Pasadena 1

Lessons from 21 Lutetia

M.A. BarucciLESIA - Observatoire de Paris

Journey to comet Churyumov-Gerasimenko

• 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, 2nd, 2004

ESA Rosetta mission

First rendezvous to a comet, ambitious ESA mission, cornerstone aimed at the deciphering of our origins

500.000 km -9:30h 400.000 km -7:30h

300.000 km –5:30h 215.000 km -4:00h

160.000 km -3:00h

81.000 km -1:30h

63.000 km -1:10h

40.000 km -0:46h

Is (21) Lutetia a C-type or M-type asteroid?

• 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

(Barucci et al. 2005, A&A 430, 313)

OSIRIS data

V albedo = 0.19±0.01

Opposition Images

26.000 km -0:30h

20.000 km -0:22h

17.000 km -0:19h16.000 km -0:18h

α = 4.1° α = 2.0° α = 0.6° α = 0.15°

(Sierks et al. 2011)

Surface age: 100 Ma-3.6Ga

by S. Marchi (OCA)

Matteo Massironi, UPD

grooves

Fascinating area with multiple cross-cutting and incising of craters

Cut the groove-like structure - depressions

A

Regolith Thickness

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:Þ ~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

Temperature map from VIRTIS

Thermal Inertia : I ~20-30 SI unitsÞ Thick regolith

(Coradini et al. 2011)

Temperature Vs Morphological Features

Spectroscopy of Lutetia: VIRTIS-M

Extremely homogeneous, less than 5% variabilityNo obvious spectral signature

No 1 µm band (pyroxenes)

Spectroscopy of Lutetia: VIRTIS-HCalibration in progress…

No 2 µm band (pyroxenes)

No 3 µm band (hydrated minerals)

No 3.6 µm band (C-H in organics)

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

Conclusions from VIRTIS

MIRO : Microwave Instrumentfor 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 m2 s0.5) (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.

25

Complementary informations Herschel observed Lutetia !

O'Rourke, L. et al.

PACS70, 100 & 160 µm

21 dec. 2009

SPIRE250, 350 & 500 µm

11 jul. 2010

Inhomogeneities on the surface of 21 Lutetia

Aqueous altered materials ?ferric iron spin-forbidden absorptions, phyllosilicates (jarosite…)?

(Perna, D. et al. 2010, A&A 513, 4)

Lazzarin et al.2006

(Nudelcu et al. 2007)

CV3 (red)

E6 (Blue)

CI (green)

(Birlan et al. 2006)

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, A&A, 454, 677

Birlan et al., 2006

(Rivkin et al. 2011, Icarus)

CV meteorite

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

CO3 carb. chondrite

CV3 carb. chondrite

___0-20 micron. --- 50-100 micron.

… 20-50 micron.

___100-150 micron.--- >150 micron.

___0-20 micron.

(Barucci et al., 2008)

21 LUTETIA: Emissivity - SPITZER

Enstatite chondrites C peak at 8.3 µm(Izawa et al. 2010)

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)

COMPARISON WITH METEORITES

0.0

0.5

1.0

1.5

2.0

2.512 14 16 18 20 22 24 26 28 30

H3H4 H4

H5

H5L4

L4B(0.12)L4B

L5

L5B(0.10)

L6

LL

LL5

E4(0.09)

E6 E6 AubAubCh

HowHow

UrFe

FeFe VSS

S SS LS

K

M

SS

S SSS

S SS

S

E

S

E

KS

R

SS

SS

A

S

E

M MM

iron meteoritesenstatite chondritesordinary chondritesachondrites

Inversion angle, deg

Barbara

Lutetia

Pmin

, %

0.0

0.5

1.0

1.5

2.0

2.512 14 16 18 20 22 24 26 28 30

H3H4 H4

H5

H5L4

L4B(0.12)L4B

L5

L5B(0.10)

L6

LL

LL5

E4(0.09)

E6 E6 AubAubCh

HowHow

UrFe

FeFe VSS

S SS LS

K

M

SS

S SSS

S SS

S

E

S

E

KS

R

SS

SS

A

S

E

M MM

C

B

C

CC

C

PC

C

C

C

C

C

F

C

CF

C

CKCV3CK CV3

CO3CV3CO3 CO3

CM2CI1CI1 CM2

CM2CM2

CM2

iron meteoritesenstatite chondritesordinary chondritesachondritescarbonaceous

Inversion angle, deg

Barbara

Lutetia

Pmin, %COMPARISON WITH METEORITES

Lutetia ground observations on the cilindrical projection

2-3.5 µm

0.4-0.9 µm 0.8-2.5 µm 5-38 µm Barucci et al. (2011)

V albedo = 0.19±0.01

o = hemispherical + = bidirectional measurements

Lutetia density 3.40± 0.21 g/cm3

(Weiss et al. 2011)

- surface similar to chondrite; - apparent high density (exceeds that of most known chondrite meteorites)

Kaidun meteorite

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.

8-µm particle from comet 81P/Wild 2.

sulphide pyrrhotite, enstatite grain and fine-grained porous aggregate material with chondritic composition

Therefore, in a single particle, materials which formed in different regions in a protoplanetary disk can co-exist, which was not expected.

Almahata Sittaasteroid 2008 TC3

Sudan desert

Summary (21 Lutetia)Lutetia is clearly an old object with a surface age of 3.5 Ga with aprimitive 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 puzzle2) Only in situ or a Lutetia sample return will allow knowing the real

surface composition of this intriguing object.

500 1000 1500 2000 2500

0.05

0.10

0.15

0.20

E4E6

E6

E6

E6

Ref

lect

ivity

Wavelength

E4

E5

ENSTATITE CHONDRITES

• 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 12 km 53 km 31 km 17 km 0.35 km 6.7 x 5.9 x 4.3 km 126 x 101 x 73km

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 ordinarychondrite

carbonaceous chondrite

ordinary chondrite

ordinary 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-byGalileo (1993)Res=25m/px

1 year-RDNEAR (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 exploredasteroid--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