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

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

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

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

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

7. 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

8. 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.

9. 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

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

11. Temperature Vs Morphological Features

12. 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)

13. 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)

14. 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

15. 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)

16. 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

17. V albedo = 0.19±0.01

19. 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

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

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

22. 21 LUTETIA: Emissivity - SPITZER CV meteorite Iron meteorite
CO3 carb. chondrite
___0-20 micron.
Iron meteorite
… 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.
micron.
CV3 carb. chondrite
___ micron.
--- >150 micron.
Enstatite chondrites C peak at 8.3 µm
(Izawa et al. 2010)
(Barucci et al., 2008)

23. Lutetia ground observations
µm µm µm
5-38 µm

24. 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)

25. 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 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

26. COMPARISON WITH METEORITES

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

28. 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.

29. Almahata Sitta asteroid 2008 TC3
Sudan desert

30. 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.

31. 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 d
4.634 hr
5.267 hr hr
6.047 hr
8.168 hr
Age
200 My
2-4.5 Gy
1 Gy
2 Gy
1-100 My 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 – g/cm3

32. (Weiss et al. 2011)

33. 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°)