Pre-Rosetta Compositional Studies of Asteroid 21 Lutetia

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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
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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 Rosettas flyby of a never-before-visited
  type of asteroid is a rare opportunity to test
  the validity of our inferencesand motivate us to
  reflect on what we really know vs. what we only
  surmise.

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Rosettas 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 Es or Ps, and thus are
  part of the X colorimetric type.
• Rosettas measurements may resolve some of these
  ambiguities.

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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 chastenedbut Ill guess
  not. Here is our pre-Rosetta take.

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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
  Lutetias 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.

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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.

. . .
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The Archetypical M-Types
PDS taxonomy (Tholen, Barucci, Howell, Bus,
etc.)
21 Lutetia - M 7G M0 7I M 2I M 65A - -
Xk s X X Xc

• High quality vis/near-IR spectra of the three
  asteroids that define M-types.

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.
SMASS spectra (Bobby Bus)
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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.
• Rivkin et al. (1995) found that some M-types have
  3µm H2O bands (W sub-type), while others dont.
  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 arent
  robustly diagnostic of mineralogy, so bulk
  density and other evidence become important.

X-types are Ms, Es, or Ps, 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 Xks.
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Lutetias Mineralogy and Meteorite Analog
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. Lutetias
  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
  CAIs)
• Mixture of known meteorites (like 2008 TC3)
• Wholly unknown type (but Lutetia should be
  sampled)

X
X
X

?
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What Lessons do we Learn?

• Remote-sensing by Rosetta was at same wavelengths
  already studied from Earth given Lutetias
  spatial homogeneity, the flyby can provide only
  modest new mineralogical information.
• Even though Lutetias mass was well determined by
  the flyby, its bulk density must remain
  approximate due to shape uncertainties. Volume
  cant 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.

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• The End
• Next slide for press conference

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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 1970sand 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 Lutetias composition?
• Rather featureless spectrum rules out most common
  meteorites (OCs, CCs, achondrites)
• Bulk density (from pre-Rosetta measurements of
  mass and shape) rules out irons and CCs
• Lutetia is either an enstatite chondrite, a mix
  of meteorite types, or an unknown type.

SMASS spectrum (S. Bus)