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Review of NASA's Planned Mars Program 4 Key Issues for NASA's Mars Exploration Program In the course of COMPLEX's examination of NASA's program for the exploration of Mars, several issues arose. They are as follows: q The completion of global mapping, q Enhancing mobility, q The adaptability of the program, q The role of international partners, q The development of instrumentation, and q Sample return. These issues are discussed in the following sections. COMPLETE GLOBAL MAPPING Various science advisory groups are consistent in emphasizing the need to complete the global mapping originally planned for Mars Observer. Such mapping will place our current martian data in a global context and should lead to major revisions in our understanding of how the planet has evolved. It will accordingly provide a general framework for planning future exploration. A prerequisite for future studies of Mars is the reflight of those Mars Observer instruments not scheduled to be carried by Mars Global Surveyor, at the earliest possible date (i.e., PMIRR in 1998 and GRS in 2001). Completion of the objectives of Mars Observer does not necessarily imply that no more remote sensing will be necessary. Discoveries by the program in the next 5 years may indicate the need for further orbital observations at a later time. At present we have limited knowledge of the chemical, mineralogic, and lithologic variety present at the martian surface, the scales over which such differences occur, and the remote-sensing techniques best suited for their detection. Hydrothermal deposits, for example, are of special interest for exobiology
may be so localized as to not be detectable unambiguously by the 3-km resolution of the TES instrument to be flown in 1996. Consequently, high-resolution remote sensing data for selected areas and ground-truth measurements will be important for understanding the global data sets. The medium-range strategy should therefore be sufficiently flexible that it can adapt to new knowledge, and be amenable, for example, to schedule additional remote-sensing missions should they prove necessary. ENHANCE MOBILITY To address the main scientific goals of the Mars Surveyor program, mobility is necessary at the landing sites. While three components of the planet are available for measurement (the atmosphere, the loose material at the surface, and the solid planet), just the first two are expected to be accessible at most sites. Thus only some components of Mars can be effectively characterized without significant mobility. The atmosphere and soils are expected to furnish important information on the climatic history of the planet. The dynamics of the present atmosphere supplies clues about the behavior of past atmospheres, while the atmosphere's chemistry provides information on its original composition, losses from the upper atmosphere, and interactions with the surface. Similarly, the soil may suggest weathering conditions in the past and indicate how near-surface volatiles migrate. Unfortunately, the atmosphere and the soil have serious drawbacks in terms of contributing to understanding of biology and long-term climate. Since the records contained in the atmosphere and soil are cumulative, with each sample representing a single point on an evolutionary path, many paths could lead to today's conditions. Thus any reconstruction of planetary history from such records is ambiguous. The rock record, in contrast, displays discrete events whose time sequence can be reconstructed. Each rock unit chronicles a specific occurrence such as a volcanic eruption, a flood, a lake's evaporation, or a large impact; from the ordering of the units the events can be placed in a time sequence. A rock's lithology, mineralogy, chemistry, and other characteristics give evidence of the prevailing conditions under which the rock was deposited. Rocks are commonly preserved by deposition and burial. Through this process, past atmospheres, organics, or hydrothermal mineral assemblages may be insulated from subsequent surface conditions, thus preserving them so that they may be available for scrutiny. The solid rock record has the highest potential, therefore, for providing unequivocal evidence for past life: the most favored targets are lacustrine sediments, ancient hydrothermal deposits, and evaporites. Rocks can also indicate past climates through fluvial deposits, trapped atmospheres, and weathering horizons. Finally, the rock record supplies the best documentation for how the solid planet has evolved. In order to address the scientific goals of the Mars Surveyor program, therefore, the rock record must be accessed and, to accomplish that, mobility is essential. Unlike the atmosphere and soil, the rock record is very heterogeneous and scattered. The rock types of greatest interest for studies of water, climate, and life are likely to be present at only a few locations.
