Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 89
12 Assessment of the Mars Exploration Program INTRODUCTION Chapters 2 through 10 above review the state of knowledge of the planet Mars. Recent missions, and in particular the Mars Global Surveyor (MGS) mission, have made surprising and important new discoveries and have greatly increased our understanding of the planet. Several scientific issues have been resolved as a result of the new information, and new directions of inquiry have opened up. This chapter summarizes the science highlights and shows the extent to which the Mars science priorities will be met by currently planned U.S. and foreign missions. Following the discussion of the importance of sample return in Chapter 11, this chapter also discusses aspects of the sample-return missions toward which the U.S. program is building, as well as several other issues affecting the Mars Exploration Program. The evolution of thought about Mars science priorities since 1978 has been methodical and thorough. The science priorities recommended by COMPLEX and other groups remain fully valid in light of the new discoveries, although the degree to which NASA and other space agencies have responded to the priorities has been uneven. The new NASA Mars Exploration Program (MEP), announced in October 2000, is a science-driven program that seeks to understand Mars as a dynamic system and to understand whether life was ever part of that system. As part of this program of global understanding, NASA’s goals emphasize: Understanding the martian climate; Understanding the role water plays in the environmental history of Mars; Understanding Mars’s biological potential and its connection to the climate record; and Understanding the interactions between the surface, atmosphere, and interior and how they are related to water. The NASA philosophy “Seek, In Situ, Sample” is designed to advance learning about the global dynamics of the Mars system and to narrow down the search to focus on its biological potential. The MGS and Mars Odyssey missions were designed for global reconnaissance, and the Mars Exploration Rovers (2003) and Mars Reconnaissance Orbiter (2005) missions, which will carry out field geology and remote sensing, will focus on finding environmental indicators that suggest the possibility of life. These missions will identify a suite of sites for
OCR for page 90
intensive surface analysis by Mars Science Laboratorya (2007),b in principle permitting the best sites for sample return to be located. Overall the “Seek, In Situ, Sample” strategy is a sound one, and NASA has built up a strong, risk-attentive program focusing on the understanding of Mars. This is an excellent approach, and in the areas that focus on answering questions about past or present life, NASA is doing a good job of addressing priorities for Mars science that have been recommended by COMPLEX and other groups since 1978. However, it is important to insert a strong note of caution regarding this plan. The effort must focus on answering the question, Did life ever arise on Mars?, and not on searching for evidence of the life that “must surely be there.” If life ever arose, its signatures may only be evidenced through detailed geochemical or isotopic investigations, and separation of biotic from abiotic signatures will require an extensive understanding of all of Mars’s systems and their histories. From that perspective, COMPLEX notes that the current NASA strategic plan does not address all of the priorities needed to understand Mars as a dynamic system suitable for life. NEAR-TERM MISSIONS—MARS RESEARCH OPPORTUNITIES NASA’s near-term Mars missions (presented in Table A.1 in Appendix A) includes a lineup of vehicles and investigations that will address a wide range of important scientific goals, while remaining within the program’s overall budgetary constraints. A summary of the priority science that will be met by these missions appears below. Non-U.S. missions are also included in the list, as they are complementary to NASA’s missions. A later section of this chapter contains COMPLEX’s assessment of MEP and its congruence with recommended science priorities. 1998—Nozomi (Japan) The Japanese Nozomi mission (formerly called Planet B) carries 14 scientific instruments (including a NASA neutral mass spectrometer) designed to investigate the structure of Mars’s upper atmosphere, ionosphere, magnetic field environment, and solar wind processes. Because the spacecraft used up too much fuel in a trajectory correction maneuver, it will arrive at Mars in 2004, 5 years later than originally planned (see Chapter 10 in this report). The mission was not designed to be long-lived, and some of the spacecraft systems and instruments may suffer damage during the long and unplanned cruise. Nozomi is one of the few missions to address important upper-atmospheric science priorities; unfortunately, it is considered to be compromised. None of the near-term NASA missions has upper atmospheric/ionospheric mission objectives. 2001—Mars Odyssey Mars Odyssey, an orbiter, carries a payload consisting of the GRS instrument, which was part of the original Mars Observer payload, THEMIS, and MARIE (a radiation environment experiment that will characterize Mars’s galactic cosmic radiation environment). GRS should provide valuable global-scale maps of Mars’s elemental composition with a spatial resolution of 300 km. It will also produce maps of the near-surface hydrogen abundance, which should be interpretable in terms of the abundance of water ice and adsorbed water in the near-surface regolith. The THEMIS instrument is a high-spatial-resolution multichannel infrared radiometer and imager. The primary goal of THEMIS is to map the global surface emissivity of Mars in 10 spectral bands at a spatial resolution of 100 m, much higher than has been accomplished by previous infrared instruments such as Mariner 9’s Infrared Interferometer Spectrometer and MGS’s TES. THEMIS may be able to identify some minerals as well, but for the most part it will define broad surface emissivity and petrologic units that will be useful for choosing future landing sites. The imager on THEMIS will acquire 20-m-spatial-resolution images in five wavelength bands covering the visible to near-infrared region, but of only 5 to 10 percent of the planet. The GRS will provide a critical global data a Also referred to as the Mars Smart Lander, the Mobile Science Laboratory, and by a variety of other names. b Following the completion of this study, NASA announced that it was delaying the launch of the Mars Science Laboratory until 2009 to allow time to develop an advanced, radioisotope power system for this mission.
