Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust

C/2006 P1 McNaught (M. Druckmuller, Argentina)

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Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust
A.Chantal Levasseur-Regourd
UPMC Univ Paris 6 / IPSL-CNRS, France

Outline1. Introduction
2. Key facts about cometary dust
3. Polarization of solar light scattered by dust
4. Cometary dust polarimetric observations
5. Interpretation through numerical simulations
6. Interpretation through laboratory simulations
7. Optical properties, interplanetary & asteroidal dust
8. Perspectives and conclusions
2

Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust

1. Introduction

C/2006 P1 McNaught (M. Druckmuller, Argentina)

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1. Introduction: Dustin small bodies
Cometary
nuclei
Eros (NEAR)
Itokawa (Hayabusa)

Tempel 1 (Deep Impact)

Asteroids

Hale-Bopp (N. Biver)
Dust trails (W. Reach)

Regolith

Mathilde (NEAR)

Comae and dust tails
Optically thin media

Interplanetary dust (M. Fulle)
Optically thin dust cloud

4

IDPs (Nasa)
Irregular, >1 µm

1. Introduction: Motivation• Dust partly revealed to thenaked eye by the solar
scattered light• Limited amount of informationfrom a few unique in-situ
missions• “Ground truth” from sample
return missions, up to now
limited to dust particles collectedin the coma of comet Wild 2
 Numerous clues to the bulk
properties of the dust fromremote observations• Spectroscopy providing
information on composition• Optical properties forclassification and bulk properties

Holmes (© M. Jäger)

McNaught (McNaught)
Halley (Giotto)

Wild 2 dust container
5(Stardust)

Optical properties of Dust Particles
in the Solar Systemwith emphasis on cometary dust

2. Key facts about cometary dust

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2. Key facts about cometary dust: Evidence for dust
Un-polarized solar light scattered by particles partially linearlypolarized by scattering
An old “technique”, already used by:• Arthropods (e.g. honeybees) to navigate, with their “eyes” (ommatidia)
sensitive to polarisation by Rayleigh scattering on atmosphericmolecules• Arago to establish the presence of dust in cometary tails, throughobservations of a bright comet in 1819 and of comet Halley in 1835
(with a ‘polariscope’)no7

2. Key facts about cometary dust: Composition
1P/Halley flyby• Vega 1 and Giotto dust mass spectrometers
 Dust ≈ 50% rocky silicates and 50 % light elements “CHON”• Vega 2 spectrometer Tentative identification of PAHs
• Giotto plasma analyzers
 Heavy CHON ions (possibly POM)
Remote observations in the infrared domain
 Silicate emission feature for some comets (very structured for Hale-Bopp) Fits requesting amorphous and crystalline silicates (Crovisier et al., 2000)
81P/Wild 2 (Stardust flyby and samples analysis)
 Dust also rich in CHON; heterogeneous distribution of organics;
amorphous & crystalline silicates (Sandford et al., 2006; Zolensky et al., 2006, 2008)
H C

N O
Mg Si

Kissel et al., 1986

Hanner et al., 1999

Wild 2 (Stardust)
82. Key facts about cometary dust: Bulk properties
1P/Halley (Giotto) Dust fluence versus mass
 Local brightness & polarization showing significant changes within coma Dynamical model best fit of a dynamical model (Fulle et al., 2000)
Albedo ≈ 0.04, dust size distribution ≈ s-2.6, density ≈ 100 kg m-3
 Very fluffy dust particles
81P/Wild 2 (Stardust flyby, samples tracks and impacts)
 Fragmentation in the coma; evidence for aggregates(Burchell et al., 2006, 2008; Hörz et al., 2006)

no9

Nucleus distance (km)Levasseur-Regourd et al., 1999

Green et al, 2004

Hölz et al, 2006

Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust
3. Polarization of solar lightscattered by dust

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3. Polarization of light scattered by dust: Definitions• For randomly oriented particles, solar lightscattered by an optically thin media linearlypolarized, with the electromagnetic wave
predominantly oscillating perpendicular orparallel to the scattering plane.
• Linear polarisation P, dimensionless ratio,
allowing comparisons for objects at different
distances to the Sun and the observer
Noticed by Lyot for lunar regions
Of major importance for comets, whose
intensity is not proportional to 1/R2, but(normalized light-curve variations) nor to 1/Δ2
(development of a bright coma)• P varying only with- phase angle α- wavelength λ- properties of the scattering medium

