An Exobiological Strategy for Mars Exploration (Part 4)
PLANNED MISSIONS: ANTICIPATED MEASUREMENTS AND OBSERVATIONS
This section is based on the mission scenario currently envisioned by the IMEWG. It is a plan for launching probes to Mars at every launch opportunity from 1996 to 2003. The plan assumes multiple launches, with the U.S. component being two launches at each opportunity so that failure or delay of one element will not result in a total failure for that opportunity. The plan also assumes that missions currently in their implementation phase will be launched as planned. The early missions are predominantly U.S. and Russian, so the scenario is strongly dependent on sustained funding for Mars exploration in both the U.S. and Russia.
THE 1996 OPPORTUNITY.
Three missions are approved for launch in 1996: the U.S. Mars Global Surveyor (MGS) and Pathfinder missions, and the Russian mission, Mars-96 (formerly Mars-94). The first of these is designed to recover part of the Mars Observer objectives.
Mars Global Surveyor
The U.S. MGS, the first element of the proposed multimission Mars Surveyor Program, is an orbital mission intended to recover part of the Mars Observer science objectives. However, the spacecraft, to be launched on a Delta, is considerably smaller than the Mars Observer spacecraft and the original payload cannot be fully accommodated. MGS will carry the following instruments of relevance to exobiology:
Camera. A camera incorporating both wide-angle and narrow-angle capability will provide planet-wide surface imagery and monitor global atmospheric and surface changes at 7.5 km resolution, partial coverage of the surface at a few tens of meters resolution, and selected targets at 1.4 m resolution. These cameras will provide some of the necessary information for geomorphology, geologic mapping (including establishment of relative chronologies), site selection and mission planning.
IR spectrometer. A thermal emission (mid-IR) spectrometer will map variations in the mineralogy of the surface and obtain temperature profiles of the atmosphere. This will permit identification of important surface lithologies of interest to exobiology, including evaporites and those characteristic of hydrothermal activity, mapping their global distribution at about 3 km spatial resolution.
Altimeter. A laser altimeter will determine surface topography. These data will permit definition of drainage divides and basins, necessary for evaluating the potential for aqueous sedimentary deposits. These data are also of crucial importance for planning of future landed missions, because topography places constraints on engineering and systems for landing.
Radio science. A multi-purpose radio-science experiment is expected to yield a greatly improved determination of the martian gravity field, needed for accurate interpretation of the altimeter data. It is also envisioned that the spacecraft would carry a communication antenna to relay back to Earth data from the Mars-96 penetrators and small landers (see below).
Pathfinder (Lander)
The U.S. Pathfinder mission will be launched on a Delta. It is primarily an engineering demonstration. Its main objectives are to develop and demonstrate a low-cost entry, descent and landing system that could be used for subsequent missions. It does, however, carry some instruments of scientific, and even exobiological, interest and will be landing in exobiologically interesting terrain. The primary landing site for Pathfinder is located on Chryse Planitia near the terminus of Ares Vallis (Latitude 20 deg. N Longitude 34 deg. W). The periodic and catastrophic outflows that created these channels could have transported detrital thermal-spring materials downstream from the thermokarst sources located in areas of chaos near their head reaches, depositing them at the lander site. This type of “grab bag” site has the advantage of providing access to a wide variety of lithologies from the surrounding regional geologic terrain, and in the case of Ares Vallis, may also provide samples of aqueous minerals that have a high priority for exopaleontology.
The instruments carried on Pathfinder include the following:
Camera. This will provide color stereo images of rocks near the lander, at about the same resolution as the Viking Lander cameras, but with 12 color filters for each camera rather than the 6 filters of Viking. These filters will permit a limited assessment of mineralogy, especially for iron oxides and pyroxene. In addition, there will be two filters for water-vapor detection and one for atmospheric dust.
Magnetic susceptibility experiment. In principle, this could supply some information about the mineralogy of the regolith close to the lander.
Pathfinder (Rover)
Mars Pathfinder will also carry a small, solar-powered rover that will be able to move within several to tens of meters from the lander, limited mainly by line-of-sight communications. The rover will carry the following instruments:
Camera. The camera used for navigation of the rover will also be used for scientific imaging. This will permit examination of local lithologies at a resolution of about 3 mm at 1 m range, giving some information about rock textures and fabric.
Alpha-proton-x-ray spectrometer (APX). This instrument can be placed in close contact with rocks and soils reachable by the rover, permitting analysis of the surface regions of those lithologies for the major rock-forming elements (O, Na, Mg, Al, Si, S, K, Ca, Ti and Fe) and carbon.
Mars-96 (Orbiter)
The Mars-96 spacecraft is to be launched on a Proton. It consists of an orbiter, two penetrators and two small landers. The penetrators and landers will be released to the surface shortly before the orbiter is injected into Mars orbit. The orbiter will carry a wide array of instruments of exobiological relevance:
Cameras. Three separate cameras are designed, respectively, for navigation, for low-resolution surface and atmospheric monitoring, and for high-resolution color and lower-resolution stereo surface mapping. As with Mars Global Surveyor, these images will provide basic information about martian geology, and will be invaluable for site selection and mission planning. Optimum spatial resolution will be ~10 m/pixel on the ground, with stereo “pixel height” resolution of ~20 m.
UV, visible and IR spectrometers. A variety of such instruments will provide information on the global distribution of major lithologic units, including aqueous mineral deposits. Spatial resolution of the visible and IR imaging spectrometer will be between 0.4 and 4.0 m/pixel over the spectral range 0.32 to 5.2 m.
IR radiometer. The mapping radiometer will operate over a spectral range of 8.5 to 12 m at 0.1 km/pixel and will map thermal anomalies at a resolution of 0.5 km/pixel.
Long-wave radar. This experiment will probe beneath the martian surface in a global search for subsurface water ice. It will operate with a range measurement uncertainty of 3.5 km.
Neutron and gamma-ray spectrometers. These instruments will map variations in the surface distribution of a number of key elements (H, C, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Fe, Th and U). Although the spatial resolution of this experiment is poor, essentially equal to the height of the spacecraft above the planet’s surface, information on the global distribution of water is of fundamental importance to exobiology, as well as to many other disciplines. The data for the rock-forming elements will usefully complement the mineralogical mapping performed by the UV/Vis/IR spectrometers as well as being necessary for accurate derivation of water abundance from the spectrometer data.
