The following discussion includes measurements required at global scales, at local specific sites, and by means of sample return, in order to understand and explore prebiotic chemistry, possible extinct life, and possible extant life. An important principle underlying the proposed strategy is that it is essential to understand the martian environment before deploying biologically specific experiments. In what follows, where specific instruments are mentioned, these should be regarded as illustrative and based on current technology; they should not be taken as excluding the possibility of new approaches and technologies.

ORBITAL EXPERIMENTS

The primary focus of global-scale measurements is to characterize and select sites having exobiological interest. The emphasis should be on estimating the size of global reservoirs of volatiles such as water, carbon, nitrogen, etc., and also on assessing the global consequences of the action of liquid water. In addition, sites are to be identified where deposits might have preserved a record of the early environment, including, perhaps, a record of an ancient biosphere. This leads to an approach that rests heavily upon the search for (a) near- surface water, in either liquid, solid, or bound form, and (b) mineralogy and morphology indicative of the presence of liquid water or of present or past aqueous mineral deposits exposed at the surface. In general, measurements are not specifically assigned to prebiotic chemistry, extinct life, or extant life because, to a great extent, the required measure-ments, and site selections, cross over among the several topics and are not distinct to any single one. Specific observations or measurements are as follows:

Global geologic mapping
Essential baseline information for any detailed exploration of Mars consists of global imaging at an appropriately high resolution (about 10 m, with selected sites imaged at about 1 m resolution) combined with corresponding topographic data.

Stereo imaging would greatly enhance the interpretation of geomorphic features and topography, and is useful as an adjunct to laser altimetry. Not only is this information required for site selection and mission planning, but geomorphologic evidence is still a key guide to the evolutionary history of specific regions of the martian surface. In particular, topography is necessary for defining the drainage patterns that have controlled the depositional environment at different sites. Consequently, a high- resolution camera and an altimeter are required.

Ages of surfaces
An important aspect of site selection based upon surface imagery in the visible range is understanding the age of the particular site that will be sampled. For example, the search for extinct life would focus on older sites, while that for extant life would focus on the younger sites. Using cratering chronology and other relative dating methods, appropriate relative ages can be determined. Imaging of specific locations at 10 m resolution would provide the required information. Note that placing this relative chronology on an absolute age basis will require highly sophisticated landers, or possibly sample return missions.

Globally mapped mineralogy
For the minerals of exobiological interest, that would be indicative of the presence of water-deposited sediments or hydrothermal systems, this can best be done with a mid-infrared spectrometer capable of measuring thermal emission between about 5 and 50 *m. Spatial resolution should be the highest possible consistent with global-scale reconnaissance (e.g., a few kilometers), supplemented by higher-scale resolution of sites of potential interest (e.g., better than 0.1 km).

Globally mapped elemental abundances
Global characterization of elemental abundances, particularly for the rock-forming elements, is a prerequisite for understanding the local-scale abundances, mineralogy, and evolution of the surface. For example, ratios of elements such as Ca/Al can be used to help identify sites where aqueous alteration of the crust might have created carbonates or clay-rich deposits. Also, elemental abundances might indicate where hydrothermal processes have played a role. Although high spatial resolution would be of immense value, measurements are limited to global scale by technique. Using gamma-ray spectroscopy, mapping can be done with a resolution equal to the altitude of the orbiter, which would be approximately 300 to 500 km.

Globally mapped near-surface water
Water in this context includes liquid water, water ice, and physically adsorbed or chemically bound water. The former might occur on small spatial scales when activated by heating as a result of volcanism, impact, or other processes. Ice in permafrost regions is a possible site for finding non-living evidence of recently living organisms, as well as a potential source for transient occurrences of near-surface liquid water. IR spectroscopic evidence for chemically bound water would usefully complement spectroscopic evidence for surface occurrences of aqueously altered lithologies. Near-surface water can be mapped on a global scale at 300-500 km resolution using neutron or gamma-ray spectroscopy.

