[meteorite-list] Re: The Composition of Asteroid 433 Eros

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:02:24 2004
Message-ID: <3C7FBBC9.88398873_at_lehrer1.rz.uni-karlsruhe.de>

Herbert wrote:

> R chondrites (another type of carbonaceous chondrite) fall within or
> close to the Eros ranges and remain viable candidates.Are they talking
> of CR (Renazzo-type carbonaceous chondrites) or R (Rumuruti)
> chondrites (which are more closely to ordinary chondrites rather than
> to carbonaceous chondrites, I thought)? Best greetings, Herbert Raab


Hello Herbert and List,

Unfortunately you don't quote any references but here are two extracts
from what the experts have to say with regard to the composition of
Eros:

EVANS L.G. et al. (2001) Elemental composition from gamma-ray
spectroscopy of the NEAR-Shoemaker landing site on 433 Eros
(MAPS 36-12, 2001, pp. 1639-1660):

In comparing the chemical composition of Eros to that of asteroids, the
first question we might ask is whether Eros has the same composition as
an ordinary chondrite, given the abundance of both S-type asteroids and
ordinary chondrites. Almost all meteorites have Si/O ratios in the range
of 0.35-0.6 so while the GRS (= Gamma-Ray Spectrometer) results are
within this range this ratio cannot give us much information concerning
meteorite comparisons. Figure 12 plots the Mg/Si vs. K for a wide array
of meteorite types, as well as the GRS-derived value for 433 Eros (with
the 40% uncertainty ellipse). We find that the composition of Eros
overlaps that of H, L and LL ordinary chondrites, although the error
ellipse would also allow other types of chondrites (carbonaceous,
enstatite, R) and primitive achondrites (acapulcoites, brachinites,
winonaites). Most meteorites from fully-differentiated bodies (e.g.,
howardites, eucrites, diogenites, aubrites) are excluded.

McCOY T.J. et al. (2001) The composition of 433 Eros: A mineralo-
gical-chemical synthesis (MAPS 36-12, 2001, pp. 1661-1672):

Several groups of meteorites are consistent with the x-ray data, but
inconsistent with the MSI/NIS (Multi-Spectral Imager / Near-Infrared
Spectrometer) constraints. R chondrites have Mg/Si, Fe/Si and Al/Si
ratios within the range of the x-ray data. However, band center and
area parameters derived from laboratory-based spectra of the R-chondrite
Rumuruti (Burbine and McCoy, unpubl. data) are inconsistent with those
for Eros. Rumuruti has a band area ratio consistent with a nearly-pure
olivine assemblage. Among the groups of primitive achondrites -
meteorites that have experienced limited degrees of partial melting
and melt migration - the acapulcoites, lodranites, brachinites and
winonaites all have bulk chemical compositions which are broadly
consistent with the x-ray and gamma-ray derived composition for 433
Eros (Nittler et al., 2001). However, the mineral assemblages inferred
from the spectral band parameters are inconsistent with a link between
433 Eros and these meteorites (Burbine et al., 2001b). Acapulcoites,
lodranites and winonaites all appear to have more pyroxene-rich
compositions than derived for Eros, although individual members of these
groups, particularly the lodranites, may match Eros. Further, the
spectral heterogeneity one might expect from the parent bodies of these
groups (McCoy et al., 2000) is not observed on Eros. In contrast to
these other groups, brachinites are dominated by olivine and pyroxene
features are essentially absent in their spectra. This is in sharp
contrast to Eros, which has pyroxene spectral absorption bands and,
thus, Eros is not parental to the brachinites.


Best wishes,

Bernd
Received on Fri 01 Mar 2002 12:35:05 PM PST