Neutral mass spectrometer. This is designed to determine the elemental and isotopic composition of hydrogen, oxygen neon and argon in the upper atmosphere. These data, in conjunction with a variety of plasma experiments designed to characterize the solar wind and its interaction with the planet, will aid in reconstruction of the long-term evolution of the light-element inventories on Mars.
Mars-96 (Landers)
The objectives of the small landers are to determine the vertical structure of the atmosphere at the landing sites, to determine the chemistry of the materials at the sites, and to make prolonged magnetic, seismic, and meteorological measurements. To meet these objectives each lander carries a variety of instruments, of which the following will generate data of exobiological relevance:
Alpha-proton-x-ray (APX) spectrometer. Similarly to the case for the Mars Pathfinder rover, this instrument will yield the abundances of the rock-forming elements, and carbon, in the close vicinity of the lander. However, on Mars-96, resulting data will be mainly derived from regolith samples rather than from individual rocks.
Mars oxidant experiment (MOx) . By exposing a series of coated fiber-optical detectors to the regolith close to the lander, the MOx will attempt to study the oxidant(s) apparently responsible for elimination of organic matter from the upper martian regolith.
Mars-96 (Penetrators)
Each penetrator consists of two parts. A forebody will penetrate the surface to a depth of a few meters carrying with it several instruments of interest to exobiology:
Gamma-ray spectrometer. This will measure the concentrations of a number of elements (H, Na, Mg, Al, Si, K, Cl, Ca, Ti, Mn, Fe, Th and U) in the regolith within a meter or so of the penetrator.
Neutron spectrometer. This is designed to monitor the water content of the regolith within about 30 cm of the penetrator.
APX. This instrument will measure the composition of the regolith adjacent to the penetrator for the elements listed above for the lander APX.
The aft body, which remains at the surface, contains several instruments of which two are of particular exobiological relevance:
Camera. A camera operating in the visible range will image the terrain surrounding the impact site and local lithologies exposed at the surface near the penetrator.
Gamma-ray spectrometer. This will analyze the martian surface adjacent to the penetrator for the elements listed above in connection with the forebody instrument.
In conclusion, Mars-96 is a very comprehensive mission addressing a broad range of science questions, and making a variety of pilot measurements on the surface that will be useful for the design of subsequent more definitive experiments. It is also an international mission, the instruments being provided by a large number of nations.
THE 1998 OPPORTUNITY
Planet-B
In 1998 Japan plans to launch Planet B, its first nonlunar planetary mission. This will be a Mars mission launched on an M-5 rocket. The objectives of the mission are two-fold: (1) to study the martian upper atmosphere and its interaction with the solar wind, and (2) to develop technologies for future planetary missions. The scientific objectives are threefold. The first set of experiments will measure the structure, composition and dynamics of the ionosphere, the effects of interaction of the upper atmosphere with the solar wind, and the escape of atmospheric constituents. The second set of experiments will measure the intrinsic magnetic field, the penetration of the solar-wind magnetic field, and the structure of the magnetosphere. The third set of experiments will measure dust in the upper atmosphere and in orbit around Mars. Numerous instruments, which need not be detailed here, will be employed to perform this wide range of measurements, but of particular exobiological interest is a U.S.-supplied neutral mass spectrometer which will measure the elemental and isotopic composition of the upper martian atmosphere.
Mars Surveyor
The payload of this U.S. orbiter has yet to be determined but there is a strong desire for it to carry the two Mars Observer experiments not included on Global Surveyor:
Infrared radiometer (PMIRR). This instrument will map the three-dimensional structure of the martian atmosphere, including the distribution of water vapor, and will follow the transport of this water throughout the current martian system for a martian year. It has the capability to discover localized sources of water on the martian surface, if such exist.
Gamma-ray/Neutron spectrometer (GRS). As in the case of Mars-96, this instrument will map the chemistry of the surface, including the all-important distribution of near-surface hydrogen, i.e., hydrated minerals or ice. The sensitivity of this spectrometer for near-surface water is likely to be somewhat greater than that of the Mars-96 instrument.
The orbiter will likely also carry a camera, an ultrastable oscillator and a relay antenna to service landers on the surface.
Current NASA plans call for launch of the 1998 Mars Surveyor orbiter on the proposed Med-Lite launch vehicle. The limited throw weight of this vehicle would probably restrict the payload to only one of the remaining Mars Observer instruments. If such an unfortunate circumstance transpires, exobiological priorities would strongly favor selection of the GRS.
Neolander
A Pathfinder-derived Mars lander is currently being considered by NASA for the 1998 opportunity. This “Neolander” would be sufficiently reduced in weight (175 kg landed mass vs. 270 kg for Pathfinder) so that it could be launched on a small launch vehicle, a Med-Lite instead of a Delta. The capabilities and instrumentation have yet to be defined, but the mission theme is planned to be Volatiles and Climatology. This could result in measurement of light-element isotope ratios and volatile compounds in the regolith, both topics of significant exobiological interest.
Joint U.S.-Russian Activities
In addition to the above missions, the U.S. and Russian space agencies are exploring possible cooperative arrangements for Mars opportunities from 1998 on. Details of these activities remain to be defined at this time, but exobiology may well be one of the topics of Mars exploration of mutual interest to both parties.
THE 2001 OPPORTUNITY
The possibilities for 2001 are being evaluated. The U.S. Mars Surveyor program calls for at least two launches to Mars at every launch opportunity for the life of the program. The current plan is for the U.S. to use Med-Lite launches to place a network of small meteorological stations on the martian surface.
There is a similar uncertainty with respect to Russian plans. A Proton could be used to launch an as yet undetermined combination of previously developed vehicles.
THE 2003 OPPORTUNITY
2003 is the first opportunity at which an international network of multiple stations could be established on the martian surface. The ESA Intermarsnet concept would use an Ariane 5 booster to launch a Rosetta-derived carrier to Mars. The ESA carrier could deliver an ASI-furnished communications satellite into orbit around the planet and four landers built by an international consortium would be deployed on the surface. At the same time the Russians are exploring the possibility of using a Proton to send four Mars-96-derived small stations and two penetrators to the surface, and the U.S. could send on a Med-Lite, two Mini-geolanders similar to those developed for the 2001 opportunity. Thus the possibility exists of having 12 simultaneously operating stations on the surface, thereby achieving the seismology and meteorology goals of a global network, and adding substantially to the number of sites at which we would have in situ measurements and observations.
