The Scientific Context for Exploration of the Moon: Interim Report (2006)

Table 1 provides a summary of scientific objectives for early lunar exploration and their principal implementation modes. These objectives and implementations provide a rich basis for the extraction of specific objectives that could be organized into a science program. The committee provides the following prioritization of these lunar science goals that flow from the 10 themes discussed in Chapter 1 and that can be accomplished by lunar measurements and analyses. The committee has used the prioritization criteria adopted by the decadal survey New Frontiers in the Solar System: An Integrated Exploration Strategy (NRC, 2003) as a guideline: scientific merit, opportunity, and technological readiness.

1. Fundamental Solar System Science

   • Characterize and date the impact flux (early and recent) of the inner solar system.

   • Determine the internal structure and composition of a differentiated planetary body.

   • Determine the compositional diversity (lateral and vertical) of the ancient crust formed by a differentiated planetary body.

   • Characterize the volatile compounds of polar regions on an airless body and determine their importance for the history of volatiles in the solar system.

1. Planetary Processes

   • Determine the time scales and compositional and physical diversity of volcanic processes.

   • Characterize the cratering process on a scale relevant to planets.

   • Constrain processes involved in regolith evolution and decipher ancient environments from regolith samples.

   • Understand processes involved with the atmosphere (exosphere) of airless bodies in the inner solar system.

1. Other Opportunities (additional information is required for these)

   • Utilize data from the Moon to characterize Earth’s early history.

   • Determine the utility of the Moon for astrophysics observations.

   • Determine the utility of the Moon as a platform for observations of Earth.

   • Determine the utility of the Moon as a platform for observations of solar-terrestrial processes.

Several fundamental solar system science issues are given higher priority because of their broad importance to understanding the way in which the Moon and planets have evolved as solar system bodies, especially in the first 500 million years of solar system evolution. These goals are readily addressed in the time frame specified by this report. Although still fundamental and having implications for the same processes on other planets, several other planetary processes are given somewhat lower priority because they are (1) more highly linked to later Vision activities or events and (2) focus on the Moon as a specific body. A few, such as cratering processes and ancient regolith sampling, appear to require components that extend considerably into the period when human involvement is significant. Other opportunities are listed as lower priority at this time primarily due to the lack of input to the committee for assessing these opportunities.

The committee will provide a more complete prioritization of, and increased specificity for, these opportunities in its full report if relevant information becomes available.

Many science objectives will require investigation by several techniques—orbital observations, in situ analysis, astronaut field observations, and analysis of returned samplesso a simple prioritization of individual objectives alone is not feasible. The committee has assumed that an “opportunity” criterion is specified by its statement of task, which directed it to consider LPRP robotic precursor missions and early human “sortie” missions similar to Apollo capabilities. These provide constraints of mass, power, and technology development on what can reasonably be available for implementation. “Technology readiness” is ambiguous for the more distant exploration schedule, which may allow many capabilities, currently unavailable, to be developed by the time of the missions. Nevertheless, some capabilities, such as deep drilling, are probably beyond the time horizon addressed in this report, and the committee has tried to accommodate this perspective. Although its task focused on science objectives and mission capabilities, the committee thought it necessary to consider the degree of preparation required for conducting the science that would be accomplished by the Vision.

With this introduction and the above discussion of science themes and priorities, the committee provides the following list of findings and recommendations, divided into two categories. The first category directly addresses the scientific objectives. In Chapter 5, related findings and recommendations are derived from the committee’s consideration of the challenges facing NASA in the implementation of a lunar science program of the type envisioned by the science objectives and implementation approach recommended here.

PRINCIPAL FINDING: Lunar activities apply to broad scientific and exploration concerns.

Lunar science as described in this report has much broader implications than simply studying the Moon. For example, a better determination of the lunar impact flux during early solar system history would have profound implications for the evolution of the solar system, the early Earth, and the origin of life. A better understanding of the lunar interior would bear on models of planetary formation in general and the origin of the Earth-Moon system in particular. And exploring the possibly ice-rich lunar poles could reveal important information about the history and distribution of solar system volatiles. Furthermore, although some of the committee’s objectives are focused on lunar-specific questions, one of the basic principles of comparative planetology is that each world studied enables researchers to better understand other worlds, including our own. Improving our understanding of such processes as cratering and volcanism on the Moon will provide valuable points of comparison for these processes on the other terrestrial planets.

