POLARIMETRY OF THE SMALL BODIES OF THE SOLAR SYSTEM

Stefano BagnuloArmagh Observatory

Alberto CellinoINAF Torino

3rd SOLARNET WORKSHOP Granada, 27 May 2015

Ludmilla KolokolovaUniversity of Maryland

Irina BelskayaKharkiv National University

Gian Paolo TozziINAF Arcetri

POLARIZATION BY REFLECTION

- > 0

http://background.uchicago.edu/~whu/intermediate/polarization/polar1.html

POLARIZATION BY REFLECTION

Incident light (unpolarized)

Reflected light (polarized)

PHASE ANGLE

= -

Negative branch

Positive branch

~ albedo

~ albedo

pV ≈ 0.09

pV ≈ 0.20

pV ≈ 0.50

THE ALBEDO-POLARIZATION

RELATIONSHIP

Log(pv) = C1 Log(h) + C2

Log(pv) = C3 Log(Pmin) + C4

C1 ≈ C2 ≈ C3 ≈ C4 ≈ -1

THE ALBEDO-POLARIZATION

RELATIONSHIP

Cellino et al. (2015), MNRAS (in press)

Polarimetric slope (%/DEG)Pmin (%)

1

0.1

1

asteroids Trojans

Alb

ed

o

|Pmin

|, %

20.5

0.2

0.5

0.05

0.2

THE ALBEDO-POLARIZATION

RELATIONSHIP

WHAT ELSE POLARIMETRIC CURVES TELL

US?

EMPIRICAL FIT TO THE POLARIZATION

CURVES TAXONOMY (PENTTILA ET AL

2005)

THE BARBARIANS

THE BARBARIANS

Cellino et al. (2015)

TRANSNEPTUNIAN OBJECTS

TRANSNEPTUNIAN OBJECTS

(IXION)

Boehnhardt et al. (2002)

Belskaya et al. (2015)

TRANSNEPTUNIAN OBJECTS

Bagnulo et al. (2008)

CENTAURS

TRANSNEPTUNIAN OBJECTS + CENTAURS

Bagnulo et al. (2006), Belskaya et al. (2010)

NUCLEUS OF COMET 2P/ENCKE

a0 ~ 13o

(Boehnhardt et al. 2008)

(Belsakya et al. 2015)

NUCLEUS OF 133P/ELST-PIZARRO

Bagnulo et al. (2010)

COMETS

COMETS

Stinson et al. (2015, in prep.)

COMETS

Nucleus Dust Tail

Coma Ion Tail

COMETS

CONSTRAINTS FROM THE POLARIMETRIC

COLOUR

AGGREGATES

T matrix code for cluster of spheres(Mackowki & Mischcenko 2011)

AGGREGATES

• Radius of the monomers ~ 0.1 µm• Number of monomers > 256• Refractive index (based on in-situ data for

comet Halley): 1.88-i0.47 at l=0.45 µm and

1.98-i0.48 at l=0.60 µm• Composition: silicate 31.76%, iron 2.56%,

carbonaceous materials 65.68% (1/3 organic

refractory, 2/3 amorphous carbon)

0.01

0.1

1

10

100

Ap

180120600

(deg)

1.0

0.8

0.6

0.4

0.2

0.0

P

10-14

10-13

10-12

10-11

10-10

S1

1 /k2 (m

2)

0.01

0.00

P

100

BCCAN=256

CONSTRAINTS FROM THE POLARIMETRIC

COLOUR

CIRCULAR POLARIMETRY

Rosenbush et al. (2007)

CIRCULAR POLARIMETRY

Violation of the mirror symmetry in the medium

• Asymmetry of the medium where light experience multiple scattering

• Asymmetry of the optical properties of the material (e.g., asymmetry in crystal structure of the material, or in the structure of the molecules)

THE BUILDING BLOCKS OF LIFE (AS WE

KNOW) ARE CHIRAL

All amino acids of biological origin are left-handed

Gregory & Raps (1974)Gregory et al. (1975)

Wolstencroft et al. (2004)

Sparks et al. (2008)

CIRCULAR POLARIMETRY

Violation of the mirror symmetry in the medium

• Asymmetry of the medium where light experience multiple scattering

• Asymmetry of the optical properties of the material (e.g., asymmetry in crystal structure of the material, or in the structure of the molecules)

• Asymmetry of the scatterers themselves (alignment of particles due to some mechanism)

AN ALTERNATIVE EXPLANATION

AN ALTERNATIVE EXPLANATION

ASTEROID SPECTROSCOPY

ASTEROID SPECTROSCOPY

Bagnulo et al. (2015)

PRELIMINARY QUESTION

How polarization spectra depend on phase-angle?

DOES THEIR SHAPE CHANGE WITH PHASE-ANGLE?

NORMALISING POLARIZATION SPECTRA

IF PQ(λ,α) = A(λ) * B(α) THEN pq = pQ(λ)

ORIGINAL NORMALIZED

Bagnulo et al. (2015)

DO ASTEROIDS WITH SIMILAR

REFLECTANCE SPECTRA SHOW SIMILAR

POLARIZATION SPECTRA?

NO!

S-type

THE UMOV EFFECT: P ~ 1/R

REFLECTANCE SPECTRA

POLARIZATION SPECTRA

VIOLATION OF THE UMOV EFFECT (P ~ R)

236 Honoria (L-type)

REFLECTANCE

POLARIZATION

YES! Low albedo:

dp(λ)/dλ >0 in the positive branch

dp(λ)/dλ <0 in the negative branch

YES! Intermediate albedo:

dp(λ)/dλ <0 in the positive branch

dp(λ)/dλ >0 in the negative branch

Results fully consistent with the analysis carried out by Lupisko & Kiselev (1995) and by Belskaya et al. (2009) based on multi-colour broadband polarimetry

CONCLUSIONS

Spectro-polarimetry offers a new degree of constraint for the characterization of the atmosphere-less bodies of our solar system

Future attempts of modeling the surface structure of asteroids should take into account both I and Q spectra!