Assessing Nonliving Contaminants of Concern
Technological developments are leading to increasingly sensitive and sophisticated instruments that may be applied to the search for life on Mars (see Chapter 6). As a result, the scope of the search has expanded to encompass signs of past as well as extant life, and the issues that could complicate or compromise the search have increased accordingly.
Planetary protection of Mars has previously emphasized the need to restrict a spacecraft’s burden of living organisms to prevent biological contamination of Mars and to avoid jeopardizing experiments designed to detect life there. But because many of the newer techniques for detection of life depend on the measurement of trace quantities of specific molecules, it is important that planetary protection measures also address the level of nonliving contamination of spacecraft that could confound with false positives the results obtained with such techniques.
The issue of contamination of Mars from nonliving sources was noted briefly by the NRC in 1992 (NRC, 1992, pp. 38-39). More recently, the challenges posed to life-detection experiments by nonliving contaminants were addressed by the Organic Contamination Science Steering Group (OCSSG) in a report (Mahaffy et al., 2003) that greatly aided the deliberations of the Committee on Preventing the Forward Contamination of Mars.1
This chapter describes six categories of nonliving materials as potential contaminants that could compromise the search for extant or extinct life on Mars. Examples of such contaminants introduced by spacecraft to Mars include organic molecules derived from living and nonliving matter, nutrient elements such as nitrogen, and particulates (dust). At worst, such nonliving contaminants potentially could (1) confound the detection of life by instruments carried on a particular mission to Mars, (2) contribute to the contamination of particular regions of interest that could complicate future life detection there, (3) promote or assist the growth of contaminating Earth microbes transferred concurrently or previously by spacecraft, and (4) compromise measurements of Mars atmosphere trace gases of significance for life detection.
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
of co-transported terrestrial organisms. With the exception of nitrogen, all of these elements are transmissible primarily as particulate material or as dissolved ions. Salts, dust, aerosols, and fingerprints are all of concern for the control of contamination, both on the surfaces and within the interior spaces of spacecraft, as well as for the potential transfer of contaminants from spacecraft components to Mars. Nitrogen as a contaminant could take many forms in addition to particulate matter: it could also be present in nitrogenous salts and in volatile phases, including as N2 gas; the reduced species ammonia, hydrazine, and cyanide; and the oxidized forms N2O and nitrogen oxides (NOx). Nitrogen and phosphorus are fundamental nutrient elements critical for the growth of all organisms; thus, they are potential contaminants to be avoided.
Simple Organic Molecules
Simple organic molecules include formaldehyde, cyanide, acetate, urea, simple sugars, amino acids, benzene, polycyclic aromatic hydrocarbons (PAHs), and light hydrocarbons. In addition to serving as potential substrates for microbial growth and as substrates for the formation of more complex macromolecules, these compounds are generated by organisms and are potential indicators of the presence of life. Contamination of Mars by exogenous material could compromise the ability to make the necessary distinctions; sources of such exogenous material could include the inevitable meteoritic input but could also be greatly enhanced locally by contamination from spacecraft.
Simple organic molecules can be produced through the breakdown of substances of biological or synthetic origin. Such materials are both commonplace in and problematic for life-detection experiments, because their high volatility can lead to their migration to surfaces previously uncontaminated by these molecules. The recoating of previously cleaned surfaces exposed to volatilized and migrating organic molecules requires diligent monitoring. As recommended in the OCSSG report (Mahaffy et al., 2003), approaches to monitoring the migration of contaminants throughout a spacecraft include the use of witness plates, development of transport models appropriate to the atmospheric pressure of Mars, and maintenance of an archive of all organic-containing materials used during spacecraft assembly.
Biopolymers include the polysaccharides, proteins, lipids, and polynucleotides mentioned above, as well as lignin, chitin (amino-sugar polymer), and humin, all of which are found in terrestrial soils and therefore are possible components of dusts incorporated into spacecraft during assembly. Synthetic polymers include the plastics, plastic residues, and Teflon; they are typically present as spacecraft and payload component materials. Other nonvolatile complex molecules in this category include pigments and porphyrins (biological) and epoxies and adhesives (usually synthetic). Biopolymers that could compromise searches for life might be targeted for removal by spacecraft cleaning (Mahaffy et al., 2003).