Intensity
I = Ip + Inp
I = (I⊥+ I//)
Linear polarisation
P = (I⊥- I//) /(I⊥+ I//)P = - Q/I
α = π - θ (θ scattering angle)

Moon
Lyot 192911

3. Polarization of light scattered by dust: Small bodies• Phase angle α changing with time (observations’ geometry), providing Pλ(α)
‘Smooth’ polarization phase curves, typical of scattering by irregularparticles with sizes greater than observational wavelength, i.e. a few µm
Mie solutions to Maxwell equations not acceptable here
• Empirical laws, e.g. albedo - slope at inversion, not necessarily acceptable(possibly only derived for one given class of objects)• Tentative interpretation of the observed variations of Pλ(α)(e.g. αmin, Pmin, α0, h, αmax) and Pα(λ) in terms of physical properties,
from experimental and numerical simulations with realistic (?) particles
Observational constraints• Moving objects in a fixed star-field• Narrow interference filters mandatory
for comets (to avoid depolarization fromgaseous emissions)• Interplanetary data needing inversion• Asteroids usually at small α

Levasseur-Regourd et al. 1999

Positive branchwith Pmax
Inversion α0

Slope hNegative branch with Pmin
Backscattering region

12Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust

4. Cometary dust polarimetric observations

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4. Cometary dust polarimetric observations:
Spatial changes within the coma
• In situ measurements (OPE/Giotto)
 Significant variations of P in inner coma
• Remote P imaging technique (since 1990)
Circumnucleus ‘halo’ and jet-like features(specially for active comets)• P reaching an asymptote for increasing aperture Significance of whole coma polarisation data

Halley, α = 73°
α = 44°

Levasseur-Regourd et al, 1999
Hale-Boppf-o-v 83000 km

α = 46°

Tempel 1
f-o-v 70000 km

α ≈ 41°
Adapted from
Hadamcik et al, 2007

I

<I>

P

Hadamcik & L-R, 2003

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4. Cometary dust polarimetric observations:
α dependence (whole coma)
Pred(α)xx
Levasseur-Regourd et al, 1996

α0 ≈ 20°

Hadamcik & Levasseur-Regourd 2003

• Smooth polarisation phase curves,
suggesting 2 (or 3) classes of comets from their dust properties• Pmax higher whenever silicate emission feature• P possibly increasing in jet-like features or after an outburst
15

4. Cometary dust polarimetric observations:
λ dependence (whole coma)
Pα(λ)
50°°
40°°
30°°

Levasseur-Regourd & Hadamcik, 2003

Hadamcik et al, 2003

Levasseur-Regourd et al, 2005

• P usually increasing with λ for α > ≈ 30°, at least in the visible domain• Some exceptions- innermost coma (e.g. 1P/Halley)- disruption events, tentatively due to different dust properties inside thenucleus (e.g. C/1999 S4 LINEAR)
164. Cometary dust polarimetric observations:
Need for simulations• Significant amount of observational data for JCF comets, as well
as from comets probably originating from the Oort cloud• Existence of empirical laws (mostly deduced from asteroidal data,
e.g. slope at inversion - albedo relation)• Interpretation of the observed variations of the polarization
properties in terms of physical properties, requesting both numericaland experimental simulations, with tentatively realistic dust particles
In-situ observations
and samples

Remote light scattering polarimetricobservations

Changes in the dust properties
Presence of aggregates

Irregular particles
Estimation of the albedo

Light scattering numerical and experimental simulations
on irregular particles and aggregates(Realistic particles? Realistic light scattering? )Clues to the physical properties of different dust media

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Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust
5. Interpretation throughnumerical simulations

C/2006 P1 McNaught (M. Druckmuller, Argentina)