OCR for page 91
set, and THEMIS will be important for scaling thermal data from the surface to orbit and for identifying mineral-ogically interesting landing sites. However, considering the difficulty experienced in extracting mineralogical information from the data obtained by previous and ongoing orbital infrared experiments, the results from THEMIS may not be as definitive as expected. The mission will also help refine the record of geologic stratigraphy on smaller scales than was possible before. 2003—Mars Exploration Rovers The two Mars Exploration Rovers (MERs) scheduled for launch in June 2003 will each carry the Athena integrated payload, which had originally been selected for the 2003 sample-collection mission. The MERs will provide improved surface mobility relative to Mars Pathfinder’s Sojourner rover, as well as a significantly enhanced capability for characterizing rock mineralogy. A goal is to link chemistry and mineralogy at the surface with that surmised from orbital observations and to find conclusive evidence of water-affected surface materials, and thus regions where conditions may have been favorable to the preservation of biotic processes. One concern about MER is that its landing ellipse is too large and its roving range is too small to guarantee access to the most exciting geological sites that have been identified in MGS data. This, combined with engineering concerns regarding landing safety, may ultimately result in the selection of landing sites that limit the overall scientific potential of the 2003 missions. (Some landing ellipses under consideration have interest that is regional in extent; in these, limited rover range is less damaging.) 2003—Mars Express (European Space Agency) The payload of the European Space Agency’s Mars Express represents an attempt to recover some of the science lost with the failure of Russia’s Mars 1996 mission. The payload includes the HRSC, which will obtain global color images at resolutions between 10 and 30 m and topography at the same spatial resolution, and 2-m images of 1 percent of the planet. Global observations of visible/near-infrared reflected light at an average resolution of 1 km will be acquired by OMEGA and will be used to investigate surface mineralogy and the atmosphere. Global circulation measurements; high-resolution mapping of atmospheric composition (including water vapor); density, temperature, and pressure profiles in the atmosphere; and interaction of the atmosphere with the interplanetary medium will be studied by the Planetary Fourier Spectrometer (PFS), SPICAM, ASPERA, and the Radio Science Experiment (MaRS). In addition, MARSIS will map subsurface structures at kilometer scale, looking for subsurface liquid and solid water. 2003—Beagle 2 (United Kingdom) The United Kingdom’s Beagle 2 is a small but heavily instrumented spacecraft, and it represents the landed component of Mars Express. The mission goals are to examine the geology, surface composition, oxidation state, and mineralogy of rocks at the landing site and to make detailed atmospheric-composition measurements, including determination of isotopic fractionation. These exobiology measurements will be made with a suite of instruments including panoramic, wide-angle, and microscope cameras, a gas chromatograph-mass spectrometer, a Mössbauer spectrometer, an alpha-proton-x-ray spectrometer, and an environmental surface weather station. 2005—Mars Reconnaissance Orbiter Currently in the early stages of development, Mars Reconnaissance Orbiter (MRO) promises to fill a number of gaps in our knowledge concerning the search for compelling environmental indicators related to the action of liquid water and possible biological processes as well as to provide significant new discoveries. Instruments that are expected to be on the payload are the PMIRR and MARCI instruments from the failed Mars Climate Orbiter (MCO), an extremely high resolution imager (called HiRISE) with a surface resolution of tens of centimeters, a visible/near-infrared mapping spectrometer (known as CRISM) with a spatial resolution of 50 m, and SHARAD,
OCR for page 92
a radar sounder to be provided by the Italian space agency which will acquire multiple vertical atmospheric profiles over 1 martian year. The purpose of this mission is to recover the critical atmospheric measurements lost with Mars Observer and Mars Climate Orbiter, and to investigate in detail hundreds of potential landing sites with high biological potential, using the high-resolution, high-data-rate instruments. With its polar orbit and high-resolution imaging capability, this is one of the few NASA missions with the ability to obtain information on climate, by looking at the polar layered terrain. 2007—Mars Science Laboratoryc The advanced rover mission known as the Mars Science Laboratory (MSL) is now in the definition stage. The goal of this mission is to develop technologies required for future sample-return missions, as well as to conduct landed science focusing on surface and subsurface materials potentially linked to life. The MSL will be a pathfinding mission to the most biologically interesting sites. The lander’s engineering goals of improved landing precision, terminal hazard avoidance, high rover mobility, and long surface lifetime are responsive to the needs of sample-return missions. The main concerns relate to whether all technical challenges can be met in addition to the challenges of the science activities, and to the limitations imposed by the solar power supply. (See the subsection “Power Supply for Landers and Rovers” below in this chapter.) One experiment that deserves cautious consideration is a proposed drilling system designed to provide subsurface access to depths of 2 m. Such a system could consume a large fraction of the mission’s resources, and it may not be compatible with the goal of high mobility. 2007—Mars Scout A principal-investigator-led Mars Scout mission will be selected in 2003 for a launch opportunity as early as 2007. The Mars Scout program offers NASA an excellent opportunity to fill gaps in our knowledge, to react to recent scientific discoveries, and to increase community involvement. As such, Mars Scout needs to be open to all science investigations of Mars that are not addressed within the scope of the existing program. Topics beyond the current emphasis on “water and life” should be encouraged. (See the subsection “The Scout Program,” below in this chapter.) 2007—NetLander (France) The Mars NetLander mission is sponsored by a consortium of European nations led by France. The mission objective is to perform simultaneous surface measurements in order to investigate Mars’s internal structure, its subsurface layering, and the atmosphere. During the baseline 1 martian year mission, the NetLander payloads will conduct simultaneous seismic, atmospheric, magnetic, and ionospheric measurements, with a surface meteorology package, a ground-penetrating radar, an electric field package, a magnetometer, the NetLander Ionosphere and Geodesy Experiment, a panoramic camera, a seismometer, and a soil properties package (SPICE). This is the one near-term mission that significantly addresses issues related to the deep interior of Mars. Another important mission objective is to search for subsurface reservoirs of liquid or frozen water. The Longer Term—Sample-Return Missions Returned samples from Mars will be extremely powerful tools for answering many of the most important scientific questions about the planet and its history (see Chapter 11). In 1995—prior to the ALH84001 Mars meteorite discoveries in 1996—NASA had begun planning a mission to collect and cache Mars samples, to be c Following the completion of this study, NASA announced that it was delaying the launch of the Mars Science Laboratory until 2009 to allow time to develop an advanced, radioisotope power system for this mission.
