The U.S. and the world have the opportunity, within the next decade, of obtaining samples of rock, soil, and atmosphere from Mars. A Mars sample return mission in 2005 is now being planned by NASA and its international partners. Returned samples will provide unprecedented opportunities to learn about Mars, and by analogy the Earth, using the diverse and sophisticated instruments and procedures available in terrestrial laboratories. Although spacecraft (orbiters and landers) are a critical part of any strategy for solar-system exploration, sample return has unique advantages. There are no limits to the quality or number of analyses, no limits to the flexibility and scope of investigations, and no limits on future use as new techniques are developed and new theories are proposed. These advantages contrast starkly with the power and mass limitations inherent in remote, robotic investigations.
However, a returned Mars sample raises serious concerns and issues, beyond the obvious public worry about biological contamination of the Earth. Some of these issues, as yet unresolved, include the following: (1) What investigations will be done on Earth (i.e., what are the scientific goals of sample return)? (2) What changes (including contamination) will arise from handling and transporting the samples? and (3) What kinds of samples should be returned?
These issues bring up many strategic and engineering decisions that will have profound consequences for the usefulness of the returned samples. In a "worst-case scenario," a seemingly trivial engineering decision early in mission planning could nullify a critical chemical or biological analysis. Thus, it is imperative that strategic and engineering planning for Mars sample return be guided by the advice of the relevant scientific disciplines (e.g., biology and geology). Mission decision makers should be aware of the potential scientific consequences of their decisions. Conversely, scientific advisors must recognize that their advice has potential implications for the cost and scheduling of the mission.
The Value of Returned Samples
Why should we return samples of Mars? Aren't remote analyses by spacecraft on Mars good enough? These are excellent questions in light of the public's concerns about planetary protection and the costs involved in returning samples. And answers to these questions are critical to understanding the importance and immense value of returning samples to Earth.
Today's spacecraft can indeed analyze rocks and other materials on planets or asteroids as recently demonstrated by Pathfinder on Mars. Pathfinder obtained color images, chemical analyses, and analyses of the magnetic properties of rocks and soils on Mars. The Mars Pathfinder results have refined our understanding of martian floods, the compositions of rocks and soil, and left some tantalizing hints about the early history of Mars. But we could have learned much more with returned samples, including whether or not the Pathfinder rocks contained traces of ancient martian life.
The need for sample return to Earth arises from the fact that robotic spacecraft analyses are severely limited compared to Earth-based analyses:
Analytical quality is limited, because of inherent restrictions on instrument sizes and mass, power requirements, and data transmission rates.
The scope and flexibility of investigations is limited, because only a few instruments will fit on a spacecraft; and these instruments can be used in only certain ways.
Remote analyses must "make do" with very limited sample preparation.
Future, further investigations are severely limited or impossible.
Because of these underlying restrictions and limitations, robotic spacecraft analyses will always lack state-of-the-art accuracy and precision, and these investigations may also be limited by our preconceptions about what we expect to find. In contrast, studies in Earth laboratories can be of the highest quality possible and can be tailored to fit the samples exactly. In addition, studies of returned samples could take advantage of the instruments and knowledge available on Earth and would have nearly infinite flexibility to respond to what is actually in the sample. We only need to get samples from Mars to Earth.
Quality. The quality of analyses (precision and accuracy) that can be achieved in modern Earth-based laboratories is phenomenal. For example, Fig. 1 shows the oxygen-isotopic ratios of selected meteorites, analyzed in laboratories on Earth, compared with the Viking lander analysis of oxygen-isotopic ratios in the martian atmosphere, analyzed on Mars. From the error bars in Fig. 1, it is clear that oxygen-isotopic analyses on Earth are thousands of times more precise than the Viking lander's.
Oxygen isotopes are the "industry standard" for classifying extraterrestrial materials, including the affinities and sources of meteorites. Figure 1 shows that the Viking oxygen analyses are so imprecise that they have little value. From the Viking analyses alone, we cannot tell if the martian atmosphere was like martian meteorites, martian water, the Earth, or any other meteorite group (i.e., parent asteroid) on Fig. 1.
Of course, robotic instrumentation has improved since the Viking landers, but Earth-based instrumentation has also improved and continues to benefit from the advantages of essentially unlimited power, essentially unlimited mass, durability, availability of outside resources (e.g., liquid nitrogen), and "on-site technical support." Similarly, Earth-based laboratories can readily do complex sample preparations, whereas the technical challenges of remote sample preparation (e.g., making a microscope slide) can be daunting. Indeed, the process for analyzing the complete isotopic composition of oxygen is so difficult that only one or two laboratories in the entire world routinely perform such analyses on geologic materials.
