Coordinated Laboratory Studies of Aubrite Meteorites for Rosetta flyby of Asteroid 2867 Steins
Résumé
A coordinated study of candidate meteoritic material supporting many space missions to asteroids has been developed to provide information for interpretation of in situ measurements. ESA's Rosetta spacecraft is scheduled to flyby 2867 Steins, an E-type asteroid, September 5, 2008. There is evidence that Steins is related to aubrite meteorites based on its high albedo, 0.45, its reflectance spectrum with absorption band at 0.5 µm attributed to CaS, oldhamite, and no absorption bands in the 1 and 2 micron regions consistent with the presence of low-Fe or end-member enstatite. Two 5-gram samples of aubrites ALHA 78113 and ALH84007, comprised of nearly iron-free pyroxene (enstatite), small amounts of Ni-Fe and sulfides with some iron oxide halos, were provided by the Antarctic Meteorite Working group. The reflectance spectrum from 300 - 2600 nm of the whole rock shows an absorption band at 430 nm (probably iron oxide) and no absorption around 500 nm that is seen in the Steins spectrum. At Kharkiv Astronomical Observatory photometry and polarimetry at very small phase angles measures the brightness as a function of phase angle and polarization at two wavelengths (472 and 670 nm). To simulate Stein's flyby, an enstatite mineral, not from the meteorite but of the same albedo and granulation as aubrite samples, has been measured showing 25% increase in brightness between 0.0076-1°. Results from scheduled observations using PROGRA2 measuring linear polarization on a range of phase angles at two wavelengths (543.5 nm and 632.8 nm) of powdered sample and with the imaging spectrometer and goniometer facility at INAF-Roma will be presented. Additional plans include measurements with PROGRA2 during the CNES and ESA parabolic flight campaigns in October 2008, with Southwest Ultraviolet Reflectometer Chamber (SwURC), and characterization of the meteorite before and after irradiation simulating solar wind exposure at Experimental Astrophysical Laboratory, Catania.