Pre-Rosetta Compositional Studies of
Asteroid 21 Lutetia
Clark R. Chapman1, W.J. Merline1, B. Carry2,
H.A. Weaver3, A. Conrad4, and J.D. Drummond5
1Southwest
Research Inst., 2Paris Obs., 3JHU/APL, 4Keck Obs., 5AFRL
42nd AAS/DPS Meeting
Pasadena CA
“Lutetia and Other Main-Belt Asteroids” #46.03,
Ballroom E, Thurs. a.m. 7 Oct. 2010
Role of Remote Sensing in
Asteroid Exploration
 Spacecraft missions to asteroids (flybys, landers,
sample returns) can visit only a handful of bodies…
so we must continue to rely on Earth-based
observations of these countless small bodies.
 Lacking in situ calibration of our compositional
inferences based on telescopic studies from Earth,
it is challenging to understand asteroid
compositions from remote-sensing data (e.g.
reflectance spectra).
 Even after 40 years, only one asteroid, Vesta, has an
inferred meteoritic compositional analog that is
unanimously accepted (even that has issues).
 Thus Rosetta’s flyby of a never-before-visited type
of asteroid is a rare opportunity to test the validity
of our inferences…and motivate us to reflect on
what we really know vs. what we only surmise.
Rosetta’s Flyby of 21 Lutetia was the
First Visit to an Enigmatic M-Type.
 Lutetia is the archetype M-Type asteroid: the M
taxonomic type was defined based on
properties of Lutetia and two other asteroids.
 M-types have non-diagnostic or ambiguous
features (e.g. absorption bands are absent or
few and weak).
 Yet M-types are fairly rare and potentially
fascinating: although their colors suggest a
metallic composition, many show water bands.
 In the absence of albedo data, M-types cannot
be distinguished from many E’s or P’s, and thus
are part of the X colorimetric type.
 Rosetta’s measurements may resolve some of
these ambiguities.
Purpose of this Talk
 To establish a synthesis of the best Earth-based
evidence concerning the composition and
meteoritic analog for Lutetia, with estimates of
uncertainties.
 In the previous session, we learned some
preliminary results from Rosetta, which should
more strongly constrain the correct answer
because:
Much better estimate of mass
Stronger constraints on size, shape, hence density,
given the mass measurement
 Spatially resolved reflectance spectra (we could
disentangle the Earthbased hemispherical averaging of
spatially varying colors [from the UV to the near-IR], if
they do vary…)


 If the constraints were strong enough, some or
most of us have now been chastened…but I’ll
guess not. Here is our pre-Rosetta take.
Taxonomy vs Mineralogical
Composition vs Meteorite Analog
 They are not the same thing, though related.
 Taxonomic Class: Lutetia is an M-type, by definition.
 Mineralogy:
 The vis-to-3µm absorption bands most diagnostic of
mineralogy are weak or absent for Lutetia.
 Not proven reliably diagnostic of asteroid mineralogy: spectral
“shape”, UV reflectance, mid-IR emissions, polarimetry.
 Extremely high or low bulk density would be a strong
constraint; but lacking knowledge about Lutetia’s porosity, its
intermediate density [Drummond et al., 2010] is not diagnostic.
 Many minerals are ruled out, except in minor quantities, by
lack of bands and by bulk density.
 Meteorite Analog:
Enstatite chondrite is most likely (ord. chondrites have bands).
 Nickel-iron meteorite ruled out by density.
 Most carb. chondrites ruled out by density, albedo, spectra.
 Mixture of types (like TC3) or unrepresented type possible.