Propulsion Exhaust Products of Space Vehicles
Materials in the five categories of potential Mars contaminants outlined above all release trace amounts of matter through, for example, outgassing, migration of microbial degradation products, and incidental contact. A sixth category of materials involves the exhaust products of Mars lander or vehicle propulsion systems, as well as exhaust intentionally released onto the martian surface and into the atmosphere. Little research has been conducted to assess the potential for such contamination to confound future scientific investigations. The concern about such contamination will grow, as the number and size of lander missions is expected to increase as the Mars Exploration Program evolves.3
Any Mars mission that enters the martian atmosphere and reaches the surface will emit solid, liquid, or gaseous nonbiological materials that are foreign to the Mars environment, including propulsion exhaust products in amounts ranging from as little as a few kilograms (as was the case for Pathfinder) to as much as 30 metric tons for a Mars excursion vehicle with a crew of four people and a surface payload of 25 metric tons (MSFC, 1991). Two concerns arise: (1) Could contamination caused by these exhaust products confound the measurement capabilities of the mission’s science payload? (2) Could such contamination affect the global environment so as to alter or mask important atmospheric markers of past or current conditions potentially relevant to biological investigation of Mars?
Since the Viking missions in the mid-1970s, Mars missions with soft landers (landers that touch down on the surface at speeds below a few meters per second) have used purified hydrazine fuel for the terminal descent phase (Husted et al., 1977). Use of purified hydrazine avoids contamination of the landing site with water and assorted carbon compounds, because the primary exhaust products are hydrogen, nitrogen, and ammonia. (See also Appendix G, Table G.1.) Other landers using passive devices such as airbags to absorb surface impact at higher landing speeds (e.g., Pathfinder and the Mars Exploration Rovers (MER) Spirit and Opportunity) may use solid retrorockets to more crudely control their terminal velocity. Solid rocket plumes contain aluminum oxide and carbonaceous compounds arising from pyrolysis of the ethylene propylene diene monomer rubber case insulation. To date, these solid rocket plume exhaust products have not been assessed as possible contaminants because it has been assumed that any impact they might have on the environment would occur well away from the final resting place of the landers.
Future mission concepts such as Mars Sample Return (MSR), Mars airplanes, and human missions entail more landed mass and/or higher total mission impulse (i.e., energy dissipation). These missions imply the use of propellants with higher energy content, and the exhaust products from this class of propellants usually contain measurable amounts of water and various other compounds, including carbon monoxide and carbon dioxide. One example of this type of propulsion is the first stage of the Mars Ascent Vehicle (MAV) for the planned MSR. Current designs for MSR use a solid motor first stage that ignites at the surface and burns to an altitude of approximately 15 km, emitting tens of kilograms of H2O, N2, CO, HCl, and Al2O3. Whether or not such contamination is tolerable, given that the martian samples to be returned will already have been collected, will depend in part on any subsequent surface investigations planned after the ascent stage launch, as well as the nature of the atmospheric contamination from the solid motor first stage occurring during launch. These issues are considered in greater detail in Appendix G.
DETERMINATION OF ACCEPTABLE LEVELS OF CONTAMINATION
Determining the background level allowable for each of the foregoing categories of materials requires an evaluation of the analytical limits of detection of current instruments, as well as of improvements that may be expected in future generations of instruments. The OCSSG report (Mahaffy et al., 2003) addresses these concerns with respect to mass fraction, dividing categories of molecules into components of high concern (1 to 100 ppb threshold) or medium to low concern (>100 ppb to ppm threshold).4 The implication of such mass-based standards is the requirement that a scientifically targeted molecule of “high concern” would have to be detected in a sample at a threshold of >100 ppb to be considered relevant. Simultaneously, it would be established or assumed that the contamination background was substantially <100 ppb. Such standards represent challenges both for the development of monitoring efforts and for the selection of appropriate instrument technologies.