The remote-sensing data will suggest where best to go, but landers will need to carry components that are able to move about and search areas of interest, rather than be restricted to examining whatever happens to be close to the lander upon its arrival. In addition, some sites of high interest for the study of environments conducive to life, such as lacustrine/fluvial sediments and mineralized zones around hydrothermal vents, are very localized and virtually impossible to target with the existing navigational capabilities. Thus, in order to have a reasonable chance of successfully addressing questions of climatological and biological interest, the landers need to be equipped with rovers that move about, explore, and sample candidate sites. The distances required to move cannot be stated precisely without more information on Mars, but in view of the heterogeneity of the martian surface, the landing errors anticipated, and the prospects for future rover development, mobilities of tens of kilometers appear necessary (see Table 1 for the specifications of Sojourner, Marsokhod, and a strawman advanced rover). Such rovers must carry appropriate instruments since the purpose of mobility is to make measurements at different places away from the immediate landing site. Development of a roving capability is, therefore, intimately connected with the need for miniaturization of scientific instruments, discussed below. These mobility requirements may be mitigated partly by improving the accuracy of landing; nevertheless, even with a capability to do a pinpoint landing, some mobility will be essential to sample a variety of sites. A major issue for the Mars Surveyor program is that the small landers planned for the next decade are likely to have very limited mobility. The 1998 lander, for example, has a total payload mass of 22 kg. Owing to the need to accommodate science instruments and devices for the manipulation of samples, little mass is left to enable significant movement around or from the landing site. COMPLEX considers this inadequate to meet the stated objective of accessing the information contained in the martian rock record in a variety of terrains. 2 Because of the essential information provided by detailed analyses of rock fragments at different sites, NASA must aggressively pursue ways to enhance the mobility of landers and other vehicles in order to allow measurements to be made on a variety of rocks and terrains. This development can evolve as lander designs and technologies advance. International participation-for example through the use of Russian launch vehicles and Marsokhod minirovers, and French balloons-would also improve the opportunities for mobility on, or close to, the martian surface. ASSURE AN ADAPTABLE PROGRAM At present the Mars Surveyor program is highly structured, with guidelines calling for pairs of launches at each launch opportunity, and for all U.S. launches to be on Med-Lite vehicles. Because of launch energy constraints, any lander planned for the 2001 launch opportunity is likely to be a downscaled version of the 1998
lander, and the 2003 landers could, perhaps, be even smaller. The main opportunities for adaptation to scientific findings and new concepts are in the choice of landing sites and, possibly, improved instruments. Nevertheless, within the guidelines various mission types can be accomplished, including small landers as planned, balloon missions, and networks of miniature meteorology stations. NASA's experience with Discovery proposals demonstrated that the space science and engineering communities are extremely resourceful in designing a wide variety of highly innovative and cost-competitive mission concepts. COMPLEX has stressed the virtues of flexibility in small mission programs. 3 Given the relatively long life of the Mars Surveyor program, advances in both technology and our knowledge of the martian surface and atmosphere should permit revolutionary changes in how program goals might be best achieved. In view of today's incomplete understanding of Mars, plus the complex and dynamic nature of the planet, it is highly desirable to maintain maximum flexibility in the program to allow for adaptation to any new discoveries and unpredicted opportunities. The program should be sufficiently flexible to incorporate such changes, in addition to the incremental improvements envisaged for the small landers as they evolve from 1998 through 2003. COMPLEX is concerned that strict adherence to the guidelines of multiple launches at all opportunities may be too constraining. The planetary science community accepts the necessity to stay within the mandated cost cap, but also recognizes that different strategies may be followed within the cost constraint. The current plan of having multiple flights during each launch window has the advantage of distributing risk, thereby reducing the chance of complete failure at any launch opportunity. However, in some circumstances, this benefit may be offset by the scientific advantages of placing a single larger capability at Mars, as mentioned in the preceding section, where mobility and scientific capability are both seen to be required for