The longest running argument in asteroid science concerns mineral composition and meteoritic association of the asteroids assigned to taxonomic type S. The approaching flyby of the S-type asteroid Gaspra by the Galileo spacecraft will drag an even larger section of the space science community into this argument. The basics of this problem are presented and discussed. The various proposed S asteroid compositions are presented and summarized in roughly the order in which they appeared

Bell, Jeffrey F.

English

NASA Technical Reports Server

N92-10882
Will Galileo Resolve the S-Asteroid Controversy?
Jeffrey F. Bell (Dept. of Geology and Geophysics, Univ. of Hawaii, Honolulu HI 96822)
The longest running argument in asteroid science concerns the mineral composition and meteoritical
association of the asteroids assigned to taxonomic type S. Over the past 20 years this controversy has occu-
pied an immense number of abstracts, funding proposals, telescope time requests, progress reports, workshop
preceedings, Arizona "Blue Book" chapters, NASA Sis, CIS, TMs, JPL internal documents, and occasion-
ally even refereed journal articles. The approaching flyby of the S-type asteroid Gaspra by the Galileo
spacecraft will drag an even larger section of the space science community into this turgid debate. This
review is intended to introduce these unfortunate newcomers to the basics of the problem. Below are sum-
marized the various proposed S asteroid surface compositions in roughly the order in which they appeared.
A) ORDINARY CHONDR1TES: It was known long before asteroid spectroscopy began that ordinary
cbondrites (OCs) make up more than 75% of observed meteorite falls. Some (but not all) meteoriticists of
the 1960s came to believe that these fall statistics must reflect the proportions of mete_te parent bodies in
the asteroid belt. Thus when the first asteroid colors and albedos were obtained in the early 1970s there
was a strong expectation that many asteroids would resemble OCs. Indeed, the spectral class "S" originally
was intended to stand for "silicaceous" as "M=metal" and "C=carbonaceous" (1). Thus the original 3
asteroid types neatly accounted for ordinary chondrites, irons, and carbonaceous chondrites. For a short
time everyone was happy.
B) STONY-IRONS: Later, when spectra of ordinary chondrites were measured in the lab (2) it
became uppment that they actually had little similarity to S asteroids other than having olivine and pyroxene
absorption bands. The asteroids have a steep red continuum totally unlike that of the OCs, and the details
of the silicate bands vary wildly, implying mineralogies usually far outside the OC range. To explain these
facts it was suggested that most S-typo surfaces are differentiated assemblages of metal, orthopyroxene, and
olivine, similar to stony-iron meteorites such as pallasites, lodranites, and siderophyres. This material would
be the product of melting in the deep interiors of the asteroid parent bodies, subsequently exposed by the
collisional disruption. Advocates of this hypothesis have proposed various alternate source bodies for OCs.
Probably the least objectionable of these is the Q-class asteroids, which conveniently are all tiny Earth-
crossing asteroids with low-quality spectra. But current collisional models of the asteroids require that this
population be constantly replenished from a reservoir in the main belt, where no Q-types have been found.
C) WEATHERED ORDINARY CHONDRITES: Upon discovery of the continnuum slope problem,
advocates of interpretation A) proposed that the red continuum of S asteroids is created by some "space
weathering" process which alters the spectrum of the uppermost regolith. Usually they propose that this is
associated with the metal component of chondrites, because it is obvious that pure-silicate asteroids of
classes V, R, A, and E do not have any reddening. This proposal has inspired investigations of both syn-
thetic metal-rich regoliths derived from OCs (3) and natural OC parent body regolith material preserved in
some meteorite breccias (4). All these studies demonstrate that "weathered" OC material does not redden,
but rather becomes spectrally flatter and in extreme cases approximates a C-type spectrum, never an S-type.
In fact this similarity leads some to propose that the OCs actually come from C-type asteroids (5).
D) CARBON-LESS CARBONACEOUS CHONDRITES: When the first near-IR spectra of S asteroids
revealed that most had higher ol/pyx than any OC, it was proposed (6) that they represented unknown types
of cbondrites, specifically material with the silicate composition of carbonaceous chondrites but no carbon.
But since no such meteorites have ever fallen on Earth, this hypothesis requires its advocates to abandon the
very fall-statistics argument that had originally inspired the chondritic interpretation of S-types in the first
place. Furthermore, the asteroid Flora which was cited in (6) as the most OC-like of the S-types was later
shown to have large variations in silicate mineralogy between different regions of its surface, far outside
the range of OCs (7). As a result, this theory is almost forgotten, except by die-hard supporters of interpre-
tation A) above who mistakenly cite (6) as supporting them.
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E) EVERYTHING: The mounting spectral evidence for wild variations in composition between
different S asteroids and even across the surface of individual ones leads some workers (7) to wonder if
both schools might be right. It is impossible to rule out some chondritic areas on the surfaces of S asteroids
with the current data, if one allows the other areas to be made of extreme differentiated mineralogies (e.g.
pure metai or pure olivine). Since we observe an entire "hemisphere" at once with Earth-based telescopes
the chondritic areas could not be separated from the differentiated areas. Current ideas about asteroid