OCR for page 93
launched in 2003, even before a plan to convey the samples back to Earth was fully formulated. As events have unfolded, the prospect of sample return has receded farther and farther into the future. According to the current program (2002), we are more than a decade away from the first potential launch of a sample-return mission. This scaling back of ambitions aptly demonstrates that Mars sample-return missions are nontrivial in their engineering scope, cost, and risk, and that they will require significant investments in technology, and possibly international cooperation, to be successful. The results of the analysis of Mars Pathfinder and MGS observations provide some potential guidance in sample return strategies. Before those missions, it was considered sufficient to target landers to “grab bag” landing sites that were hoped to contain a wide variety of rocks that could be accessed at a single location, with small requirements for mobility. Now it appears that Mars’s geologic and climatic history is best exposed in widely separated, isolated locations, and that a complete picture of the planet’s history probably cannot be obtained from samples collected at a single location. Instead, a series of samples will be required from diverse locations on the planet, and this will require multiple sample-return missions to accomplish (discussed below in the section “Mars Sample Returns”). Thus, the first sample return should be seen as a “pathfinder” for future sample-return missions, and it should be used to develop the key technologies, procedures, and infrastructure necessary to embark on a future program in which samples are returned from many locations on the planet. STATUS OF HIGH-PRIORITY SCIENCE QUESTIONS Mars Pathfinder and Mars Global Surveyor have greatly improved our knowledge of the interior of Mars (see Chapter 2 in this report). The former gave us a significantly improved moment-of-inertia factor, leading to a better estimate of the bulk composition of the planet. Topographic data from MGS have shown that the Tharsis Plateau region predates the fluvial channels, opening up the possibility that volcanic gas release from the plateau’s formation would have created suitable conditions for the creation of the channels. The topographic data likewise ruled out the impact hypothesis for the hemispheric dichotomy. In order to fully understand the interior structure and composition, however, knowledge of the moment-of-inertia factor must be coupled with the core size, which is still a free parameter. When it is known, the bulk composition of Mars will be much more closely constrained, which will have important implications for the raw material of Mars and cosmochemical models for the pressures and temperatures in the solar nebula that gave rise to that raw material. We will understand whether Mars has an oxidation state higher than that of Earth, and if the temperature and pressure in the accretion zone for Mars allowed significantly different mineral condensation there. The composition of Mars’s surface materials is coupled with the interior structure of the planet, yet recent missions have provided information only about the major element compositions of rocks and fines at a few sites (see Chapter 3 in this report). Beyond the suggestion by MGS’s TES that a distinction can be drawn between basaltic and andesitic areas of the Mars surface, we are largely ignorant of the surface mineralogy. TES data provide a tantalizing glimpse of an area of gray hematite near the equator suggestive of large-scale water interactions, but no direct measurements of hydrated minerals exist to date because of insufficient resolution. The Mariner through MGS missions have shown that water has played a significant role in the evolution of the planet, from evidence of standing water, to large outflow channels, to valley networks, and most recently very youthful channels (see Chapter 6). The higher resolution of the MGS images has returned contradictory evidence of a northern hemisphere ocean and has made the understanding of the valley networks more complex, since the data suggest that the sources of the eroding fluid are not from surface runoff. The discovery of channels on very steep slopes makes the hypothesis of possible subsurface water sources more complex, and has suggested the possibility that channels were created by other volatiles. The MGS discovery of remnant crustal magnetic anomalies suggests an early dynamo, but because of the lack of absolute dating of the surface, it cannot be determined when the dynamo ceased (see Chapter 2). Similarly, while the MGS mission established a good relative stratigraphic record for Mars, models using the lunar cratering curve can link this to absolute ages only to within a factor of two (see Chapter 4). This is insufficient accuracy for understanding the global dynamic system of interactions among the interior, surface, hydrosphere, and atmosphere during the periods of most interest to the question of the search for life.
