Rationale for Sample Return
Chapters 2 through 10 briefly summarize current knowledge of Mars. This chapter has a different purpose: It focuses on the concept of Mars sample return—the immediate goal toward which the Mars Exploration Program is building. Until recently, NASA had been planning for the first element of a sample-return mission to be launched in 2005. Currently, however, the first such mission is to be no earlier than 2011. Although this delay is unfortunate from a scientific perspective, technological and fiscal reasons probably dictate it.
Mars has experienced a complicated history that has created a wide variety of surface and subsurface environments. To select among these during the search for life, much more will have to be known about their origins, histories, and relationships. It will be practically impossible to develop that kind of information at the required level of detail without having samples returned to Earth for study with the full range of laboratory instruments and methodology available here. Many studies have shown the advantages of bringing back samples for study in laboratories on Earth,1,2,3,4 but it is worth revisiting the issue.
IMPORTANCE OF SAMPLE-RETURN MISSIONSIN THE FRAMEWORK OF NASA’S MARS EXPLORATION PROGRAM
The importance of analyzing returned samples is considered below in two separate sections. The first is concerned with understanding the nature of the samples themselves—for example, their elemental, mineralogical, and isotopic composition. This information addresses science objectives discussed above relating to geochemistry and petrology, chronology, and climate (Chapters 3, 4, and 9, respectively). The second section deals with understanding the nature of any organic or biological material that the samples may contain, topics discussed in Chapter 7. As these preceding chapters show, ongoing developments have consistently raised the priority for early return of samples.
Some martian samples have already been “returned” to Earth. The SNC meteorites (discussed in Chapter 3) have provided both a tantalizing view of a few martian rocks and a demonstration of how much can be learned when samples can be examined in Earth-based laboratories. These meteorites represent, however, a highly selected subset of martian materials, specifically, very coherent rocks of largely igneous origin from a small number of sources. The samples that could provide the most information about martian climate history are something different—namely, sediments and soil samples (SNC meteorites represent the other end of the rock spectrum). Taking Yosemite Valley as a terrestrial analog, the SNC meteorites represent the cliffs rather than the river muds
and the sediments from the outwash stretching into California’s Central Valley. It is the latter materials that could provide information about timing, chemical conditions, and biological processes, and it is their martian analogs that are sought in sample-return missions.
Geochemistry, Petrology, Chronology, and Climate
Understanding the nature and origin of a rock involves examination of its many properties in great detail, using a variety of techniques. Usually the bulk elemental composition of the rock is determined both for the major elements and for several tens of trace elements that provide strong clues about and constraints on the nature of the differentiation events that led to the formation of the rock. This information is much more valuable when combined with microscopic studies, since rocks contain a nearly infinite amount of information on a microscopic scale, some of it crucial to an understanding of the rock’s origin and history. Detailed petrographic examination of the rock is needed to precisely determine the compositions, amounts, and textures of all the minerals present. Measurement of the isotopic composition of a variety of elements in separated mineral grains allows the age of the rock to be determined, and provides constraints on the differentiation history of the system that gave rise to it. Combination and comparison of these chemical, petrographic, and isotopic signals can provide information about relationships between the components of the system.
If the analytical results can be placed in a planetary context, the informational returns can be much greater. For example, it might be shown that the chemical and isotopic composition of a particular mineral indicated clear relationships to a process or source area already recognized elsewhere on the planet. Alternatively, the interpretation of the initial analytical results might have indicated that a fluid—no longer present—had altered the minerals at some point in their history. Either that fluid itself or further evidence for its presence might show up in materials from other sites. As such data accumulate, a detailed understanding of the evolution and significance of a complex rock can be obtained.
Often, in laboratory studies of important extraterrestrial samples, a team of investigators using different analytical techniques is organized into a “consortium” to study a particular rock. Most notable for Mars studies was a consortium organized by J.C. Laul to investigate the Shergotty (Mars) meteorite.5 The results of this consortium study were published together as a collection of 19 manuscripts in a single issue of a journal, which probably did as much as any other body of research published to date to increase our understanding of the nature of the geochemical processes on Mars. This type of detailed investigation of a sample, where coordinated analyses are made on carefully separated microscopic mineral grains, cannot be done in situ by a robotic spacecraft.
A few related types of measurements can be made in situ, but they do not provide the information needed to thoroughly understand a body as complicated as Mars. For example, it may be possible to determine in situ rock ages based on K-Ar dating, and some researchers believe that these ages may be good enough to calibrate the cratering record on Mars, a very important scientific objective (see Chapter 4 in this report). However, it is known from the study of martian meteorites that their ages are not easily understood. When the data are examined in detail, a complex history of formation followed by multiple disturbances is revealed.6 This understanding was arrived at only by long and arduous studies of different mineral separates made from rocks that were studied by a wide variety of isotope chronometers. K-Ar dating alone could not have provided it.
