NIR REFLECTANCE SPECTROSCOPY: THE PROMISES AND CHALLENGES FOR FUTURE REMOTE STUDY OF ASTEROIDS

As a practicing scientific discipline, near-infrared reflectance spectroscopy of asteroids has been evolving for more than 35 years. Theoretically grounded in crystal field theory and facilitated by low-resolution astronomical spectrographs, NIR spectroscopy remains the primary method to remotely study the geology of asteroids. Some notable accomplishments to date include creation of extensive mineral and meteorite spectral catalogs, NIR spectral surveys of the brighter main-belt asteroids, the development of successful observational, data reduction, and quantitative mineralogical calibrations or protocols, the pairing of specific meteorite type(s) with an asteroid (i.e., 4 Vesta/Vestoids, 6 Hebe), the discovery of weak spectral features on “featureless” asteroid spectra, and the convergence of interpretations in the asteroid and meteorite communities.

Despite these accomplishments, NIR asteroid reflectance spectral work is still in its infancy. With only ~10 full-time scientists, two observatories (IRTF and TNG), and more than 200,000 asteroids to study, a vast amount of knowledge has yet to be gleaned from main-belt and near-Earth asteroids. Efforts to ameliorate this situation include attracting more students into this field, equipping more observatories with the appropriate spectrographs, and providing more funding opportunities.

Improving quantitative spectral calibrations for different minerals can impose more rigorous constraints when interpreting NIR asteroid spectra. The relationship between opx/Type B cpx band parameters with Fe2+ content is well-established, but quantitative calibrations are either less rigorous or non-existent for important minerals such as olivine, spinel, and plagioclase feldspar. Studying the spectral behavior of binary and trinary mineral mixtures (opx, Type A cpx, Type B cpx, olivine) can provide insight into the spectral behavior of such mixtures, even if no systematic trends emerge with changing mineral chemistry.

Application of current and improved interpretive calibrations allows for diverse investigations such as physical characterizations of near-Earth asteroids, inferring asteroid thermal histories to test early solar system heating mechanisms, and probing additional links between specific asteroid-meteorite combinations.