OCR for page 94
Many of the advances in the understanding of the potential for life on Mars have come not from recent missions but rather from studies of life in extreme environments on Earth and from studies of the SNC meteorites. On the basis of those studies, coupled with the evidence from MGS and earlier missions that the past climate on Mars was very different from what it is now, a better understanding is being built of the types of environments on Mars that could constitute suitable life habitats. MGS data have shown striking detail in the polar deposits (see Chapter 9), hinting that there may be records of quasi-periodic climate variations recorded there. Surface features give clear evidence of past climate differences, but it is unknown if there was a sustained warm, wet climate or just episodic nonequilibrium events. These questions are crucial to future progress in understanding Mars as a habitable planet. A key to understanding the past climates is an understanding of the lower and upper atmospheres and their evolution (see Chapters 8 and 10). The Viking and MGS missions have provided a good estimate of global seasonal water vapor variations but have been poor at monitoring daily variations. Similarly, MGS, Mars Pathfinder, Viking, and Hubble Space Telescope have provided much data on dust loading in the atmosphere. This information has given us a good basic understanding of the pole-to-pole general circulation patterns, but the seasonal circulation patterns are unknown, and the evolution of the atmosphere will not be understood until we know the crustal history and composition, the interaction of volatiles with the near-surface layer, and the loss mechanisms at the top of the atmosphere. Table 12.1 summarizes the outstanding science issues connected with Mars exploration that are discussed in Chapters 2 through 10. MARS SCIENCE PRIORITIES AFTER MARS GLOBAL SURVEYOR The Mars science priorities recommended by seven NRC reports since 1978 and by two NASA reports—the 1996 Mars Expeditions Strategy Group report and the 2000 MEPAG report—are assembled in Appendix B. The recommendations reprinted in Appendix B are discussed in Chapters 2 through 10 and are summarized in Table 12.2. Organized in the same sequence and under the same topics as discussed in Chapters 2 through 10, the science priorities are grouped by subject in Table 12.2, proceeding from the interior of the planet outward. Solid circles in the column titled “Panel Recommending” identify the questions that are recommended for study in each report. The column in Table 12.2 labeled “Inclusion in Missions” shows which missions will address these questions. (Solid circles signify missions that will concentrate on each science objective, and open circles signify a lesser level of attention to that objective.) Missions in NASA’s Mars Exploration Program are listed separately from the missions projected by other nations. ASSESSMENT OF NASA’S MARS EXPLORATION PROGRAMAND ITS CONGRUENCE WITH RECOMMENDED SCIENTIFIC PRIORITIES Table 12.2 shows a high degree of complementarity between the NASA missions and those of other nations. Table 12.2 shows that in many but not all cases, planned missions (foreign as well as NASA) will address the high-priority Mars science issues. However, understandably because of budget constraints, some important areas of Mars science will remain underserved: The Mars Exploration Program recognizes the importance of gaining information about the surface, atmo-sphere, hydrosphere, and interior of the planet to arrive at an understanding of the Mars dynamic system and a global context for assessment of the biological potential of Mars. However, NASA has no plans for missions that address high-priority questions about the interior of Mars. Absolute dating of planetary surfaces by isotopic techniques is not contemplated for any NASA or foreign mission prior to sample return, yet understanding the absolute chronology attached to the Mars stratigraphic record is essential to understanding the history and evolution of the planet, water, and climate, and the geological context of the sites visited. There is an absence of NASA missions that specifically address Mars’s atmosphere, climate, polar science, ionosphere, and solar wind interactions. Direct measurements related to volatiles—for example, the distribution
OCR for page 95
and behavior of near-surface water—are needed. Some but not all of these goals will be addressed by upcoming foreign Mars missions such as Nozomi, Mars Express, Beagle 2, and NetLander. COMPLEX urges NASA to continue its support for U.S. participation in Mars missions conducted by NASA’s international partners. COMPLEX has identified 10 measurements that could be made, which would contribute knowledge in the underserved areas outlined. In most cases it is fairly clear why these measurements have not been adopted by the Mars Exploration Program. The first four are considered most important: A gas mass spectrometer in a long-lived surface package, to study the chemical dynamics and isotoperatios of C, H, O, and noble gases in the Mars atmosphere (see Chapter 8 in this report). This would greatly refine the measurements of the Viking aeroshell mass spectrometer. Time variability of isotopic compositions could be interpreted in terms of sources, sinks, and reservoirs of volatiles, and atmospheric evolution. Contra this concept: A related experiment is included on the 2003 Beagle 2 mission. There is also a potential problem in providing power to a surface package for a long time; see the subsection “Power Supply for Landers and Rovers,” below. Passive seismometers to study the interior of the planet (see Chapter 2). This has been consistently endorsed by advisory groups since 1978, but it is not addressed in the current NASA Mars Exploration Program. Contra: A passive seismology experiment is projected for the 2007 NetLander mission, described above and in Chapter 2; again, there is a potential problem in providing power to seismic stations long enough to register naturally generated seismic signals—see “Power Supply for Landers and Rovers,” below. In situ age determination of rocks at selected sites (see Chapter 4). Even if only a low order of accuracy of radiometric dates (±20 percent, by the 40K-40Ar technique) is achievable, more rocks can be (approximately) dated in situ than through sample return, increasing the potential for calibrating the cratering rate and the stratigraphic column on Mars. An understanding of the evolution of the planet’s surface—the timing of events and the coupling of different systems—requires that an absolute chronology be established with significantly better than the factor-of-two uncertainty that now exists for surfaces of intermediate age. Contra: More MER/MSL-type missions than are currently projected would be needed to fully exploit this technique; further, it has not been proven that robotic age-dating can be made to work at all. Global mapping of subsurface ice and water in the upper crust by radar (see Chapter 6). The distribution of ice and water in the crust is consistently identified as one of the most important goals of Mars studies. In principle, it can be learned by pulsing the surface with radar at different frequencies from orbit, to detect materials with contrasting dielectric properties. Contra: The cost would be high and the results might be equivocal, i.e., not subject to unique interpretation. Mars Express’s MARSIS and Mars Reconnaissance Orbiter’s SHARAD experiments will constitute a feasibility study of this concept. Six other experiments that COMPLEX deems worthy of consideration are these: Long-lived landed humidity sensors. These would study the exchange of atmospheric water with the regolith. Further mapping of the magnetic field of Mars, to fill in gaps in the MGS magnetic map, along with plasmamapping. However, these require an elliptical orbit, which is not ideal for other orbital science. Combined calorimetry and precision gas analysis of surface material in the polar regions. This may be the only opportunity to learn in detail about polar materials, which probably will not soon be targeted for sample return. Measurements from orbit of the dynamics of the middle and upper atmosphere of Mars, and atmosphericescape, using a Fabry-Perot interferometer. Detection of ice in the martian regolith by neutron spectroscopy (epithermal neutrons from H), using anairborne instrument. Synthetic aperture radar investigations of the surface to quantify roughness and particle size, and topenetrate below the surficial dust layer to reveal buried morphologies.
