Pasadena
M.A. Barucci
1
LESIA - Observatoire de Paris

ESA Rosetta mission
Journey to comet Churyumov-Gerasimenko
First rendezvous to a comet, ambitious ESA mission, cornerstone aimed at the deciphering of our origins
• Comet RdV maneuver : 2014/05
• Insertion into comet orbit : 2014/09
• Lander : 2014/11
• Mission end : 2015/12
• Stein flyby: 2008/9/5
• Lutetia flyby: 2010/7/10
Launch by Ariane 5G+
March, 2 nd , 2004

500.000 km -9:30h
300.000 km –5:30h
160.000 km -3:00h
400.000 km -7:30h
63.000 km -1:10h
81.000 km -1:30h
215.000 km -4:00h
40.000 km -0:46h

(Barucci et al. 2005, A&A 430, 313)
• Spectrum : Moderately red slope (0.3-0.75
 m), generally flat (0.75-2.5
 m), possible absorption band at 3
 m.
• Albedo = 0.16-0.22

OSIRIS data

V albedo = 0.19
0.01

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

(Sierks et al. 2011)

by S. Marchi (OCA)

Matteo Massironi, UPD grooves

Cut the groove-like structure
- depressions
A

First estimation of d/D for different "old" regions between 0.13 and 0.3, similar to what has been measured on other planetary surfaces.
"Young" region shows craters completely buried under the regolith blanket.
If the region was similar to the rest of the asteroid before the resurfacing, these craters must be at least 600m deep, which gives a lower limit on the regolith thickness.
Crater diameter: 70 pixels ~ 4.5 km
Blanket thickness:

~600 m (for d/D = 0.13)
Work by Jean-Baptiste Vincent, MPS

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

Thermal Inertia : I ~20-30 SI units
 Thick regolith
(Coradini et al. 2011)

Spectroscopy of Lutetia: VIRTIS-M
Extremely homogeneous, less than 5% variability
No obvious spectral signature
No 1 µm band (pyroxenes)

Spectroscopy of Lutetia: VIRTIS-H
Calibration in progress…
No 2 µm band (pyroxenes)
No 3 µm band
(hydrated minerals)
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)
LESIA coIs: J. Crovisier, E. Lellouch, ,
D. Bockelee-Morvan, T. Encrenaz, N. Biver
Radio-telescope of 30 cm:
190 GHz (1,6 mm) : continuum
563 GHz (0,5 mm) : continuum + spectro
Small thermal inertia:
I ~10-30 J/(K m 2 s 0.5
)
(comparable Moon regolith: ~25 SI)
Subsurface (depths from ~ 2 mm to ~ 2 cm) temperatures ranged from ~ 193 K on the sunlit hemisphere to ~ 60 K on the dark hemisphere.

Complementary informations
Herschel observed Lutetia !
O'Rourke, L. et al.
SPIRE
250, 350 & 500 µm
11 jul. 2010
PACS
70, 100 & 160 µm
21 dec. 2009
25

Inhomogeneities on the surface of 21 Lutetia
(Perna, D. et al. 2010, A&A 513, 4)
Aqueous altered materials ?
ferric iron spinforbidden absorptions, phyllosilicates
(jarosite…)?
Lazzarin et al.2006

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

(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
CO3 carb. chondrite
___0-20 micron
.
Iron meteorite
•
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
Enstatite chondrites C peak at 8.3 µm
(Izawa et al. 2010)
… 20-50 micron.
___0-20 micron.
--- 50-100 micron.
CV3 carb. chondrite
___100-150 micron
.
--- >150 micron.
(Barucci et al., 2008)

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)

P min
, %
0.0
COMPARISON WITH METEORITES
0.5
1.0
Ch Aub
LL5
R
E
L5
Aub
Fe
Fe
S
S
S
S
S S
S
Fe
K S
M M
S
E4(0.09)
L4B
L4B(0.12)
Lutetia
Barbara
1.5
2.0
iron meteorites enstatite chondrites ordinary chondrites achondrites
2.5
12 14 16 18 20 22 24 26 28 30
Inversion angle, deg

P min
, %
0.0
COMPARISON WITH METEORITES
Ch Aub
E E
LL5
R
E
L5
Aub
0.5
1.0
F
Fe
Fe
S
S
CK
S
S
S
S
Fe
K S
S S
L4B
L4B(0.12)
CV3
CV3
1.5
F
B
C
C
C
C
C
C
C
C
Lutetia
CM2
CM2
Barbara
CO3
CO3
2.0
C
CM2 iron meteorites enstatite chondrites ordinary chondrites achondrites
C
CI1
CM2 carbonaceous
2.5
12 14 16 18 20 22 24 26 28 30
Inversion angle, deg

Lutetia ground observations on the cilindrical projection
0.40.9 µm 0.8-2.5 µm
2-3.5 µm 538 µm
Barucci et al. (2011)

o = hemispherical + = bidirectional measurements

3
(Weiss et al. 2011)
- surface similar to chondrite;
- apparent high density (exceeds that of most known chondrite meteorites)

8µm particle from comet 81P/Wild 2. sulphide pyrrhotite, enstatite grain and finegrained 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.

A
asteroid 2008 TC
3
Sudan desert

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.
1) We need to put together all the pieces of puzzle
2) Only in situ or a Lutetia sample return will allow knowing the real surface composition of this intriguing object.

ENSTATITE CHONDRITES
E6
E6
0.20
0.15
0.10
E6
E6
E4
E5
E4
0.05
500 1000 1500
Wavelength
2000 2500
• crushed meteorites with grain sizes less than 500 µm (Gaffey 1976)
• spectral feature at 0.87-0.90 µm

Asteroid (Type) Gaspra (S) Mathilde (C) Ida (S) Eros (S) Itokawa (S) Steins (E) Lutetia (M? C?)
Diameter
Period
Age
Density
Porosity
Meteorite
Objective
Science return
12 km
7.09 hr
200 My
2.7g/cm 3 (b)
?
ordinary chondrite
53 km
17.406 d
2-4.5 Gy
1.3 g/cm 3 (a)
55 – 63 % carbonaceous chondrite
31 km
4.634 hr
1 Gy
2.6 g/cm 3 (b)
18 – 24 % ordinary chondrite
17 km
5.267 hr
2 Gy
2.67 g/cm 3 (b)
16 – 21 % ordinary chondrite
0.35 km
12.132 hr
1-100 My
1.95 g/cm 3 (b)
39 – 43 % ordinary chondrite
6.7 x 5.9 x 4.3 km
6.047 hr
100-150 My
? ( c )
?
aubrite
126 x 101 x 73 km
8.168 hr
0.1-3,6 Gy
3,4 g/cm 3
?
condrite
(CK/CO/CV +EC)
Fly-By Galileo
(1991)
Res=54m/px
Fly-by NEAR
(1997)
Res=180m/px
Fly-by
Galileo (1993)
Res=25m/px
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 Stype
First constraints on mechanical properties
1 year-RD
NEAR (2000)
Res=cm/px
Larger amount of boulders than expected
Lack of very small craters
First evidence of thick regolith
Hovering
Hayabusa
(2005)
Res<1cm/px
First evidence of rubble-pile structure
First S-type with low bulk density
Large boulders
Lack of small craters (<10 m) requires unknown process
Fly-by Rosetta
(2008)
Res<80 m/px
- First chunk of e highly differentiated object
-First visit to a body shaped by the YORP effect?
Fly-by Rosetta
(2010)
Res >60 m/px
-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