A preliminary set of science goals has been established for the ESA-launched landers. The internal structure and dynamics of the planet will be determined by a network of simultaneously operating, broad-band seismometers, flux-gate magnetometers and transponders. The dynamics of the atmosphere and the atmospheric boundary layer will be studied using a network of simultaneously operating meteorology stations, and the vertical structure of the atmosphere at each landing site will be determined during descent. The geology and geochemistry of the landing sites will add further to our knowledge of Mars’ diversity. All these objectives would be significantly enhanced if the ESA-launched probes were supplemented by or complemented by Russian- and/or U.S.-launched landers.
NECESSARY MEASUREMENTS/OBSERVATIONS MISSING FROM CURRENT PLANS
Current mission plans, either approved or proposed, of the U.S., Russia and Japan go a long way towards satisfying requirements for the early stages of the exobiological exploration of Mars. In particular, much of the global information required by exobiology will be met by existing mission plans, with two caveats. The first is that it is not yet certain that the GRS will be included in the 1998 orbiter. The second is that the present design of mid-IR spectrometer, TES, is restricted in its spatial resolution to about 3 km. This may well be inadequate for detection of outcroppings of aqueous mineral deposits by means of their characteristic mineralogy, a major goal in the survey phase of exobiological exploration. Also, there is currently no plan for instrumentation which could detect contemporary volcanic or hydrothermal activity, such as the release of characteristic gases (e.g., H2S, H2, SOx, HCl).
Among landed-science plans, there are more-conspicuous shortcomings from an exobiological perspective. Most serious of those, on a short to intermediate timescale, is the lack of a capability for either in situ mineralogical identification or the acquisition of subsurface samples from either regolith or rocks. Neither of these represents a uniquely exobiological requirement, in fact the former would presumably be of widespread geological value. It is important to note that both of these capabilities will be needed early in the landed-science phase, to aid in planning of subsequent missions.
It is also notable that no specifically organic analysis is included in present mission plans. Ultimately, analytical instruments of considerable sophistication will be needed for characterization of any martian organics that may be present. However, before such narrowly focused experiments are deployed, we will need instrumentation capable of both identifying oxidant-free locations on the martian surface and then assaying them for the presence of organic matter. These capabilities will be needed on an intermediate timescale.
We conclude that current mission plans represent a generally sound basis for the progressive exploration of martian exobiology, but with a number of important exceptions. We identify the most immediate instrumentation needs as the following: Improvement in the spatial resolution of orbital mid-IR spectrometry; Design and construction of instrumentation for in situ mineralogical identification, description of the microenvironment (including oxidant distribution), and detection and characterization of organic matter; Development of a capability for acquiring samples from a depth of several meters in the regolith and from the interiors of rocks, and; Early deployment of the above.
Note that the above assessment is based on the assumption that all of the missions discussed earlier fly as planned. This may well be an overly optimistic assumption. In our opinion, the key experiments most vulnerable to programmatic changes are the orbital and landed gamma-ray spectrometers, and the EM sounders. If these fall victim to changes in the present plans, we urge their early incorporation into alternate missions.
SITE SELECTION
INTRODUCTION
Mars site selection, or knowing where to go on Mars to accomplish particular landed-science objectives, is fundamental for sound planning of future exploration efforts for exobiology. In the broadest sense, the search for martian life is guided by what we perceive to be the basic requirements for the existence of living systems, or for their preservation as fossils.
Site selection for Mars exobiology will be discussed in terms of the search for extant and fossil life. However, it should be noted that the same rationale that applies to the search for fossils on Mars, also holds for a prebiotic chemical record. On the Earth, this early precursor record has been lost, recycled by weathering and tectonics, and destroyed by emerging life. But such compounds are much more likely to have persisted within the more stable martian crust, particularly if life failed to emerge on Mars. In exploring Mars for precursor organic compounds, the most important targets are the same as those identified for fossil life, namely stable aqueous mineral deposits in ancient cratered terrains. Such deposits may have sequestered and preserved such prebiotic organic molecules for several Gyr. Thus, even if martian life never developed, aqueous minerals, and especially the fluid inclusions they contain, remain as primary targets for exobiology.
The fundamental requirement for living systems is liquid water. Without it, metabolic activities of living cells would be impossible. Thus, to a large extent we equate the search for extinct or extant life with the search for liquid water, past or present. Given present martian surface conditions, stable, or non-transient, liquid water today can only be deep beneath the surface where higher temperatures and pressures favor its stability. But the deep subsurface of Mars is unlikely to be accessible for some time given the logistics and cost of “deep” (>few meters) drilling. Consequently, exobiological site selection is driven by the search for environments favorable either for the preservation of ancient life or for natural processes that bring extant subsurface life into surface environments.
HIGH RESOLUTION ORBITAL IMAGING
In planning for upcoming missions, the most pressing need is the selection and prioritization of targets for high-resolution orbital imaging. Terrestrial analog studies indicate that a resolution exceeding 30 m/pixel will be needed to resolve many important lower-order geomorphic features on Mars. Martian landforms visible at nominal Viking resolution (~200 m/pixel) are at a scale much larger than comparable features on Earth. To what extent this reflects major differences in process or small differences integrated over long time spans, is presently unknown. Thus, present site priorities are likely to need refinement as higher-resolution imaging becomes available for key sites and the criteria for geological processes and duration are reassessed.
Once exobiology sites have been imaged from orbit at high resolution, the goal is to search for appropriate aqueous mineral deposits using visible imaging to refine geological interpretations, and infrared and gamma-ray spectroscopy to interpret mineralogy. Such data will provide a basis for refining site priorities for future landed missions and for developing site-specific exploration strategies to explore for evidence of life. In developing exploration strategies for landed missions, it is important that we incorporate predictions from depositional facies models based on studies of terrestrial analogs, particularly during the early stages of exploration.