OCR for page 96
TABLE 12.1 Outstanding Mars Exploration Science Issues Topic Priority Issues Current Plan Future Possibilities 1. Deep interior Size of the core Interior activity Solidity of core Passive seismic network deployed by NetLander Crust Thickness Structure None Active seismic experiments Heat flow Geothermal gradient None Thermal probes in drilled sample holes Gravity field More detailed gravity map than that of MGS Measured by MRO Magnetic field Complete survey of spatial distribution None Low altitude magnetic mapping Rock magnetization None In situ rover magnetic field and mineralogy Study of returned samples 2. Geochemistry, petrology Rock compositions at selected localities MER, Beagle 2, MSL Extend coverage of sampling Study of returned samples Rock compositions beneath near-surface altered zone None Drilled samples Areal geochemistry Outline geochemical provinces, relate to rock compositions Measurements by MO, ME, MRO 3. Chronology, stratigraphy Crystalline rock ages None Dating of returned samples, rocks in situ Tie cratering record to dated surfaces Crater record improved by MGS, ME, MRO, CNES Orbiter, but dependent upon rock dating Dating of returned samples, rocks in situ Tie stratigraphic column to dated samples Stratigraphic data from MGS, ME, MRO, CNES Orbiter, but dependent upon rock dating Dating of returned samples, rocks in situ 4. Surface processes Effects of water Detailed MGS, ME, MRO observations of channels Correlation with rock compositions by MER, Beagle 2, MSL Study of returned samples Effects of wind Detailed observations of eolian deposits by MGS, ME, MRO Study of returned samples Volcanism Detailed MGS, ME, MRO observations of volcanic morphology Correlation with rock compositions found by MER, Beagle 2, MSL Study of returned samples Impact cratering MGS, ME, MRO cratering record improvement Surface alteration MER, Beagle 2, MSL in situ observations Correlation with spectral mapping by MO, ME, MRO Study of returned samples
OCR for page 97
Topic Priority Issues Current Plan Future Possibilities 5. Water Water in the atmosphere Measurements by MO, ME, MRO, NetLander Reflight of MVACS payload from lost Mars Polar Lander Near-surface water in the planet GRS on MO Reflight of MVACS payload from lost Mars Polar Lander Deep water in the planet Radar Sounder on ME, MRO, NetLander Active seismic and electromagnetic studies Evidence of water in the past Detailed observations of channels by MGS, ME, MRO Study of returned samples 6. Life Extant life None Study of returned samples Fossil life None Sample return (especially sedimentary rocks) Organic material, oxidants MER, Beagle 2, MSL Study of returned samples 7. Atmosphere Composition ME (PFS, SPICAM) Beagle 2 (ATMIS, GAP) MRO (PMIRR-MkII, MARCI) In situ isotopic composition surface material measurements Diurnal compositional measurements from orbit Meteorology network; Mars surface upward-looking spectroscopic measurements General circulation Earth-based observations Meteorology network History From geologic and geochemical evidence (see 3, 4, 6, and 8) 8. Climate change Interannual variability ATMIS on NetLander Long-lived surface stations Quasi-periodic variability High-resolution MRO observations of layered polar deposits Landed studies of polar layered deposits Long-term climate change Stratigraphic studies (see 3 above) Geomorphic studies (see 4 above) Mineralogic studies (see 2 above) Study of returned samples 9. Thermosphere, exosphere, ionosphere Dynamics of upper atmosphere None Optical remote sensing or in situ measurements of winds from a future (to be determined) orbiting spacecraft Escape of hot atoms Nozomi, ME Nozomi, a compromised mission; additional measurements needed Escape of ions Nozomi Nozomi compromised; need additional measurements Interaction of crustal and interplanetary magnetic field Nozomi Nozomi compromised; need additional measurements Energetics of the ionosphere Nozomi Nozomi compromised; need additional measurements
OCR for page 98
TABLE 12.2 Comparison of Recommended Science Priorities with Experiments on Projected Flight Missions
OCR for page 99
MARS SAMPLE RETURNS Sample-return missions are technologically very challenging. Much must be accomplished in the years between now and the putative earliest date (2011) when the first sample-return mission might be launched. COMPLEX urges that NASA redouble its efforts to develop the essential technologies and infrastructure necessary to make the first sample return a reality in that time. The sample-return strategy is ambitious and exciting, but as currently defined, it depends on numerous technologies and strategies that have not been attempted before. These include precision landing and surface operations, a robust Mars sample collection and containment capability, a Mars ascent vehicle, a strategy for reliable sample recovery and Earth return, and an Earth-based quarantine facility with plans for sample handling and sample distribution to the sample analysis community. Timing of the First Sample-Return Mission An important question concerns the timing of sample return. There are two different points of view about when the first sample-return mission should be flown. From one perspective, because the technological and cost requirements are so great for sample-return missions, it is essential for the first sample returned to contain vital information relative to the biological potential of Mars. The other perspective is that a state of diminishing returns has been reached (after the missions through 2005) in acquiring data to identify promising sites; enough is known now to select fruitful sites, and the best strategy is to move to sample return as quickly as possible to guide future Mars exploration. COMPLEX elaborates below on these disparate viewpoints. The first viewpoint argues that it is likely that the costs of sample return will be high, both in spacecraft resources and in the Earth-based infrastructure to receive and house the samples, and therefore that the number of sample-return missions flown will be small. Sample return should be deferred, therefore, until everything has been done that can be done with remote sensing and through numerous in situ measurements of key indicators such as reduced carbon, to ensure that the samples with the most compelling potential to answer the question, Did life ever arise on Mars?, are obtained. Underlying these arguments, to some extent, is the fear that without the reassurance of exhaustive remote-sensing surveys, the first samples returned may turn out to be indistinguishable from SNC meteorites, and that in such a case, further support of sample return would be jeopardized. Those with the opposing point of view hold that enough will be known from remote sensing and in situ measurements by 2011 to mount a fruitful mission, and that sample return should be expedited. By the end of the 2005 missions, most of the critical measurements that are needed to guide surface sampling will have been made from orbit. However, the true meaning of the remotely acquired data will not be fully understood because of the lack of