effective landers. Under the present guidelines, it may not be possible to place a long-range, well-instrumented rover on the surface of Mars, particularly one equipped with, for example, sophisticated exobiologic instruments. Sample return is another example: despite major advances in determining how this long-time goal of Mars exploration might be accomplished at modest cost, sample return still appears to need larger payloads at Mars than can be delivered currently with Med-Lite launch vehicles. How can the Mars Surveyor program retain the adaptability to take advantage of scientific and technological opportunities as they become available? Ideally, mission profiles should not be fixed earlier than two launch opportunities prior so that advisory groups can monitor progress and propose necessary program changes. NASA should investigate the realism of this suggestion and also study the use of different permutations in the number of launches and launch vehicles. ENGAGE INTERNATIONAL PARTNERS At the present time NASA has three potential international partners for joint missions to Mars: Japan, Russia, and the European Space Agency. In addition,
many other partners are available for cooperation at lesser levels, such as provision of science instruments. Russia is supplying some components of PMIRR to be flown in 1998 and a lidar system for the 1998 polar lander. NASA is building the neutral mass spectrometer for the Japanese mission Planet-B to be flown in 1998. Talks are under way concerning the possibility of a joint U.S.-Russian mission that might involve the use of a Russian launch vehicle and a Marsokhod minirover. ESA and NASA conducted a joint study of the Intermarsnet mission, for which NASA would supply three or four small landers whose data would be relayed to Earth via an ESA orbiter. These landers would form a small seismology and meteorology network, as well as make geochemical measurements. The meteorology network could be supplemented by a U.S.-only launch of very small meteorology landers. These cooperative missions provide an important means for substantially improving the overall science return of the Mars Surveyor program and, more importantly, for filling in important scientific gaps (e.g., global seismology and aeronomy) in NASA's program. COMPLEX commends NASA for aggressively pursuing these opportunities and encourages additional efforts to engage international partners, both at the missions level and in mutual provision of other capabilities such as relay links and science instruments. DEVELOP MICROINSTRUMENTS The success of the Mars Surveyor program beyond completion of the Mars Observer goals will depend largely on the capability of the landers deployed on the martian surface. In this regard, the program is seriously constrained by the performance of the proposed Med-Lite vehicle. Expectations are that the useful mass of NASA's Mars landers will decline substantially between 1996 and 2003. It is possible, however, that reductions in spacecraft mass may be counterbalanced by developments outside the Mars Surveyor program. In addition to Mars Surveyor 1998's pair of microlanders, the New Millennium program will, 4 for example, develop and test new materials, avionics, and spacecraft designs that could lead to significantly smaller spacecraft. Nevertheless, the ability to reduce the mass needed to achieve a successful landing on Mars will have limited value to the Mars Surveyor program unless accompanied by a parallel development of microinstruments, that is, instruments significantly smaller than those currently available. It serves little purpose to deliver small spacecraft to the surface of Mars unless they have useful analytical capabilities. Sophisticated microinstruments will be increasingly required as the Mars Surveyor program matures. Not only will lander masses likely decline, but the relatively easy, exploratory observations also will have been performed. Later landers are likely to be smaller and yet be required to conduct more sophisticated observations. As emphasized in the section above on mobility, the rock record must be examined to enable progress toward understanding the climatic and exobiologic history of the planet, and this effort will require significant mobility. In addition, to do anything other than remote sensing, eventually we will need devices to acquire and prepare
samples. All these capabilities must be included within the payload allocation. Landers will need to measure the mineralogy of primary silicates and low- temperature volatiles, and isotopic compositions of atmospheric gases as well as of stable and radiogenic species in secondary minerals such as clays and carbonates; to detect and characterize organic matter; and to determine various trace elements. It will also be necessary to examine microscopically both in situ and prepared samples, and to carry out various kinds of mineralogical and chemical analyses under the microscope. Many analytical techniques require sample preparation. The development of sample-acquisition and sample- preparation techniques must therefore be a part of NASA's instrumentation program. Funding within the Mars Surveyor program itself is too limited to foster significant advances in instrument designs. Although NASA does have a relevant program for this, the Planetary Instrument Definition and Development Program (PIDDP), it is relatively modest and concentrates on instruments that are likely to be chosen for flight in the near term. Previous reports by COMPLEX 5 and other National Research Council committees 6six have recommended that NASA devote more attention to the provision of instruments for small-spacecraft missions. The long- term development of microinstrumentation could be a part of, or independent of, the New Millennium program. MAINTAIN GOAL OF SAMPLE RETURN Sample return has long been a goal of Mars exploration. 