Scientific Themes, Goals, and Questions
This chapter is a brief summary of the important scientific themes and goals that can and should be addressed during implementation of the initial phases of the Vision for Space Exploration, the Lunar Precursor and Robotic Program (LPRP), and the early phases of human activities. In this new age of exploration, the most scientifically compelling objectives include elucidating solar system bombardment history using the Moon as a unique, singularly important “Rosetta stone,” understanding the origin and evolution of the Moon and rocky planets, evaluating the nature and stock of volatile elements and other potential resources on the Moon, and assessing the utility of the Moon as an observational platform. The science goals and themes in this report derive from these objectives, based on the current status of scientific knowledge, and a summary of implementation options presented for each. Many of the science themes use the Moon to understand our solar system, while others address understanding of the Moon itself; both are valuable pursuits. A common set of basic science goals is identified along with priorities for research associated with each theme. Several of these science goals and themes encompass broad and compelling science and require multiple approaches and integrated analysis. Others themes need further study to determine their full merit. All themes will be documented in greater detail in the committee’s full report.
The bombardment history of the inner solar system is uniquely revealed on the Moon.
The heavily cratered surface of the Moon testifies to the importance of impact events in the evolution of terrestrial planets and satellites and the exceptional ability of the lunar surface to record them all. Lunar bombardment history is intimately and uniquely intertwined with Earth’s, where the role of early intense impacts and possible periodicity in large impact events in the recent past on the atmosphere, environment, and early life underpin our understanding of habitability. The correlation between surface crater density and radiometric age discovered on the Moon serves as the basis for estimating surface ages on other solid bodies, particularly Mars. Significant uncertainties remain in our understanding of the lunar cratering record and our ability to extend it to other planets and other solar systems. Returning to the Moon provides an unparalleled opportunity to resolve some of the fundamental questions that relate to these goals.
Assess the early impact flux. The radiometric ages of impact melt lithologies in the returned sample collection have been used as an argument for a late cataclysm, that is, a spike in the cratering rate around 3.9 billion years ago, just about the time life on Earth was emerging. However, the geologic setting of the Apollo landing sites led to a selective sampling of material from Nectaris, Serenitatis, and Imbrium. With currently available data, it is impossible to decide whether a cataclysm occurred or whether the cratering rate smoothly declined with time since lunar origin. Determining the ages of impact-melt rocks from the South Pole-Aitken (SPA) basin (the stratigraphically oldest lunar basin) and major impact basins within SPA will go a long way toward resolving this issue. The precision required to accurately date these events requires isotopic analysis of well-chosen samples in terrestrial laboratories.
Assess the recent impact flux. Variability in the recent lunar and terrestrial impact flux may be related to singular solar system dynamic events, such as asteroid breakups, and may have significant effects such as impact-induced mass extinction on Earth (e.g., the Cretaceous/Tertiary (K/T) boundary and currently hotly debated Permian/Triassic (P/Tr) boundary events). On Earth, there is a greater relative number of younger craters, possibly suggesting a recent increase in projectile flux, but the diameters and especially the ages of most terrestrial craters are so poorly known that the terrestrial impact flux is uncertain. The Moon records the projectile flux in the Earth-Moon system over the past ~3.5 billion years, and researchers can use it to determine whether this flux has been approximately constant,
or has exhibited shorter-term variations or periodicity, by determining crater densities on known young surfaces, such as crater ejecta deposits, and radiometric ages of lunar soil spherules.
Determine the exact ages of key craters. Impact basins and young craters serve as stratigraphic benchmarks for determining the relative age of lunar surfaces that have not been or cannot be directly accessed. There is still considerable debate about the ages of individual impact basins on the Moon. For example, the Orientale basin cannot be dated precisely because no samples in the current collection can unambiguously be attributed to the Orientale formation event. While radiometric ages of Apollo 12 samples suggest a narrowly constrained age of 800 million to 850 million years for Copernicus, crater counts on the ejecta blanket of Copernicus indicate a significantly older age of up to 1.5 billion years. Again, to accurately date these events requires analysis of carefully chosen samples in terrestrial laboratories.