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5. Interpretation through numericalsimulations: principles• First light scattering simulations by Mukai et al (1987), on 1P/Halley
observations, by applying Mie theory to spheres with Vega size distribution
 Complex refractive index, 1.385 + 0.035 i
Also simulations with spheroids, polyhedrons… (Lumme, 2000)• From the late 90s, simulations with fractal aggregates of grains with DDA
codes (Xing & Hanner, 1997; Levasseur-Regourd et al, 1997; Nakamura & Okamoto, 1999;
Haudebourg et al, 1999; Petrova et al, 2000; Tiskovets et al, 2000, Kimura et al, 2003, Mann etal, 2004… ) or ray tracing for large grains (Okada et al, 2006…)• Example: Light scattering simulations (T-matrix, ray-tracing, DDA codes) for
spheroids of astronomical silicates and more absorbing organics(equivalent radius a) and fractal aggregates thereof (BCCA & BPCA, 500
dipoles per grain, 1500 orientations, up to 256 grains)
BCC

BPC

19

Lasue et al, 2009

5. Interpretation through numericalsimulations: Hale-Bopp in two colorsm ≈ 1.62+0.003i

m ≈ 1.88+0.1i• Observations fitted for only a few freeparameters (e.g. slope s of the size distributionas, minimal a, maximal a, silicates /organics ratio)• For Hale-Bopp, from fits in two colors
Size distribution ≈ a-3
0.1 µm < a < 20 µm
In mass ≈ 40% silicates & 60% organics(up to 65% silicates, down to 35% organics)
20
Lasue etetal,al.
2009
Lasue
2009

5. Interpretation through numericalsimulations: ‘predictions’ for other colorsm ≈ 1.62+0.003i

m ≈ 1.88+0.1i• Observations fitted for only a few freeparameters (e.g. slope s of the size distributionas, minimal a, maximal a, silicates /organics ratio)• For Hale-Bopp, from fits in two colors
Size distribution ≈ a-3
0.1 µm < a < 20 µm
In mass ≈ 40% silicates & 60% organics(up to 65% silicates, down to 35% organics)• With identical parameters,also excellent agreement in UV and near IR

21
Lasue
et et
al, 2009
Lasue
al. 2009

5. Interpretation through numericalsimulations: summary• In Hale-Bopp, significant amounts of compact grains and aggregates(more than ≈ 20%), as well as of silicates (indices comparable toastronomical silicates) and organics (absorbing material),in agreement with Stardust results on Wild 2(loosely-bound tiny-grains aggregates & cohesive-compact grains)• Similar approach possible for other comets
observed on a large enough range of α and λ,e.g. 1P/Halley Size distribution ≈ a-2.8
0.13 µm < a < 19 µm,
Similar silicates/organics ratioLess satisfactory fit on the negative branch- larger aggregates ?
- different refractive indices ?
- more compact aggregates ?
• Need also for experimental simulations, withmeasurements on quite realistic particles

22
Lasue
al. 2009
Lasue
et et
al, 2009

Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust
6. Interpretation throughlaboratory simulations

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5. Interpretation through experimental
simulations: principle• Measurements with PROGRA2-vis experiment (at 543.5 & 632.8 nm) in thelaboratory and during parabolic flights campaigns, avoiding sedimentationand multiple scattering on gravity packed layers (Worms et al, 1999, Renard et al. 2002)• Calibration with spheres through Mie computations

Hadamcik et al, 2008

24
ESA

5. Interpretation through experimental
simulations: main results• Good match for mixtures of fluffy Mg-silicates, Fe-silicates and C

aggregates, with some compact Mg-silicates grains(fluffy chains = negative branch, C compounds = polarization colour)• Excellent match with new sets of NASA samples, e.g. porous aggregatesof subµm (MgSiO + FeSiO + C) grains, and compact Mg-silicates

Hadamcik et al., 2007 and 200825

Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust
7. Optical properties
Interplanetary and asteroidal dust

C/2006 P1 McNaught (M. Druckmuller, Argentina)

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7. Optical properties:
Zodiacal light observations• Interplanetary data needing an inversion
Iλ(ε, R) = F(λ) 䌶 [n(r) σλ(r, θ) / r2] dl• Local polarisation P retrieved- rigorously for l-o-s tangent to the direction of
motion of the observer and for the section of
the l-o-s where he is located (Dumont, 1973)- through inversion techniques, in the nearecliptic symmetry plane of the cloud(Dumont and Levasseur-Regourd, 1985; LevasseurRegourd et al., 1991; Lumme, 2000)

