics, and explosives, provided that water is also present (e.g., Lovley et al., 1994; Bradley and Chapelle, 1996; Esteve-Nuñez et al., 2000). Solvents may be present on spacecraft as residues from cleaning procedures; hydrocarbons may be used as lubricants for mechanical components; plastics, including Teflon, Kevlar, and other composites, are used as structural components and in instruments; and benzene and other polyaromatic compounds are formed during the thermal breakdown of other carbon residues. In the case of microbial oxidation of these reduced organic materials, ferric iron [Fe(III)] is frequently the oxidant. On Mars, contaminant organic materials could be exposed to abundant oxidizing power in the form of ferric minerals, including nonspecific iron oxides and hematite crystals (Morris et al., 2004). Preventing the development of “microbial islands” or growth pockets aboard spacecraft and/or landers will depend in part on the ability to limit the contact between microbial cells, organic substrates, oxidants such as Fe(III), and water.
Light Elements Critical for Microbial Growth
In addition to carbon, energy sources, and water, nutrients critical for growth include nitrogen, phosphorus, and sulfur. Also included in this category are the trace elements selenium, molybdenum, copper, zinc, iron, cobalt, and nickel. Although it is unlikely that future Mars science missions will rely strictly on the analysis of elements to determine biosignatures, it is the case that excessive contamination by these elements could expedite the growth
An organic material is one that includes carbon covalently bonded with hydrogen and perhaps other atoms. Therefore, most carbon-based molecules are organic molecules, but CO, CO2, and certain others are not, despite their potential importance in the biochemistry of life.