Scope and Flexibility. To show the value of scope and flexibility, consider the rock named Barnacle Bill that Pathfinder imaged and analyzed. The APXS instrument on the Sojourner rover produced an X-ray chemical analysis (rather imprecise by terrestrial standards) indicating a composition like an andesitic volcanic rock. Lander and rover images of Barnacle Bill show no internal structures like layers, veins, or rock fragments; high-resolution images do show numerous small cavities. Barnacle Bill appeared as reddish-black, like basalt lava rock on Earth. Comparison of this chemical analysis with that of local soils suggests that the analysis was severely contaminated by soil stuck on Barnacle Bill.
This is all we know of Barnacle Bill. We do not know if it is igneous or sedimentary, or whether it represents a melt generated by a meteor impact. We do not know if it formed near the landing site, or was transported from far away. We do not know why it has cavities in it, and whether it has any tell-tale structures smaller than the Pathfinder camera could see. We do not know if Barnacle Bill is young or ancient. And we do not know if Barnacle Bill's chemical composition implies that water was abundant on early Mars, as some scientists have suggested.
However, if a few grams of Barnacle Bill could have been returned to Earth, the scope and flexibility of Earth-based analyses would have allowed us to truly understand Barnacle Bill, including when it was formed. Earth-based analyses could even have given us an extended geologic history of the sample site. High-precision chemical analyses and optical microscopy might have shown, for instance, that Barnacle Bill was truly an andesite, a volcanic rock from a water-bearing magma.
Then, if Barnacle Bill were truly an andesite, electron-microbeam instruments could reveal its thermal and chemical history and whether it formed (as did many andesites on Earth) as a mixture of lavas. Radioisotope chronologies would give the age of its eruption, and perhaps allow Barnacle Bill to be traced to a particular volcano. Trace-element analyses coupled with radiogenic isotopic studies would allow us to understand its source what melted inside Mars to form the lava and when that source itself formed. If, as seems likely, Barnacle Bill is from the martian highlands, these analyses might tell us when the highlands formed, what in general they are made of, and whether significant amounts of water were involved in their formation.
In addition, chemical and mineralogical studies of late (postigneous) alteration materials or salts in Barnacle Bill would help us to learn about more recent water in the martian crust and perhaps the composition and age of the floods that scoured Chryse Planitia. Similar studies of the outer surface of Barnacle Bill would tell about weathering at the martian surface, and perhaps whether Mars' climate was once warmer and wetter than it is now. And, of course, biological and organic analyses (which were not possible on Mars Pathfinder) would search for traces of extant or ancient martian life, with far greater sensitivity than could be done on the martian surface.
Future Investigations. A returned planetary sample is a gift that keeps on giving. This is amply demonstrated by the Apollo samples from the Moon, which keep revealing their secrets to techniques that were not even imagined in the 1960s and 1970s. At least five radiochemical tracer and age-dating methods have been developed since Apollo 17 (187Re-187Os, 190Pt-186Os, 176Lu-176Hf, 146Sm-142Nd, 182Hf-182W), several of which have been used on the Apollo samples. From them, we have refined the age of the Moon, learned more of when and how its crust formed, learned more of how it is related to the Earth, and learned how fast planets (and the Moon) formed early in the solar system. If the Apollo missions had not returned samples, none of these results would have been possible. And although not a substitute for additional sampling missions, having returned samples means that a new mission is not required whenever a new analytical technique is invented. A returned sample can be studied now and for generations to come.
Summary. Sample return is a long-term investment in knowledge; returned martian samples will be an inheritance that will pay off for decades to come. Robotic spacecraft analyses are, of course, critical to exploration of the solar system, but cannot address all of NASA's goals for understanding planetary evolution. Only through careful study of returned samples can we realize the full potential of the instrumentation and analytical skills that are available here on Earth. These studies will define the agenda for future Mars exploration what we need to learn and where best to learn it and will be crucial to any human habitation of Mars by the next generation of explorers.
What Types of Investigations Should Be Conducted?
Mars is unique in planetary science as the world most similar to our own. Mars offers us a comparison and contrast to the Earth, an opportunity to learn the history of a planet that was once suitable for life as we know it (according to current theories), and may yet harbor living organisms. Under standing the contrast, why Mars is now a cold dry desert and not a wet living world, will involve studies from a broad range of natural sciences: biology, climatology, geology, space physics, hydrology, etc. Our overall strategy for Mars sample studies must recognize this ecology of sciences. The study of any particular aspect of Mars will benefit enormously from coordinated investigation of its many aspects.