Quality control for the detection and monitoring of contamination levels can be achieved in at least four ways:
Monitoring of the spacecraft and facility throughout assembly. Witness plates, Teflon swabs, and alcohol wipes can be used both to maintain cleanliness (see Appendix C) and to assess the mass and composition of residues removed from the spacecraft and instruments (Mahaffy et al., 2003).
See Tables 2 and 3 and Appendix A in the OCSSG report (Mahaffy et al., 2003).
Monitoring of volatiles during bakeout. Volatile compounds can be monitored during a thermal bakeout procedure in which spacecraft components are heated to release volatile compounds and thus reduce the rate of escape and migration of such volatiles during the mission (Mahaffy et al., 2003). This outgassing of volatiles can be monitored and quantified by sample collection and identification of the volatiles. Models generated from these data can be used to predict the behavior of volatile compounds in ambient Mars conditions and assess whether a given material will cause an unacceptable background level of either total carbon or individual compounds.
Incorporation of “blanks” into the instrument payload. Use of blanks—carefully selected control materials (e.g., mineral samples) that are free of an analyte of interest (e.g., individual amino acids)—can help in the determination of background levels of contamination acquired throughout the launch, transit, and landing process of a lander deployed to Mars. This process requires that the standard appropriately mimic the texture and structure of the Mars samples to be analyzed but that it contain levels of the target analyte that are lower than can be detected with existing technology. The use of blanks could be particularly effective for verifying the authenticity of martian organics; however, the use of blanks could be difficult to implement in a practical manner, possibly requiring planning and preparation by the scientific community of a set of standards applicable to the anticipated wide range of scientific experiments associated with any particular mission.
Use of terrestrial standards. Standards containing rigorously calibrated concentrations of potential analytes of interest also could be included with a payload to Mars to support accurate determination of the concentrations of analytes found in Mars samples. The benefits of such a technique include the ability to recalibrate instruments and achieve greater analytical precision and accuracy in the analysis of Mars samples. However, as noted in the OCSSG report (Mahaffy et al., 2003), the potential contamination hazards associated with transporting these standard materials on spacecraft appears to be significant. If materials deliberately containing above-threshold levels are transported and analyzed in situ, accidents or equipment failures potentially could allow for insurmountable contamination of both the instruments and the martian sample environment.
The assessment of nonliving contaminants of concern is complex. The materials identified in this chapter are likely to be transferred to Mars by different processes and at different mass scales; they have varying potential to compromise experiments designed to detect life on Mars; and the acceptable levels of contamination may have different thresholds depending on the class of contaminant, as well as on the scientific demands of individual Mars missions. Preventing the microbial contamination of Mars, however, includes consideration of such nonliving material. In the presence of living cells and liquid water—either on a spacecraft or transferred to the Mars surface environment—several classes of organic and inorganic compounds could contribute to the growth of microbes and limit the ability to accurately detect their presence. This includes the transfer of labile biomolecules, both monomeric (e.g., nucleotides, amino acids, sugars, acetic acid, and fatty acids) and polymeric (DNA, proteins, polysaccharides, lipids). It also includes the potential to contaminate Mars with the most critical nutrient elements—nitrogen and phosphorus. Other organic contaminants such as polymers, plasticizers, and combustion products (PAHs) may be less likely to confound life-detection experiments, but they are ubiquitous and correspondingly harder to avoid introducing as contaminants. Finally, on a total mass basis, contamination of the Mars atmosphere by the exhaust products of rocket propellants is possibly the greatest source of exogenous material. However, it also is less clear to what extent these gases might be interpreted as indicators of biological processes.
Clearly, for all the reasons presented, comprehensive consideration of nonliving contaminants should be an integral part of spacecraft cleanliness requirements and the processes implemented in mission development and flight to comply with these requirements.
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