Title: Clark R. Chapman
1 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
2The 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?
350 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
4Asteroid 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?
5Early 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
6Remote 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.
7Vesta 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)
8In 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)
9Does 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.
10Structure 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)
11Stony-Iron Meteorites S-Type Analogs? (The
paradigm, 1975-1990)
12Matching 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...
13Some 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.
14Azzurra 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!
15Idas 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 !
16Eros 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
17My 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.
18Spacecraft 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)