ground-truth. Landed science missions will provide some ground-truth, but they are no substitute for the laboratory examination of surface materials. Experience on Earth and on the Moon has shown that significant advances in exploiting remotely sensed data require integration with samples and a knowledge of surface properties. Early sample return will be essential for optimizing future efforts to find the most compelling biological sites. Experience must be gained in the technically complex and risky activity of collecting and returning martian samples, in the process maximizing the value of the remotely acquired data sets for future exploration. Those in this camp argue that it is by the second or third sample-return mission, with attendant periods during which data are evaluated, that the most compelling sites are likely to be revealed and sampled. COMPLEX emphasizes that answering the question, Did life ever arise on Mars?, must be approached from a broad understanding of the planet and its history. Investigators must be as prepared for an answer of no as well as for yes. It is the committee’s view that the goal of the “wait for a perfect sample” viewpoint is too narrow in scope; that approach risks failure if an uninformative sample is in fact returned, and it risks interminable delays if the remotely acquired orbital and in situ data are held by some to be equivocal. COMPLEX notes that sample return “as soon as possible” will not occur all that soon (=2011), and in the interim there is time to make additional remote-sensing measurements in support of the first sample return. The committee argues (see Chapter 11) that, with or without additional remote-sensing studies, there is no danger that the surface samples returned by the first mission will be identical to SNC meteorites, or that they will be uninteresting, whether or not they contain evidence bearing directly on the question of martian life.
OCR for page 100
The committee considers this to be a measured strategy, one that provides an opportunity to learn and to focus on the best sites through experience. It is very possible that studies of the returned samples will reveal new ways of using existing remotely sensed data to seek promising environments for future sampling. COMPLEX fully endorses a 2011 launch date for the first sample-return mission in NASA’s plan. Since the study of martian samples in terrestrial laboratories will advance our understanding of Mars to a new level, this date should not be allowed to slip (in the absence of some extraordinary technological problem). Even if the 2005 missions provide essentially no additional information about possible landing sites, we will have visible images at very high spatial resolution, maps made in both near- and far-infrared wavelengths, and maps of elemental composition. This is enough information to define a half-dozen or more sites that would be excellent starting points for an undoubtedly long campaign of martian field sampling. Moreover, nothing will help with landing-site selection as much as experience gained at the first site, wherever it is. Developments in flight technology can and should continue beyond 2011, but those developments also will be most significantly aided by experience with the realities of an active sample-return program. Successful and productive programs usually involve a series of missions. Flights at the beginning emphasize—and catalyze—technology development. Those later in the series are more productive scientifically both because the technology is better and because the scientific questions and strategies are continually refined. At Mars, a further constructive interaction can be foreseen between a sample-return program and orbital and in situ robotic investigations. (Though the program strategy does not discuss plans for a continuation of orbital and in situ investigations concurrently with sample return, allowance for such missions as new ideas develop should be part of the plan.) Characteristics of the returned samples will suggest objectives and methods for remote-sensing techniques and identify optimal objectives for robotic missions. It will be far better and more realistic to plan for multiple missions and to begin them as early as possible, than to concentrate on attempting to design and manage one or two “perfect” sample-return missions. Recommendation. Because returned samples will advance Mars science to a new level of understanding, COMPLEX endorses the high priority given to sample return by earlier advisory panels, and it recommends that a sample-return mission be launched at the 2011 opportunity. How Many Sample-Return Missions Are Required? Since sample-return missions are expensive, a key question is how many of them are needed. The following factors, partly based on earlier experience with the Apollo program, are important to consider in planning a program of Mars sample return. Chronology Measuring the ages of Mars rocks is a crucial goal of the sample-return missions. Ages are needed to calibrate the temporal significance of crater densities on Mars and to attach ages to units in the stratigraphic column of Mars (see Chapter 4), as has been done for the Moon. To accomplish this, more than one site should be sampled in areas representing each of the major martian time periods (Noachian, Hesperian, Amazonian). The need for this duplication of missions lies partly in the uncertainty that a particular mission will be able to sample bedrock and partly in the fact that multiple objectives will be set for each mission, which will make accomplishment of the chronological goal less certain. Ages measured on returned samples are far more precise and can provide more kinds of information than can the in situ age measurements discussed above in this chapter. Sample Provenances The association of detached rocks collected at a landing site with the bedrock underfoot may not be straight-forward. After two lander missions (Viking and Mars Pathfinder) to Chryse Planitia, uncertainty still remains as to
OCR for page 101
whether the rocks there are from the local substrate or were delivered hundreds or thousands of kilometers laterally by outflow channel floods. Thus, the association of rock ages with ages of the surfaces on which they are collected can be uncertain, and multiple sample sites may be needed to verify the relationship. Life on Mars The best environments for extant Mars life are probably below the cryosphere (see Chapter 6), which will be very hard to sample. It will likely be hard or impossible to find live or recently-live organisms in most martian surface environments. However, samples from below the cryosphere may be obtainable in special settings (e.g., among outflow channel deposits), and samples from surface expressions of hydrothermal or aqueous systems may also be obtainable. Intelligent analysis and multiple attempts will be required to collect samples with martian life in them, if there is any. Technology Development It is not known how effectively the surface of Mars can be sampled or how readily the rocks collected can be returned. It is worth remembering