Methodologies for Advancing Astrobiology
ORBITAL MEASUREMENTS AT GLOBAL AND REGIONAL SCALES
Measurements at global and regional scales are a necessary and important component of a Mars astrobiology strategy. Such measurements allow global, regional, and local characterization of properties as well as the elucidation of global processes that affect habitability.
Although liquid water is not stable at the surface today, it could persist in the subsurface for extended periods of time. There, chemical reactions involving water and the surrounding regolith could provide energy that could support metabolism of organisms or, conceivably, the chemical reactions that might lead to life. Researchers are learning that Earth’s subsurface biosphere may be comparable in size to that at its surface, and it is expected that any extant biosphere on Mars would involve organisms in the subsurface. Subsurface liquid water could be detected from the surface or from orbit by radar and microwave techniques, based on the distinct dielectric behavior of water compared to ice, rock, or regolith. Wavelengths can be selected that would allow sensitivity to different depths beneath the surface, allowing subsurface sounding and profiling at various length scales. The anticipated results would include the potential discovery of subsurface deposits of water and determination of its geographical distribution. The next logical step would be direct, in situ access to the subsurface via drilling.
Measurements from the Mars Exploration Rovers and from the Mars Express orbiter, combined with other evidence, increasingly point to a warmer and/or wetter ancient environment compared to the present cold and dry climate. Determining the processes by which the atmosphere evolved is central to understanding the nature of martian habitability in particular and planetary habitability in general, and provides a key boundary condition to the possible occurrence of martian life. Direct measurements from Mars Express show that atmospheric atoms and molecules are being lost to space today, and isotopic measurements indicate that this loss has been an important process through time and may have been the dominant loss mechanism. A thorough understanding of the structure, composition, and dynamics of the upper atmosphere today, and of the solar- and solar-wind-driven loss processes and how they operate today, and an understanding of how they vary with properties of the incoming solar and solar
wind inputs will allow a determination of how these processes operated through time and how important loss to space has been over time. These measurements will require global-scale observations that can be made through a combination of in situ and remote-sensing approaches from orbit. They would address the current inadequate information in this area today.
If Mars has active geological processes that involve gas exchange between the surface and the subsurface (such as outgassing associated with volcanism or gas exchange between the atmosphere and a deep aquifer), there should be evidence of this in atmospheric trace gases. There also is likely to be trace-gas evidence if Mars has a large, active surface or subsurface biosphere. Methane has been discussed in this context recently, due to its reported detection by three different groups. However, atmospheric methane can result from either of these sets of processes. Distinguishing between the two would require measurements of other trace gases (e.g., formaldehyde) that would be produced by only one of the processes. Thus, global-scale measurements of atmospheric gases, with sufficiently high precision and accuracy to allow detection and characterization of trace gases, would be a valuable astrobiology objective. There also is the possibility that observations can be made in a nadir-pointing mode—the preferred approach when trying to identify a localized source—that would provide high-spatial-resolution mapping of trace gases, which could in turn allow identification of a localized source of trace gases and their potential temporal variability.
Other regional- and global-scale measurements would contribute to astrobiological science goals. These might include high-resolution spectroscopic mapping that would allow identification of mineralogy associated with water- or volcanic-related geological features, geophysical measurements that would determine the thermal history and subsurface structure, and mapping of polar deposits at all wavelengths that would provide information on their history and the potential occurrence of liquid water.
GEOLOGICAL AND ENVIRONMENTAL CONTEXT AT LOCAL SITES
The plausibility of life existing at a given landing site depends on the location having a habitable environment. The recognition and characterization of present environments are straightforward. If such an environment existed in the past, then the geological history of that site must have allowed preservation of an environmental record and of traces of organisms that populated it. Past environments and historical geology can be retrieved from rocks using in situ instruments, as demonstrated by rovers such as Mars Pathfinder’s Sojourner and the Mars Exploration Rovers, Spirit and Opportunity.1,2 These missions clearly demonstrate that the keys to unraveling geological and environmental context are mobility and a complementary suite of instruments for observations and measurements.
The geologically active surface of Earth constitutes a major hurdle for recognizing the remains of its ancient life. Surface rocks on Mars, as far as we know, have not experienced the thermal (metamorphic) and deformational (tectonic) events that so commonly obscure the record of life in terrestrial Precambrian rocks. However, several processes can potentially complicate astrobiological studies of Mars rocks:
Volcanism. Much of the planet’s surface is covered with lava flows that would destroy any surface or near-surface organisms. The limited number of impact craters on some terrains and the young ages of many martian meteorites indicate that volcanic activity has continued throughout Mars’s history. Understanding the timing of volcanism at a local site, relative to the age of any putative life forms, is critical to any hypothesis for martian life.
