M.A. Barucci, I. Belskaya, S. Fornasier, C. Leyrat Overview on Lutetia surface composition M.A. Barucci, I. Belskaya, S. Fornasier, C. Leyrat Pasadena

Albedo variation up to 30% (Sierks et al. 2011)

complex surface morphology flat spectra albedo variations; deep regolith young and old surfaces (100My – 3.6 Gy)

IR Spectra during scan phase Cube I1_00237394252 - IR 15 consecutive scans; 27min duration Started 65min before CA S/C distance 48000 – 25000 km Pixel size between 12km and 6.5km Hyperspectral Images cover a longitude max range of ~20° SOSTITUIRE

Reflectance uniform within < 5% All the variation is limited to the thermal contribution above 3500nm

Lutetia IR Reddening The plot is built using all the available data Wavelength ratio 2076nm/1132nm Large spreading of the data points is related to the irregularity of the target and to emergence angle effects

VIRTIS False color (Blue=2μm, Green=4μm, Red=5μm) image taken in ‘pushbroom mode’ at the closest approach phase. Resolution (2km at the top to 0.75km at the centre: North pole) VIRTIS-M is a slit spectrometer and the image is a collection of successive slit acquisitions (time increases from top to bottom) taken at a fixed repetition rate, while S/C speed relative to the asteroid varied from beginning to the end.

Virtis-M, closest approach Time Point of view / sampling distance change continuously over the picture! Oversampling area On n’obtient pas exactement des images puisqu’on obtient une ligne à la fois: ça devient bizarre si on se déplace trop vite ou si l’écartement des lignes est incorrect

Temperature map from VIRTIS Thermal Inertia : I ~20-30 SI units Thick regolith (Coradini et al. 2011)

Temperature Vs Morphological Features

Spectroscopy of Lutetia: VIRTIS-M Extremely homogeneous, less than 5% variability No obvious spectral signature Seule l’émission thermique varie de façon notable No 1 µm band (pyroxenes)

Spectroscopy of Lutetia: VIRTIS-H Seule l’émission thermique varie de façon notable Pas de minéraux hydratés, contrairement à certains rapports d’observations télescopiques No 3 µm band (hydrated minerals) No 2 µm band (pyroxenes) No 3.6 µm band (C-H in organics)

Conclusions from VIRTIS No spectral signature identified • No Fe-rich pyroxene / olivine • No hydrated minerals • No organics • No unexpected absorption => Mostly matches some primitive meteorites (chondrites) Thermal studies • Temperature map + reflectance spectrum & variability Max T ~ 245K • Thermal map implies low thermal inertia (I ~20-30 SI units) => thick regolith at surface

MIRO : Microwave Instrument for Rosetta Orbiter P.I. S. Gulkis (JPL) Radio-telescope of 30 cm: 190 GHz (1,6 mm) : continuum 563 GHz (0,5 mm) : continuum + spectro Small thermal inertia (I ~10-30 SI ) (comparable Moon regolith: ~25 SI)

Herschel observed Lutetia ! 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

V albedo = 0.19±0.01

Inhomogeneities on the surface of 21 Lutetia (Perna, D. et al. 2010, A&A 513, 4) Aqueous altered materials ? ferric iron spin-forbidden absorptions, phyllosilicates (jarosite…)? Perna et al. 2010 Lazzarin et al. 2004

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

(Rivkin et al. 2011, Icarus) Birlan et 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. Birlan et al., 2006

21 LUTETIA: Emissivity - SPITZER CV meteorite Iron meteorite CO3 carb. chondrite ___0-20 micron. Iron meteorite … 20-50 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)

Lutetia ground observations 0.4-0.9 µm 0.8-2.5 µm 2-2.35 µm 5-38 µm

Polarimetric properties of Lutetia’s surface Lutetia’s has particular polarimetric properties as compared to all asteroids observed so far. Large inversion angle is indicative of small particle size and/or high refractory material or inclusions Only few asteroids (mainly L-type) have wider negative branch of polarization. (Belskaya et al., 2010, A&A 515, 29)

Possible reasons of large inversion angle of Lutetia Fine-grained regolith High refractory material Mixture of particles with high contrast in albedo 1 2 3 Inversion angle vs particle size Bare rock 50-100 µm 1-5 µm Refractive index 1- bare rock 2 – lunar fines 3- CV3-CO3 Laboratory data from Dollfus et al. 1979, Belskaya 1987, Shkuratov 1994

COMPARISON WITH METEORITES

Lutetia density 3.4± 0.3 g/cm3 (Weiss et al. 2011)

Kaidun meteorite 8-µm particle from comet 81P/Wild 2. sulphide pyrrhotite, enstatite grain and fine-grained porous aggregate material with chondritic composition 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.

Almahata Sitta asteroid 2008 TC3 Sudan desert

21 Lutetia is a mixture of CCs and ECs Summary (21 Lutetia) Lutetia is clearly an old object with a surface age of 3.5 Ga with a primitive chondrite crust and a possible partial differentiation with a metallic core. The surface is a mixture of "incompatible'' types of materials: carbonaceous chondrite (for the majority) and enstatite chondrite (in minor percentage). This are the consequence of impacts that are at the origin of the present composition. 21 Lutetia is a mixture of CCs and ECs Only in situ or a Lutetia sample return will allow to know the real surface composition of this intriguing object.

carbonaceous chondrite Asteroid (Type) Gaspra (S) Mathilde (C) Ida (S) Eros (S) Itokawa (S) Steins (E) Lutetia (C?) Diameter 12 km 53 km 31 km 17 km 0.35 km 6.7 x 5.9 x 4.3 km 126 x 103 x 95 km Period 7.09 hr 17.406 d 4.634 hr 5.267 hr 12.132 hr 6.047 hr 8.168 hr Age 200 My 2-4.5 Gy 1 Gy 2 Gy 1-100 My 100-150 My 0.1-3,6 Gy Density 2.7g/cm3 (b) 1.3 g/cm3 (a) 2.6 g/cm3 (b) 2.67 g/cm3 (b) 1.95 g/cm3 (b) ? ( c ) 3,4 g/cm3 Porosity ? 55 – 63 % 18 – 24 % 16 – 21 % 39 – 43 % Meteorite ordinary chondrite carbonaceous chondrite ordinary chondrite aubrite condrite (CK/CO/CV +EC) Objective Fly-By Galileo (1991) Res=54m/px Fly-by NEAR (1997) Res=180m/px Fly-by Galileo (1993) Res=25m/px 1 year-RD NEAR (2000) Res=cm/px Hovering Hayabusa (2005) Res<1cm/px Fly-by Rosetta (2008) Res<80 m/px Fly-by Rosetta (2010) Res >60 m/px Science return First asteroid with young age (200 Myr) Absence of large craters First asteroid with low density Large craters (5 with D> 20 km) suggest porous bodies have much higher impact strength than expected First discovery of a satellite (Dactyl) Age estimate (1 Byr) First estimate of density of S-type First constraints on mechanical properties Larger amount of boulders than expected Lack of very small craters First evidence of thick regolith First evidence of rubble-pile structure First S-type with low bulk density Large boulders Lack of small craters (<10 m) requires unknown process - First chunk of e highly differentiated object -First visit to a body shaped by the YORP effect? -Larger, older explored asteroid -high density - heterogeneity - Very large craters (D>40 km) - Landslides -Fields of large boulders (>60 m) (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

(Weiss et al. 2011)

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°)