Within the span of Mars science, biology is now in the forefront. The most visible force behind Mars sample return is the search for life, extant or fossil. But because life and its history are inextricably connected with the physical factors of its environment (climate, hydrology, and geology), the study of Mars as a possible home for life requires the integration of the full range of physical and biological sciences. In this way, a mandate to understand martian life or the potential for martian life is a mandate to conduct balanced interdisciplinary studies.
It is not our place and it is not possible to dictate what kinds of investigations will or should be done using returned samples. However, we strongly advise that no one type of study be advanced over all others. In this regard we are not too different from a financial advisor who recommends having a diverse investment portfolio. And, as advisors, we recommend a mix of risky and safe investments.
Perhaps the riskiest study, in the sense of having an assured positive result, is the search for extant life on Mars. Returned Mars samples will be quarantined on the presumption that they contain viable life forms, but in actuality there is very little chance that viable organisms will be found in returned near-surface samples. The reasons for this are twofold: (1) water appears to be a universal requirement for life, and there has apparently been no liquid water near Mars' surface for a few billion years; and (2) highly oxidizing chemicals and ionizing radiation, both of which are harmful to organic molecules and to life, are abundant at Mars' surface. However, the potential scientific gains from a search for extant life are so great that it certainly should proceed.
Less risky, in terms of assured results, is a search for indirect signs of life. These signs are collectively called biomarkers and range from fossils to subtle chemical and isotopic signatures. Biomarkers can persist long after the life it represents has vanished for instance, chemical and isotopic markers of life are preserved in some Earth rocks nearly 4 billion years old. In this way, a search for biomarkers is a search for an answer to the question of whether Mars ever supported life. Mars may never have had living organisms, so the search for biomarkers does not have an assured result. But again, the potential scientific gains from the recognition of biomarkers are so great that the search must take place.
Much safer, in terms of assured results, is the study of the past and present physical environment of Mars. The martian environment is the stage on which life may have arisen, evolved, and then either survived or perished. Whether or not life arose and flourished on Mars, physical and chemical analyses of returned martian samples will teach us about Mars' current environment and its past climate, its formation, and the ebbs and flows of its water and ice. From our experiences studying meteorites and lunar samples, we know that samples of any sort can yield insights both important and unexpected, and that even small samples (~0.5 g) can yield enormous insights into a planet's origin and history. We see no reason why returned Mars samples would be any less useful.
In short, the search for life is exciting but may not be fruitful. The harsh conditions of the martian surface may have proved inhospitable to life or even to organic molecules. However, physical and chemical studies will elucidate the history of Mars (exciting in itself) and define the ecological contexts for martian life (whether it is discovered or not).
Thus, we see diversity as the prudent approach to investigating returned Mars samples. There will likely be pressures to use all returned samples in one type of study or another (e.g., biochemical analysis or geologic age dating). These pressures must be resisted so that sample return is assured a positive result, which will be important as the mission is judged in the court of public opinion. If the presence of life cannot be determined, we can at least gain a better understanding of the environment that might have succored it.
What Types of Samples Should Be Returned?
Two factors will influence what types of samples will be returned from Mars: the scientific problems we wish to investigate and the engineering constraints on the mission. An ensuing consideration is that the types of investigations that will be possible will depend on how the samples are obtained and how they are treated subsequently.
In the following sections, we will discuss possible sampling, sterilization, and quarantine procedures that may affect scientific investigations on returned samples. For the limited purposes of this discussion, we assume that samples will be returned in such a way that all investigations are possible. We further assume that they will be collected by a rover within the capabilities likely available in the next ten years, at landing sites ±15° from the equator.
We have already concluded that broad-based scientific investigations will be required to search for life (extant and fossil) and to understand the environments and history that may have permitted life. What kinds of samples are needed for such a broad scientific program? Where should the lander set down to find these samples?
Broad investigations require a broad suite of samples, so we recommend returning as large and diverse a suite of samples as possible. Because complete diversity is not possible in a single mission, and because current mission rationales emphasize martian biology, sampling priority may well be given to those regions that are likely to retain a record of habitable (aqueous) environments. On Earth, such deposits are most commonly sedimentary rocks deposited at the bottoms of seas and lakes, but rocks altered by hydrothermal processes (e.g., hot springs) may also have hosted lifeforms. On Mars, the oldest rocks are key to the search for traces of life, as water was more abundant and more available for life at one time than it was in later times.
It is a challenge for mission planners to reliably locate deposits like these, and it is a challenge for mission engineers to develop landing methods capable of precisely deploying a lander to recover such a deposit. Therefore, it is conceivable that biological considerations might temporarily bow to concerns for a safe, predictable landing. But these are difficult issues that will require great deliberation in the scientific and engineering community. And possibly there will be a landing site that will easily satisfy both scientific and mission priorities.