Learning about the past climate on Mars is another important objective of Mars science, and returned samples offer the best way to understand an important product of past climates. Ultimately it may be possible to return ice cores from the martian poles that directly address the planet’s climate history, but even the first samples collected will contain information about the climate in the layer of weathering products that one expects to find on rock samples. These products will almost certainly be very complex minerals or amorphous reaction products that will tax our best Earth-based laboratory techniques to understand. It is very unlikely that anything but a highly qualitative and ambiguous description of the weathering products could be made by robotic instruments operating on the martian surface.
For these data on weathering products to be most useful for understanding Mars, they need to be considered in the light of remote and in situ observations of stratigraphic layers on Mars (see Chapter 5 in this report). The returned sample will provide a valuable synergism to remote and in situ spacecraft studies, both by providing
ground-truth and by allowing for the design of better instruments in the future that are optimized for detection of the properties that samples are found to have.
Biology and Paleobiology
To date, a single set of robotic studies has searched for extant life on Mars: the Viking life-detection experiments, which were designed to test for organisms that used as their carbon source either carbon dioxide or organic molecules. Although the results obtained from the three sets of experiments are regarded as having shown the materials tested to be devoid of both organic compounds and evidence of life,7,8 this interpretation has been subject to debate.9 The lack of agreement highlights the difficulties inherent in the detection of viable microorganisms by robotic means. Indeed, even were there unanimity that the Viking experiments did not show the presence of life, the experiments could still be criticized as being overly “geocentric” in that they showed a lack of evidence of metabolism only of those types particularly common among terrestrial microbes, not of all conceivable metabolisms (nor even of various redox-reaction-based microbial metabolisms well known on Earth). Moreover, the problem of distinguishing between biological and nonbiological organic material is complicated. The carbon-aceous chondrites, interplanetary dust particles, and probably other bodies within the solar system contain abundant organic material that is structurally similar to biological products. Definitive resolution of the differences between biotic and abiotic organic molecules requires highly sophisticated techniques well beyond any that could be managed robotically.
Thus, at the present state of knowledge, results obtained from any life-detection experiment carried out by robotic means seem likely to be ambiguous: (1) results interpreted as showing an absence of life will be regarded as too geocentric or otherwise inappropriately limited; (2) results consistent with, but not definitive of, the existence of life (e.g., the detection of organic compounds of unknown, either biological or nonbiological, origin) will be regarded as incapable of providing a clear-cut answer; and (3) results interpreted as showing the existence of life will be regarded as necessarily suspect, since they might reflect the presence of earthly contaminants rather than of an indigenous martian biota. Finally, the detection of life robotically is unlikely to be accomplished by a search for either of the two categories of fossil life that might be brought to bear on the problem: stromatolites and microfossils.
Formally defined, a stromatolite is an accretionary organosedimentary structure, commonly thinly layered, produced by the activities of mat-building communities of mucilage-secreting microorganisms (see Figure 11.1). Unfortunately, however, on Earth true stromatolites can be confused with nonbiologically deposited look-alikes— cave rocks, such as stalactites and stalagmites, and hot spring deposits such as those formed where minerals build up in thin, sometimes wavy layers as they crystallize from solution. On Earth, microbes are so widespread that there is practically no place where stromatolitic look-alikes form without life playing at least a minor role. But on a planet where life never got started, there could be many places veneered by thinly layered stromatolite-like deposits unrelated to life—laid down, for example, by repeated wetting and drying or freezing and thawing of mineral-charged salt pans or shallow lagoons. Moreover, it is useful to recall that stromatolites were known on Earth for more than a century before their microbial origin was firmly established.10 Were stromatolite-like structures to be photographed on the surface of Mars, it seems certain that there would be widespread questioning as to whether the objects detected were in fact produced by life.
In a similar vein, it seems unlikely that robotic detection of fossil-like objects in, or on the surfaces of, rocks on Mars would prove sufficiently convincing to demonstrate to an acceptable level of certainty that past life existed on that planet. Although it is likely that optical studies of robotically prepared petrographic thin sections could overcome problems of establishing whether objects detected are indigenous, the crucial problem of demonstrating the biogenicity of the objects in question would remain. Here lessons learned from the search for ancient (Precambrian) microbes on Earth would certainly apply—lessons that well illustrate the error of assuming that microstructures “unlike known mineral forms” can be regarded as “fossils” simply for want of any other explanation, as they have been repeatedly in the past.11,12
In summary, at the present state of knowledge and technological expertise—and, probably, for the forthcoming several decades—it is unlikely that robotic in situ exploration will prove capable of demonstrating to an
acceptable level of certainty whether there once was or is now life on Mars. For the foreseeable future, it is reasonable to expect that such studies can be performed only on samples retrieved from Mars and brought to Earth for detailed investigation in appropriately equipped laboratories.