M-Types are DEFINED by Lutetia!
1975 to
1978
...
 There really can be
no dispute: Lutetia
is one of 3 asteroids
that define the M
class on the basis of
vis/near-IR colors
and albedo.
 And M-types do not
necessarily mean
nickel-iron core!
As early as 1973,
enstatite chondrites
were cited as
equally plausible
meteorite analogs.
The Archetypical M-Types
 High quality vis/near-IR
PDS taxonomy (Tholen, Barucci, Howell, Bus, etc.):
21 Lutetia - M 7G M0 7I M 2I M 65A - - Xk s X X Xc
spectra of the three asteroids
that define M-types.
SMASS spectra (Bobby Bus)
Why have people set up this
dichotomy for Lutetia
between C-type and metallic
M-type? We KNOW the type!
It is the mineralogy that
remains uncertain.
M-Type Complications and Caveats
 Taxonomic types are broad regions in parameter
space; they contain asteroids spanning
somewhat diverse compositions.
 Assignment of an asteroid to a type may be
ambiguous if it is near a boundary in parameter
space and/or it has large observational error bars
(but these are not true for Lutetia).
 Observations beyond wavelengths that defined a
type, have led to sub-types.
X-types are
M’s, E’s, or
P’s, when
albedo info is
lacking. In
the visible,
Lutetia is an
Xk (X but
trending
toward K), but
it lacks the IR
band common 
in Xk’s.
Rivkin et al. (1995) found that some M-types have 3µm
H2O bands (“W” sub-type), while others don’t. Lutetia
seemed to have a weak band, but recent groundbased
spectra (Birlan et al., this mtg.) show no 3µm band.
 Some M-types have radar reflectivities suggesting high
metal content, but many (including Lutetia) do not.
 Newer colorimetric taxonomies subdivide X-types.
Lutetia is an Xk.

Most remote-sensing traits of M-types aren’t
robustly diagnostic of mineralogy, so bulk
density and other evidence become important.
Lutetia’s Mineralogy and
Meteorite Analog
X
X
X
~X
 Metallic core: NO. Lutetia has only modest radar al



~
bedo, bulk density too low (even for large %-tage void).
Stony ordinary chondrite or common achondrite: NO.
Lutetia lacks silicate absorption bands.
Common carbonaceous chondrite: NO. Lutetia’s
visible albedo is too high, spectral shape wrong, bulk
density too high.
CV, CO carbonaceous chondrite: Probably NO. For
Lutetia, 1µm band is missing, near-UV/violet is too
high, albedo is somewhat high.
Enstatite chondrite: Probably YES.


?
3µm water band is probably absent, no longer an issue
Mid-IR and polarimetric issues are poorly understood
 Unsampled meteorite: MAYBE.
 Akin to known meteorite (e.g. Allende with more CAI’s)
 Mixture of known meteorites (like 2008 TC3)
 Wholly unknown type (but Lutetia should be sampled)
What Lessons do we Learn?
 Remote-sensing by Rosetta was at same
wavelengths already studied from Earth;
given Lutetia’s spatial homogeneity, the
flyby can provide only modest new
mineralogical information.
 Even though Lutetia’s mass was well
determined by the flyby, its bulk density
must remain approximate due to shape
uncertainties. Volume can’t be well
determined because of poor visibility of the
“back side” and of the side deep in shadow
due to obliquity.
 Lutetia remains enigmatic…. It may be an
enstatite chondrite, a mixture of meteorite
types, or possibly an unsampled meteorite.
 Asteroid remote sensing still has limitations.
 The End
 Next slide: for press conference
Lutetia: It is an “M-Type” for
Sure, but what is it Made of?
 Lutetia (and 2 other asteroids) defined
the M class in the 1970s…and it has
been classified as M-type ever since.
 M-type does NOT mean “metallic core”



Some M-types are metal-rich
For others, a spectral band means hydration
Still others could be stony (enstatite chondrite)
 What is Lutetia’s composition?
Rather featureless spectrum rules out most
common meteorites (OC’s, CC’s, achondrites)
 Bulk density (from pre-Rosetta measurements
of mass and shape) rules out irons and CC’s
 Lutetia is either an enstatite chondrite, a mix
of meteorite types, or an unknown type.

SMASS spectrum (S. Bus)