 |
| Titel |
Synthetic enstatite for the comparison with ground-based and VIRTIS-M/ROSETTA reflectance spectra of asteroid 2867 Šteins in the VIS and IR |
| VerfasserIn |
Kathrin Markus, Gabriele Arnold, Harald Hiesinger, Arno Rohrbach, Fabrizio Capaccioni, Gianrico Filacchione, Federico Tosi, Maria Cristina De Sanctis, Maria Teresa Capria, Eleonora Ammannito, Stephane Erard |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097730
|
| Publikation (Nr.) |
 EGU/EGU2014-13341.pdf |
|
|
|
|
|
| Zusammenfassung |
| Asteroid 2867 Šteins was observed by the Rosetta spacecraft on September 5, 2008. During
this close encounter, the VIRTIS spectrometer (Visible and Infrared Thermal Imaging
Spectrometer) aboard Rosetta acquired spectra of Šteins in the wavelength range from
0.2μm to 5μm [1]. We compare ground-based VIS and NIR reflectance spectra of Šteins
[2-7] with reflectance spectra obtained by OSIRIS (Optical, Spectrocopic and Infrared
Remote Imaging System) [8] and VIRTIS-M during the fly-by and with laboratory
reflectance spectra of enstatite chondrites (aubrites) and minerals (enstatite, oldhamite,
albite).
Ground-based and fly-by observations show an overall flat spectrum with the typical
E[II]-type absorptions bands at 0.49μm and ~0.9μm. E-type asteroids have been
associated to aubrites due to their high reflectance and overall featureless spectrum. The lack
of absorption bands at 1μm and 2μm indicates that Šteins’ surface has no Fe-bearing
pyroxenes or olivines.
It is well known that even small amount of Fe in enstatite lead to prominent absorption
features in the IR at 1μm and 2μm. Terrestrial samples of enstatite usually contain
several mol% of FeO and therefore exhibit the absorption features. Pure enstatite is
extremely rare. A sample of enstatite with 2.5mol% of FeO already shows distinct
absorptions features at 1μm and 2μm [9]. To have available sufficient amounts of
pure enstatite for our spectral studies, we developed a technique for synthesis of
enstatite.
Enstatite has 3 stable polymorphs with clinoenstatite, orthoenstatite, and protoenstatite
being stable at low (600°C), and high (>1000°C) temperatures
[10]. Orthoenstatite and protoenstatite are orthorhombic, while clinoenstatite is monoclinic.
Orthoenstatite is abundant in terrestrial rocks and in meteorites. Clinoenstatite is known from
meteorites [10, 11]. Shear deformation and quenching of orthoenstatite or protoenstatite
induces inversion to clinoenstatite [10]. Clinoenstatite in enstatite chondrites and aubrites
formed presumably by crystallization from a melt and subsequent quenching and mechanical
deformation (brecciation) [11].
We synthesized powders of orthoenstatite and clinoenstatite. Following the synthesis we
used XRPD to discriminate between the polymorphs. The grain sizes of the samples were
determined using SEM pictures of the samples and are comparable to the |
|
|
|
|
|
|