Understand the limitations in extending the lunar flux curve to other planets. There are several outstanding issues that merit detailed further study in understanding the lunar flux curve and being able to use it to date other planetary surface features. Examples include the abundance of secondary impact craters and how they can be distinguished, potential latitudinal and hemispherical asymmetries in the number of formed impact craters, and the exact shape of the size-frequency distributions of solar system projectiles.
In summary, the overarching science requirement is to characterize and date the impact flux (early and recent) of the inner solar system.
The structure and composition of the lunar interior provide fundamental information on the evolution of a differentiated body.
The origin and evolution of the Moon. One of the key motivations for studying the Moon is to better understand the origin of the planets of the inner solar system in general, and that of Earth in particular. The origin of the Moon is inextricably linked to that of Earth. The precise mode of formation affected the early thermal state of both bodies and, therefore, affected the subsequent geologic evolution. The leading hypothesis at present is that the Moon formed as the result of the impact of a Mars-sized object with the growing Earth. However, the details of the process are not clear, and even its validity is not proven. Because the Moon’s geologic engine largely shut down long ago, its deep interior is a vault containing a trove of information about its initial composition, differentiation, and crustal formation, and subsequent magmatic evolution. During and immediately after accretion, the Moon underwent primary differentiation involving the formation of a (presumably) iron-rich core, a silicate mantle, and a light, primordial crust. The initial bulk composition, as well as the pressure and temperature conditions during this separation, will be reflected in its current chemistry, structure, and dynamics. Although researchers have some information on the composition of the outermost layers of the Moon’s crust, that of the bulk crust is less well known, and even its thickness is not well established. The composition of the mantle can only be vaguely estimated, and the presence of compositional stratification, bearing on the late stages of differentiation and the efficiency of subsequent convective mixing, cannot be confirmed or denied. Furthermore, the size and the composition (e.g., Fe vs. FeS, or even FeTiO3) of its core are unknown, except for loose bounds on its diameter.
The value of new data. Data concerning interior structure and dynamics are difficult to obtain but are worth considerable effort to achieve. Direct samples of mantle rocks, from the deeply excavated South Pole-Aitken (SPA) basin or xenoliths, can provide a detailed glimpse into upper mantle composition. Geophysical measurements are the best, and in some cases the only, way to obtain information about the composition and structure of the deep lunar crust, mantle, and core. Researchers know from terrestrial experience that seismology is the most sensitive tool for determining internal structure. The waves produced by seismic events can provide essential information on crust and mantle structure and the size and nature of the core. These measurements can be augmented with analysis of rotational dynamics from precision tracking of surface reflectors. The flow of heat from the Moon’s
interior is a primary indicator of the global energy budget in terms of sources (e.g., radiogenic, accretional) and the mechanisms that control its release (convection, conduction, volcanism). Knowledge of the heat flow provides important constraints (through inferences about internal temperatures) on the rheology and dynamic behavior of deeper layers of the Moon, on both global and regional scales.
Thus, a variety of geophysical and compositional analyses of the Moon will enable researchers to determine the internal structure and composition of a differentiated planetary body.
The Moon’s crust is much more complicated than are the mare and highlands.
Understanding of the formation of the lunar crust and mantle is framed by the lunar magma ocean hypothesis, whereby the Moon melted as it accreted and then followed a planetwide crystallization sequence, resulting in a floating feldspathic crust underlain by a dense, mafic mantle. The concept of a planetary magma ocean, though founded on lunar science, has become the one applied to the history of all the terrestrial planets.
Though the concept of the lunar magma ocean continues to serve us well, geophysical, remote sensing, and sample analyses reveal a lunar crust that varies both laterally and vertically in composition, age, and mode of emplacement. The traditional, dichotomous mare-highland classification developed from Apollo experience is inadequate in describing the structure and geologic evolution of the lunar crust.
From the global remote sensing coverage of the Clementine and Lunar Prospector missions of the 1990s and the study of lunar meteorites, researchers now know they have an incomplete sampling of the lunar crust, including unique materials of great interest to science and in situ resource utilization (e.g., high-Ti basalts, pyroclastic glass deposits). Understanding the composition and structure of the lunar crust underpins many other science goals but is also first-order lunar science. By integrating global remote sensing, detailed regional geology, and precise sample studies, researchers gain a predictive capability that allows them to make smarter choices about where to send future robotic and human missions.