DURATION OF HYDROLOGICAL SYSTEMS
Prioritization of targets for orbital imaging is an important aspect of site selection because we will not be able to image all areas of interest at high resolution. In prioritizing sites for orbital imaging, emphasis is placed on geological features considered to be indicative of prolonged hydrological activity, as well as depositional processes that favor the long-term preservation of biological information. Table 1 provides an example of a comparative framework that emphasizes the level of integration of water-related geomorphic features within drainage basins as a basis for assessing the relative duration of hydrological systems in fluvial-lacustrine settings. We are presently limited in using this approach because we are not really sure how analogous martian landforms are to those on Earth. In part this stems from a general lack of high-resolution imaging at critical sites and an inability to resolve many smaller-scale landforms that may be critical to interpretations of process and duration. As higher resolution imaging (<30 m/pixel) becomes available, it is important that we carry out comparative geologic studies of key sites to evaluate relative duration. Of course, our estimates of duration based on comparative geomorphology will eventually need to be calibrated to absolute time scales based on radiometric dating of martian samples. Table 2 lists a number of high-priority sites representing a range of geological settings and exploration objectives.
HIGH-PRIORITY SITES FOR EXTINCT MARTIAN LIFE
In selecting sites for exopaleontology (i.e., the search for fossils or biomarkers), priority is given to landing sites in ancient terrains where hydrological systems involving liquid water appear to have been long-lived and which exhibit a high probability of having surficial aqueous mineral deposits. We also give preference to mineral deposits that are likely to have had a long residence time in the martian crust, namely, those that are diagenetically stable and resistant to chemical weathering. Examples include silica (as chert) and apatite (as phosphate). It should be noted that fine-grained detrital sediments also provide a suitable host for fossils and organic matter. Especially favored are clay-rich sedimentary deposits formed in environments that were reducing, with rapid sedimentation and compaction, and where permeability was further minimized by early cementation. Given the absence of plate tectonics on Mars and the attenuated hydrological cycle there, ancient terrains (>3.0 Gyr) are probably much more widespread and better preserved than on Earth.
In exploring for evidence of an ancient biosphere, site selection is guided by what we have learned about fossilization processes through studies of ancient (Precambrian) rocks on Earth, and studies of terrestrial environments regarded to be good analogs for the early Earth and Mars. Because the development of a martian biosphere in surface environments is likely to have been interrupted very early (~3.0 Gyr), we believe the microbial record of the Precambrian provides a reasonable proxy for Mars, allowing for obvious differences in geological history and environment.
The Precambrian terrestrial record reveals that the long-term preservation of microbial fossils requires rapid entombment of organisms by fine-grained aqueous mineral phases that are stable and which retain biological information through diagenesis. In many Precambrian examples, mineralization occurred very rapidly, prior to cellular decomposition, and probably while organisms were still viable. The best preservation is observed where organic materials were rapidly perfused with fine-grained silica or phosphate. Other potential host minerals include carbonates, which are less stable, but which also have long crustal residence times. Evaporites, another group of potential host minerals, are quite soluble and tend to dissolve in an active hydrologic cycle. But the crustal residence times of aqueous minerals on Mars are likely to be longer due to the dry climate and the absence of tectonic overprinting. Thus, while Precambrian evaporites are rare on Earth, they may be abundant on Mars where the hydrologic cycle was interrupted early. As noted previously, environments that are especially favorable for microbial fossilization include mineralizing subaerial and subaqueous springs, evaporites, and certain hard-pan soils.
Thermal-Spring Deposits
Subaerial thermal-spring deposits are regarded as excellent targets in the search for a fossil record on Mars because of the high biological productivity and pervasive early mineralization typically associated with such systems. Volcanic terrains are widespread on Mars and some possess outflow channels that are likely to have formed by spring sapping. The association of such features with potential subsurface heat sources, such as volcanic constructs or thermokarst features, indicates the possibility for past hydrothermal activity on Mars. Thermokarst features and related chaotic terrains that may have been formed by hydrothermal processes are also prime targets for hydrothermal mineralization and a fossil record. For example, many of the outflow channels comprising Simud, Ares and Tiu Valles systems originate from chaotic terrains of probable thermokarst origin, or from the floors of chasmata related to the vast Vallis Marineris system (e.g. Echus Chasma). Target deposits in these areas include the common thermal-spring minerals, silica, carbonate, and iron oxides, as well as clay-rich hydrothermal alteration halos associated with shallow igneous intrusives.
Dao Vallis-Hadriaca Patera (Latitude: 33.2 deg. S, Longitude: 266.4 deg. W)
This is a broad outflow channel of simple form that originates from an amphitheater-shaped source area on the southern flank of Hadriaca Patera, an ancient highland volcano. The outflow channel is believed to have formed where a localized subsurface heat source melted ground ice. This process is likely to have been associated with sustained hydrothermal activity. The process probably created not only Dao Vallis but similar outflow channels to the south (e.g., Harmakhis Vallis). The large size of the outflow channel suggests that the interval of activity may have been sustained long enough for extensive hydrothermal mineralization, favorable for the preservation of fossils and organic chemical fossils.
Dao Vallis clearly meets several important criteria as a site for exopaleontology and will be a recommended target for high-resolution visible imaging during upcoming missions. Such information is needed to evaluate fully the origin of important small-scale features (such as the knobby terrain on the floor of Dao Vallis, potential spring mounds) and shed light on both the nature of hydrological processes and their duration. But to assess accurately the potential of this site for exopaleontology, high-resolution infrared spectral data are also needed to explore for hydrothermal mineral deposits, such as silica, travertine, or iron-oxide sinters.
Spring outflows may have also transported thermal-spring minerals to the channeled plains of the Hellas Basin, a potential site for landed missions, and also to the Pathfinder landing site in Ares Vallis.
Sublacustrine Spring Deposits and Carbonate Cements
In arid lake basins on Earth, coarse-grained, nearshore facies are often a locus for extensive carbonate mineralization. This process is of special interest to exopaleontologists because such mineralization typically enhances the preservation of microbial fossils and organic matter. For example, in many alkaline lakes in the Great Basin (western United States), microorganisms living on the surfaces of submerged tufa mounds associated with subaqueous springs, or living interstitially within coarser sediments of lake-margin facies (e.g., fan delta deposits), are commonly entombed by precipitating carbonate minerals. In ancient tufas, evidence of microbial activity is preserved as cellular microfossils and stromatolites, as well as disseminated organic matter. Such deposits are regarded as excellent targets in the search for a fossil record on Mars.
