The other model notes that the size-frequency distribution of the highland craters on the Moon is the same as that in the main belt and distinct from the modern population of near-Earth asteroids. The near-Earth asteroids seem to be responsible for cratering the lunar maria, i.e., more recently than 3.9 billion years ago.5 Strom et al. postulate that Neptune, and possibly Uranus, were formed 500 million years after the formation of the other planets, around 4 billion years ago.6 The resultant migration inward of Jupiter caused resonances to sweep through the main belt with the same consequences.

Conclusions

The committee concludes that the South Pole-Aitken Basin Sample Return mission remains a very scientifically important mission that should be considered for the New Frontiers Program. Although the committee is concerned that NASA should not be too specific in defining how New Frontiers missions should be conducted, it has concluded that in this case, given the maturity of the science questions and the precise design of the mission as stated in the decadal survey, studying new samples from the Moon is a reasonable and irreducible requirement of the mission. Furthermore, the South Pole-Aitken Basin is the preferred lunar region to target for this mission. Other sample return sites may exist that can address the preponderance of the objectives for this mission; however, it is the responsibility of the proposer to convincingly defend the merits of an alternative site.

After exhaustive and extended laboratory analysis of the Apollo and Luna lunar samples and meteorites from the Moon, no evidence of water in any form has been found in lunar rocks or soils.7 While the search for water on the Moon is not a science objective for this mission, returned samples from the deep crust or upper mantle may contain trace water. Discovery of water in returned lunar samples, even in the minutest quantities, would constitute a major scientific discovery.

Mission-Specific Recommendations

A South Pole-Aitken Basin Sample Return mission is tenable under the New Frontiers Program and can address many decadal survey objectives. The committee recommends that the South Pole-Aitken Basin Sample Return mission as described in the decadal survey remain a high priority for the New Frontiers Program. The committee has no changes to the decadal survey’s scientific objectives or engineering implementation of this mission. However, the committee recommends that NASA not be overly prescriptive about specific approaches to address the scientific objectives. Instead, NASA should allow proposers to develop their own innovative approaches.

The committee believes that the following science goals, not in priority order, should be established for this mission:

  • Elucidate the nature of the Moon’s lower crust and/or mantle by direct measurements of its composition and of sample ages;

  • Determine the chronology of basin-forming impacts and constrain the period of late, heavy bombardment in the inner solar system, and thus, address fundamental questions of inner solar system impact processes and chronology;

giant planets of the solar system in the gaseous protoplanetary disk and their relationship to the current orbital architecture, The Astronomical Journal 134:1790-1798, 2007.

5

See R.G. Strom, R. Malhotra, T. Ito, F. Yoshida, and D.A. Kring, The origin of planetary impactors in the inner solar system, Science 309(5742):1847-1850, 2005.

6

See R.G. Strom, R. Malhotra, T. Ito, F. Yoshida, and D.A. Kring, The origin of planetary impactors in the inner solar system, Science 309(5742):1847-1850, 2005.

7

As the committee was finishing its report, it learned of a presentation at the fall meeting of the American Geophysical Union that may indicate the presence of significant water in lunar volcanic glasses. See A.E. Sael, E.H. Hauri, M. Lo Cascio, J. Van Orman, M. Rutherford, and R. Cooper. Volatiles in the lunar volcanic glasses: Evidence for the presence of indigenous water in the Moon’s interior, AGU Fall 2007 Meeting.



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