Determine the variety and origin of rock types. Large lunar terrains have distinct geochemical characteristics inferred to be the result of asymmetry in the crystallizing lunar magma ocean or later large impact events. The Apollo and Luna samples came largely from a single, unique terrain (mare), and there are several other types of terrains yet to be fully characterized. Sample return from well-characterized sites representing new terrains allows high-precision laboratory analyses of petrology, composition, and radiometric ages and the ability to continue experiments for decades. Global compositional information from remote sensing extends the knowledge gained from samples collected from across the entire Moon.
Assess the vertical stratigraphy of the crust. Geophysical models of the lunar crust are highly dependent on assumptions about the type and distribution of materials across the crust and at depth. A regional, active seismic network can probe the depth of the megaregolith, which is potentially important in lunar base construction, and will determine whether a compositionally distinct lower crust exists. Samples from, and detailed geologic maps of, regions that have exhumed materials from depth provide further constraints, which in turn can highlight new areas of interest for future exploration.
Determine the composition of the lower crust and bulk Moon. Current understanding of the origin and early evolution of the Moon depends on its bulk composition, which is poorly constrained until researchers can determine the types and extent of lunar crustal rocks. The South Pole-Aitken basin may have excavated or melted the lower crust of the Moon or may possibly even provide a window to the lunar mantle. Lunar pyroclastic flows may bring deep-seated rocks to the surface. Returned samples from these types of sites will enable detailed petrologic and compositional analyses to determine the lower crustal composition, and by inference, the bulk Moon.
Thus, the Moon provides an exquisite opportunity to determine the compositional diversity (lateral and vertical) of rocks formed in a differentiated planetary body.
Lunar volcanism provides a window into the thermal and compositional evolution of the Moon.
Volcanism is a well-documented and information-rich phase in the evolution of terrestrial planets. For the Moon, several key questions about its volcanic history have yet to be answered:
What are the origin and variability of basalts? Many different types of mare basalts are represented in both the Apollo and Luna collections and in lunar meteorites, yet key basalt flows, identified from orbit, remain unsampled and their detailed chemistry and absolute ages are unknown.
How old is the youngest mare basalt? Recent crater counts suggest that some of the basalts in Oceanus Procellarum might be as young as 1.2 billion years—an age unrepresented anywhere in the sample collection and an important calibration point for understanding lunar volcanism, thermal evolution, and the impact cratering flux.
What are the compositional range and extent of lunar pyroclastic deposits? Pyroclastic volcanism offers the most direct sampling of the lunar mantle; within the existing sample collection, the range of composition is large, and it is likely that this range will grow even larger as researchers discover and sample new deposits.
What is the flux of lunar volcanism and how did it evolve through time? The magma production rate through time is not known in detail, nor is the chemical evolution of these magmas and the thermal evolution of the Moon overall. The link between basalt composition and age requires better definition.
Understanding the history of the Moon and its thermal and magmatic evolution entails understanding the origin of the earliest crust, the thermal and dynamic evolution of the lunar mantle, how magma production rates changed with time, and the processes that formed highland igneous rocks—all of which can be addressed wholly or in part by answering the questions above. Namely, samples of basalts that erupted on the lunar farside will help elucidate at what depths melting occurred, when eruptions happened, and whether the composition of the mantle is uniform from the nearside to the farside. Samples of the youngest basalts will help constrain how basaltic processes have evolved over time. A range of subsurface sounding methods will permit determination of the thickness and structure of individual benchmark basalt flows. Investigating the thermal state and history of the interior (e.g., through careful measurements of the interior heat flow) will establish the thermal constraints on magma production through time.
Thus, the character of volcanism on the Moon allows us to determine the time scales and compositional and physical diversity of volcanic processes.
The Moon is an accessible laboratory for studying the impact process on planetary scales.