differentiation lean toward such a complex pattern of heating. The fatal objection to this theory is that the
actual OC breccias do not contain differentiated clasts, which would be sure to exist in the regolith of a
"patchwork asteroid".
F) NOTHING: Alternatively one may take the wide variety of S spectra to indicate that there is really
no such thing as a unified "S-type asteroid", but a variety of different objects with different origins and his-
tories which we have not yet properly distinguished. For instance, the Eos asteroid family contains objects
formally classified S in most systems, but with IR spectra that closely match those of CO or CV chondrites.
A new class "K" was recently created to contain these objects (8). But this probably does not herald the
beginning of the end for Class S. There does seem to be a hard core of well observed objects with classical
S properties that will always remain even if some of the fainter objects which have only incomplete spectral
data later turn out to be something else.
THE CURRENT POSITION: At present almost all scientists actively involved in research on asteroid
composition appear to hold some version of interpretation B. In fact, no full journal article defending any
other view has appeared for at least 10 years (the closest approximation being (9)). Yet some of them
(especially C) continue to be defended vigorously in less formal situations. None can be rigourously
excluded on the basis of current data, and very little new data is being collected due to funding and person-
nel shortages.
GASPRA: ROSETI'A STONE OF THE ASTEROID BELT?. Will the Galileo flyby of Gaspra end the
Great Debate? Most attention has focused on spectral observations to be made by the NIMS IR spectrome-
ter and SSI camera. But the speclral studies to be performed by these instuments will be essentially the
same as those already being done from Earth. The whole debate outlined above has been between those
who accept the fundamental usefulness of mineral spectroscopy, and those who reject it in favor of meteor-
ite fall statistics. More spectral data is unlikely to convince the second group, who have been remarkably
ingenious in proposing reasons why the hundreds of existing S-type asteroid speclra don't really mean what
they obviously do. Alternatively, it is possible that some striking morphological evidence of a metal-rich
composition will apppear in the images of Gaspra. For instance, impact experiments in nickel-iron targets
produce craters with rigid, "frozen" ejecta flaps, usually with many sharp prongs of metal visible. If similar
structures were seen on Gaspra, they would slrongly suggest that the bedrock was dominated by metal.
Probably the least ambiguous datum would be an accurate density for Gaslra. Any density significantly
above 3.8 would rule out a chondritic composition. The stony-iron model implies a density between 4.0 and
7.8. This determination is independent of any quibbles about spectral interpretation and would sense the
entire body of the asteroid rather than a superficial layer. Obtaining accurate values for the mass and
volume of Gaspra should be the highest priority during the flyby.
REFERENCES: (1) Chapman C. R., Morrison D., and Zellner B., Icarus 25, 104-130. (2) Chapman C. R.
and Salisbury J. W., Icarus 19, 507-522; Gaffey M. J., J. Geophys. Res., gl, 905-920. (3) Galley M. J.,
Icarus 66, 468-486. (4) Bell J. F. and ]Ceil K., Proc. LPSC 1801, 573-580. (5) Britt D. et al., LPS XX,
p.lll-l12. (6) Feierberg M. et al., Astrophys J. 257, 361-372. (7) Gaffey M. J., Icarus 60, 83-114. (8)
Bell J. F., Meteoritics 23, 256-257; Tedesco et al., Astron. J. 97, 580-606. (9) Wetherill G. W. and Chap-
man C. R., in Meteorites and the Early Solar System, Univ. of Arizona Press, 1988, p.35-67.
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Cruikshank, D. P., D. J. Tholen, W. K. Hartmann, J. F. Bell, and R. H. Brown, Three basaltic asteroids and the
source of the basaltic achondrite meteorites. Icarus 89, 1-13 (1991).
Britt, D. T., D. J. Tholen, J. F. Bell, and C. Pieters. Classification of asteroid and meteorite spectra by principal
components analysis. In NASA TM-4210, p. 118-119, July 1990.
Bell, J. F., K. Keil, and D. T. Britt. On the relationship between shocked ordinary chondrites and asteroids. In
NASA TM-4210, p. 124-126, July 1990.
Britt, D. T., D. J. Tholen, J. F. Bell, and C. M. Pieters. Comparison of asteroid and meteorite spectra by princi-
pal component analysis. In Lunar and Planet. Sci. XXI, 131-132, March 1990.
Hawke, B. R., P. D. Spudis, P. G. Lucey, and J. F. Bell. The composition of the crust in the Orientale region of
the Moon: A pre-Galileo view. In Lunar and Planet. Sci. XXI, 473-474, March 1990.
Gaffey, M. J., J. F. Bell, R. H. Brown, and T. Burbine. Mineralogical variations within the S-asteroid popula-
tion. In Lunar and Planet. Sci. XXI, 399-400, March 1990.
Bell, J. F., "Igneous and metamorphic processing of asteroids: An observational perspective." EOS Trans. Am.
Geophys. Union, 71, p. 1432-1433.
Bell, J. F., and K. Keil, "Reflection spectra of shocked ordinary chondrites." Bull. Am. Ast. Soc. 22, 1124 (Oc-
tober 1990).
Granahan, J. C., and J. F. Bell, "On the reality of some recently proposed asteroid families." Bull. Am. Ast. Soc.
22, 1125 (October 1990).
Burbine, T. H., M. J. Gaffey, and J. F. Bell, "Principal component analysis of asteroid and meteorite spectra
from 0.3 to 2.51am." Bull. Am. Ast. Soc. 22, 1124 (October 1990).
Veeder, G. J., J. Goguen, D. Matson, E. Tedesco, J. F. Bell, and J. C. Gradie, "The A-class asteroid 1951 Lick."
Bull. Am. Ast. Soc. 22, 1114 (October 1990).
Gaffey, M. J., J. F. Bell, R. H. Brown, T. Burbine, J. Piatek, K. L. Reed, nEvolutionary implications of composi-
tional variations within the S-type and A-type asteroid populations." Bull. Am. Ast. Soc. 22, 1114 (October
1990).
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