7 Many critical measurements are too complicated and interactive to perform remotely on a distant planet in the foreseeable future. Determinations of absolute ages, for example, strain the capabilities of terrestrial laboratories yet are essential for understanding how a planet has evolved. Only very recently have techniques been developed for determination of D/H and oxygen isotopes on different temperature extracts from Shergotty-Nakhla-Chassigny meteorites that are believed to be from Mars; these measurements have proved invaluable in improving our understanding of the evolution of water on the planet. Because trace elements are so strongly fractionated in geologic processes, they are excellent markers of past processes. Yet many of the determinations are very difficult measurements to make even in sophisticated terrestrial laboratories. Microscopic techniques for detection of Archean life on Earth have been developed only in the last two decades and would be extraordinarily difficult to employ remotely at Mars. The challenge presented in conducting these critically needed analyses on a remote planet is a primary reason for requiring returned samples. Furthermore, the diverse nature of Mars argues that samples will need to be returned from many sites, presumably necessitating several flights or long-range rovers before a fairly complete inventory will have been taken. Sample return can also be justified for operational reasons. Spacecraft to Mars will be able to carry only a limited number of instruments and do a restricted set of
measurements. It is not possible, a priori, to identify the most critical measurements to make. Having samples on Earth enables a wide range of measurements to be made and permits adjustment of the measurement strategy in response to previous analytical results. New techniques can be developed in response to results obtained from the samples themselves. Experience with the Moon emphasizes the enormous value of returned samples when placed in the context of global data. The lunar sample data provide the basis for almost all our ideas about how the Moon evolved. With lunar samples in hand, the analytical approach became wide-ranging and flexible, such that the emphasis could shift as the meaning of each set of results became clear. These advantages should be even greater on Mars, given the more complex geology and the possibility of past life. The above ideas are not new. The Committee on Human Exploration of the Space Studies Board recently stated that "to take best advantage of human capabilities in scientific exploration, it will be desirable, some argue essential, to return reconnaissance samples from Mars prior to human exploration." 8 The reasons for this are those above-an improved knowledge of martian processes and history, which "will permit a more informed choice of the landing sites . . . and the types of investigations [to conduct]" 9nine-as well as concerns about planetary quarantine and soil toxicity. The projected costs of sample return and the launch masses required have both declined dramatically in the last decade. Despite this, it is still not clear whether sample return can be achieved within the mass and cost constraints placed on the Mars Surveyor program. This is especially true if rock samples, as argued above, are to be returned. COMPLEX is encouraged that NASA continues to retain sample return as a goal and that the agency has recently indicated that the number of launches and launch vehicle constraints originally placed on the Mars Surveyor program may be relaxed provided that there are compelling reasons. COMPLEX re- emphasizes the importance of returning samples of martian soil, atmosphere, and, especially, rocks and commends NASA for retaining sample return as part of the Mars Surveyor program. Taking this option, however, places the onus on NASA to explore ways that will allow samples (including rocks) to be returned within the constraints of the Mars Surveyor program. If studies show that this is not possible, then Mars sample return will need to be considered as a stand-alone, high-priority scientific program that must compete against NASA's other scientific goals. REFERENCES 1. Exobiology Program Office, Exobiological Strategy for Mars Exploration, NASA, Washington, D.C., 1995. 2. Space Studies Board, National Research Council, 1990 Update to Strategy for Exploration of the Inner Planets, National Academy Press, Washington, D.C., 1990, p. 22.