Neal, and 49 co-authors submitted this paper to The Inner Planets Panel, NRC Decadal Survey for the Planetary Sciences Division, Science Mission Directorate, NASA in 2010. It allows that the Moon is our closest celestial neighbor and represents a nearly complete picture of the processes that have influenced the inner solar system over time, especially the period >3 b.y. ago, which has been obscured/lost on Earth. The Moon also represents the only other planetary body that humans have visited. Investigations carried out on the lunar surface, coupled with returned samples and lunar meteorites, and data from orbital missions, have allowed sophisticated scientific questions to be posed regarding the formation and evolution of the Moon and the inner
Solar System. Many of the discoveries from our studies of the Moon have become the paradigms for the evolution of the terrestrial planets. Three fundamental scientific concepts emerged:
1) lunar origin by giant impact,
2) the existence of an early lunar magma ocean, and
3) the potential of an impact cataclysm at 3.9 billion years ago.
As often occurs with scientific discovery, however, these ideas have raised more questions than they have answered.
For example:
• The Giant Impact theory proposes that the Moon formed as a result of a collision between a
large protoplanet and the growing Earth [3,4]. Although the idea that the Earth-Moon system
owes its existence to a single, random event was initially viewed as radical, it is now believed
that such large impacts were commonplace during the end stages of terrestrial planet formation
(e.g., [5,6]). Sophisticated numerical models suggest that a giant impact can indeed produce a
disk of rocky/vapor material orbiting the Earth. However, we do not yet know whether such a
disk can evolve into the Moon observed today, particularly given the existence of significant
volatile reservoirs in the lunar mantle.
• The Late Heavy Bombardment (LHB) refers to a period ~4.0-3.8 Ga during which several
large lunar basins formed. The nature of the LHB is debated: one view is that the LHB marked
the end of a steadily declining bombardment from remnants of planet accretion [7]. Another
proposes that the LHB was a short-lived "cataclysm" of dramatically increased impact rates
[8], and still another proposes that large basin formation was continuous or episodic throughout
the period 4.5-3.8 Ga [9]. We are still unable to observationally distinguish between these three
fundamentally different models. The NRC [1] report ranks testing the cataclysm hypothesis
and constraining the early lunar impact record as its two top science goals.
• While fundamental questions remain, we believe that over the last decade, there has been a
revolution in our understanding of the origin of the Solar System. Theoretical and computational
breakthroughs make it possible to realistically model the Moon-forming impact on the proto-Earth
and to track the collisional and dynamical evolution of planets and small bodies for billions of years
to evaluate the lunar and Solar System bombardment history, and its subsequent differentiation
and evolution. In addition, significant improvements in laboratory techniques for dating and analysis
of rocks, coupled with improvements in crater counting due todigital image processing, allow us to
better assess the compositions and ages of lunar terranes, and to understand their origins and
relationships to the deep lunar interior.
With these gains in hand, we now stand on the brink of new fundamental insights into the formation and evolution of the Moon. However, the implications of further lunar exploration go far beyond the Moon as constraints on the lunar origin and evolution can also be used to constrain Solar System evolution. If the Moon is given a high priority by this decadal survey, we will be able to address high priority lunar and broader Solar System science questions, which
have significant traceability to planning documents from the 1980’s to the present (e.g., [1,2, 10- 18]). The Moon can also be used to develop mission-enabling technologies and protocols for wider Solar System exploration (both robotic and manned). Thus, the Moon holds the promise of being the "Rosetta Stone" for understanding the evolution of the Solar System as a whole.
Many “Solar System” science objectives can be studied in more detail on the Moon with human-robotic partnerships than they can elsewhere because of the largely pristine exposures on the Moon and ease of operations there, relative to more distant locales. Therefore, we need to use the Moon to not only understand how the Moon and the Solar System have evolved, but also in understanding how to keep humans safe in space and develop exploration and safety systems so that human Solar System exploration can become a reality. In doing so, the major inner Solar System science questions can be addressed through lunar exploration and this adds to the designation of the Moon as the “Rosetta Stone” for Solar System science exploration. Here, the main science objectives are outlined and their applicability to better understanding the solar system (and in some cases, beyond) are briefly described. This white paper does not advocate any specific missions or mission types, but emphasizes the unique opportunity lunar science affords for studying the Solar System. It does highlight specific lunar white papers that go into more detail about the subjects than is possible in this contribution.
The authors concluded that a coordinated lunar science exploration plan allows for significant progress to be made in our understanding of the Moon and also the Sun, Earth, Solar System, and beyond (see also the white paper entitled “Calming the Gathering Storm with a Long Term Lunar Program”). The
topics briefly outlined above demonstrate the relevance of the Moon as a “Rosetta Stone” for Solar System science and further demonstrate that Solar System science can be done by studying the Moon in situ with human and robotic systems as well as remotely with robotic spacecraft, before moving on to far away Mars.
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