Physics – Optics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..838l&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Physics
Optics
Scientific paper
The flyby of Steins by the Rosetta spacecraft provides the first in-situ study of an E-type asteroid. Such objects are assumed to be the source of aubrite meteorites. Laboratory experiments, mostly developed to infer the properties of cometary dust from comparisons between light scattering observations of comets and measurements on analogues, are to be used to possibly confirm the possible abovementioned relationship. Asteroidal taxonomy and polarization properties Remote observations of asteroidal spectra provide taxonomic classifications, mostly related to the composition of the surface material. Besides the broad classes of S, C or M asteroids, a number of less documented classes (e.g. E-type) have been noticed. Remote observations of the linear polarization of solar light scattered by asteroidal surfaces provide a clue to the properties of the dust (i.e. regolith) on the surface [1, 2]. The polarization is actually a dimension-less ratio, which only varies with the geometry of the observations, the wavelength, and the properties of the scattering medium (mostly albedo, size distribution and porosity). The polarization dependence upon the phase angle gives access to the albedo, as well as to a classification that is reminiscent of the taxonomic classes [3]. The polarization dependence upon the wavelength spectral gradient (for phase angles above about 25°) shows a positive gradient for bright S-types and a negative gradient for dark C-types [4]. Asteroid Steins On its journey to rendezvous with comet 67P/Churyumov-Gerasimenko, the Rosetta spacecraft will flyby two asteroids, 2867 Steins on September 2008 and 21 Lutetia on June 2010. Steins is the first E-type asteroid [5] to be studied by a space probe. E-type asteroids are expected to be related to aubrites, which are differentiated, stony meteorites lacking chondrules, composed primarily of enstatite (Mg2Si2O6). Aubrites have mostly been found in Antarctica, and are unusual in their lack of iron. They formed in a very reduced environment in the solar nebula. Steins is a small main belt object. Polarimetric remote observations have been used to derive its albedo (about 0.45) and its size (about 4.6 km), large enough to allow the presence of some regolith [6]. Polarimetric observations also suggest a negative spectral gradient, unexpected for such a bright object. Simulations of the regolith properties An extensive programme of laboratory and numerical simulations with various irregular compact grains and aggregates has been developed in the past years in our group, with emphasis on cometary and interplanetary dust [7]. Comparisons of the results with remote observations (tentatively on a large range of phase angles and wavelengths) provide information on dust properties, which fairly agree with the results of insitu studies and sample return analysis. The PROGRA2 experiments allow us to measure the linear polarization on a large range of phase angles and for two wavelengths (543.5 nm and 632.8 nm), either on layers or on levitating dust particles, in the laboratory and during parabolic flight campaigns in board the CNES dedicated aircraft [8, 9]. Numerous samples have been used in the past, including dust sample from Orgueil meteorite. A programme of laboratory simulations with PROGRA2 is now being developed, with powders of an aubrite meteorite (e.g. ALH84007) provided for an investigation entitled "Integrated Laboratory Studies of Meteorites to Interpret in situ Space Observations of Asteroids", for a NASA research programme on Planetary geology and geophysics If the data derived from laboratory simulations of the polarimetric properties confirm the possible association of Steins with aubrites, then Rosetta flyby is that of a fragment of a larger and highly differentiated and reduced parent body. References [1] Levasseur-Regourd, A.C. and Hadamcik, E. (2003) J. Quant. Spectros. Radiat. Transfer, 79, 903-910. [2] Pentillä, A. et al. (2005) Astron. Astrophys., 432, 1081-1090. [3] Cellino, A., et al. (2006) Advances in Geosciences 7, 21-32, 2006. [4] Belskaya, I. et al. (2008) Icarus, submitted [5] Barucci, A. et al. (2005) Astron. Astrophys., 430, 313-317. [6] Fornasier, S. et al. (2006) Astron. Astrophys., 449, L9-L12. [7] Levasseur-Regourd, A.C., et al. (2007) Planet. Space Sci., 55, 1010-1020. [8] Renard, J.-B., et al. (2002) Appl. Opt., 41, 609-618. [9] Hadamcik, E., et al. (2008) Experimental optics of light scattering media, Praxis, in press
Hadamcik Edith
Levasseur-Regourd Anny-Chantal
McFadden Leslie
Renard Jean-Baptiste
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