Shock metamorphism. Meteor impacts transmit large shock pressures to target rocks, transforming minerals, pulverizing rocks, and sometimes melting them. Shock metamorphism is pervasive in martian meteorites, and the high density of craters in ancient Noachian terrains (arguably the most intriguing sites for life) argues that most of these rocks have experienced shock. Cratering also excavates materials, thus disrupting the outcrop stratigraphy that is so useful in reconstructing geological history and environments.
Weathering. The spectral identification of readily weathered igneous minerals (olivine and pyroxene) at regional and local scales suggests that weathering processes on Mars might be dominated by physical rather than chemical weathering. However, alteration processes in soils and alteration rinds on rocks reveal chemical dissolution reactions that could potentially obscure evidence for life.3 In that case, mechanisms for accessing fresh rock
interiors (MER rovers utilized a rock abrasion tool) are required. It is also possible that weathering could produce chemical gradients at rock surfaces that might actually be exploited by microorganisms, and so examination before and after grinding or drilling may be desirable.
Global-scale observations by orbiting spacecraft also have a role in the geological exploration of local sites. As an example, orbital imagery of channels was instrumental in interpretations of the geological history of the Mars Pathfinder landing site. As another, chemical biomarkers must be detected in the context of the global environment. The 13C biomarker is a good indicator of life, but only if nonbiological fractionations in 13C in the carbon dioxide background are known (see Chapter 3).
Determining the geological and environmental history of a local site provides a critical filter for assessing the plausibility of life at that site. Remote sensing by instruments on rovers and orbiters can provide adequate characterization in most cases, although advances in the identification of minerals and measurement of trace elements and isotopes (which constrain environments) and in geochronology (which constrains geological history) are needed.
IN SITU ANALYSES RELATED TO LIFE DETECTION
From the lessons learned from ALH 84001 and the early life on Earth debate (see Chapter 2), a multi-instrument, multi-measurement strategy must be used in robotic exploration and analyses to be able to make statements about the presence of biomarkers with any confidence. Although the focus of this section is in situ measurements, the same capabilities apply to the collection and analysis of samples returned from Mars. In situ astrobiological instruments must provide for the following tasks:
Acquire appropriate samples,
Understand the context,
Identify the best place on the sample for further analysis, and
Perform a number of mutually confirming independent measurements.
Accomplishing these tasks requires that any mission be able to identify suitable samples from a distance; perform contact instrument analysis to confirm suitability of the sample for further investigation (sometimes called sample triage); and utilize a suite of instruments to analyze a selected portion of the sample, either in situ on Mars or in terrestrial laboratories on returned samples. These instruments should be capable of making the following observations and measurements:
Comprehensive imaging. Image each investigation scene to assess the variety of local environments expressed in surface features such as outcrops.
Definitive mineralogy and chemistry. Determine mineralogical and chemical (elemental) composition at all scales of investigation: site/scene surface reconnaissance scale (range: infinity/horizon to meter; resolution: kilometer to centimeter); hand-sample scale (range: meter to centimeter; resolution: centimeter to millimeter); and acquired subsample scale (bulk measurement of a few grams or milligrams of subsample with high accuracy).
Redox potential. Assess the redox potential and oxidation chemistry of materials in the near-surface environment.
Fine-scale surface analyses. Investigate the surfaces of selected exposed or acquired samples at fine scales for morphological, chemical, and molecular signatures suggesting preservation of prebiotic or biotic organic compounds. This may include directly detected compositional markers, evidence of minerals formed in or altered by liquid water, or particular sample textures. Color optical (microscopic) imaging with a high resolution should also be used to provide context for any co-focused spectroscopic tools such as ultraviolet-excitation fluorescence, laser Raman, or other fine-scale techniques to perform chemical signature detection.
Subsample biosignature analyses. On selected subsamples, perform an array of high-sensitivity, mutually confirming laboratory investigations as described below.
The identity, abundance, and isomeric distribution of carbon compounds should be analyzed to low detection levels (parts per billion or below by weight within bulk ~100-mg subsamples) and to high molecular weights (hundreds to thousands of Daltons) at high peak resolutions. Mass spectrometry measurements should be configured to enable the detection of less volatile species that are relevant to the preservation of biosignatures. The isotopic ratios of C, H, N, O, P, and S should be characterized with sufficient precision to enable biogenic, environmental, or meteoritic fractionation trends to be identified. Compound-specific isotope analyses are highly desirable. Additional isotope ratios that further characterize atmospheric components and aqueous processes are also needed.
Highly sensitive tests for the presence and characteristics of specific biosignatures should be conducted on bulk subsamples or isolated extraction products (e.g., phases or concentrates). Biosignatures of particular interest include the identities or abundances of molecular compounds of distinctly biological origin as known on Earth, indicators of extant metabolic processes such as disequilibrium chemistry, and chemical or morphological traces of such compounds and processes preserved in minerals. Examples of specific tests include detection of amino and nucleic acids, lipids, and proteins; determination of chirality in amino acids and sugars; detection of enhanced concentrations of molecules that suggest selective use of specific isomers; and observations of cellular morphology or microfossils.