Levasseur-Regourd et al, 2007

P(α)α0 ≈ 15°• From thermal emission, local temperature T(R)
≈ T0 R -0.45, i.e. non blackbody (Lasue et al, 2007)• Smooth P(α) phase curve(at R = 1.5 AU near the ecliptic plane)• Decrease of P(90°)
with decreasing solar distance R

P90°(R)
27
Levasseur-Regourd et al, 1991

7. Optical properties:
Interplanetary dust simulations• Phase curve fits requesting bothtransparent and absorbing materials,
with some aggregates• At R = 1.5 AU near the ecliptic planesize distribution
a-3, 0.11 < a < ≈ 10 µm
a-4.4, a > ≈ 10 µm
in mass, 40-80% silicates, 60-20% organics• Un-fragmented aggregates likely to be ofcometary origin: at least 20% in mass at 1.5 AU• P(R) decrease with decreasing R possibly due toa progressive thermal degradation of organics(HCN polymers and other solid carbonaceouscompounds)• Drastic change below ≈ 0.3 UA possibly due todegradation of silicates with smaller particles

Lasue et al., 200728

7. Optical properties:
Observations of asteroids (α)• Observations mostly documented for S and C types and at small phaseangles (mostly negative branch), except for NEA• Slope at inversion h increasing with decreasing albedo• Trend of the positive branch providing information about taxonomic type(principal component analysis)• Example of application: 2667 Steins, first Rosetta target in Sept. 2008
 Albedo ≈ 0.45 and E-type (Fornasier et al. 2006)
10
8

Polarization (%)

6

S

C

4
2
0
-2
-4
0

20

40

60

80

Phase angle (%)

100

120

Levasseur-Regourd & Hadamcik 2003

B F C CU G CPF CP P E R M S V X
29Pentillä et al. 2005

7. Optical properties:
Observations of asteroids (λ)
Toutatis

Itokawa• Well documented for4179 Toutatis (Lupishko etal. 1995, Mukai et al. 1997,
Ishiguro et al. 1997) & 25143
Itokawa (Cellino et al. 2005)• However, from ongoingpolarimetric observations(e.g. from OHP
observatory), polarisation

likely to increase withincreasing λ for C-type3
S-type
U
I

2
1

P(%)

• Polarisation significantly
decreasing withincreasing λ for S-type

Levasseur-Regourd et al.
2003

0
-1
-2

Belskaya et al. 2008
0

5

10

15
20
Phase angle (deg)

25

30

35

30

7. Optical properties:
Asteroidal regoliths simulations

Steins (Rosetta)• Numerical simulations on irregular regolith layers not (yet?) feasible• Measurements with PROGRA2-surf on dust layers (Hadamcik et al. 2008)• Tests with huge agglomerates (filling factors of about 0.15) prepared byBraunschweig University• Tentative confirmation of the faint polarization color trend observed for
Steins on aubrites samples (expected to originate from E-type objects)

Fornasier et al, 2006

Hadamcik et al, 200831

Optical properties of Dust Particles
in the Solar System,with emphasis on cometary dust

8. Conclusions and perspectives

C/2006 P1 McNaught (M. Druckmuller, Argentina)

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8. Conclusions• Space missions, and specially sample return missions, present unique
opportunities to reveal the dust properties of a few solar system bodies• The analysis of (remote) observations of the linear polarization of thesolar light they scatter gives clues to the properties of these various media• Heterogeneities in terms of dust properties are found between- innermost comae and comae- different comets (high and low Pmax comets)- dust in the coma and “fresh” dust from the subsurface• Comparisons between the shapes of the polarimetric phase curves
provide a classification with respect to the dust properties• Extensive programmes of numerical and laboratory simulations withvarious irregular compact grains and aggregates are being developed byvarious teams…

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8. Conclusions• The programmes of numerical and laboratory
simulations with various irregular compact grains andaggregates have already established that- cometary dust particles are built of both very fluffy
aggregates and more compact grains, with asignificant proportions of both rather transparent
silicates and absorbing materials- a significant proportion of interplanetary dust is rich inabsorbing organics (evaporating close to the Sun), ofcometary origin• Such properties might suggest- significantly high micro-porosity of low-density
nuclei (Levasseur-Regourd et al., 2008)- possible survival of cometary organics embedded
in fluffy aggregates in terrestrial planets
atmospheres, by LHB time