As noted, samples from Mars are already on Earth in the form of SNC meteorites. These are, however, a very highly selected and probably altered subset of all the possible Mars samples. They include only massive, coherent lithologies that carry no information about martian surface processes. The SNC meteorites were launched from Mars by impact processes with resultant shock and heating, then drifted in space (where possibly they were involved in numerous secondary collisions), and finally fell through Earth’s atmosphere. As samples of Mars they are better than nothing, but they are far from optimal research material.
There are very important advantages to collecting samples on Mars and bringing them to Earth for study. One stems from the ability to look at ever-smaller pieces of samples, as instrumental sensitivities continue to improve. For example, it is now possible, using an ion microprobe, to determine the isotopic composition of a mineral zone or fragment as small as a micron across. It is known from studies of the Apollo lunar samples that often one can find samples of rocks present as soil particles or in breccias (compacted soils) that are derived from inaccessible parts of the planet. A single regolith sample can contain tiny samples of rocks from widely different locations on the planet’s surface. As an example, Wood and colleagues were able to infer the composition of the lunar highlands from rock fragments found in an Apollo 11 soil sample before a highlands site had actually been visited.13 Researchers’ capacity to analyze very small subsamples also means that a returned Mars sample of modest size can be divided and studied by a large number of scientists in laboratories with diverse capabilities.
Additionally, any investigation into an alien world tends to uncover as many questions as it answers. Having samples present on Earth allows investigators to develop and to answer refined, second-order questions. It also makes it possible for observations of particular importance to be checked by multiple investigators using the same or different techniques.
Though robotic missions that perform in situ analyses will continue to add incrementally to the knowledge of Mars, their advances relative to those provided by returned samples will be minor. Even if the first returned samples are not optimal in terms of siting, they will provide a greatly enhanced view of the geologic processes on Mars. Even a grab-sample of soil from a randomly chosen site on the planet will reveal the character of martian surface material: its chemistry, oxidation state, content of organic materials, mineralogy, and the history of weathering reactions that has affected it. Also, the properties of the ubiquitous martian dust will be determined, information that will allow corrections to be applied to the data sets of past and future robotic orbital and lander missions. Detailed knowledge of the surface material will permit a more intelligent choice of measurements to be made by future robotic missions. (None of this information is available from SNC meteorites, which are not surface samples.) Rocks collected at a randomly chosen site will be suitable for study by a variety of isotopic and chemical techniques that will reveal the nature and chronology of the planetary fractionating events that produced them. They will also contain an isotopic record of the integrated effects of the martian surface radiation environment.
The exercise of even a modest amount of selectivity in landing sites will open additional doors: samples from a formerly fluvial environment, for example, may be found to include rocks with diverse compositions and ages, sedimentary rocks that contain a record of aqueous activity on the martian surface, conceivably even fossil evidence of life.
Observations made by robotic orbiters and landers can provide no more than tantalizing hints and glimpses of the information we want: answers to the questions of whether life ever started on Mars, what the climate history of the planet was, and why Mars evolved so differently from the way Earth did. The definitive answers to these questions will come from the study of Mars samples in laboratories on Earth.
1. M.J. Drake, W.V. Boynton, and D.P. Blanchard, “The Case for Planetary Sample Return Missions: 1. Origin of the Solar System,”Eos68(8): 105, 111–113, 1987.
2. J.L. Gooding, M.H. Carr, and C.P. McKay, “The Case for Planetary Sample Return Missions: 2. History of Mars,”Eos70: 745, 754–755, 1989.
3. G. Ryder, P.D. Spudis, and G.J. Taylor, “The Case for Planetary Sample Return Missions: 3. Origin and Evolution of the Moon and its Environment,”Eos70: 1495, 1505–1509, 1989.
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5. J.C. Laul, “The Shergotty Consortium and SNC Meteorites—An Overview,”Geochimica et Cosmochimica Acta50: 875–887, 1986.
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8. H.P. Klein, “The Viking Mission and the Search for Life on Mars,”Reviews of Geophysics and Space Physics17: 1655–1662, 1979.
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10. J.W. Schopf, Cradle of Life: The Discovery of Earth’s Earliest Fossils, Princeton University Press, Princeton, N.J., 1999.
11. J.W. Schopf and M.R. Walter, “Archean Microfossils: New Evidence of Ancient Microbes,”pp. 214–239 in Earth’sEarliest Biosphere, Its Origin and Evolution, J.W. Schopf (ed.), Princeton University Press, Princeton, N.J, 1983.
12. C.V. Mendelson and J.W. Schopf, “Proterozoic and Selected Early Cambrian Microfossils and Microfossil-like Ob-jects,”pp. 865–951 in The Proterozoic Biosphere, A Multidisciplinary Study, J.W. Schopf and C. Klein (eds.), Cambridge University Press, New York, 1992.
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