that the Apollo 7, 8, 9, and 10 missions did little more than test out equipment on the way to the lunar landing. Apollo 11, 12, and 14 were mainly occupied with learning how to sample, move around, and explore. Only the last three Apollos—15, 16, and 17—were serious exploration missions by the criteria of stay-time, experience, crew training, and mobility. Similarly, a certain number of the Mars sample-return missions will be, effectively, technology-development missions. Increasingly Specialized Needs, Techniques, and Targets The first Mars sample-return mission should and will employ simple, minimal sample collection techniques. Lessons learned from this mission, and particular sampling needs indicated by the first samples returned (as well as continuing robotic exploration of Mars), will guide the design of increasingly sophisticated follow-on missions. This will repeat the Apollo experience, in which studies of the relatively simply collected Apollo 11 and 12 samples showed the need for more specialized sampling techniques on later missions: for example, when the importance of centimeter-sized lithic fragments in the lunar soil became apparent, sievelike collecting scoops were created for the astronauts to permit fine dust to fall through while retaining the pebbles. Serendipity The only thing we can be absolutely sure of finding on Mars is something not expected now. It will be important to keep enough slack in the schedule of sample-return missions to take advantage of the unexpected. For example, if the discovery by Pieters of spectral evidence of olivine in the central peaks of Copernicus on the Moon,1 signaling the presence there of crater debris from the lunar mantle, had been made a dozen years earlier, one of the Apollo sample-return missions probably would have been sent there. This increases the number of Mars sample-return missions that should be contemplated. Number of Sample-Return Missions How many sample-return missions are indicated by these considerations? COMPLEX believes the most useful answer it can supply is that one sample-return mission will not be enough, nor will two or three. The discussion above suggests that on the order of 10 missions may ultimately be required to learn the most important things about Mars, with perhaps three required for initial technology development and shakedown, four focused on planetary chronology, and three on major processes and exobiological sites (not necessarily in that order). Late in the sequence of missions, the committee anticipates that sampling technology may have matured to the point of including a drill capable of reaching depths unaffected by the hostile martian surface environment, and the range
OCR for page 102
of sampling operations may have expanded to include exploration of the polar caps, with the collection and return of ice and dust cores. Ten is a daunting number of sample-return missions to contemplate. However, after drawing lessons from the Apollo program, COMPLEX wishes to stress an important difference between the Mars Exploration Program and Apollo. Apollo was a “crash program,” in which the initial plan was to land 10 manned spacecraft on the Moon in the space of about 4 years. Economy dictated the crowded schedule; it was important to terminate the contracts that provided flight systems and technical support for the missions as soon as possible. COMPLEX is confident the Mars sample-return program will not be executed in this fashion. The committee anticipates, and recommends, that Mars sample-return spacecraft be acquired singly or perhaps in pairs, over a period of years, probably many years. A protracted schedule increases the credibility of the concept of 10 sample-return missions. It will also allow adequate time for the information gained by each mission to be digested and fed into the planning of the next mission, and it will permit substantial redesign of the spacecraft and sampling system between missions. The schedule of the Apollo program and the purchase of multiple copies of the same system greatly limited evolutionary development of this sort. Recommendation. It should not be anticipated that a few (two to three) Mars sample-return missions will serve the need for samples from that planet. No single site or small number of sites on Mars will answer all of the important questions about the planet, and in any case, the earliest sample-return missions will be in large part technology-development missions. Some 10 sample-return missions, spread over a substantial period of time, may be required to answer the important questions about Mars. Preparations on Earth for Sample Return A series of advisory panels have considered the special problems associated with bringing samples from Mars to Earth,2,3,4,5 and NASA has acknowledged the need to prevent forward and back contamination at every stage of the process of delivery. This includes the need to construct a quarantine facility to receive and contain the samples. NASA’s actions to date have consisted of naming a Planetary Protection Officer and sponsoring panel discussions, including those cited, but no concrete action has been taken toward the construction of such a facility. In a recent report COMPLEX drew attention to the long lead time required to prepare a quarantine facility for the reception of Mars samples once they are delivered to Earth.6 On the basis of prior experience with terrestrial biocontainment facilities and the Apollo Lunar Receiving Laboratory, the committee estimated that 7 years would be required to design, construct, and staff the facility. To this must be added the time needed to clear an environmental impact statement and to carry out several reconnaissance studies that are needed to inform the design and operation of the facility.7 The aggregate of time required will strain the schedule even of a 2011 launch (with a 2014 return). The message is plain: preparations for a sample return should not be delayed any longer than they already have been. Recommendation. Scientific research and design studies that must precede the design and construction of the Mars Quarantine Facility should begin immediately. Decisions should be made immediately about the siting and management of the facility. Design and construction of the facility should begin at the earliest possible time. (This recommendation also appears in Chapter 7.) OTHER ISSUES RELATING TO MARS EXPLORATION Power Supply for Landers and Rovers An extremely important consideration in establishing the capabilities of landed packages, static or roving, on Mars is the power supply on which they rely—the options being solar panels and radioisotope power systems. This
OCR for page 103
