1. E-type asteroids and related meteorites
S. Fornasier, A. Barucci
6 October 2008
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2. E-type asteroids Subtype I Subtype II Subtype III
bodies with high albedo ( ), 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 m
6 October 2008
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3. 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 0.5m 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 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
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4. 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) ±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
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5. EII-type asteroids: meteorite analogues
Fornasier et al., 2008
Subtype II (64 Angelina like): strong absorption at 0.5m 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
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6. 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
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7. Meteorites analogues to E-type
Aubrites :enstatite achondrite
E4 enstatite chondrite
6 October 2008
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8. Minerals analogues to E-type
Yamada et al 1999
6 October 2008
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9. 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
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10. 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
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11. 6 October 2008
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12. 6 October 2008
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13. 6 October 2008
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14. NAC spectrophot. UT=16:39
6 October 2008
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15. 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
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16. Steins
Belskaya & Shevchenko 1999
6 October 2008
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17. Belskaya & Shevchenko 1999
The small linear slope value and high G value are typical of E type asteroids
6 October 2008
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18. Reference
Barucci M.A., Fulchignoni M.,Fornasier S., et al., Asteroids target selection for the new Rosetta mission baseline: 21 Lutetia and 2867 Steins. A&A 430,
Fornasier S., Belskaya I., et al., Polarimetric survey of asteroids with the Asiago telescope. A&A 455, 
Fornasier, S., Marzari, F, Dotto, E., Barucci M.A., Migliorini, A., 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, Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission. Icarus, 196,
Barucci A., Fornasier S., Dotto E., Lamy P. L., Jorda L., Groussin O., et al Asteroids 2867 Steins and 21 Lutetia: surface composition from far infrared observations with the Spitzer space telescope. A&A, 477,
6 October 2008
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