In Situ Flight Instrumentation Needed
Instruments that can make the observations and measurements required for astrobiology are not generally at the technology readiness level necessary to allow their inclusion in Mars missions. New developments in instrumentation are being fueled by the medical industry and by concerns about biowarfare agent detection and should be incorporated into spacecraft mission planning. Examples of new technologies include the following:
Microelectromechanical systems. This technology takes advantage of new techniques for manufacturing miniaturized components, producing smaller, sensitive instruments for portable use.
Microelectro-optical systems. These optic systems would provide more capable imaging and spectroscopy.
Microfluidics. Sometimes called “lab on a chip,” these fluidic circuits when cut into a glass or plastic wafer can transport and mix fluids, bring dried reagents into contact with fluid to perform reactions, combine liquid with electrical currents for electrophoresis or sample concentration, and perform sensitive measurements on analytes of interest.
Imaging. Atomic-force microscopy is being used on both the Rosetta and the Phoenix missions. Currently it is the only way to image in space beyond the limitations of optical devices. Other imaging technologies including interferometry, scanning near-field optical microscopy, and electron microscopy techniques should also be developed for spaceflight applications.
Imaging spectroscopy. Coupling spectroscopy to imaging is an extremely powerful tool for elucidating the chemistry of any sample. An example relevant to astrobiology is the use of imaging-Raman systems detect reduced carbon in microfossils and in ALH 84001.4-7 The use of multiple/tunable laser systems, light-emitting diodes, and advances in miniaturization and detector design have direct relevance to spacecraft spectrometers.
Mass spectrometry. Mass spectrometry (MS) coupled to some form of gas chromatography (GC) system is the method of choice for unambiguous identification of organic molecules. The chromatography is essential to resolve and obtain the characteristic spectra of isobaric compounds and determine their relative abundances. Instruments used for Mars exploration would likely exploit the well-established technologies of electron impact ionization and quadrupole or time-of-flight (TOF) mass analyzers. Given the low expected abundances of organic molecules in Mars samples, attention should be paid to improving sensitivity over that of existing methodologies.
Biotechnology. Key technologies for life detection involve specific recognition of a target of interest using a probe molecule. A probe molecule is one that has a site that interacts specifically with the target, allowing its con-
centration and detection by various means. Common probes are antibodies, FAb fragments, DNA/RNA aptamers, and molecular imprinted polymers or capture resins. Detection of the target molecules can occur via optical means (fluorescence or colorimetry), mass spectrometry, or surface plasmon resonance.
Sample-handling technology. A major hurdle for in situ investigations of biomarkers is the availability of robust and flexible sample-handling systems. Most measurements for biosignatures require that the sample be pretreated in some fashion. The extraction of biomolecules and their introduction into instruments without cross-contamination or biasing the concentrations of molecules of interest, allowing the concentration of the sample to increase the likelihood of detection, and ensuring that no poisoning compounds or ions come into contact with the sample, is critical.
MARS METEORITE COLLECTION AND ANALYSIS
There are currently more than three-dozen examples of the so-called SNC (shergottite, nakhlite, and chassignite) meteorites believed to have originated on Mars.8 All of these meteorites are igneous rocks whose source localities on Mars are unknown. Most of the known martian meteorites have crystallization ages younger than ~1.3 billion years.9 Only one, the orthopyroxenite ALH 84001 with its crystallization age of ~4.5 billion years, is likely to represent a sample of the ancient crust of Mars that is most likely to have had environments that could have hosted life.10,11
Given the lack of geological context for the martian meteorites, their origin from a limited number of mostly young volcanic terrains on Mars, alteration by shock and cosmic irradiation during their ejection from Mars and delivery to Earth, and subsequent weathering and contamination by organisms in a terrestrial environment, the martian meteorites are not ideal for astrobiological investigations. Nevertheless, some of them contain clear evidence for low-temperature alteration by aqueous fluids, and investigations of these samples have provided significant constraints on the near-surface environment and the history of water on Mars.12,13 Ancient ALH 84001 in particular has been the focus of numerous (often controversial) studies that may have implications for past biogenic activity on Mars.14–17 Therefore, although not optimal, the martian meteorites still provide a relatively inexpensive means of investigating the potential for past and/or present life on Mars.
