The interpretation of asteroid spectra is closely tied to surface structure and composition. Asteroid surfaces are usually assumed to be covered with a regolith, which is a mixture of mineral grains ranging from micrometers to centimeters in size. The inverse problem of deducing the characteristics of the grains from the scattering of light (e.g., using photometric and polarimetric observations) is difficult. Meteorite spectroscopy can be a valuable alternative source of information considering that unweathered meteoritic 'falls' are almost pristine samples of their parent bodies.
Reflectance spectra of 18 different meteorites were measured with the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO) covering a wavelength range of 350-2500 nm and a zenith angle of reflection range of ± 60 degrees (Suomalainen et al., Sensors 9, 3891, 2009). Principal Component Analysis (PCA) performed on the spectra tends to separate the undifferentiated ordinary chondrites and the differentiated achondrites. The measurements expand the database of reflectance spectra obtained by Paton et al. (JQSRT 112, 1803, 2012) and Gaffey (NASA PDS, 2001). The spectra of meteorites found in all the data sets are consistent. For the FIGIFIGO measurements, the principal components suggest a discrimination between the spectra from ordinary chondrites of petrologic grades 5 and 6. The distinction is not present when the data are supplemented with the spectra from the two other data sets obtained with differing measuring techniques.
Single-scattering albedos for meteoritic fundamental scatterers were derived with a Monte Carlo radiative-transfer model. In the derivation, realistic scattering phase functions were utilized. The functions were obtained by fitting triple Henyey-Greenstein functions to the scattering matrix elements of olivine powder for two different size distributions (Muñoz et al., A&A 360, 777, 2000; JQSRT 113, 565, 2012). The simulated reflectances for different scattering phase functions were fitted to the measured meteorite spectra. The single-scattering albedos for the analyzed ordinary chondrites range from 0.65 to 0.9, which is in line with the largest single-particle albedo values of 0.50 ± 0.25 for the ordinary chondrite Bjurböle measured by Piironen et al. (PSS 46, 937, 1998). Based on the analysis, meteorites with higher petrologic grades have higher single-scattering albedos across the wavelength range. Using the larger single scatterer results in a lower albedo but a wider range in albedo values.