Regions of high heat flow
An expected surface expression of hydrothermal systems and/or areas of high heat flow would be elevated surface and near-surface temperatures. These could be mapped globally using either thermal infrared or microwave observations. In either case, some wavelength measurements would be required, as would high spatial resolution. Again, the spatial resolution should be consistent with the ability to obtain global maps, and higher spatial resolution should be obtained for selected sites. The lowest useful resolution would be of the order of 100 km, while regions of interest should be mapped at 10 m resolution.

Ratios of atmospheric stable isotopes
These are of value in understanding the evolution of the volatile-element reservoirs and in distinguishing biological from nonbiological influences on isotopes. Measurements of D/H, 18O/17O/16O, 13C/12C, 15N/14N in the bulk atmosphere, in the region between the homopause and the exobase in the upper atmosphere, and in species escaping to space, are required. Observations of properties relevant to escape processes are also important, in order to understand the context of the isotopic data, as are the ratios for elements of non-biological interest such as 38Ar/36Ar and 22Ne/20Ne. The isotopic ratios would require a mass spectrometer, while the related information would require instruments of the type that would fly on a Mars aeronomy orbiter.

Regions of subsurface water
At depths greater than can be explored by neutron or gamma-ray techniques, liquid water can be detected using active and/or passive microwave techniques, especially EM sounding. Instruments that can detect the frequency response of the subsurface might be able to show the characteristic behavior of liquid water, possibly down to depths of kilometers.

Degree of mineral crystallinity
For clays, the degree of crystallinity can be used as an indicator of the intensity of chemical weathering. This may be detectable from orbit using reflectance spectroscopy, covering the wavelength range of 0.3-3.0 *m. Again, coarse-scale mapping of global properties, followed by higher-resolution observations of specific sites would be of most value.

Trace gases
Methods for determining trace atmospheric constituents, particularly if these can be made to estimate near-surface constituents, could provide clues to geothermally active areas and possible subsurface regions of biological activity. Biologically important trace gases like H2, H2S, CH4, SOx, NH3 and NOx are of particular interest in this connection.

LANDED EXPERIMENTS

These refer to in situ measurements or observations made by landers or rovers placed on the martian surface. Such observations are needed for specific sites in order to characterize surface chemistry, local geological processes and biological potential.

Preservation and texture of surface rocks
Even with careful site selection, rocks preserving a record of either extinct or extant life may be rare at a landing site, and the same is likely to be true for prebiotic chemical evolution. Consequently, a detailed assessment of rock diversity at a landing site is a necessary early step in the search for either extinct or extant life and is also of importance in the study of prebiotic chemical evolution. Imagery with sub-mm spatial resolution would be required, thus putting a premium on mobility in order to bring the instruments as close as possible to the target rock. Proper characterization of rocks at a landing site would require mobility within a 10- to 100-m radius of a lander. Regional characterization would require mobility on a multi-kilometer scale.

Elemental abundances of surface materials
In addition to imagery, chemical character-ization of the materials at a local site is fundamental. Of interest are the elemental abundances in surficial deposits of fine materials and in rocks. This would focus on the rock-forming elements and carbon and can be done with X-ray fluorescence spectroscopy or alpha-proton-x-ray experiments. Some data on major rock-forming elements can be obtained by means of gamma-ray spectroscopy, coupled with data on naturally radioactive elements and hydrogen, i.e., water. Sensitivity should be of the order of 0.1 wt%. Again, an understanding of the diversity of composition among surface materials will be of major importance, particularly in the assessment of aqueous chemical activity and the search for evidence of extinct or extant life.

Near-surface water abundance
Because of the intimate connection between water and any plausible martian biology, it will be of importance to determine the abundance of hydrogen at any sites to which we obtain access. In most cases, this water will be present in chemically combined form as a hydrated lithology, though it may be possible to find a location where subsurface ice is accessible by drilling beneath a landing site. Alternatively, penetrators may be used to probe beneath the martian surface. Hydrogen abundance can be determined using either passive neutron or gamma-ray spectroscopy or pulsed-neutron gamma-ray spectroscopy, as is used for logging oil wells on Earth. These techniques detect hydrogen within about half a meter of the detector.