In recent years, many new members of this clan have been recovered from hot and cold desert regions of the world. Although the terrestrial residence time of martian meteorites collected in hot and cold deserts is about the same, the former are much more subject to terrestrial weathering and biological contamination than the latter.18,19 Because those meteorites collected in Antarctica have spent most of their time on Earth fully encased in ice, they do not, in general, display the veins and cracks filled with alteration minerals commonly seen in meteorites found in hot deserts. Consequently, a concerted effort for the collection of martian meteorites in Antarctica should continue, given the potential scientific return from the recovery of new samples, particularly if they could provide a record of ancient and/or water-rich environments on Mars.
ANALYSES OF RETURNED SAMPLES AND STATEGIES FOR MAXIMIZING ASTROBIOLOGICAL POTENTIAL
In the development of a shorter-term strategy for the investigation of Mars from an astrobiological perspective, sample return must also be accorded a higher priority than indicated by current NASA planning.20 Returned samples, which can be analyzed in Earth-based laboratories, offer significant advantages (Box 5.1) over remote analyses of samples by landers and rovers.21 For the foreseeable future, the capabilities for remote analyses cannot hope to match those available in Earth-based laboratories in terms of sensitivity, accuracy, and precision. Moreover, given their severe mass- and power-limitations, spacecraft can only be equipped with a limited suite of analytical instruments, whereas the entire range of analytical capabilities available in Earth-based laboratories can be brought to bear on returned samples. Many analytical techniques require extensive sample preparation, which cannot be duplicated in remote spacecraft protocols. Analysis of chemical gradients in rocks requires spatial (depth) resolution that is difficult to achieve on Mars but would be relatively easy if done on Earth.
If properly stored and isolated from the terrestrial environment, returned samples can be examined by more sensitive analytical techniques and methodologies that are likely to be developed in the future. Returned samples
also offer great flexibility; in any scientific investigation it is advantageous to be able to alter the analytical strategy as new information emerges. This is particularly critical in astrobiological investigations, where the characteristics of extraterrestrial organisms or prebiotic chemistry cannot be confidently predicted. Some analytical flexibility has been demonstrated in Mars Exploration Rover (MER) missions, in which adaptations of instrument protocols have been employed to analyze unexpected rock compositions, but changes in sample-handling capabilities and instrumentation are clearly impossible for investigations involving remote analysis by spacecraft.
Pros and Cons of Sample Return and In Situ Analysis
Mars sample return and in situ analyses both have advantages and disadvantages, as summarized below. The two are actually quite complementary.
Mars Sample Return
Pros. The factors favoring sample return include the following:
Cons. The factors arguing against sample return include the following:
Finally, sample return is particularly important for astrobiology investigations on Mars, since it is certain that any significant finding with potentially far-reaching implications will require corroboration by multiple replications of the same analyses (ideally in different laboratories and by different investigators) and by different types of analyses. Moreover, investigations with an astrobiological focus are likely to be a significant component of any future human exploration of Mars. In addition to being of the highest scientific priority in its own right, sample return by a robotic spacecraft has been identified in several NRC reports as being an important if not essential
In Situ Analysis
Pros. The advantages of in situ analyses on Mars include the following:
Cons. The disadvantages of in situ studies of martian samples include the following:
Value of Complementary Approach
In situ measurements conducted in the context of a sample-return mission have the following desirable characteristics:
Various methods have been proposed to acquire martian samples. These include grab sampling (using a scoop or arm on a lander or collection of airborne dust during a flyby through the upper atmosphere), sampling a variety of rocks and soils by using a rover, obtaining subsurface samples using a drill, conducting a Phobos sample return that might contain Mars ejecta from large impacts, or collection by astronauts.
To maximize the astrobiological potential of a sample return from Mars, it will be important to recover a wide range of materials from a well-characterized site of astrobiological interest. This requirement immediately raises a longstanding strategic question: Is it appropriate to wait until sufficient knowledge has accumulated to identify the “right sample” or to get a sample from the best place identifiable today? Proponents of the “right sample” approach typically argue that spending billions of dollars to return the “wrong sample” would be a major setback to the Mars Exploration Program. The biggest concern about getting the “right sample” is that, once this approach
has been accepted, the only viable sample is one for which there already is compelling evidence for life or at least a high probability of there having been life. This would seem to require obtaining a sample from a place at which detailed in situ astrobiological analysis has already been done.
That approach also presupposes that the only astrobiological value in returning a sample is determining whether life is or has been present. As already pointed out in this report, the astrobiology science goals for Mars are much broader than this, and researchers can make major progress in understanding the history of Mars, its volatiles and climate, and its geological and geophysical history by returning samples.
Related to the question of the right sample is the number of sample-return missions likely required to achieve the identified scientific goals. While some groups have attempted to estimate appropriate values for this number,25 programmatic and fiscal realities are likely to dominate purely scientific considerations. Returning samples from Mars will cost many billions of dollars, and the history of NASA’s space science activities clearly demonstrates that each major scientific community gets only one multibillion-dollar mission per decade. Thus, the only realistic number of sample-return missions that can be contemplated within the predictable time horizon is one.