Impact cratering is a fundamental process that affects all planetary bodies. Understanding of cratering mechanics is heavily biased by observations of craters on Earth and in Earth-based laboratories. Though this understanding has been scaled as much as possible for lunar gravity, there are many untested hypotheses about lunar cratering, including the detailed structure and rim diameter of multi-ring impact basins, the effects of target composition on crater morphology, the amount of central uplift within craters, the existence and extent of impact melt sheet differentiation, the mixing of local and ejecta material, and scaling laws for oblique impacts. In this context, the Moon provides unique information because it allows the study of cratering processes over several orders of magnitudes, from micrometeorite impacts on glassy lunar samples to the largest basin in the solar system, the South Pole-Aitken basin. The large number of lunar impact craters over a wide range in diameters provides the basis of statistically sound
investigations, such as, for example, depth/diameter ratios, which in turn have implications for the possible layering and strength of the lunar crust but can also be extrapolated to other planetary bodies. Thus, the Moon is a valuable, easily accessible, and unique test bed for studying impact processes throughout the solar system.
The decadal survey report New Frontiers in the Solar System: An Integrated Exploration Strategy (NRC, 2003) asked, “How do the processes that shape the contemporary character of planetary bodies operate and interact?” Cratering is one of several such processes, affecting the lunar surface, the crust, and possibly even the mantle; each advance in understanding of cratering mechanics moves researchers closer to answering that key scientific question.
Current hypotheses and assumptions about cratering processes underpin many of the hypotheses about the composition and evolution of the lunar crust, and thus the rest of the solar system. For example, answering the question of whether or not impact melt sheets can differentiate will either open or close a door on the range of potential origins of igneous rocks found on the Moon. Some cratering hypotheses are rarely questioned and have become sufficiently accepted that they are now “rules of thumb.” An example of this involves the amount of central uplift within craters. Models of crustal structure and character have been derived from data on the composition of central peaks of lunar craters; if these peaks did not originate from the depths currently assumed, then these models might require re-evaluation. Thus testing and validating both the wildest and the most accepted hypotheses about the cratering process are critical to being able to correctly interpret lunar geology, and ultimately that of the solar system.
To achieve this, sample return from craters and basins, including a vertical sample of a basin melt sheet, is needed. In addition, the walls, rim, and central peaks of compositionally diverse major complex craters need to be mapped in geologic detail, beginning with orbital measurements and followed by selected field studies of at least one crater. The structure of large multi-ring basins needs to be mapped through drilling programs or geophysical measurements.
In summary, implementation of the Vision for Space Exploration presents an opportunity to characterize the cratering processes on a scale relevant to planets.
The Moon is a natural laboratory for regolith processes and weathering on anhydrous airless bodies.
Regoliths, exemplified by the lunar regolith, form on airless bodies of sufficient size to retain a significant fraction of the ejecta from impact events. The regolith contains representative rocks from both local and distant sources, the alteration products induced by meteoroid and micrometeoroid impacts, and modifications due to the implantation of solar and interstellar charged particles, radiation damage, spallation, exposure to ultraviolet radiation, and so on. Knowledge of the processes that create and modify the lunar regolith is essential to understanding the compositional and structural attributes of other airless planet and asteroid regoliths in general. Because the regolith collects the products of the interaction of impactors and radiation with the surface, the composition of ancient regoliths, protected by overlying layers of volcanic materials, may yield information on the time-history of the Sun and interstellar particle fluxes in the inner solar system. Understanding the “space weathering” processes that affect the regolith, particularly the distribution of materials volatilized by impacts, is essential to the interpretation of spectral data used to map the distribution of rock types on the surface. Because the effects of space weathering depend on both the composition and the exposure history of the surface, new samples that represent materials of different initial composition and age should be prime candidates for study.
Layers of interspersed volcanic rocks and ancient regolith can be observed or inferred in the maria, where the periods between successive volcanic flows were periods of new regolith formation. Sampling of these ancient regolith layers can be carried out by drilling through the rock column or by collecting rocks in the walls of impact craters or along rilles. If these ancient regolith layers have been indurated through the thermal effects of the overlying lavas, they might also be discovered in the rock
fragments that surround impact craters. Samples may be available at many mare locations, but targeted collection would benefit from on-site human field observations to identify and retrieve the desired sample materials. For example, a sampling device (e.g., a rover) could sample the stratigraphic column in an impact crater wall or in a rille, with an astronaut making critical observations of the properties of the layered sequence using handheld sensors.