Margaritifer Sinus-Parana Vallis (Latitude: 22 deg. S, Longitude: 11 deg. W)
This site is located within an ancient cratered terrain that has been heavily dissected by several major dendritic valley networks. Channel networks surround a central basin that may have been a depocenter for fluvial-lacustrine sedimentation. Most of the valleys debouch along the southeast margin of the basin. In general, basin-floor sediments appear to be exposed at the surface, although in places hummocky features may be an aeolian mantle.
Formation of the valley networks surrounding the basin was apparently preceded by an early period of mostly larger impacts, evidenced by dissection of the rims of many of the older craters by headward erosion of the channels. The period of hydrologic activity that produced the valleys was followed by a period of smaller impacts, some of which were superimposed on the older craters and valleys. That the intervening period of hydrologic activity that created the valleys may have been of relatively long duration is indicated by the presence of two or more levels of tributaries in several of the longer channel systems and varying degrees of channeling on the rims of the older craters.
Gusev Crater (Latitude: 15.5 deg. S, Longitude: 184.5 deg. W)
A second high-priority fluvial-lacustrine site is Gusev Crater, an impact basin of ~135 km diameter that is located in ancient cratered terrain. The system consists of a single, ~800 km long channel (Ma’adim Vallis) that flowed north, debouching along the southern margin of Gusev Crater. Several different levels of stream terracing, present within the steep-walled canyon, likely record rapid changes in base level. In addition, the lower end of the valley is deeply incised by a much smaller channel that formed by headward erosion late in the history of Ma’adim. Base level changes could have been controlled by drops in the level of a paleolake that resided within the Gusev Crater, or perhaps by local tectonic uplift. These observations support a prolonged hydrologic history for the Ma’adim-Gusev system, although distinction of the processes responsible for the observed changes in base level will require higher spatial resolution than is presently available from Viking.
Just basinward of the terminus of Ma’adim Vallis are lobate deposits that were channeled by late stage downcutting of outflows from Ma’adim Vallis and subsequently wind-eroded. Terracing above and below these deposits suggest they are fluvial-deltaic in origin. The depositional units of the delta are deeply channeled, and stand in high relief above the surrounding crater floor, suggesting they are well-indurated. Deltaic and marginal lacustrine deposits are commonly a locus for precipitation of carbonate cements and sublacustrine spring tufas, processes that favor the preservation of fossils and organic matter. In addition, coarser-grained channel deposits may contain fossiliferous clasts derived from older formations upstream. Ma’adim Vallis originates within an extensive chaotic terrain to the south that appears to have formed by thermokarst processes. Thus, throughout its long history, hydrothermal minerals may have been carried to the floor of Gusev Crater from the source areas of Ma’adim Vallis. Refinement and testing of this scenario will require high-resolution visible-range imaging, and infrared spectral data to assist in the search for carbonates or other aqueous minerals.
Evaporites and Lacustrine Shales
Terminal lake basins in arid environments on Earth are usually ephemeral in nature and eventually dry up, depositing their dissolved salts while forming flat playa basins. Evaporite minerals formed in such environments frequently incorporate micro-organisms within fluid inclusions during crystallization. Such deposits have been suggested as potential short-term repositories for viable organisms, or longer-term repositories for cellular fossils and biomolecules. In addition, the fine-grained, clay-rich shales often interbedded with evaporites in these settings provide good repositories for organic matter, particularly where early cementation occurs. Given the numerous paleolake basins that have been identified in ancient terrains on Mars, such deposits hold much interest for exopaleontology.
White Rock (Latitude: 8 deg. S;Longitude: 335 deg. W)
High-priority targets on Mars for evaporites include an unnamed 80 km crater within the Sinus Sabeaus Quadrangle. Numerous channels resembling terrestrial dendritic drainage systems surround the crater basin and may have sustained a paleolake for some undetermined interval of time. The crater floor exhibits patchy albedo of varying intensity. Of particular interest is a spindle-shaped mound of relatively high albedo called “White Rock”. This feature exhibits two sets of irregular, wind-eroded fractures and is similar in form to terrestrial yardangs. It has been suggested that White Rock is a playa deposit consisting of chemically precipitated evaporite minerals. This interpretation implies that a hydrological system operated here for a long period of time, first concentrating soluble salts by chemical dissolution and then removing them by evaporation and precipitation. Similar high-albedo features can be found on the floors of other impact craters on Mars (e.g., crater Becquerel, Latitude:21.3 deg. N; Longitude: 8 deg. W), some showing irregular stratification under high resolution. This suggests that martian evaporites may be fairly widespread.
HIGH-PRIORITY SITES FOR EXTANT LIFE
As mentioned previously, if life exists on Mars today, it is likely to be a chemosynthetic form residing in subsurface habitats where liquid water may be present. Despite the inaccessibility of the deep subsurface during upcoming missions, it is possible that recent outflows of subsurface water have brought such organisms into near-surface environments where they may have been cryopreserved in ground ice. Thus, areas of stable ground ice associated with very recent outflow channels are probably our best hope for discovering extant life. It has been argued, both on empirical grounds as well as theoretical evidence, that ground ice is presently unstable on Mars at latitudes <40 deg. . Therefore, as noted previously, exploration for cryopreserved martian life should be focused at higher latitudes.
In prioritizing sites for extant life, we should emphasize the very youngest martian terrains (e.g., preferably those completely lacking impact craters). It has been suggested that micro-organisms may survive in ground ice for possibly millions of years and in evaporite deposits for perhaps hundreds of millions of years. Such suggestions should not be ruled out, despite the likelihood that the normal background radiation in such deposits, integrated over geologic time, may severely restrict long-term organism viability by destroying (via mutagenesis) the genomic integrity of cells. The survival time of dormant, but viable organisms under martian conditions is poorly constrained at present, as are the factors that affect preservation during the transition to the fossil record.