Finally, reasoned and thoughtful determinations can be made today concerning where to obtain samples that would have good chances of providing valuable information on whether life is or has been present. These include likely sites of longstanding liquid water and of the geochemical potential for life. While additional data always will provide better information, sufficient data already exist to choose important sites.
The combination of the high astrobiology science value in returning samples, the seemingly impossible task of defining a single “right sample,” the likely number of sample-return missions, and the ability to choose sites today that have a high potential for providing details about life suggests that the appropriate strategy is to return samples at the earliest possibility rather than continuing to wait for more data.
The selection of promising landing sites using orbital data is not infallible, and in all likelihood surprises will be encountered at every landing site. The MERs have demonstrated the ability to traverse significant distances and to analyze the diverse materials encountered on the surface and in the subsurface (exposed in craters). That demonstrated ability suggests a plausible strategy for collecting samples to be returned to Earth: Utilize all the highly mobile, well-instrumented rovers (and possibly stationary landers) in the NASA Mars exploration strategy to cache collections of interesting samples for possible future sample-return missions. A subsequent sample-return mission could then land near (possibly guided by a beacon on the cache) and retrieve a sample cache, perhaps using a tethered rover (minimizing mobility, communication, and navigation requirements) and eliminating the need for analytical instruments to assess the nature of the samples. The storage of samples should involve very simple mechanisms. Such a strategy would increase the likelihood that astrobiologically interesting samples could be obtained, and it obviously would decrease the cost of a sample-return mission (or multiple missions) significantly.
Planning for the 2009 Mars Science Laboratory is too far advanced to allow adding even a simple sample-caching capability, but this strategy should be considered for any subsequent rover (or lander, if it is to acquire subsurface samples). It may also be desirable to coordinate with the European Space Agency (ESA), to see if this strategy could be adopted for other (non-NASA) Mars rovers and landers.
While sample caching can help facilitate a Mars sample-return mission, this strategy may, according to some observers, raise issues relating to the implementation of current planetary protection policies. These concerns can be divided into those associated with forward contamination and those associated with back contamination. The former concerns arise in particular because of the potential for contaminating the martian materials collected with organisms that hitchhiked from Earth in the sample-acquisition and sample-handling systems or the caching container.26 The latter concerns are associated with ensuring that the caching container, the samples within it, and any other spacecraft subsystems exposed to the martian environment and scheduled for return are completely sealed within the Earth-return canister prior to ascent from the surface of Mars.27
None of these concerns is unique to a sample-caching strategy. Indeed, all Mars sample-return scenarios must contend with all of the above planetary protection issues. Caching causes complications because some of the procedures that would only have to be implemented on a sample-return mission must now be implemented on the preceding missions that are caching samples for later return to Earth. Of particular concern is the need to ensure that the sample acquisition and handling systems undergo appropriate bioload-reduction prior to launch. If this is not done then martian materials might be contaminated with organisms from Earth as they are collected
and might later cause false-positives during subsequent biological examination in a terrestrial laboratory. Yet the necessary bioload-reduction procedures are likely to be similar to those that would be required if the same sample acquisition and handling system was processing materials for an in situ detection experiment.
NASA’s 1995 report An Exobiological Strategy for Mars Exploration advocated sample return only after Mars had been thoroughly studied at global and regional scales, and following detailed local investigations that would ensure that any returned samples would have astrobiological relevance. NASA has implemented those recommendations in its exploration planning, with the unfortunate result that Mars sample return has been repeatedly pushed to (or beyond) the end of any planning cycle. The committee suggests that the strategy advocated by the 1995 report should not delay sample caching at multiple sites; NASA should cache samples at every opportunity, and return the most interesting collection as expeditiously as possible. The scientific advantages for astrobiology of a diverse collection of rocks and soils returned to Earth from any promising site outweigh the promise of just the “right” sample at some time in the indefinite future.
This is not to say that continued characterization of the global environment and of regional and local environments is not critical to the selection of astrobiologically relevant sites. An understanding of the regional geological context of the samples at any particular site is essential for interpreting their formation histories and thus the martian paleoenvironments represented by these samples. The synergy resulting from ground-based observations and measurements, coupled with orbital data (sometimes obtained synchronously), has proven critical in investigations by the twin rovers of the MER mission at Meridiani Planum and the Gusev crater. The current suite of orbiting spacecraft at Mars (NASA’s Mars Odyssey and Mars Reconnaissance Orbiter and ESA’s Mars Express) have already provided and will continue to supply global and regional mapping data for Mars at unprecedented resolution. As is evident from the ongoing site selection process for the 2009 Mars Science Laboratory, there are numerous promising candidates for sites of astrobiological interest on Mars based on the available mapping.