Table 2 shows linkages between the science goals for the recommended lunar exploration program and broader scientific and applied concerns.

FINDING 1: Enabling activities are critical in the near term.

A deluge of spectacular new data about the Moon will come from four sophisticated orbital missions to be launched between 2007 and 2008: SELENE (Japan), Chang’e (China), Chandrayaan-1 (India), and the Lunar Reconnaissance Orbiter (United States). Scientific results from these missions, integrated with new analyses of existing data and samples, will provide the enabling framework for implementing the Vision’s lunar activities. However, NASA and the scientific community are currently underequipped to harvest this data and produce meaningful information. For example, the lunar science community assembled at the height of the Apollo program of the late 1960s and early 1970s has since been depleted in terms of its numbers and expertise base.

FINDING 2: Explore the South Pole-Aitken basin.

The answer to several high-priority science questions identified can be found within the South Pole-Aitken basin, the oldest and deepest observed impact structure on the Moon and the largest in the solar system. Within it lie samples of the lower crust and possibly the lunar mantle, along with answers to questions on crater and basin formation, lateral and vertical compositional diversity, lunar chronology, and the timing of major impacts in the early solar system.

Missions to South Pole-Aitken, beginning with robotic sample returns and continuing with robotic and human exploration, have the potential to be a cornerstone for lunar and solar system research. (A South Pole-Aitken sample return mission was listed as a high priority in the 2003 NRC decadal survey report New Frontiers in the Solar System: An Integrated Exploration Strategy.)

FINDING 3: Determine the composition and structure of the lunar interior.

Determining the structure and composition of the interior of the Moon, from the outer layers of the crust to the deep core, will provide enormous insight into many of the highest-priority scientific questions. Long-duration geophysical stations (with broad-band seismometers, heat flow measurements, and precision tracking capability) implemented at multiple (six or more) sites are required to provide comprehensive subsurface information.

FINDING 4: Maximize the diversity of lunar samples.

Laboratory analyses of returned samples provide a unique perspective based on scale, precision, and flexibility of analysis and have permanence and ready accessibility. The lunar samples returned during the Apollo and Luna missions dramatically changed understanding of the character and evolution of the solar system. We now understand, however, that these samples are not representative of the larger Moon and do not provide sufficient detail and breadth to address the fundamental science themes outlined in Table 1. Laboratory analyses of returned samples provide a unique perspective based on scale, precision, flexibility, and permanence/accessibility.

FINDING 5: Proceed with lunar surface mission development and the site selection process.

Plans to return to the Moon will involve the selection of surface exploration sites. The selection process will involve, among many factors, scientific potential, ease of access, potential to conduct traverses, options for future build-up to an outpost or base, resource utilization potential, and explorer safety. Many of the science goals listed in Table 1 depend critically on site selection, although a given site might satisfy several requirements of several different goals. In the period under consideration for this report, both robotic and human sample return capabilities can be considered.

FINDING 6: Understand the lunar polar deposits and environment.

Almost nothing is known about the sources of volatiles at the lunar poles and the processes operating on these volatiles. Additionally, there is almost no information about the physical properties of the lunar soils in permanently shaded regions. Finally, there is no knowledge of how robotic and human activities on the Moon will affect and change the unique and possibly fragile lunar polar environment.

FINDING 7: Understand and characterize the lunar atmosphere.

The lunar atmosphere is the only surface boundary exosphere (SBE) in the solar system that is sufficiently accessible to study in detail. Despite many Apollo and Earth-based measurements, numerous aspects of the lunar atmosphere (e.g., composition, interaction processes) remain unknown. Furthermore, the lunar atmosphere is so extremely fragile that robotic and human activities can completely transform the nature—possibly permanently—of this pristine environment.

FINDING 8: Evaluate the Moon’s potential as an observation platform.

Observation from the Moon may be useful to astrophysics, studies of the Sun-Earth connection, Earth science, and some parts of astrobiology. There are uncertainties in the environmental benefits, disadvantages, potential mitigations, and costs of the Moon as an observation platform relative to alternate space sites, requiring a thorough study.

FINDING 9: Establish strong ties with international programs.

The current level of planned and proposed activity indicates that almost every space-faring nation is interested in establishing a foothold on the Moon. Although these international thrusts are tightly coupled to technology development and exploration interests, science will be a primary immediate beneficiary. NASA has the opportunity to provide leadership in this activity, an endeavor that will remain highly international in scope.