Mineralogy of surface materials
Materials that have been altered by hydrothermal activity or weathering sometimes have elemental abundances that are very similar to the unaltered materials. For this reason, specific determination of mineralogy is important in the search for evidence of aqueous processes and for potentially fossil-bearing lithologies such as carbonates, cherts, evaporites or phosphates. This can be done using an infrared spectrometer to do a quick survey of the materials at a given site (with ability to isolate specific small-scale features on the surface, for example with a spot size 1 cm across at several meters distance from a lander), followed up by x-ray diffraction/fluorescence on individual samples. The latter step may require excavation of samples from the interior of rocks. An additional goal of mineralogical investigations on the martian surface is the search for minerals that might have been produced as a result of biological processes, such as phosphates, manganese oxides, and certain carbonates.

Distribution of the surface oxidant
It is important to map the distribution, in three dimensions, of the oxidant(s) identified on the martian surface by the Viking mission. The goal will be to find oxidant-free regions, either at depth in the regolith or at locations where pristine material has been exposed too recently for the oxidant to be present. On a microscale, one possible oxidant-free environment might be the interiors of aqueously altered sedimentary rocks. The first step in determining the distribution of the oxidant(s) is clearly to define its/their chemical nature. This can be achieved by deploying on the martian surface a series of sensors designed to be sensitive to specific oxidants. Probing the vertical distribution of the oxidant(s) will presumably require drilling into the regolith, whereas determining the horizontal distribution will probably involve some kind of compound-specific analysis whose character will depend on the chemical nature of the oxidant(s). Ideally, a chemical signature would be sought whose global distribution could be determined from orbit.

Physical/chemical characterization of the microenvironment
To understand the conditions for survival of putative extinct or extant life forms, a number of physico-chemical measurements must be made. These include assaying the available chemically reactive species in the upper surface, as well as the nature of the environment when moistened or wetted, including pH, Eh (oxidizing potential), ionic strength, presence of micronutrients, and other aspects of the soils and soluble minerals.

Stable isotopic measurements of surface materials
Determination of stable-isotope ratios for the biogenic elements (C, H, O and N) in surficial mineral deposits, e.g., evaporites, provides an additional constraint on volatile history and reservoirs. However, such measurements would probably require significant sample preparation prior to mass spectrometry.

Presence of organic carbon
A stepwise approach is preferred. At the first level, a procedure for quantitative analysis of organic (= noncarbonate) carbon is needed. A system employing a reactive carrier gas and a carbon-sensitive detector should be adequate. Additional information could be obtained by employing temperature-programmed techniques that provided information about temperature of pyrolytic release or combustion and about energy produced or consumed by such processes.

Elemental and isotopic analyses of bulk organic material
If any organic material is found, it is likely that characterizable molecules will be rare relative to total organic carbon. Moreover, most techniques of molecular analysis are applicable to substances with particular levels of polarity or types of functional groups, and these will not be known in advance. For both of these reasons, a second stage of organic analysis should focus on the elemental and isotopic composition of bulk organic material. The elemental information, in the form of atomic ratios, will allow optimization of subsequent molecular techniques, and knowledge of isotopic compositions (for nitrogen and hydrogen as well as carbon) will be of immediate and independent interest, since they will provide information on the origin of the organic matter. A robotic variant of the conventional laboratory procedure of combustion, gas purification and mass spectrometry seems the most likely approach.

Molecular identity of organic carbon
Spectroscopic instruments capable of providing information about bond types and even specific molecular identities should be flown when evidence for analyzable species is found. Resulting data would yield important information about synthetic mechanisms, in the case of prebiotic evolution, and about possible biomarkers, in the case of extinct or extant life. Key compound classes for which evidence should be sought include lipids, amino acids, and carbohydrates. The analytical system should include chromatographic or other techniques of separation. New technologies like the polymerase chain reaction, and variations of it, may provide a basis for amplification of genetic material (and thus increasing sensitivity), and with appropriate experimental design, might provide simple automated tests which would be highly informative. While these approaches involve major assumptions about the nature of martian life, they are becoming automated and miniaturized to the point that they should be included in such studies.