STUDIES OF EARTH ANALOGS FOR ENVIRONMENTS ON MARS THAT MAY HARBOR LIFE
The search for extraterrestrial life begins here on Earth, with studies of both ancient rocks containing the earliest traces of terran life, and modern systems that serve as analogs for conditions and environments present on modern-day and ancient Mars. Studies of these Earth analog systems and also other Earth environments characterized by conditions of extreme temperature, pH, radiation, salinity, water activity, and so on, have greatly expanded the known range of environments that harbor life and the metabolic pathways and conditions under which life can thrive. Earth analog studies are primary drivers of astrobiological research and are also crucial for defining zones of habitability and targets for astrobiological exploration in extraterrestrial systems. However, it must be emphasized that terrestrial locations can only be used as analogs of various aspects of Mars, not as perfect analogs of an entire martian environment. Thus, when using a terrestrial analog, it is important to understand what might be similar and what might be different, and to ensure that the differences do not affect the analysis for which the analog is being used.
Studies of Earth analog sites should continue to be a fundamental aspect of Mars astrobiological research, because they provide several critical functions:
Provide Mars-like environments for testing and development of instrumentation (e.g., via investigations supported by NASA’s Astrobiology Science and Technology for Exploring Planets and Astrobiology Science and Technology Instrument Development programs) to be used on missions and a testbed for crucial real-time problem solving during missions (e.g., recent MER mobility issues).
Allow testing of sample-collection and sample-handling protocols under Mars-like conditions, an often overlooked but critical aspect of Mars astrobiology exploration.
Provide environments for development and validation of biosignature techniques, including establishing baseline abiotic signatures and assessment of biomarker preservation potential and alteration.
Allow discovery of novel organisms and metabolisms and the chemical/isotopic imprints of these metabolisms on Mars-like environments. Recently identified metabolisms—not all necessarily from Earth analog sites—include nitrate-Fe respiration, H2O radiolysis, and ammoxidation-PO3– respiration.
Extant life at or near Mars’s surface would have to be able to endure present-day conditions of low temperature, low water activity, and intense radiation, as well as a lack of organic matter (unless methane is a factor). Such organisms may manifest only briefly, seasonally and in restricted zones where liquid water exists. Subsurface environments, such as aquifers, groundwater, and the sediments discussed above, would still be characterized by low temperature.
Cold, High, and Dry Environments
Several studies have been conducted in Earth environments with conditions closest to modern-day conditions on Mars, including high-altitude regions in South America (Altiplano) and Australia (Paralana Springs) that receive intense ultraviolet radiation, the Arctic (Svalbard/Spitsbergen-Arctic Mars), Antarctica’s Dry Valleys, the Atacama Desert, basaltic rocks in the cold, dry deserts of Idaho and Oregon, and geological terrains that contain Earth’s oldest rocks and earliest traces of life. The types of microorganisms associated with these environments span a range of phylogenies but are largely characterized by their ability to survive desiccation and, for those exposed at the surface, ultraviolet radiation. Organisms related to Deinococcus radiodurans are one example of the type of organism commonly found in many of these types of habitats.28 The ability to withstand the damaging affects of drying and radiation, often leading to similar types of cellular damage, is a common trait. In addition to yielding information on the occurrence and survival of extremophilic microorganisms, these environments have also been used to test various analytical and sample-handling instruments proposed for in situ applications on Mars. Continued studies in Mars surface analog environments are critical for new discovery and for testing biosignature techniques and biosignature preservation.
As noted above, the extreme oxidizing conditions and high radiation flux at the surface of Mars provide conditions that are generally not conducive to extant life or the preservation of biomarkers. Under this premise, subsurface habitats may be the most likely refuge for life on Mars and may also provide conditions favorable for the preservation of biomarkers.
Some of the more intriguing analogs are deep crystalline rocks where the presence of autotrophic microbial populations is supported solely by H2 generated via abiotic reactions such as weathering of Fe(II)-bearing silicates. The concept of a subsurface lithoautotrophic microbial ecosystem, or “SLiME,” was first described for the deep basalt aquifers within the Columbia River Basalt (CRB) of south-central Washington state. The CRB microbial communities were described by Stevens and McKinley as the first discovered that are completely independent of solar-derived energy.29 The importance of the result was not that this particular microbial ecosystem was independent of the Sun. Rather, it was the fact that communities can, in principle, exist independently of photosynthesis.