The regolith within permanently shadowed regions may have special properties, such as cementation, extreme brittleness, or grain size effects, especially if the suspected volatile deposits, such as water ice, have interacted with the material. The chemical, mineralogical, and physical properties of these deposits could be representative of similar deposits on Mercury or elsewhere in the solar system.
Most lunar resources will be derived from the regolith. Understanding the mineralogy, volatile concentrations, and physical properties of the regolith and having a better understanding of regolith formation and history will be crucial to exploring for and developing extractive techniques for regolith-based resources. This is particularly true for the polar regions, for which there is no current basis for understanding these properties in detail.
Physical properties and many volatile concentrations can be best studied through in situ surface investigations. The details of chemical weathering and charged particle interactions require the collection of samples for study in terrestrial laboratories. The collection of samples of ancient regolith requires field observations carried out by astronauts.
In summary, multiple opportunities will be present during implementation of the Vision for Space Exploration to constrain processes involved in regolith evolution and decipher ancient lunar environments from regolith samples.
The Moon may provide important information about the early Earth and the origin of life.
Using the Moon to study Earth’s history. Studying Earth’s biologic history is a major topic for NASA and for astrobiology. To understand how life may have originated requires an understanding of Earth’s distant past and its relationship to the history of the Sun and of the cosmic-ray and cosmic-dust environment. These topics are a portion of the goals of NASA’s Astrobiology Roadmap, namely to understand how life emerges from cosmic and planetary precursors, and to understand how past life on Earth interacted with its changing planetary and solar system environment.
Studies of four key eras are of import:
The characteristics and environment of Earth at the time when life originated, a period currently believed to have spanned the time from 4.2 billion to 4.4 billion years ago.
The period when the Late Heavy Bombardment (LHB) may have occurred, around 3.9 billion years ago. There are suggestions that during this period the number of different forms of life was reduced sharply going through a bottleneck where only organisms that thrived at high temperatures survived.
The period from 2.1 to 2.9 billion years ago, when oxygenic photosynthesis developed and when snowball-Earth and hothouse-Earth sequences are proposed to have occurred.
Late Proterozoic, the period from 0.55 billion years ago to 1.2 billion years ago, the era in which multicellular eucaryotes came to prominence. It was also the period when a hypothesized “snowball Earth” may have occurred and a period in which the oxygen abundance in Earth’s atmosphere increased dramatically.
For these eras, terrestrial analyses of zircons are starting to give clues to the thermal and aqueous history of early Earth. But information on solar variations, cosmic dust input, or unmetamorphosed samples of the early Earth surface are not currently available from Earth. There is high interest in comparing the lunar bombardment history with Earth’s geologic record. This connection can be explored by studying the lunar cratering rate (as discussed in Theme 1). There is interest in studying cosmic dust
infall rate changes and solar activity changes that can be related to terrestrial events. Fragments of early Earth might also have been transported to the Moon during the LHB. Study of ancient lunar regolith (discussed above in this report) can provide information in each of these areas of interest. However, the search for pieces of early Earth will involve looking for a minute and unusual fraction of the regolith material and should logically be deferred until in situ resource utilization (ISRU), which will require large amounts of lunar regolith material for processing, is implemented.
Significant opportunities exist, interwoven with other themes, to utilize lunar observations to characterize Earth’s history.
The lunar poles are special environments that may bear witness to the volatile flux over the latter part of solar system history.
The lunar polar environment and its importance. The Moon and Mercury share a microenvironment at their poles that is unique in the solar system. The very small obliquity of these small planets causes topographic depressions near the poles to be permanently shaded from sunlight, allowing them to achieve extremely low temperatures ranging between 50 and 80 K; these temperatures are not expected elsewhere in the solar system within the orbit of Neptune and nowhere else on exposed silicate surfaces. The presence of these cold surfaces adjacent to the hot surfaces of the Moon and Mercury may allow cold trapping of volatile material that has impacted or otherwise been introduced to the surfaces of these objects. Any water or other volatile molecule that encounters a cold trap surface will be permanently trapped with respect to sublimation, depending on the temperature of the trap and the vapor pressure of the species. The trapping process suggests that the lunar (and Mercurian) poles may record a history of volatile flux through the inner solar system over the lifetime of the traps. The importance of these regions for understanding solar system volatile materials is analogous to the importance of meteorites in Antarctica as a valuable resource for studying solar system refractory materials. The committee also notes that the polar regions may provide critical resources, such as high concentrations of hydrogen and possibly water for future exploration. By answering the science issues described here, important information will be provided for potential ISRU opportunities.
