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NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL
June 18, 2004 Current Operating Status
5
The Moon
PROGRESS
The Moon is far more complicated than was generally appreciated when the 1978
report was written. This appreciation of the complexity of lunar history has come
from continued study of lunar samples (including meteorites from the Moon),
ground-based remote sensing data, sophisticated use of the Apollo orbital remote
sensing data, and general advances in our understanding of geological and
geophysical processes.
Besides being intrinsically interesting in its own right, the Moon provides a unique
window into solar system history. Its origin is intertwined with that of Earth, its
craters preserve a record of meteoroid fluxes through time, and it preserves at
least a fragmentary record of its early (pre-four-billion-year) evolution. It is one
testing ground for our ideas about the origin and evolution of small planets. The
Moon is an ideal body on which to study the processes, such as exogenic impacts,
that have shaped the other solid bodies in the solar system and perhaps even
caused extinctions of some forms of life on Earth. The Moon is also the only
extraterrestrial body from which we have samples from a known geologic context,
thereby providing a much more quantitative understanding of its history. The lunar
soil preserves a four-billion-year-old record of the Sun's history. Finally, the Moon
is a readily accessible body, making its scientific exploration easier to achieve.
Lunar science has evolved in several stages. The late 1960s and early 1970s
focused largely on surface exploration, centering on the Apollo program. The mid-
1970s through the 1980s have emphasized reflection on those results in the
context of new concepts and continued analysis. The future will address unsolved
problems in lunar science and prepare for advanced studies from a lunar base.
During the mid-1970s, there was no consensus about how the Moon formed. Now
the idea that it originated as a result of a giant impact on Earth has caught hold and
has passed the tests given it so far. Nevertheless, this hypothesis is far from
proven; assessing it requires a better understanding of the Moon's bulk chemical
composition.
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The idea that the primitive Moon was surrounded by an immense magma system
known as the "magma ocean" continues to be a central tenet of lunar science, but
vigorous debate is taking place about its nature and the processes that operated in
it. Some investigators are even questioning whether there was a magma ocean.
Proof of the magma ocean hypothesis hinges on the composition of the Moon's
crust and the nature and ages of lunar anorthosites. Whether there was a magma
ocean or not, it has become clear that there was a period prior to four billion years
ago of intense igneous activity that modified the primordial lunar crust. In contrast
to the narrow range of rock types defined during the mid-1970s, continued sample
analysis has revealed a vast array of rock types in the lunar highlands, and remote
sensing has shown that rock types rare in the Apollo collection are nevertheless
abundant on the Moon. We need many more data from remote sensing and
sample returns to determine the full range of rock types and how they relate to
each other and to the products of the magma ocean.
Our understanding of mare basalts has advanced tremendously. After the Apollo
missions ended, it was generally believed that mare basalt volcanism took place
3.2 to 3.8 billion years ago. Subsequent photogeological and lunar sample studies
have shown that this type of volcanism occurred over a much greater time period,
from 4.3 to possibly 1.0 billion years ago. This discovery has great implications for
the Moon's thermal history. Furthermore, we now know that we sampled only about
half of the full range of mare basalts. Because basalts contain information about
the interior, we have an incomplete knowledge of the nature of the lunar mantle.
The time and rate of formation of large craters and the great lunar basins are still
uncertain. During the mid-1970s, the consensus was that they formed during a
narrow time interval 3.85 to 4.0 billion years ago—the so-called lunar cataclysm.
The consensus now is that the bombardment rate declined gradually and that only
a few basins, which we happened to sample during Apollo, formed during the
period from 3.85 to 4.0 billion years ago. The question will remain open, however,
until samples are obtained and dated from basins far removed from those on the
near side. In addition, the compositions and sources of the projectiles that made
large craters and basins are largely unknown. Understanding these factors will
contribute to our understanding of the later stages of planetary accretion with
implications for the early history of the Earth.
Unanswered questions about the Moon abound. These problems can be
addressed by global surveys from orbit, installation of a network of geophysical
instruments, sample-return missions, and detailed field studies from a lunar base.
SCIENTIFIC OBJECTIVES
It is very important that the Moon's entire surface be adequately imaged and
mapped geochemically, mineralogically, and geophysically. To meet this
requirement, COMPLEX recommends that a spacecraft or series of them, be
placed in a lunar polar orbit. The measurements would benefit greatly if two
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spacecraft were in orbit simultaneously. The second spacecraft, which could be
provided by another nation, would allow electromagnetic sounding of the interior
and mapping of the far-side gravity field. Besides contributing to the solution of
fundamental questions in lunar science, orbital measurements will provide critical
information about where to locate a lunar base, regions containing potential
resources, sites for sample-return missions and intensive field work, and
emplacement of a network of geophysical stations.
To contribute to significant advances in lunar research, orbital measurements
ought to include the following:
1. Abundances of major rock-forming elements (O, SL Fe, Mg, Al, Ti, and Ca) and
of selected minor and trace elements (K, U, and Th). This would yield the average
composition of the surface and, if basins were used as natural drill holes, an
estimate of the chemical composition of deeper crustal layers.
2. Spectroscopic .measurements to obtain mineralogical and chemical data at high
spectral and spatial (<500-m) resolution. This would provide information on the
distribution of lunar rock types.
3. Topographic data (combined with gravity data) to. address problems involving
density distributions and lithospheric loading.
4. Measurement of the gravity field on both the near and far sides to allow
calculation of crustal thicknesses and densities.
5. Measurements of the Moon's magnetic field to shed light on the characteristics
and origins of magnetic anomalies and to place constraints on the size of the core.
6. Imaging data to obtain global, digital photographic coverage with a line-pair
resolution of 25 m and high resolution (1 m) of selected areas. These data would
provide information about impact cratering, volcanism, and tectonism and would
provide the geologic context needed to interpret other types of data.
7. Measurement of the average global surface heat flow to constrain the Moon's
thermal history and provide indirect measurement of the bulk content of heat-
producing elements.
A thorough understanding of the Moon will be impossible without knowledge of its
interior. Better constraints on the size of the core will shed light on the origin of the
remanent lunar magnetic field and, hence, on the origin of planetary dynamos in
general. This will require the installation of a geophysical network of at least eight
stations. Each station should include a seismometer, heat-flow probe, and
atmospheric sensors. Such a seismic network would also be able to monitor the
present meteoroid flux, using the entire Moon as a collecting surface. Deployment
scenarios include automated landers or rovers, penetrators, or, eventually,
astronauts. COMPLEX recommends that NASA develop the technology to deploy
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geophysical stations.
The lunar regolith contains a 4-billion-year-old record of solar particle emission
history; deciphering this record will improve our understanding of how the Sun
varies with time and improve the basis of predictions of how it will vary in the
future. Lunar craters can be dated to provide a test of the hypothesis that mass
extinctions of life on Earth were caused by periodic increases in the impact rate on
our planet.
UPDATED RECOMMENDATIONS
The committee endorses the recommendations in the 1978 report. Measurement of
the Moon's global chemical composition remains a high priority, but the committee
recommends that global mineralogical measurements at high spatial and spectral
resolution also be given a high priority. Much more information about the nature of
the lunar interior is needed as well; acquisition of the appropriate geophysical data
from orbit and on the lunar surface remains a high priority. This will require
instrument development and research on how to deploy instruments on the
surface.
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