Clark R. Chapman

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Evolving Perspectives on Space Weathering of
Asteroids

• Clark R. Chapman
• Southwest Research Inst.
• Boulder, Colorado

Solar System Remote Sensing Symposium Honoring
Bruce Hapke Univ. of Pittsburgh, 21 Sept. 2002
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The Prime Question
(One of Bruce Hapkes early accomplishments)

• Given early (post-Apollo) demonstration that the
  lunar surface is space weathered
• Why has it taken so long for it to become
  accepted that asteroid surfaces are space
  weathered?
• Indeed, is it even yet accepted?

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50 Years of Early Physical Studies of Asteroids
1929-1978

• UBV colors (reviewed by Hapke, 1971),
  lightcurves, and Bobrovnikoffs spectra...
• Then, spectral and radiometric studies
  proliferated

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Asteroid Remote-Sensing Trying to Deduce
Mineralogy
This has been the game, from the 1950s until
now...
Its as if a geologist had to pick up stones in a
streamand try to associate them with colorful
layers in the distant mountains
An astronomer measures asteroid spectra a
cosmo-chemist studies meteorites that have
fallenthey try to determine the associations
How successful has this endeavor been?
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Early progress 1960s, 1970s

• First good UBV photometry, but spectral
  resolution inadequate to reveal minerals
• Comparison of asteroid spectrophotometry/
  radiometry with lab spectra of meteorites
• Matching of spectral traits reveals
  (dis)similarities
• Quantitative, physics-based mineralogical assay
• Apollo lunar rocks found to differ dramatically
  from telescopic spectral reflectance data,
  implicating a space weathering process
• Early debates about what exact lunar processes
  acted
• Early discussion of why asteroids might or might
  not be affected by lunar-like space weathering

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Remote sensing sees surfaces ...

• A basic philosophical point is that we see only
  optical surfaces
• And surfaces are most susceptible to being
  affected by exogenous processes (cf. Hapkes
  1960s pre-Apollo simulations of the solar wind)

Strange South American fruit can you guess what
its insides taste like by looking at its outside?
Given our knowledge (by 1970) of bombardment of
airless bodies by micrometeorites and the solar
wind, of lunar regolith processes, and of
gas-rich meteorites we should always have been
skeptical, in asteroid remote sensing, that what
we see represents the bulk make-up of the body.
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Vesta Proposed to be HED P.B. in 1970Still the
Accepted Model

• The first and greatest success has enduredbut is
  it true?
• Some Vestoids are in strange orbits were there
  other Vesta-like bodies?
• We really dont know anything about Vestas
  interior (except gross bulk density)

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In retrospect, Vesta gave us a false sense of
confidence...

• Vesta is the brightest asteroid, with high
  albedo, and the deepest absorption bands every
  other asteroid is more difficult to observe and
  interpret
• Like the lunar highlands (compared with the
  maria) achondritic Vesta is depleted in mafic
  materials that are most subject to optical
  modification by space weathering

In the subsequent three decades, no other
asteroid has been as reliably associated with a
meteorite type as was Vesta (despite some claims
to the contrary)
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Does Asteroid Mineralogy Vary with Solar
Distance? (Do Meteorites come from the Belt?)

• Understanding the compositional structure of the
  asteroid belt has been only so good as the
  mineralogical interpretation of the spectral
  data.
• In a 23 March 1971 letter to Bruce Hapke, Ed
  Anders wrote
• I have plotted the four Hapke-Gehrels color
  groups as a function of a, and was delighted to
  find a correlationfrom your data on meteorites
  it appears that Group I is richer in Fe2 than
  Group II, which in turn suggests that the
  asteroid belt gets more reduced with increasing
  a.
• Anders believed that the NEAs and meteorites MUST
  come from the main asteroid belt, even if
  dynamicists hadnt figured out the exact
  mechanism.
• George Wetherill believed that the hard physics
  needed to be done (and he tutored his students to
  work the problem) meanwhile, he considered that
  meteorites probably came from comets, because
  comets -- at least -- cross the Earths orbit.