However, we reiterate that sample diversity is critical to understanding Mars, and it seems reasonable that many potential landing sites on Mars will present a useful diversity of samples: (1) All sites will permit a sample of the atmosphere, which would tell us much about the history of volatiles on Mars and, therefore, much about the climatic history as well. (2) All sites will permit a sample of windblown dust, which is perhaps an average sample of Mars' surface; on Earth such dust deposits are useful in determining the mean age and chemical composition of the continental crust and may be similarly useful for Mars. (3) Landing sites within sedimentary deposits may have rocks transported by flowing water (as postulated for the Pathfinder site). (4) Given the age of Mars' surfaces and its extensive cratering history, it is likely that impact ejecta from distant sources may be found at almost any site.
At our present level of knowledge, it is nearly a certainty that any sample of Mars will yield new insights. Most of what we know about the atmosphere, hydrosphere, and biosphere of Mars comes from studies of basaltic rocks (the martian meteorites), which are hardly ideal for understanding water and biology. A returned sample of any other sort of martian material will dramatically increase the range of martian materials in our collection, thereby increasing our understanding of Mars. Thus, whereas some martian samples would be more ideal than others, all will be useful.
Other sampling issues also need to be considered: (1) Although we have emphasized sample diversity in this discussion, we take the opportunity to address the related issue of representative sampling. The sample return mission may encounter a terrain that has a dominant rock type, as appeared to be true at the Mars Pathfinder site. It may well be that, in addition to sampling for diversity, we will also want to obtain several samples of a common rock to ensure that many different types of investigations can be performed on the same type of sample. (2) A contingency sample will likely be collected immediately after landing to ensure that some material is returned, regardless of the rover's fate. What should that contingency sample consist of? Atmosphere? Atmosphere and windblown dust? Atmosphere, windblown dust, soil, and a representative rock type? This is a decision that must be made by the time the mission lands.
To some extent, Mars sampling efforts can refer to the experience of the Apollo missions to understand issues of sample selection. In the later Apollo missions, there was a close integration between scientists, astronauts, and Mission Control, which led to the collection of a superb suite of samples. However, we recognize that, although there will be similarities in sampling strategies, there will also be differences. Lunar sampling took place over a period of hours; robotic sampling of Mars will occur over days to months. Coordination between the science team and the mission control team will, of necessity, be different. The complexity associated with Mars rocks is also likely to be greater than in the case of the Moon. No doubt this will complicate evaluation and selection, and may result in heated debate. For these reasons, it is important to have procedures in place that will serve to guide mission controllers and to act as an aid to the sleep deprived.
How Will Potential Return Samples Be Evaluated?
Before a sample is returned to Earth, it must be evaluated. Some samples are likely to be more representative than others. Some samples, though nonrepresentative, may give us important insights that typical samples would not. Some care in selection will be required. And, as we have alluded to above, there are difficulties in the evaluation of rocks when only analyses of their surfaces are available. Because the exterior of a specimen may be weathered, altered, or biodegraded, analysis of the surface may be misleading about the real nature of the rock.
Therefore, obtaining fresh, interior surfaces for analysis is important. On Mars missions, a rover should have a coring, abrading, or rock-splitting device that will provide fresh surfaces. These samples can then be evaluated using a variety of spectroscopic techniques (i.e., X-ray, optical/infrared reflectance, Mössbauer, Raman, etc.). It is important that all future Mars missions have the capability to expose and analyze fresh rock surfaces.
Of course, some samples will require only minimal, if any, evaluation. A sample of atmosphere should not require a selection strategy. The same is true (we think) for windblown dust, the composition of which seems to be very similar from site to site. However, we anticipate that care and sagacity will be required in the selection of rocks and soils. And to do this adequately for rocks requires fresh surfaces.
In conflict with the need for fresh surfaces, we also require that the sampling process not contaminate the sample. Organic contamination of Mars samples is a special concern, because potential organic biomarkers may be present only at miniscule concentrations. In addition, some inorganic materials that have highly desirable mechanical properties have chemical properties that can compromise important scientific investigations. Two prominent examples are tungsten carbide, which can grossly degrade trace-element analyses, and common (lead) solder, which interferes with age dating.
Consequently, the materials that could contact a returned Mars sample should be considered carefully. For example, the only materials allowed in the NASA Lunar Curatorial Facility are stainless steel, aluminum, and Teflon. While these materials are essentially inert against lunar materials, it remains to be seen if they are stable (e.g., resistant to oxidation) against martian materials. In addition, one might ask if Teflon could produce unacceptable organic contamination of Mars samples. Thus, the tools and containers must be chosen with care.