In searching for extant life, present knowledge suggests that we should focus on high latitudes (>40 deg. ) where stable ground ice may be present, and especially at sites where ground ice may have formed in association with recent outflows of subsurface aquifers. It follows that mapping the distribution of ground ice using gamma-ray spectroscopy has a high priority for exobiology. It is also possible that surface life may survive within undiscovered liquid water refugia near the surface (e.g., shallow hydrothermal systems), although such environments, if they exist, may be very difficult to locate. It is possible they could be identified using thermal IR. Areas of persistent fogs and frosts at the martian surface, noted previously, may provide indications of near-surface water or water ice that could be accessed by shallow drilling.
Ground ice appears to provide an excellent medium for preserving organisms and biomolecules, but only for short intervals of geological history (perhaps hundreds of thousands to possibly millions of years). Ice may inhibit oxidation of the soil and slow the breakdown of organic materials. But global climate changes, driven by obliquity or other orbital variations, have left the Earth completely ice free on numerous occasions in its history. The present ice caps probably formed during the Miocene and have waxed and waned irregularly since then. The same variations are likely to be true for Mars. Thus, while knowing the present distribution of ground ice on Mars is basic to a strategy to search for extant life there, finding stable ice that is likely to have formed in association with recent outflows of subsurface water, or water-rich pyroclastics, such as lahars, provides the most logical approach.
Ismenius Lacus (Latitude, 44 deg. N; Longitude, 333 deg. W)
Several high-priority targets for cryopreserved organic materials have been located within the Ismenius Lacus Quadrangle of Mars . This terrain lies within the Amazonian-aged “hilly unit” of Deuteronilus Mensae. This area is dominated by numerous mesa-like landforms which are surrounded by debris aprons that resemble terrestrial rock glaciers. The “softening” of mesa rims and associated features suggest the activity of near-surface ground-ice. Mid-winter ice is thought to precipitate from the atmosphere and mix with rock and soil to form masses that slowly creep downslope under the influence of gravity (rock glaciers). The latitude of the area, in combination with the various geomorphic features discussed above, indicate that near-surface ground ice, though varying seasonally, has probably been present here for some time. But the proposed periglacial origin of this terrain needs to be evaluated in more detail using gamma-ray spectroscopy to confirm the presence of ground ice. Unfortunately, young outflow channels that may have delivered a subsurface biota to the ground-ice environment remain to be discovered.
North Polar Cap
Potential targets for indirect evidence of extant life are the polar regions, with their ice caps and layered terrain. Molecular, and even morphologic, signatures for life could have found their way into polar ices which probably act as cold traps for organic molecules in the atmosphere. This has the added advantage that we already know where polar ice is located. A landed mission to the water-rich north polar cap could be revealing, provided that it combined capabilities for subsurface drilling with in situ organic analysis and microscopic examination.
PLANETARY PROTECTION FOR MARS MISSIONS
NASA’s planetary protection policy and its implementation demonstrate to the public that NASA is safeguarding the planets, including our own, during space exploration. The U.S. is signatory to a 1967 international agreement, monitored by COSPAR, which establishes the requirement to avoid forward and back contamination of planetary bodies during exploration. To help implement requirements, NASA established a Planetary Protection Office and has issued a document, NHB 8020.12, that delineates planetary-protection requirements for all NASA robotic extraterrestrial missions. This document is specifically directed to: 1) the control of terrestrial microbial contamination associated with robotic space vehicles intended to land, orbit, flyby, or otherwise be in the vicinity of extraterrestrial solar-system bodies, and 2) the control of contamination of the Earth and Moon by extraterrestrial solar-system material collected and returned by such missions.
The increasing interest in Mars exploration and the long time elapsed since consideration of the scientific rationale for such exploration, have prompted a new look at the planetary protection requirements for forward contamination. In 1992, the Space Studies Board of the U.S. National Academy of Sciences recommended changes in the requirements for Mars landers that significantly alleviated the burden of planetary protection implementation for these missions. The recommendations were published in, “Biological Contamination of Mars: Issues and Recommendation” and presented at the 1992 29th COSPAR Assembly in Washington DC. In 1994, a resolution addressing these recommendations was adopted by COSPAR at the 30th Assembly and has been incorporated into NASA’s planetary protection policy. As we learn more about Mars, the requirements may change again to reflect current scientific knowledge.
The academy’s recommendations, subsequently adopted by COSPAR, recognize the very low probability of growth of microorganisms on the martian surface. With this in mind, the policy shifts from probability of growth considerations to a more direct and determinable assessment of the number of microorganisms with any landing event. For landers that do not have life-detection instrumentation, the level of biological cleanliness required is that of Viking prior to heat sterilization, which can be accomplished by class 100,000 clean-room assembly and component cleaning. This is a conservative approach that minimizes the chance of compromising future exploration. Landers with life detection would be required to meet Viking post-sterilization levels or levels dictated by the experiment. It is recognized that the sensitivity of a “life-detection” instrument may impose the more severe constraint on the mission.
Included in the changes to the COSPAR policy is the option that an orbiter is not required to remain in orbit for an extended time if it can meet the standards of a lander without life-detection experiments. Also, the probability of inadvertent early entry has been relaxed compared to previous requirements.
The policy for samples returned to Earth is directed toward containing potentially hazardous martian material. Concerns have included a difficult-to-control pathogen capable of directly infecting human hosts (extremely unlikely) or of a life form capable of upsetting the current natural balance of Earth’s ecosystem. It is of paramount importance to address the potential public perception that might attribute an epidemic, personal illness, or unusual event to an introduced martian contaminant.
For a Mars sample return mission, all samples would be enclosed in a hermetically sealed container; the contact chain between the return vehicle and the Mars surface must be broken in order to prevent the transfer of uncontaminated surface material via the spacecraft exterior; and the sample would be subjected to a comprehensive quarantine protocol to investigate whether or not harmful constituents are present. It should be noted that even if the sample return mission has no exobiological goals, the mission would still be required to meet the planetary protection sample return procedures and the life-detection protocols for forward contamination, not only to mitigate concern of potential contamination but also to prevent a hardy terrestrial hitchhiker from masquerading as a martian life form. In today’s environment, public concern and legal requirements (in multiple jurisdictions) would be significant drivers in mission planning and planetary protection implementation. It may be worthwhile to consider maximizing those experiments that could be done on the martian surface, thereby extending the time before a sample return and perhaps relaxing fears of back contamination, leading to delayed, and possibly reduced, cost.