Lasue et al. 2009

Atmospheric ablation
for fluffy aggregates34

8. Perspectives…
• More on-going remote observations of comets• More zodiacal light remote observations with Akari
Also in-situ observations, parallel to Planet C trajectory• In 2014-2015, Rosetta rendezvous with and landingon 67P/Churyumov-Gerasimenko
dust in the coma (MIDAS, GIADA, OSIRIS…)
nucleus structure (CONSERT)ESA• More experimental simulations to take place in the lab
Also possibly on board the ISS with ICAPS precursor?
• Marco-Polo mission to a primitive body (ESA-JAXA) ?

ICAPSCONSERT35

8. Perspectives
Some important aims for future studies• Numerical simulation of light scattering through irregular dust
particles layers (i.e. multiple scattering)• Laboratory simulations on realistic ices and dust mixture, possibly
in low gravity conditions• Better links with interstellar dust and dust debris studies
 Useful information on formation and physical processes, e.g.
accretion and agglomeration,
dust ejection and fragmentation,
sublimation of organics, collisions,gravitational and non-gravitational effects

36

Acknowledgements
R. Dumont, E. Desvoivres, B. Goidet-Devel, E. Hadamcik,
V. Haudebourg, W. Kofman, P. Lamy, J. Lasue, J.B. Renard, J.C. Worms…
I. Belskaya, J. Blum, A. Cellino, M. Fulle, S. Green, M.S. Hanner,
L. Kolokolova, I. Mann, N. McBride, J.A.M. McDonnell, K. Muinonen,
J. Nuth, A. Pentillä, W.T. Reach, F. Rietmeijer, Y.G. Shkuratov, M. Zolensky…

H. Kimura, T. Mukai, Y. Okada, and all my Japanese colleagues

37

References on recent papers to which we have contributed on these topics
A.C. Levasseur-Regourd, T. Mukai, J. Lasue, Y. Okada, Physical properties of cometary and interplanetary dust, Planet.
Space Sci. 55, 1010-1020, 2007
E. Hadamcik, A.C. Levasseur-Regourd, V. Leroi, D. Bardin, Imaging polarimetry of the dust coma of comet Tempel 1
before and after Deep Impact, Icarus, 190, 459-468.
J. Lasue, A.C. Levasseur-Regourd, N. Fray, H. Cottin, Inferring the interplanetary dust properties from remoteobservations and simulations, Astron. Astrophys., 473, 641-649, 2007
E. Hadamcik, J.B. Renard, F.J.M. Rietmeijer, A.C. Levasseur-Regourd, H.G.M. Hill, J.M. Karner, J.A. Nuth, Light
scattering by fluffy Mg-Fe-SiO and C mixtures as cometary analogs (PROGRA2 experiment), Icarus, 190, 660-671,
2007
A.C. Levasseur-Regourd, M. Zolensky J. Lasue, Dust in cometary comae: present understanding of the structure andcomposition of dust particles, Planet. Space Sci., 56 1719-1724, 2008
I.N. Belskaya, A.C. Levasseur-Regourd, A. Cellino, Y.E. Efimov, N.M. Shakhovskoy, E., Hadamcik, P. Bendjoya,
Polarimetry of main belt asteroids: wavelength dependence, Icarus, 199, 97-105, 2009
J. Lasue, A.C. Levasseur-Regourd, E. Hadamcik, G. Alcouffe, Cometary dust properties retrieved from polarization
observations, Application to C/1995 O1 Hale-Bopp and 1P/Halley, Icarus, 199, 129-144, 2009
E. Hadamcik, A.C. Levasseur-Regourd, Optical properties of dust from Jupiter Family Comets, Planet. Space Sci., inpress
A.C. Levasseur-Regourd, E. Hadamcik, E. Desvoivres, J. Lasue, Probing the internal structure of the nuclei of comets,
Planet. Space Sci., in press
E. Hadamcik, J.B. Renard, A.C. Levasseur-Regourd, J.C. Worms, Laboratory measurements of the light scattered byclouds of solid particles by an imaging technique, Light scattering review 4, Bremen University, in press
Also Space Sci. Rev., 128, 2007 papers on Rosetta, dust, and structure of the nucleus

Thank you for your attention38

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