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Structure of the Asteroid BeltVariation of
Taxonomic Types...
with Distance from the Sun
with Diameter
Despite voluminous data acquisition, no
bias-corrected statistical studies have been
published since the 1980s despite the fact that
compositional types show dramatic differences in
their distributions.
Gradie, Chapman Tedesco (1988)
Of course, taxonomic types reflect different
mineralogical assemblages, so their variations
with other properties are fundamental...
Gradie, Chapman Tedesco (1988)
Gradie Tedesco (1982)
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Stony-Iron Meteorites S-Type Analogs? (The
paradigm, 1975-1990)
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Matching worked OKbut it didnt satisfy
cosmochemists

• Nearly every asteroid matched some meteorite
  type
• But the most common meteorites in museum
  collections (ordinary chondrites) were
  represented by almost no asteroids
• That didnt bother astronomers, who claimed that
  the OC parent bodies were small/unobserved and
  that highly selective collisional/dynamical
  processes caused OC meteorites to be
  over-represented
• It did bother cosmochemists, who had reasons for
  believing that OC material was more fundamental
• Physics (of collisions, dynamics) could explain
  some varia-tions in representation, but not such
  great a disparity

Passing like ships in the night, researchers in
the three sub-disciplines of planetary science
generally failed to communicate with each other...
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Some mid-1970s Perspectives
Thus we are left with two possibilities. Either
the S asteroids are stony ironsor they are OCs,
in which case we must explain why the spectral
reflectivity data tell us otherwiseI, at least,
find it easier to believe that the spectral
reflectivity data mislead us than to accept the
alternative that the most abundant meteorite
class (OC) has no asteroidal equivalent, and the
second most abundant asteroid class (stony-iron)
has no xenolithic and only rare meteoritic
equivalents. -- Ed Anders (1978)
Matson et al (1977), among others, doubted that
lunar-like space weathering could affect
asteroids. The prevailing lunar paradigm invoked
agglutinization, which was known to be a minor
process in the asteroid belt.
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Azzurra Ejecta from a Fresh Large Crater on Ida

• Geissler et al. (1996) modeled ejecta
  distribution
• The crater and ejecta have bluer, fresher
  un-space-weathered colors

What this means Geologically recent features
and deposits are associated with different colors
from the general, older terrains. SO SOME PROCESS
IS CAUSING COLORS TO REDDEN WITH TIME!
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Idas Evidence on the S-type/OC Conundrum
Space Weathering

• Fresher terrains on Ida (and Dactyl) look more
  like ordinary chondrites
• Ida and S-types in the Koronis family could be
  parent bodies for OCs
• Binzels find spectral gradation be-tween S- and
  Q-types among NEAs
• Impetus for laboratory simulations (cf. Moroz et
  al, Sasaki et al)

Long-standing debate Should one take spectra of
the common S-types literally, or do the colors of
asteroidal surfaces exposed to solar wind and
micrometeorites change with time? Opinions
started to change after Ida... But we needed an
orbiter for proof !
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Eros shows very little spatial color variability,
unlike Ida

• Even small rocks are usually the same color as
  the rest of Eros
• Possibilities
• Coated with electro-statically levitated dust?
• Maturely space-weathered while in near-Earth
  orbit?
• Maybe Beth Clark has the answer (next talk)
• In many ways, Eros resembles Idabut not in color
  heterogeneity
• Probable composition OC (favoring L/LL but not
  yet secure)

IDA Enhanced color
EROS Black white image
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My Answer to the Prime Question (Why so long to
accept asteroidal space weathering?)
There are two different philosophies of
scientific proof

• Detailed physics/chemistry of hypothetical
  processes must be understood before the process
  is accepted.
• Multiple, plausible hypotheses should be
  evaluated (with due weight, but not blind
  acceptence, being given to interdisciplinary
  insights) while details are research-ed... so
  long as the laws of physics arent violated.

C
Space weathering is alive and well But advances
are still being made (reported at this meeting)
in understanding lunar space weathering. And
Eros teaches us that we have much to learn about
space weathering processes on asteroids.
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Spacecraft studies of asteroid colors and spectra

• Spectral variations on Ida (from Galileo SSI
  images) suggest space weathering
• Early NEAR colorimetry of Eros shows bland
  colors NIS spectra suggest ordinary chondrite
  composition (also implied by X-ray spectra,
  though calibrations uncertain)