Indeed, the Stevens and McKinney results were subsequently challenged by Anderson and colleagues.30 However, the original results are now supported by more recent results reported by Chapelle and co-workers following studies at another site.31 The microbial communities associated with the CRB are numerically dominated by autotrophic microorganisms, bacteria capable of growth by oxidizing H2 and fixing CO2, including high populations of acetogens.32 Subsequent cultivation-independent molecular analysis revealed that Archaea also accounted for 1 to 2 percent of the population of the CRB.33 Due to the common occurrence of similar rocks on Mars—identified by their spectroscopic and morphological characteristics—and the likelihood of liquid water in the subsurface, SLiME is an attractive analog in the search for microbial life on Mars. This same basic concept has been extended from the CRB to hydrothermal waters circulating through igneous rocks in southern Idaho and to the deep groundwater of the Fennoscandian Shield. Radiolysis of water has been implicated in the millimolar concentrations of H2 observed in the groundwater of several Precambrian Shields. These concentrations are well
above those required to support H2-utilizing microorganisms. More recently, geochemical, microbiological, and molecular analyses of alkaline saline groundwater at 2.8 kilometers depth in Archaean metabasalt in South Africa revealed a microbial community dominated by a single phylotype of sulfate-reducing bacteria (SRB) belonging to Firmicutes.34 These SRB were reported to be sustained by geologically produced sulfate and H2 at concentrations sufficient to maintain activities for millions of years. The discovery of hyperthermophilic autotrophic methanogens in conjunction with H2 at millimolar concentrations beneath an active deep-sea hydrothermal field in the Central Indian Ocean Ridge further extended the SLiME concept to hyperthermophilic marine systems.
Deeply Buried Sediments
For reasons presented above, the subsurface is the most likely environment to harbor extant life and preserved tracers of life (biomarkers) on Mars today. Deeply buried sediments such as those beneath the seafloor on Earth are another important subsurface analog system that has yielded a wealth of new information on lithologic controls on microbial activity, especially at very low organic carbon concentrations, and on the ability of microbes to persist under extremely low nutrient conditions, leading to the requirement for refined definitions of life and death for very-slow-growing microorganisms. Parkes and colleagues have established that there is a diverse and active microbial community in deeply buried marine sediments35 and that these organisms can persist for extended periods in spite of a relative lack of circulating fluids. Although relatively little is known about the phylogeny of these organisms, molecular analyses of deeply (~ 200 m) buried sediments from the Peru Margin indicate that the communities were dominated by bacteria in the gamma-Proteobacteria, Chloroflexi (green nonsulfur bacteria), and Archaea in the Miscellaneous Crenarchaeotic Group and South African Gold Mine Euryarchaeotic Group, and that the community composition changed with depth.36
Direct in situ evidence for hydrothermal activity on Mars is forthcoming; however, chemical studies of SNC meteorites suggest some history of aqueous or hydrothermal alteration in Mars’s past, and current juxtaposition of water-rich and volcanic systems combined with geomorphological evidence points strongly to the existence of hydrothermal systems on Mars today or in the past. Martian hydrothermal systems, be they cold, warm, or hot, could have created environments conducive to the development and support of life. Critical studies in hydrothermal Earth analog systems (especially the low-temperature systems found in the Arctic) include studies of the initial imprinting, preservation, and subsequent alteration of potential biosignatures; determination of fossilizable components of hydrothermal deposits that may harbor biosignatures; and characterization of biogenic patterns recorded in a suite of related chemical/isotopic measurements. Hydrothermal systems on Earth have been the sites of discovery of numerous novel extremophilic organisms as well as the most ancient organisms yet found on Earth. For example, hydrothermal systems harbor many deeply branching microorganisms such as members of the Aquificales37 as well as many anaerobic thermophilic archaea of the genera Pyrococcus, Archaeoglobus, and Methanococcus.38 Additionally, hydrothermal systems may also preserve prebiotic compounds and serve as sites of prebiotic chemical synthesis.
Acidic and Alkaline Aqueous Systems
Findings from Mars exploration over the past decade have yielded data to suggest the occurrence of evaporative sedimentary environments and both alkaline and acidic aqueous environments during Mars’s history. Accordingly, evaporative systems hosting microbial mat communities and aqueous alkaline and acid systems—groundwater and lakes (e.g., Mono Lake and Rio Tinto), and terrestrial hot springs and marine hydrothermal systems (e.g., seafloor serpentinization at Lost City, mid-ocean ridge hydrothermal deposits and seamounts)—are currently being explored for the presence of microorganisms and biosignatures preserved within the fossilizable (mineral) portions of these systems. A molecular-based study of Mono Lake microbial communities39 revealed that most of the 212 sequences retrieved from the samples fell into five major lineages of the domain Bacteria: alpha- and gamma-Proteobacteria
(6 and 10 percent, respectively), Cytophaga-Flexibacter-Bacteroides (19 percent), high-G+C-content gram-positive organisms (Actinobacteria; 25 percent), and low-G+C-content gram-positive organisms (Bacillus and Clostridium; 19 percent). Twelve percent were identified as chloroplasts. The remaining 9 percent represented beta- and delta-Proteobacteria, Verrucomicrobiales, and candidate divisions. In contrast to the alkaliphilic Mono Lake, the acidic Rio Tinto River is host to communities that are dominated by microeukaryotes.40 It is very important to study the behavior of chemical and isotopic biosignatures over the full range of possible environmental conditions identified on Mars in order to best apply biosignature methods to Mars samples, both in situ and returned samples.
