Remote Compositional Analysis: The Coming of Age

Remote mineralogical analysis of planetary surfaces was attempted more than a century ago. This involved spectroscopy of regions on, mostly, the lunar surface, using groundbased telescopes and of rocks and minerals in the laboratory. However, it was not until the 1960s that science and technology developed to the point of allowing reflectance spectroscopy to become a quantitative technique. Some of us were luck enough to appear on this scene, young and energetic and with supporting funds available to take advantage of these advances to further the knowledge of molecules and minerals in the solar system. Electronic light detectors became available and the near IR portion of the spectrum was quantitatively accessed so that specific absorption bands could be detected in the reflectance spectrum, begging interpretation. At the same time, the physics of the interaction of light and minerals was becoming much better understood, allowing interpretation. Geochemists and geologists became interested and helped place these discoveries in the context of solar system science. Major successes resulted mostly from a few scientists who accomplished some expertise in all three areas. This allowed identification of many minerals and their crystal state using the reflectance spectra. The early emphasis was on the Moon because of its proximity to Earth and the Apollo Program. Reflectance spectra of the Moon were obtained in the late 60s and early 70s that showed absorption features and these features were interpreted, for example, to suggest a basaltic composition for the maria with high titanium content in some places. The Apollo Program produced samples and their reflectance spectra were measured in the laboratory. Comparisons with telescopic measurements indicated very good agreement and confirmed remote mineralogical interpretations. With confidence gained, we proceeded to explore the mineralogy of the Moon and derived interpretations therefrom. This success gave us confidence to proceed to other solar system objects, including asteroids and satellites, work that some of us carry on to this day and hopefully well into the future.