How Will Selected Samples Be Sealed For Return?
In the present plan, Mars samples selected for return to Earth are to be brought back by the 2005 mission. No matter which scenario leads to sample return to Earth, the samples must be stored safely throughout their journeys. How will the samples be stored? What types of containers will be used? Will the samples, or fluids liberated from them, react with the container? Will the samples be sealed and isolated from the martian environment and from each other? If so, will the seal be of a type that ensures quarantine or will it be a temporary barrier, anticipating tighter isolation at a later date? Seals that are "airtight" may not meet quarantine standards, and many of the higher-quality seals may possibly contaminate the samples, either organically or inorganically.
These are difficult issues that must be confronted early in the mission design process, because the science that can be done on Earth will depend on how the samples are captured. Further, if the sample return mission is to retrieve samples cached on an earlier mission, the return mission should be designed before designing the sampling mission.
If procedures for collection and storage on the martian surface do not include sealing for return, this procedure will have to be performed later. Of some concern is the dust that is present in the martian atmosphere. Windblown dust may be deposited on exposed seal surfaces, and thereby compromise quarantine conditions. And the process of placing soils and rocks into any container may have the same result, if adhering soil or dust inadvertently drops onto the seal surface. Because planetary protection and quarantine are so important, the question of sealing is critical and requires thoughtful consideration.
How Will Samples Be Preserved During Their Return Trip?
After the samples have been evaluated, sealed, and liberated from Mars, there is the issue of preservation. Will the samples be maintained at ambient martian temperatures and pressures? If not, what scientific information will be lost as a consequence? Conceivably, the samples could be frozen at temperatures lower than those ambient on Mars, preserving delicate structures and minerals en route. On the other hand, this may not be feasible for a reasonable cost.
Temperature control has implications for sample integrity. If the samples are heated above Mars' ambient temperature, they will likely liberate fluids that could interact with other samples in the cache, influencing later analytical results. And liberated gas could conceivably generate enough pressure in the sample cache to threaten the containment vessel or quarantine.
The temperature environment of space is very different from that of the Earth's surface. Once the capsule has reentered the Earth's atmosphere, how long can it be kept at terrestrial temperatures before it thermally equilibrates? This is an issue of insulation as well as sealing. What insulating material will be used to keep the sample cold for as long as possible?
Thus, the issues surrounding sample preservation are complex. A careful cost-benefit analysis should be performed on the optimal thermal conditions for sample return.
Will Sterilization Be Necessary?
For planetary protection, it is likely that the exterior surfaces of the return capsule will need to be sterilized. How will this be accomplished? Heat treatment? Intense gamma radiation? Corrosive chemicals? How will this treatment affect the samples? For example, if heat treating is the sterilization method of choice, will the samples inside be heated as well? What science will be lost if this occurs?
If the exterior of the return capsule is sterilized, should the interior and the samples be sterilized as well? This procedure would negate the possibility of culturing living martian organisms, but it would prevent any pan-planetary infection. What would be the risks and benefits of such a choice? Will there be scientific information lost because of the sterilization process? Should some material be sterilized, while other material is kept pristine?
We advocate an approach that minimizes the potential hazards to the Earth's population and maximizes the retention of the returned samples' scientific value. For biological studies, at least part of the returned sample mass must be spared any sterilization procedure. Most geological studies may still be possible on sterilized samples, provided that a suitable technique is developed.
For geological studies, the most promising sterilization technique so far appears to be irradiation by gamma rays. This method can be performed at low temperature and does not heat the sample. It is the standard means by which the Centers for Disease Control and Prevention and the food industry routinely kill dangerous bacteria. Studies are now underway to determine the effects of intense gamma irradiation on geological and geochemical investigations. The results of these studies should be followed closely by mission planners.
Summary: Samples of rocks, soils, and atmospheres are valuable for studying the origin and evolution of planets. And understanding the biological and geological evolution of planets is of intrinsic interest to our species and may be important to our survival. Sampling Mars will tell us much about that planet and will allow us to evaluate the pervasiveness of life in our solar system. But the sampling act itself will have consequences that must be explored. In order to extract the maximum information from returned martian samples, their acquisition, storage, transport, retrieval, and quarantine must be carefully thought out in advance. We have raised more questions here than we can answer, but we are confident that remaining questions can be answered prior to the mission launch. However, it may take time to find the optimal solutions to these issues, and the time to address them is now.
This article was orignally prepared as a report for the Curation and Analysis Planning Team for Extraterrestrial Materials, which represents the interests and needs of scientists to NASA program and mission designers.
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