Scientific questions about the polar environment. The most important scientific questions deal with determining the compositional form of the polar volatiles, their sources, their alteration during transport and sequestration, and the nature of any processes that may have altered them in situ. The potential sources are solar wind gas, volatile-rich comets, asteroids, and interplanetary dust particles, and even volatile material from giant molecular clouds through which the solar system periodically passes. The possible processes operating on these volatiles range from mass fractionation during transport, to retention from burial and chemical alteration processes, to losses from solar wind and galactic ultraviolet radiation and micrometeorites. Studies of transport and alteration processes are also needed for understanding how robotic and human missions to the Moon can affect the pristine lunar polar environment.
Current knowledge and future opportunities. The complexity of the lunar polar environment is matched by a near-total lack of data to constrain understanding. Radar results for Mercury and additional neutron data for the Moon have achieved the zero-order existence proof that cold trapping of volatiles does occur, but the chemical identity of the polar materials, their sources and evolution, and the explanation for the differences between Mercury and the Moon are entirely unknown. Without new data, the value of the polar deposits for addressing larger issues in planetary science is unknown.
Analysis of existing orbital data (Clementine, Lunar Prospector) and data from planned orbital missions will contribute significantly to understanding of the lunar polar volatile deposits. For example, the Lunar Reconnaissance Orbiter (LRO) will provide photometry, morphology, topography, and temperature information that will improve knowledge of the polar environment. LRO will also provide information about the hydrogen distribution near the poles and possible surface frosts if they are present. The M3 infrared spectroradiometer on Chandryaan-1 could potentially detect water or hydroxyl features
through measurement of surfaces illuminated by sunlight scattered by nearby topographic highs. Future measurements that make high-spatial-resolution volatile measurements (<10 km) at latitudes down to 70 (the lowest latitude where most permanently shaded regions are expected) would be valuable to characterize the location and total inventory of the volatile deposits.
Even so, a positive result by upcoming orbital remote sensing missions will not answer the most compelling scientific questions; these can only begin to be addressed by robotic in situ measurements. (A negative orbital result does not diminish the potential scientific value of the poles, but does complicate their further investigation if candidate landing sites are not identified.)
First, the localized existence and lateral distribution of volatile deposits within permanently shaded regions needs to be determined, since there are indications that these regions may be patchy within the shaded regions.
Second, when such deposits are found, the key measurements to answer the major science questions are measurements of elemental and isotopic composition, and physical and mineralogical characteristics of the volatile deposits. These measurements may need to be made as a function of stratigraphy since there are indications that these deposits are layered.
In addition, by making these measurements, more information will be gained about the physical properties of the regolith in permanently shaded regions for which there is almost no known information.
The results of these initial measurements would guide further investigations. If it can be shown that polar cold traps preserve volatiles with high fidelity, such that source characteristics can be inferred or in contrast reveal significant degrees of processing (e.g., to complex organics), subsequent more detailed analysis is warranted, likely using cryogenically preserved returned samples.
In summary, a unique opportunity exists to characterize the volatile compounds of polar regions on an airless body and determine their importance for the history of volatiles in the solar system.
Further exploration can vastly improve understanding of the fragile lunar atmosphere.
The lunar atmosphere is the nearest example of a surface boundary exosphere (SBE)—the most common type of satellite atmosphere in the solar system. SBEs are tenuous atmospheres whose exobase is at the planetary surface. Because the individual atoms and molecules rarely collide in SBEs, kinetic chemistry is all but nonexistent, but important structural, space physics, and dynamical issues can be studied. The lunar atmosphere is the only SBE atmosphere in the solar system that is sufficiently accessible that researchers can expect to study it in detail.
SBE atmospheres are known to exist on Mercury, Europa, Ganymede, Callisto, and Enceladus; they are expected to exist on many other satellites and perhaps even Kuiper Belt objects. They are the least studied and least understood type of atmosphere in the solar system. They offer to teach us new insights into surface sputtering, meteoritic vaporization processes, exospheric transport processes, and gas-surface thermal and chemical equilibration. The lunar atmosphere is the only SBE atmosphere in the solar system that is sufficiently accessible that researchers can expect to study it in detail using both landed and orbital techniques.
