 |
| Titel |
Compaction and possible differentiation of asteroid 21 Lutetia |
| VerfasserIn |
Wladimir Neumann, Doris Breuer, Tilman Spohn |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250081319
|
|
|
|
|
|
| Zusammenfassung |
| The Rosetta flyby at the asteroid 21 Lutetia revealed an unexpected high bulk density of
3400±300 kg m-3 from the mass measurements and a global shape model (the dimensions
are 121±1 by 101±1 by 75±13 km). This high density indicates a bulk composition
which is enriched in heavy elements like iron. Furthermore, spectroscopic data
suggests that Lutetia formed from enstatitic material[1] and the complex geology and
an ancient surface infer that Lutetia may be a primordial planetesimal[2]. It has
been even suggested that Lutetia is partially differentiated[3]. In the present study,
we investigate the thermal evolution of Lutetia by the means of a comprehensive
numerical model[4], which includes accretion[5], compaction due to sintering[6],
associated changes of material properties, melting, advective heat transport and
differentiation.
For the observed bulk density of 3400±300 kg m-3, we vary the present-day intrinsic
density (and hence the macroporosity Ïm) and the onset time and duration of accretion. The
initial material properties (intrinsic density, mass fractions of the components, abundances of
the radiogenic heat sources 26Al and 60Fe) are calculated assuming an enstatite chondritic
nature of the primordial material. Evolution scenarios arising from assumptions on Ïm are
examined to derive implications on the compaction of an initially highly porous material,
(partial) differentiation and the internal structure. The final bulk density is compared with the
observations in order to derive bounds on the present-day macroporosity and internal
structure.
We obtain a number of compaction and differentiation scenarios consistent with the
observations. Our results imply that the most probable macroporosities of the present-day
Lutetia are Ïm -¥0.04. Small changes arise if the possible error of ±300 kg m-3 in the bulk
density is considered. Depending on the adopted value of Ïm, the formation times
range from the formation contemporaneously with the CAIs for Ïm=0.04 to 8 Ma
after the formation of the CAIs for Ïm=0.25. If Lutetia is differentiated and has
an iron rich core, the present-day macroporosity ranges between 0.04 and 0.06
and the formation time is between 0 Ma and 1.8 Ma after the CAIs. In that case,
the size of the core is at most 25 km and the thickness of the mantle amounts to
approximately the same value. On top of the mantle is a partially differentiated layer whose
composition deviates only slightly from the primordial composition. The outer layer is
undifferentiated but compacted except the upper few radius percent. Regardless on the
degree of differentiation, the extent of the partially molten zone and the degree of
melting does not suffice for the melt to extrude through the porous layer. This is
consistent with the lack of basalt at the surface of Lutetia. For Ïm -¥0.6, differentiation
is not possible but the interior is substantially compacted below a porous outer
layer.
[1] Vernazza, P. et al., Icarus, 216, 650-659, 2011.
[2] Sierks, H. et al., Science, 334, 6055, 487-490, 2011.
[3] Weiss, B. P. et al., PSS, 66, 1, 137-146, 2011.
[4] Neumann, W. et al., A&A, 543, A141, 2012.
[5] Merk, R. et al., Icarus, 159, 183-191, 2002.
[6] Yomogida, K. and Matsui, T., EPSL, 68, 34-42, 1984. |
|
|
|
|
|
|