L ’ énigmatique astéroïde (21) Lutetia

1. L’ énigmatique astéroïde (21) Lutetia • M.A. Barucci, S. Fornasier, C. Leyrat, S. Erard, D. Perna, P. Hasselmann, M. Fulchignoni, D. Bockelée-Morvan & I. Belskaya

2. 120 121±1 x 100±1 x 75±13km3

4. (Barucci et al. 2014, Asteroids IV book) grooves Matteo Massironi, UPD

5. Surface age: 100 Ma-3.6Ga

6. Albedovariation up to 30%Spectral reddening • ( Sierkset al., 2011, Science 334, 487 B=268 nm G=536 nm R=743 nm Reddening Map North Pole • Inhomogeneous surface • Equatorial hemisphere redder than Northern one • Young regions inhomogeneous

7. VIRTIS-M & H (Coradini et al., 2011, Science, 334, 492)

8. Temperature & Thermal Inertia Tmax= 245 K Thermal Inertia : I ~20-30 SI units • Thick regolith Morphological variation • Small Thermal Inertiaconfimedby: • - MIRO (Gulkis e al. 2012, PSS Special issue) • - Herschel (PACS & SPIRE) (O'Rourke, et al. 2012, PSS) (Coradini et al. 2011)

9. OSIRIS data

10. Inhomogeneity on the surface of 21 Lutetia Aqueousalteredmaterials ? ferriciron spin-forbidden absorptions, phyllosilicates (jarosite…)? (Perna, D. et al. 2010) Lazzarin et al., 2010, MNRS 408, 1433

11. Presence of 3 micron band (Rivkin et al. 2011, Icarus, 216,62 ) (Birlanet al., 2006, A&A, 454, 677) Birlan et al. 2006 and Rivkin et al. (2000) observed the 3 micron band diagnostic of water of hydratation; new data of Birlan et al. 2010 do not confirm this detection (different observed area), new data by Rivkin et al. 2011 confirm the band.

12. 21 LUTETIA: Emissivity - SPITZER CV meteorite CO3 carb. chondrite Iron meteorite … 20-50 micron ___0-20 micron • The Lutetia emissivity spectrum is completely different from that of the iron meteorites • Low thermal inertia: I ≤ 30 JK−1 m−2 s−1/2 , typical of main belt asteroids; Lutetia is likely covered by a thick regolith layer • Lutetia is similar to CV3 and CO3 carbonaceous chondrites, meteorites which experienced some aqueous alteration ___0-20 micron --- 50-100 micron CV3 carb. chondrite ___100-150 micron --- >150 micron. Enstatite chondrites C peak at 8.3 µm (Izawa et al. 2010) (Barucci et al., 2008)

13. COMPARISON WITH METEORITES (Belskaya et al., 2011)

14. Lutetia ground observations on the cilindrical projection 0.4-0.9 µm 0.8-2.5 µm 2-3.5 µm Barucci et al. (2012) 5-38 µm

15. V albedo = 0.19±0.01

16. (Barucci et al. 2012) Lutetia o = hemispherical* = bidirectionalmeasurements

17. Lutetiadensity(Patzold et al. 2011) 3.40± 0.3 g/cm3(Weiss et al. 2011) (Weiss et al. 2012) • apparent high density (exceeds that of most known chondrite meteorites) • surface similar to chondrite + metallic core

18. Kaidun meteorite 8-µm particle from comet 81P/Wild 2. sulphide pyrrhotite, enstatite grain and fine-grained porous aggregate material with chondriticcomposition (Brownlee e al. 2006) This Kaidun meteorite (Yemen in 1980) is a mixture of “incompatible “ materials: principal carbonaceous chondrites (CV, CI, CM, CR) and estatite chondrites (EH and EL) and other peculiar materials. Therefore, in a single particle, materials which formed in different regions in a protoplanetary disk can co-exist, which was not expected.

19. Summary (21 Lutetia) • Lutetia exposes fascinating morphology with many craters, grooves, ridges, graben, scarps etc. • Old and young regions (10Ma-3.6Ga) • Surface with small grain size • Blanket in North Pole region suggests a thick layer of regolith/ejecta • Color variations • Different from any other asteroid visited so far • Different from any meteorite? (surface more similar to chondrite) • High density (exceeds that of most known chondrite meteorites) • The surface seems a mixture of different types of materials: • carbonaceous chondrite (for the majority) and enstatitechondrite (in minor percentage) • Lutetia is an old object (with a surface age of 3.5 Gy) with a primitive chondrite crust and a possible partial differentiation (Weiss et al. 2012) with a metallic core

20. (a) Average densities of meteorites for C type asteroids: 2.9 – 3.5 g/cm3 (b) Average densities of meteorites for S type asteroids: 3.19 – 3.40 g/cm3 (c) Average densities of aubrites 2.97 – 3.27 g/cm3

21. Spectral slope(989 nm/ 649 nm) Accumulation area are redderthanlandslides Landlsideswith ~flat spectrum Leyrat et al., 2011

22. Opposition Images 26.000 km -0:30h 20.000 km -0:22h 17.000 km -0:19h 16.000 km -0:18h α = 4.1° α = 2.0° α = 0.6°α = 0.15°

23. CV3 (red) CI (green) E6 (Blue) (Nudelcu et al. 2007) (Birlan et al. 2006)

24. Complementary informations Herschel observed Lutetia ! O'Rourke, Barucci, Dotto et al.... SPIRE 250, 350 & 500 µm 11 jul. 2010 PACS 70, 100 & 160 µm 21 dec. 2009

25. ALICE (Stern et al. 2011, AJ, 141, 199) A tentative model fitting the Lutetia’s albedo : 77% EH5 chondrite and 7.7% each of anorthite, H2O frost and SO2 frost.

26. DEPENDENCE ON GRAIN SIZE: CARBONACEOUS CHONDRITES Absolute reflectivity vs grains size • a strong dependence on grain sizes: albedo tends to increase when particle size decreases; • grain size of components of different hardness is not well-controlled when meteorite is crushed; • albedo of particular types of carbonaceous chondrites is well-consistent with Lutetia’s albedo (0.13 at α=5°)

27. Albedo variation up to 30% Baetica Achaia (Sierks et al., 2011, Science 334, 487)

28. Surface age: 100 Ma-3.6Ga by S. Marchi (OCA)

29. Herschel-Mach11 (O'Rourke et al.)

30. Vernazza et al. (2011) similarity to estatite chondrite two major problems on theiranalysis: - they look onlyat one spectrum (V+NIR taken by the ground) - to interpret the spectrum of Spitzer (FIR), theyused an ad hoc comparison (KBr-diluted) withdifferent grain of EC • Vernazza et al. (2011) Icarus 216, 650