Biomarkers at the poles
With respect to geochemical measurements at the polar ice cap, coring, sampling and detection of entrained gases (CH4, H2, H2S, etc.) would be important. If life ever exerted a global biogeochemical effect on the planet, and if the polar ice cap has trapped this record, it should appear. Similarly, the polar deposits should be examined for microscopic evidence of biotic activity elsewhere on the planet.

Gaseous biomarkers
In addition to measurements in polar regions, collection of data on biogeochemically significant gases with landed detectors also capable of measuring wind direction and speed might also permit locating point sources of gas emanation, though this would probably be best done using a long-range rover. Molecular analysis of these gases would probably be best performed using compound-specific sensors, many of which are already available. Of course, stable isotopic analyses of these biogenic elements would also be desirable though more difficult to achieve.

Sample acquisition
In addition to the analytical experiments that can be deployed on the martian surface, it is important not to overlook the question of procedures whereby a series of martian samples can be delivered in suitable form to an analytical device. For specifically exobiological experiments, this aspect of surface science takes on particular importance because of the necessity of penetrating whatever barrier has permitted preservation of an organic record in an environment as generally hostile as that of the martian surface. Sampling procedures can be divided into four categories. The first, and simplest, is the scooping of a regolith sample and its delivery into a hopper, as was done on the Viking landers. The second type of sampling approach involves the removal of a coherent fragment from within a rock. This technology is not yet available for space-borne experiments, but would presumably involve coring or chipping by a device mounted on a rover arm.

The third type of sampling procedure is the retrieval of a subsurface sample from within the regolith. This is one of the most commonly considered approaches to evading the pervasive surface oxidant. We follow the example of most other workers in this area and identify a rotary drill-core as the logical approach to this problem, but other possibilities such as the use of penetrators should not be overlooked. The depth to which such sampling will be needed is not yet known; some workers believe that as much as ten meters may be necessary. Robotic drills with about one-meter capability were used on the lunar surface by the Russian Luna and Lunokhod spacecraft.

Finally, it may be necessary for some applications to consider the feasibility of performing certain specific operations, such as preparing a flat, or even polished, rock surface, or cutting a thin section of a rock. Suitable technologies for these requirements are not yet available for use on planetary spacecraft.

RETURNED SAMPLES

For many reasons it will be desirable, and probably necessary, for definitive experiments of exobiological significance to await return of appropriate martian samples for analysis in terrestrial laboratories. These should include a sample of pristine martian atmosphere in addition to lithic material, to permit more accurate chemical and isotopic analyses of gaseous species, particularly those present in only trace amounts.

Among the more important reasons cited for the importance of sample return are that many different methods can simultaneously be brought to bear in the analysis of one sample; that sophisticated instrumentation readily available in ground-based laboratories would be difficult (and expensive) to develop for use on Mars' surface; that, in any case, the latest and best techniques would be available in ground- based laboratories, as opposed to techniques that needed to be developed for spacecraft years before the instrumentation could actually be deployed; that conditions can be much more rigorously controlled; and that this approach allows for flexible responses to any surprising results that may arise during examination of the martian material.

Efforts to detect metabolic activities or to cultivate the organisms responsible for these activities would certainly be made, but specific approaches cannot be detailed without a knowledge of the specific features of sites from which samples were obtained. An important lesson from recent research in microbial ecology is that we have done rather poorly in cultivating terrestrial micro-organisms. Thus, it would be appropriate also to consider seriously various types of culture-independent analysis to characterize the extant martian biota. Other obvious issues related to conducting such analyses on returned samples include planetary protection and the potential for such organisms/activities to survive transit to Earth.

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