Because of the high level of public concern over the possibility that a sample returned from Mars might contain components harmful to our health or to the Earth’s biosphere, NASA must strive for public education as well as informed public and legal consent well in advance of a sample return mission. In part, the planetary protection office provides a visible regulatory function which might mitigate concerns of forward and back contamination.
RECOMMENDATIONS
SUPPORT OF BASIC R & A
Only with spacecraft missions will we be able to answer such questions as the extent to which prebiotic chemical evolution took place on Mars, whether life ever originated on Mars, or whether life exists on Mars today. However, the intelligent pursuit of those issues also involves a significant level of ground-based activity that falls under the general heading of basic research and analysis. These activities help to improve our ability to obtain and interpret data from Mars missions, and they help in formulating more effective strategies for conducting future exobiological exploration. The definition of flight instrumentation also falls under this activity but is best discussed in the context of instrument development in the following section.
Analyses of Mars-analog and Martian Materials
Our ability to interpret data and to design future experiments depends strongly upon our understanding of the composition of Martian materials. One useful approach is to propose candidate samples, e.g., rocks or soils, whose properties are broadly consistent with those previously observed via telescopes, the Viking mission and martian (SNC) meteorites. These model materials can then be tested and modified as additional observations become available.
For example, the Viking biology experiments revealed that the martian soil was chemically very reactive in ways that were unanticipated. Earth-based laboratories have attempted to reproduce the performance of the biology experiments using materials which conform to the constraints imposed by all Viking analytical experiments. This effort has helped to restrict the number of geochemical agents, some of which are powerful oxidants, with properties consistent with the Viking observations. These studies have led to the development of an “oxidant experiment” for the Russian Mars ’96 mission, which is designed to identify these chemical agents more conclusively.
Much of our current detailed understanding of martian geochemistry stems from studies of martian meteorites (i.e., the SNC meteorites) in earth-based laboratories, recognizing that they represent a very incomplete sampling of martian lithologies. Conspicuous examples of such insights include: the entirety of our radiochronometric knowledge of the timing of igneous activity and much of what we know about the magnetic properties of martian rocks; virtually all of our observations of martian igneous petrology; our understanding of crustal and mantle differentiation processes as inferred from the distributions of elements (major, minor and trace) among different lithologies; and most of our knowledge of stable-isotope distributions among the various reservoirs, with important implications for the evolution of the martian atmosphere and hydrosphere.
Analyses of martian meteorites will continue to play a key role, even as additional in situ spacecraft measurements are made and martian samples are returned to Earth. Martian meteorites broaden the diversity of materials available. Thus the continued support of martian-meteorite research is an important component of the exobiology strategy. Support should continue for analyses of those meteorite components relevant to exobiology, improvements in relevant laboratory instrumentation, and field collection of meteorites in Antarctica. The Antarctic program has yielded several key SNC meteorites.
Biogeochemistry and paleontology of Mars-like environments
Because certain field sites on Earth serve as useful proxies for martian environments, these sites can prepare us for the challenges of Mars exploration. Such localities include the dry valleys and lakes of Antarctica, boulder fields and ephemeral lakes in deserts, and hydrothermal systems and thermal springs. All of these environments harbor ecosystems living under harsh conditions; and their study can guide our search for an extinct or extant martian biosphere. A key concern is the fossilization processes and potential in different environmental settings. We must rigorously define the true limits to life as we know it, and we must compare these limits with the full range of environments available on Mars, including those deep beneath its surface. We must learn how to recognize all evidence of martian life and its fossils, yet we must acknowledge that this evidence probably will differ in fundamental ways from the evidence we already have obtained about our own ancient biosphere. Still, basic paleontological principles and knowledge of preservation processes in extreme environments provide the basis for a strategy to explore for martian fossils. Such studies will therefore continue to be important as future Mars observations increase in their variety and sophistication.
Interdisciplinary studies
In addition to the focused research on specific aspects of the martian environment described above, a key element in developing our understanding of the planet Mars involves the integration of new results into a coherent view that is consistent with the full range of martian studies. Such an interdisciplinary approach draws from exobiological, geochemical, geological and atmospheric studies in order to understand the system as a whole. Each of these disciplines provides a different set of constraints on the nature of the system, and the system as a whole must be consistent with each of the component parts. In this sense, the field of exobiology encompasses all of the other fields to the extent that each contributes to our understanding of the requirements of, and constraints on, the origin, evolution and distribution of life. This interdisciplinary approach also includes analysis of existing data that might pertain to exobiology even if those data were obtained for other purposes.
Definition of science goals
Finally, one of the most important roles of basic R & A in any planetary-science discipline, including exobiology, is to help formulate the science goals for future planetary missions. Thus, this strategy document not only builds upon the record delivered by the Viking missions, but also reflects advances in a wide variety of non-mission-related areas, notably basic research into such topics as Precambrian paleontology, microbial ecology, impact theory, evolution of planetary atmospheres, remote-sensing instrumentation, and many others. It will remain essential to the future of planetary exploration, in general, and exobiological exploration, in particular, for such translation of basic discoveries into mission objectives to continue to receive recognition and support.
INSTRUMENT DEVELOPMENT
To reiterate a point made previously, specific instruments mentioned here should be regarded as examples and should not be interpreted as excluding definition and development of new concepts, particularly insofar as such novel technologies might permit reduction in either weight or cost.
Orbital instruments
As discussed earlier, two of the key aspects of global investigations in support of exobiology are the searches for water and aqueous mineral deposits, respectively. For water, the technique of choice for global searches is gamma-ray/neutron spectroscopy. This represents quite a mature technology but some significant improvements are worth considering. Because of their sensitivity to background radiation generated by the spacecraft structure, existing spectrometers need to be deployed on a boom that adds both weight and complexity to the system. By taking advantage of two recent developments, however, the need for a boom disappears. First, by replacing much of the aluminum in the spacecraft structure with carbon-based composites, the background radiation level can be greatly diminished. Second, by employing anti-coincidence shielding around the detector, the remaining background can be effectively eliminated. However, use of anti-coincidence shielding requires adoption of active mechanical cooling, rather than passive cooling by radiation to space, as at present. This in turn carries a weight penalty, but this weight increase is offset by elimination of the boom.