becomes important with the Mars Science Laboratory, an advanced rover scheduled for launch in 2007,d and later landers, including sample-return missions. The Viking landers operated for 7 years; they were able to do so because they were equipped with radioisotope power systems. On the other hand, the Mars Exploration Rovers (MERs) to be launched in June 2003 are equipped with solar panels, and are not designed to operate for longer than an estimated 90 days. The lifetime is limited because as the maximum elevation of the Sun in the martian sky declines, the available solar power decreases; for the same reason, the rovers get colder and need more power to keep warm. Meanwhile, dust accumulates on the panels, further reducing the power. The MERs are also constrained by the needs of their solar panels to land in the 10°N to 15°S latitude belt. Because of the 90-day time limitation, the MERs will be able to stop and make measurements on rocks no more than six times. These are very severe limitations on the utility of the MER system. The situation for MSL is similar if it is powered by solar panels. A possible solution would seem to be to increase the size of the solar array. This would be feasible on a stationary lander, but it would be problematic on a rover, which the MSL is, because solar panels that overhang the sides of the vehicle would be vulnerable to damage as the vehicle drove over a rocky surface. Thus, the MSL also will have a limited lifetime and range if it is solar-powered. A drill has been projected for the MSL, to collect samples from up to 2 m in depth; but drills are power-hungry, and they need time. Appendix A (see the subsection “In Situ Analysis and Returning Samples to Earth”) acknowledges that the use of a drill requires “reliable, long-lived power.” The other functions of MSL also require better than solar power. The power problem could have a serious impact on a sample-return mission. Reliance on solar power would mean that samples would almost certainly have to be collected at low latitudes, which excludes those parts of Mars where ground ice is stable and where other volatiles are most likely to be present. Again, if the sample-return mission has a rover to collect samples, its lifetime will be short. This, coupled with the slowness of moving across the martian surface and the necessity of looping back to the lander to deliver samples, will restrict the region of sampling to very close to the lander, probably within a radius of a few hundred meters. An option might be to acquire the samples by drilling for a long time from a stationary lander with a very large solar array. The message is that the more sophisticated stages of Mars surface exploration will be severely limited if they are forced to rely on solar power, and it seems essential that these missions be equipped with radioisotope power systems. Radioisotope power systems have been considered off-limits for the last few years because the pertinent nuclide, 238Pu, has not been produced in the United States during that time. However, a record of decision dated January 19, 2001, on a programmatic environmental impact statement concerning Department of Energy nuclear facilities, stated the intent of the Department of Energy to reestablish a domestic capability to produce 238Pu for future space missions. That record of decision also allowed for interim purchase of Russian 238Pu, if required. COMPLEX understands that even if they are available, radioisotope power systems are expensive. However, from a science viewpoint the advantages of nuclear power—long-lived missions and access to any point on Mars—are clear. COMPLEX urges the use of nuclear power sources, if at all feasible, on advanced Mars lander missions. The Scout Program The Mars Scout program provides an excellent opportunity for NASA to accommodate science topics outside the themes of water and life that are the focus of Mars missions. If the Scout program is to be modeled after the successful Discovery program, it is essential that the science goals for the Mars Scout program be directed toward the highest-priority science for Mars, and not only toward the themes of water and life. There is concern in the Mars science community that the Scout program, the youngest and smallest element of the Mars Exploration Program, may also be the most vulnerable. The fear is that the Scout program may not achieve its potential because it will be sacrificed in times of budget stringency. COMPLEX considers that this would be unfortunate. d Following the completion of this study, NASA announced that it was delaying the launch of the Mars Science Laboratory until 2009 to allow time to develop an advanced, radioisotope power system for this mission.
OCR for page 104
Recommendation. So that the Scout missions can fulfill their laudable goals of filling in gaps in the Mars Exploration Program and allowing a rapid response to scientific discoveries, COMPLEX recommends that care be taken to maintain this program as a viable line of missions when budget problems arise. Data Analysis, Ground-Based Observations, and Laboratory Analysis The Mars Exploration Program, with its missions at 2-year intervals, presents a new problem in fully exploiting the amount and variety of data that will be collected. The volume and quality of data returned by MGS alone have been extraordinary, and the analysis of these data is only beginning. With the rapid pace of Mars missions planned for the next decade, the flood of data can be expected to increase. This problem should be recognized, and NASA’s data analysis and science programs should be structured to accommodate and support the broad range of Mars science that is to come. While the Mars Exploration Program consists of flight missions, exploration and understanding of the planet as a system also depends upon other modes of data acquisition, including ground-based and Earth-orbital observations, antarctic meteorite studies, laboratory analysis, and theoretical modeling (see Chapter 1). These are all essential components of Mars science. Recommendation. A plan should be developed at the program level, not at the level of each mission, for archiving and making accessible the data to be gathered by the Mars Exploration Program. It is essential that support be provided for the study and exploitation of this body of data. Recommendation. COMPLEX endorses continued support for nonflight activities such as ground-based observing and laboratory analysis. REFERENCES 1. C.M. Pieters, “Copernicus Crater Central Peak: Lunar Mountain of Unique Composition,”Science215: 59–61, 1982. 2. NASA Astrobiology Institute, Mars Program Architecture: Recommendations of the NASA Astrobiology Institute, Ames Research Center, Moffett Field, Calif., 2000. 3. NASA, Mars Sample Handling and Requirements Panel (MSHARP) Final Report, NASA/TM-1999-209145, Wash-ington, D.C., 1999. 4. Space Studies Board, National Research Council, Mars Sample Return: Issues and Recommendations, National Acad-emy Press, Washington D.C., 1997. 5. NASA, Mars Sample Quarantine Protocol Workshop: Proceedings of a Workshop Held at NASA Ames ResearchCenter, June 4–6, 1997, NASA/CP-1999-208772, Washington, D.C., 1999. 6. Space Studies Board, National Research Council, The Quarantine and Certification of Martian Samples, National Academy Press, Washington, D.C., 2002. 7. Space Studies Board, National Research Council, The Quarantine and Certification of Martian Samples, National Academy Press, Washington, D.C., 2002, p. 2.
Representative terms from entire chapter: