REFLECTANCE SPECTROSCOPY OF INTERPLANETARY DUST PARTICLES

Reflectance spectra were collected from chondritic interplanetary dust particles (IDPs), a polar micrometeorite, Allende (CV3) meteorite mat rix, and mineral standards using a microscope spectrophotometer. Data were acquired over the 380-1100 nm wavelength range in darkfield mode using a halogen light source, particle aperturing diaphrams, and photo multiplier tube (PMT) detectors. Spectra collected from titanium oxide (Ti4O7), magnetite (Fe3O4), and Allende matrix establish that it is p ossible to measure indigenous reflectivities of micrometer-sized (>5 m u m in diameter) particles over the visible (VIS) wavelength range 450 -800 nm. Below 450 nm, small particle effects cause a fall-off in sign al into the ultraviolet (UV). Near-infrared (IR) spectra collected fro m olivine and pyroxene standards suggest that the similar to 1 mu m ab sorption features of Fe-bearing silicates in IDPs can be detected usin g microscope spectrophotometry. Chondritic IDPs are dark objects (<15% reflectivity) over the VIS 450-800 nm range. Large (>1 mu m in diamet er) embedded and adhering single mineral grains make IDPs significantl y brighter, while surficial magnetite formed by frictional heating dur ing atmospheric entry makes them darker. Most chondritic smooth (CS) I DPs, dominated by hydrated layer silicates, exhibit generally flat spe ctra with slight fall-off towards 800 nm, which is similar to type CI and CM meteorites and main-belt C-type asteroids. Most chondritic poro us (CP) IDPs, dominated by anhydrous silicates (pyroxene and olivine), exhibit, generally flat spectra with a slight rise towards 800 nm, wh ich is similar to outer P and D asteroids. The most C-rich CP IDPs ris e steeply towards 800 nm with a redness comparable to that of the oute r asteroid object Pholus (Binzel 1992). Chondritic porous IDPs are the first identified class of meteoritic materials exhibiting spectral re flectivities (between 450 and 800 nm) similar to those of P and D aste roids. Although large mineral grains, secondary magnetite, and small p article effects complicate interpretation of IDP reflectance spectra, microscope spectrophotometry appears to offer a rapid, nondestructive technique for probing the mineralogy of IDPs, comparing them with mete orites, investigating their parent body origins, and identifying IDPs that may have been strongly heated during atmospheric entry.