Near infra-red spectroscopy of the asteroid 21 Lutetia. II. Rotationally resolved spectroscopy of the surfaceReport as inadecuate

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1 Astronomical Institute of the Romanian Academy 2 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides 3 LESIA - Laboratoire d-études spatiales et d-instrumentation en astrophysique 4 EAPS - Department of Earth, Atmospheric and Planetary Sciences MIT, Cambridge 5 Astronomical Observatory 6 Institute for astronomy

Abstract : Aims. In the framework of the ground-based science campaign dedicated to the encounter with the Rosetta spacecraft, the mineralogy of the asteroid 21 Lutetia was investigated. Methods. Near-infrared NIR spectra of the asteroid in the 0.8−2.5 μm spectral range were obtained with SpeX-IRTF in remote observing mode from Meudon, France in March and April 2006. We analysed these data together with previously acquired spectra - March 2003, August 2004. I-band relative photometric data obtained on 20 January 2006 using the 105 cm telescope from Pic du Midi, France has been used to build the ephemeris for physical observations. A χ2 test using meteorite spectra from the RELAB database was performed in order to find the best fit of complete visible + infrared VNIR spectra of Lutetia. Results. The new spectra reveal no absorption features. We find a clear spectral variation slope, and a good correspondence between spectral variations and rotational phase. Two of the most different spectra correspond to two opposite sides of the asteroid sub-Earth longitude difference around 180◦. For the neutral spectra a carbonaceous chondrite spectrum yields the best fit, while for those with a slightly positive slope the enstatitic chondrite spectra are the best analog. Based on the chosen subset of the meteorite samples, our analysis suggests a primitive, chondritic nature for 21 Lutetia. Differences in spectra are interpreted in terms of the coexistence of several lithologies on the surface where the aqueous alteration played an important role.

Keywords : minor planets asteroids techniques: spectroscopic methods: observational

Author: Dan Alin Nedelcu - Mirel Birlan - Pierre Vernazza - Pascal Descamps - Richard P. Binzel - François Colas - A. Kryszczynska - Sch



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