Wendell Mendell New Views of the Moon II

Wendell Mendell
NASA - JSC

The planetary sample advisory committee to NASA, CAPTEM, has undertaken the Lunar Initiative, a project designed to refresh and revitalize lunar science studies.  To this end, they have sponsored two workshops under the title, New Views of the Moon.  The second workshop was held in Flagstaff, AZ, in late September, 1999.  Its theme was Understanding the Moon Through the Integration of Diverse Datasets.

The return of lunar samples by the Apollo missions catalyzed the creation of the discipline of planetary science, in general, and lunar science, in particular.  At the early Lunar Science Conferences, the sample investigators were kings.  A decade later, the bloom of  sample science had begun to fade and new infusions of information came from remote sensing, largely from observers on the Earth.  These new kids on the block interpreted their observations in geologic terminology but used tools and techniques regarded with suspicion by the Old Ones.  After all, a rock in the hand is worth a hundred at the end of a telescope.

The bulk of the remote observations come from the spectral analysis of sunlight reflected from the lunar surface in visible and near infrared wavelengths.   The detected ensemble of reflected photons have touched surfaces only two or three grains deep on the Moon.  Therefore, while such data can be collected from regions all over the planet, the information (like beauty) is only skin-deep.  In recent years, remote observers have begun to appreciate the effects of in-situ surface coatings and surface alterations on grains, which can produce changes in spectra that would come from samples of pure minerals in the laboratory.  Sample scientists have been wary of conclusions about lunar geochemical evolution based on such data.

At the Flagstaff workshop these two subcultures of lunar science engaged in a dialogue on the fundamentals of their research and thereby promoted a better understanding of the value and the limitations of their scientific models.  Thrown into the mix were a smattering of geophysicists and Bill Feldman, the neutron spectrometer wizard from the Lunar Prospector team.

To the eye of this reporter, one of the most interesting aspects of the workshop was the education of Feldman on the idiosyncracies of the lunar regolith and the education of the other participants on the information content of the neutron data.  For example, Feldman intended to register global iron maps of the Moon derived from neutron data with global iron maps derived by Paul Lucey from spectral data.  Pam Clark pointed out (forcefully) that the two techniques measure iron resident in different settings in the soil.  In addition, a complete explanation of the calibration of the Lucey algorithm revealed that the spectral detection of iron in crystal lattice sites is not the same as the bulk iron content of lunar soils used as the standard to interpret the spectral data quantitatively.  (The correlation is actually better that one should expect.)  Consequently, Feldman learned that differences between his maps and the Lucey maps could be information about the state of the iron in the soils rather than shortcomings of the neutron measurements.

Other interesting results were the detection of a small lunar core by two widely disparate techniques:  analysis of gravity data derived from tracking Lunar Prospector and modeling of very long term laser ranging observations using Earth-based lasers to measure the distance to reflectors left on the Moon during Apollo.  B. Ray Hawke determined that some crater rays result from deposition of bright highland material in the  ejecta while others are caused by stirring of local material by ejecta.  The two types of rays are expected to weather differently, raising questions about the validity of using ray disappearance as a uniform age measure in lunar stratigraphic classification.  Graham Ryder eloquently pleaded for a rational and consistent nomenclature for lunar rocks.  No one knew how to do that, so his plea was ignored.  Finally, it became clear to all participants that the Moon is an exceedingly lumpy planet. The incredibly deep South Pole - Aitken basin was found by the Clementine mission.  Now, Lunar Prospector’s data reveal the Procellarum KREEP Terrane, an enormous radioactive “hot spot” in the northern hemisphere.  Added to the well known nearside - farside dichotomy, these new findings place a stake in the heart of spherically symmetric evolution models of our neighboring planet.

The meeting was alive with burgeoning collaborations within and between the two scientific cultures.  This series of workshops is clearly breathing new life into lunar science.  Now, we just need a few more samples.......