With each new discovery from the exploration of Mars using orbiters and surface rovers comes the possiblility for newly defined Earth analogs. Most recently, findings from the MERs and Mars Express have expanded the suite of relevant Earth analogs to include sulfate-rich evaporite sediments, acidic aqueous systems hosting key indicator minerals found on Mars (e.g., jarosite, alunite), and Noachian-like systems.
An obvious development that would advance Mars astrobiology is precision landing of spacecraft. Landing uncertainties have decreased considerably with each landed mission (e.g., the 80- by 12-km landing ellipses of the MERs are expected to shrink to a 20-km circle for the Mars Science Laboratory (MSL)). However, astrobiology targets, such as sites of hydrothermal or fluvial activity, are likely to be small and dispersed, and better landing precision will be required to visit such locations. Similarly, implementing a robust strategy of caching samples for potential retrieval at a later date requires the development of a capability to land a spacecraft within a kilometer or less of a given point on Mars. Landing higher-mass payloads at high surface elevations requires additional development of entry, descent, and landing technology.41
The development of instruments for in situ measurements to address astrobiology goals is especially critical. Individual instruments are normally developed for specific missions, and there are few, if any, appropriate instruments at sufficient readiness levels currently “on the shelf.” Advances in appropriate technologies are proceeding rapidly, but there is a significant lag time in applying these new techniques to spaceflight missions. This stems in part from the special requirements that spaceflight imposes (miniaturization, modest power requirements, thermal and shock loading), but also from NASA’s lack of funding for instrument development, especially within the Astrobiology program.
As noted in an earlier chapter, the identification of poorly crystalline minerals or amorphous phases may require more advanced analytical techniques, building on the success of the MSL-designed ChemCam. Analysis of trace elements and stable isotope ratios in minerals and extracted organic matter, which is not possible with current flight instruments, also requires new technology. Improvements in imaging technology are needed for in situ examination of morphologies at submicroscopic scales. Especially important for implementation of the Astrobiology Field Laboratory may be further advances in sampling handling and processing dealing with the extraction and analysis of organic matter, with appropriate measures taken to ensure compliance with relevant planetary protection regulations. Analyses of gradients in chemistry and oxidation states in rocks are key measurements for astrobiology, and development of in situ methods having increased spatial resolution may be required. Other examples of instruments that could be developed for astrobiology are discussed in the section “In Situ Analyses Related to Life Detection” above.
Geochronology using radiogenic isotopes is an extremely challenging task, and remote sensing measurements are unlikely to provide precise or unambiguous age determinations. Absolute age data are necessary for assessment of a site’s geological history. Crater-density data provide only relative ages, and these may not be reliable in some cases because of cycles of burial and exhumation.42 If suitable remote-sensing or in situ techniques are not available, age determinations may require the return of samples.
Despite successes in locating global and regional subsurface hydrogen (presumably ice) using gamma-ray detection (Mars Odyssey) and potentially radar sounding (Mars Reconnaissance Orbiter), methods must be developed for local electromagnetic sounding for subsurface water at specific sites. Drilling technology or other means to access subsurface fluids and rocks may be necessary, although horizontal mobility is, perhaps, more important than vertical access.
Mars sample return requires considerable technical development, and this work cannot be postponed until a few years before launch. NASA is not currently on a path to Mars sample return, because the (admittedly daunting) technology issues have not yet been addressed. A key area that must be addressed concerns the development of the technology for entry, descent, and landing systems, both at Mars and for Earth return. Another important area is so-called go-to mobility—i.e., the ability to identify a rock and have a rover autonomously approach and collect a sample. Such a capability would provide for more efficient sampling. The MSL-designed coring device is a significant advance in sample-acquisition technology, but end-to-end sample acquisition to storage (and possibly packaging) capability must be developed. If samples are cached on the surface, as suggested above, a precision-landing capability will be needed. Retrieving cached samples, perhaps using a short-range rover with no analytical capabilities, could minimize mission complexity and cost. The development of a Mars ascent vehicle and associated mechanisms for spacecraft rendezvous, docking, and sample transfer are required if pre-collected samples are to be lofted into Mars orbit. In either case, a sample-containment mechanism that meets planetary protection requirements must be devised. Finally, Mars sample return will require that a sample-receiving facility on Earth be designed and constructed. Curated returned samples must be isolated from the terrestrial environment, not only for planetary protection, but also to preserve their scientific integrity for future studies.
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