The lunar atmosphere was long speculated about, but the first detection of species came from the Apollo Lunar Surface Experiments Package (ALSEP) and the scientific instrument module (SIM) instruments in the orbiting Apollo Service Module bay. Among the species discovered by Apollo missions were Ar, Po, Pb, Ra, and Rn, all of which emanate from the lunar interior via outgassing. As such, the lunar atmosphere represents a window into the workings and evolution of the lunar interior.
After Apollo, ground-based observers detected the alkali tracer species Na and K, which are also present in the SBE atmospheres of Mercury, Io, and other Galilean satellites, thereby strengthening the utility of lunar SBE studies for enhancing knowledge of similar atmospheres across the solar system.
Evidence for volatile species, including H2O, CO, CO2, and CH4, was also detected sporadically by Apollo sensors, but these detections remain unconfirmed. The detection and study of volatiles are of
great scientific interest and have obvious implications related to the trapping of ices that could represent resources to be exploited at the lunar poles.
Apollo ALSEP surface station instruments revealed that the mass of the native lunar atmosphere is on the order of 100 tons (3 × 1030 atoms, equivalent to ~1011 cm3 of terrestrial air at sea level [i.e., a cube of terrestrial air roughly 50 × 50 × 50 cubic meters at standard temperature and pressure]). Yet ALSEP total lunar atmosphere mass measurements failed to identify a census of species that comes anywhere close to the total mass of the lunar atmosphere: in fact, over 90 percent of the molecules in the Moon’s atmosphere are currently compositionally unidentified.
As a result of its low mass, the lunar atmosphere is incredibly fragile. A typical lunar surface access module (LSAM) landing will inject some 10 to 20 tons of non-native gas into the atmosphere, severely perturbing it. A human outpost might see sufficient traffic and outgassing from landings, lift-offs, and extravehicular activities (EVAs), for example, to completely transform the nature of this pristine environment. As a result, the committee recommends a strong early emphasis on studies of the native lunar atmosphere.
The key scientific questions to address are the following: What is the composition of the lunar atmosphere? How does it vary in time with impacts, diurnal cycles, solar activity, and so on? What are the relative sizes of the sources that create this atmosphere and the sinks (loss processes) that attack it?
Early observational studies to address these issues and the concern over human-induced modification of the ancient, native environment should include:
A complete census and time variability of the composition of the lunar atmosphere;
Determination of the average rate of volatile transport to the poles;
Documentation of sunrise/sunset dynamics;
Determination of the time variability of indigenous (e.g., outgassing, sputtering) and exogenous (e.g., meteorite and solar wind) sources; and
Determination of typical loss rates by various processes (e.g., photoionization, surface chemistry, Jeans escape, Michael-Manka mechanism).
Such studies could be completed from early surface networks, fixed or mobile landers, or orbiters, or a combination of any two with experiments that would include ion-mass spectrometers, optical/ultraviolet spectrometers, and cold cathode gauges.
Later, as rocket traffic and human activities perturb the lunar atmosphere from its native state, studies of the environmental effects of human and robotic activity would be highly illuminating, as an “active experiment” in planetary-scale atmospheric modification.
Before extensive human and robotic activity alters the tenuous lunar environment, it is important to understand processes involved with the atmosphere (exosphere) of airless bodies in the inner solar system.
The Moon may provide an excellent platform for specific types of observations.
The Moon is a platform that can potentially be used to make observations of Earth (Earth science), the interaction of the solar wind with the magnetosphere (viz, the solar-terrestrial connection), and the rest of the universe (astrophysics and astrobiology). The Moon provides both advantages and disadvantages for such observations. The optimum development of science first requires understanding the potential benefits and limitations of using the Moon as an astronomical or Earth-observing site.
The committee is in the process of studying the benefits and limitations of utilizing the Moon as an observational platform, referring to pertinent NRC studies (especially Priorities in Space Science Enabled by Nuclear Power and Propulsion, The National Academies Press, Washington, D.C., 2006). The full report will present these considerations.