Pre-Rosetta Compositional Studies of Asteroid 21 Lutetia

1. 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

2. 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.

3. 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.

4. 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.

5. 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.

6. 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. . . .

7. 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)

8. 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 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. 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.

9. Lutetia’s 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. 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) X X ~X ~ ?

10. 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.

11. The End • Next slide: for press conference

12. 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)