E-type asteroids and related meteorites S. Fornasier, A. Barucci 6 October 2008 OSIRIS MEETING
E-type asteroids Subtype I Subtype II Subtype III bodies with high albedo (0.4-0.6), are thought to be mostly differentiated and to have experienced high heating episodes (T>1500K). surface composition seems to be dominated by iron-free or iron-poor silicates as enstatite, forsterite or feldspar supposed to be parent bodies of enstatite meteorites small population (albedo needed!) ~ 25 bodies, located in the inner main belt and in the Hungaria region, but 3 different mineralogies have been identified (Gaffey & Kelley, 2004; Clark et al., 2004): Subtype I Featureless spectra Subtype II band at 0.49 m Subtype III band at 0.88--0.9 m 6 October 2008 OSIRIS MEETING
E-type asteroids Subtype I : featureless slightly reddish spectra, characteristic of aubritic pyroxene plus feldspar assemblage. 2) Subtype II (64 Angelina like): strong absorption at 0.5m and occasionally at 0.96 m, possibly due to calcium sulfide mineral oldhamite.Steins belongs to this subtype 3) Subtype III (44 Nysa like): flat or reddish spectral curve with a well define band at 0.88--0.9 m and sometimes at 1.8 μm and characteristic of enstatite pyroxene containing Fe2+ (orthopyroxene or forsterite). They derived from oxidized parent bodies Fornasier et al., 2008 6 October 2008 OSIRIS MEETING
Spectral similarity between 2867 Steins and 3103Eger The Rosetta target Steins is an E[II] well shaped 0.5 micron band, depth of 12% for Steins, ~8.5% for Eger. Steins-Eger : similar spectral slope value, the highest one of the E population: Eger 7.40±0.53 %/103 Å Steins: 7.62±0.64%/103 Å (Aug 05) 6.93±0.65%/103 Å (May 04) Fornasier et al., 2007 The spectra suggest a strong genetic relationship and numerical simulations show that they may be members of an eroded family, older than 2 Gyr formed close to the present location of Steins 6 October 2008 OSIRIS MEETING
EII-type asteroids: meteorite analogues Fornasier et al., 2008 Subtype II (64 Angelina like): strong absorption at 0.5m and occasionally at 0.96 m, possibly due to calcium sulfide mineral oldhamite (CaS) (present only in highly reduced assemblages such as aubrites). They are probably composed of basalt equivalents from enstatite chondrite-like parent bodies which underwent al least partial melting 6 October 2008 OSIRIS MEETING
Aubrites Enstatite achondrite are composed of large white crystals of the Fe-poor, Mg reach orthopyroxene, or enstatite (Mg2Si2O6 ). Other minerals present in aubrites are: feldspates (1-16%), diopside (0.2-8%), olivine (0.3-10%), Fe-Ni reach minerals (0.3-7%), the sulfides troilite and oldhamite (0.1-7%) ALH84007 6 October 2008 OSIRIS MEETING
Meteorites analogues to E-type Aubrites :enstatite achondrite E4 enstatite chondrite 6 October 2008 OSIRIS MEETING
Minerals analogues to E-type Yamada et al 1999 6 October 2008 OSIRIS MEETING
Meteorites analogues to E-type Steins spectrum tentative modelled with meteorite ALH78113 (65%) enriched with 35% of oldhamite (albedo of the model =0.38). A lot of oldhamite is needed to account for the deep 0.5 micron band a red slope Fornasier et al., 2008 6 October 2008 OSIRIS MEETING
Meteorites analogues to E-type Subtype II: aubrite ALH78113 (RELAB database) has a feature similar to the 0.49 micron one of the EII group Subtype III: aubrite PenaBlanca spectrally very similar to 317 Roxane (albedo = 0.49), and with similar albedo (0.52) 6 October 2008 OSIRIS MEETING
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6 October 2008 OSIRIS MEETING
6 October 2008 OSIRIS MEETING
NAC spectrophot. UT=16:39 6 October 2008 OSIRIS MEETING
Steins phase function (preliminary) Black: post CA, Red: pre CA Green groundbased observation Preliminary results: Linear slope β=0.0227, G=0.49 6 October 2008 OSIRIS MEETING
Steins Belskaya & Shevchenko 1999 6 October 2008 OSIRIS MEETING
Belskaya & Shevchenko 1999 The small linear slope value and high G value are typical of E type asteroids 6 October 2008 OSIRIS MEETING
Reference Barucci M.A., Fulchignoni M.,Fornasier S., et al., 2005. Asteroids target selection for the new Rosetta mission baseline: 21 Lutetia and 2867 Steins. A&A 430, 313-317. Fornasier S., Belskaya I., et al., 2006. Polarimetric survey of asteroids with the Asiago telescope. A&A 455, 371-377 Fornasier, S., Marzari, F, Dotto, E., Barucci M.A., Migliorini, A., 2007. Are the E-type asteroids (2867) Steins, a target of the Rosetta mission, and NEO (3103) Eger remnants of an old asteroid family? A&A , 474, L29-L32 Fornasier S., Migliorini A., Dotto E., Barucci A, 2008. Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission. Icarus, 196, 119-134. Barucci A., Fornasier S., Dotto E., Lamy P. L., Jorda L., Groussin O., et al. 2008. Asteroids 2867 Steins and 21 Lutetia: surface composition from far infrared observations with the Spitzer space telescope. A&A, 477, 665-670 6 October 2008 OSIRIS MEETING