The search for hydrothermal and other aqueous mineral deposits is currently hampered, as noted earlier, by the spatial resolution of the present generation of orbital mid-IR spectrometers. At a value of about 3 km/pixel, this is a factor of about 30 too large for reliable deconvolution of spectral signatures for discrete outcrops of aqueous minerals or mounded spring deposits. (A resolution of 30 m would probably be appropriate.) There seems to be no obvious physical impediment to an improvement of about that magnitude in the spatial resolution of such a spectrometer, a development that would be most desirable. It is worth noting that such detailed mineralogical information would be of value not only to the exobiological community but also to geochemists and those working on the geological and atmospheric evolution of Mars.
Landed instruments
From an exobiological perspective, the need among landed instrumentation is not so much in improvement of existing analytical instruments as in development of new technologies. However, one area in which improvement is certainly needed is the in situ identification of surface mineralogy. Two approaches are feasible here and, because they yield data in somewhat different situations, both should be developed.
The near- and mid-IR spectroscopic techniques employed from orbit for mineral identification, can also be applied on a smaller scale to yield information on the minerals exposed at the surface of a rock within the field of view of a landed spacecraft or rover. Thus, this would be a powerful technique for conducting an assessment of the mineralogical diversity at a landing site, a key tactical goal of exobiological exploration.
The survey mode of IR spectroscopy would be usefully complemented by the use of x-ray diffraction applied to individual rocks selected on the basis of IR data. Because of the unique crystallographic signature that x-ray diffraction yields for most minerals, this is a very effective means of defining martian surface lithologies, which can be made even more useful by combining it with x-ray fluorescence spectroscopy, thereby yielding elemental abundances in addition to mineral identities. Again, the acquisition of detailed mineralogical information has an importance in Mars exploration that far transcends a single discipline, such as exobiology, so that development of miniaturized IR spectrometry and combined x-ray diffraction/fluorescence would be of major benefit to Mars science.
Several analytical approaches of exobiological relevance are, however, missing from the current roster of planned experiments on the martian surface. Most conspicuous among these is the capability for molecular and/or isotopic analysis of volatile, particularly organic, species. We therefore recommend development, and eventual deployment, of the following types of instrument: Evolved-Gas Analyzer, combined with either a Differential Scanning Calorimeter or Differential Thermal Analysis; either an Isotope-Ratio Gas Mass Spectrometer or a Tunable Diode Laser Spectrometer for isotope analysis; and, possibly instruments designed for various specific classes of organic compounds, such as lipids and amino acids. In view of the need for considerably more information about the martian micro-environment before deployment of such specialized sensors for analysis of possible organic compounds, it seems unlikely that these instruments would be needed before the 2001 flight opportunity, but it would be most desirable to have them available for inclusion soon thereafter. (However, the first two types of instrument could be usefully employed on inorganic materials, such as evaporite deposits or hydrothermal alteration products, with significantly less prior investigation.)
Also missing from current plans, except for inclusion of gamma-ray spectrometers on the Mars-96 penetrators and the long-wave radar on the Mars-96 orbiter, are the means for mapping the subsurface distribution of water. Development and early deployment of EM sounding would therefore be highly desirable.
MOBILITY AND SAMPLE ACQUISITION
Optimization of the science return from instruments landed on the martian surface will require both that the instrument be in the right location to acquire the desired data and that, for cases in which analysis is performed on a discrete sample of martian material, that the sample be brought to the instrument in an appropriate form. Thus, successful Mars science places a premium on both mobility and sample acquisition.
Mobility
Considerable thought has already been invested in devising ingenious methods of moving instrument packages on Mars from their landing site, which is likely to be, at least for early missions, in geologically bland terrain, to a location of more geological, geochemical, geophysical or exobiological interest. Rovers, balloons, hoppers and aircraft have all been proposed, with both the U.S. and Russia having designed rovers for use in Mars exploration. These two rovers represent two very different concepts, the U.S. Pathfinder microrover being an extremely small, sophisticated device with limited science capability, whereas the Russian Marsokhod is much larger with a capability of carrying a diverse scientific payload. Both approaches have their merits, but we note that, if the telecommunications link between the rover and ground passes through the lander, the rover’s range is limited to line of sight, so that the larger the rover, the greater its range. Furthermore, only a large rover will provide a suitable platform for drills capable of penetrating into rocks and the subsurface regolith. Given the likely exobiological importance of such a drill-core, we therefore recommend development of a suitably large rover.
A key aspect of rover design will be navigation and control. Recent developments in telepresence and virtual reality have already shown great promise in this area and we recommend continued support of work in this field.
Sample acquisition
Approaches to sample acquisition run the conceptual gamut from a simple scoop and hopper arrangement to an automated thin-section maker for rock samples. As noted earlier, exobiology places particular demands on sample acquisition because of the common need to retrieve samples from locations shielded in some way from the local martian environment. Thus, acquiring samples both from beneath the surface of the regolith and from the interior of weathered sedimentary rocks is likely to be necessary at some stage in Mars exploration.
The need for a drill, or some equivalent means of accessing the subsurface regolith, has been apparent for some time and several concepts have been considered. One of the major unknowns at this time is the depth to which such a drill will need to penetrate. This is largely governed by the depth distribution of the surface oxidant, about which nothing is presently known except that it exceeds the depth from which the Viking Lander obtained a trench sample for the Life Detection Experiments, namely about 10 cm. The consensus is that a depth of several meters is worth striving for, with a minimum target depth of 1 m. We therefore recommend that development proceed on a coring drill capable of penetrating to a target depth of 3 m into the martian regolith.
The need for a technique capable of extracting a sample from the interior of a rock has also attracted attention, though less effort has been put into this area. Nonetheless, in view of the importance of removing the influence of martian weathering from rock analyses, and the key role that fluid inclusions and other entombed volatiles can play in geochemistry and climate research, as well as in exobiology, we recommend development of techniques that will permit access to the interior of rocks lying on the martian surface.
MISSION PLANNING
Orbital missions
Assuming that currently approved missions perform as planned, and that a gamma-ray spectrometer is included in the ’98 Mars Surveyor mission, the only presently identifiable exobiological goal requiring an orbital mission would be the identification of surface expressions of aqueous mineralization, such as hydrothermal systems or spring deposit
