Looking at the Kuiper Belt from the thermal side

Arielle Moullet, NRAO

1930

1988

1992

Slipher et al.

Courtesy of Minor Planet Center

1000 KBOs(classical, Plutinos)

200 Centaurs

200 Scattered objects

Faint sources: 22<V<28

~20 large objects ( diameter>300 km )

Large fraction of multiple systems

Icy surfaces (H2O, N2, CH4), possibly atmospheres

Nasa Images Center

Why study Kuiper Belt objects ? (and related populations)

● Physical and chemical evolution of cold/distant surfaces and atmospheres

●Pristine / unaltered objects : information on the conditions in the primitive Solar nebula 

● Analog of planetesimals in stellar debris disk

Orbital structure

Nice model in the outer Solar System, Morbidelli et al., 2008

Remarkable features :

● Sharp outer edge ● Mass depleted● Excited populations

→ indication on giants migration, approaching stars, initial disk structure

Size distribution

Identification of slope and breaks

→ direct comparison to formation models :merging, accretion, collisions

Challenging size measurements

Donnison et al., 2006

Scattered Objects

Classical Objects

Composition of early solar nebula

Great variety of bulk densities (rock/ice ratio) :

→ inhomogeneous ice/rock ratio in the outer disk ?

→ collisions on differentiatedbodies?

Very little data

Brown et al., 2012 : Densities and diameters

Surface altering

Collisional excavation

Space weathering

Volatile loss

Thermal alteration

Cryovolcanism

Radiogenic heating...

Brown et al., 2010 : Model of volatile retention on surfaces

Wide variety of surface composition (volatiles/organics), reflect surface alteration processes :

Jewitt et al. : Space-weathering

Sublimation-sustained atmospheres

Schaller and Brown, 2008

Detected on Pluto, plausible on large KBOs :sublimation of volatiles CO, CH4, N2

→ Pressure exp. dependent on surface temperature :high variations (diurnal/seasonal)

→ Condensation cycles periodically recycling surfacess

Individual characterization of the large bodies

Large-scale studies : taxonomy, families, correlations

Observing KBOs 

Doressoundiram et al, 2005 :Inclination,semi-major axis, size, spectral index

Correlations between spectral/orbital properties

Techniques

Optical – NIR photometry (>100 obj) → magnitude, spectral index

Optical – NIR spectrosocopy (>40 obj):→ icy/mineral bands

Dumas et al., 2007 : Eris spectra with water/tholin/methane/nitrogen ice model

Techniques

Optical – NIR photometry (>100 obj) → magnitude, spectral index

Optical – NIR spectrosocopy (>40 obj):→ icy/mineral bands

HST Imaging (~10 objects) :→ multiple system imaging, sizes

1''

Noll et al., 2008

Techniques

Optical – NIR photometry (>100 obj) → magnitude, spectral index

Optical – NIR spectrosocopy (>40 obj):→ icy/mineral bands

HST Imaging (~10 objects) :→ multiple system imaging, sizes

Occultations (~10 objects) :→ sizes, atmospheric height

Sicardy et al., 2012 : Eris occultation

Techniques

Optical – NIR photometry (>100 obj) → magnitude, spectral index

Optical – NIR spectrosocopy (>40 obj):→ icy/mineral bands

HST Imaging (~10 objects) :→ multiple system imaging, sizes

Occultations (~10 objects) :→ sizes, atmospheric height

IR/mm/cm continuum :→ thermal emission Moullet et al., 2008 : detection of

1999 TZ1 at IRAM-30m

KBOs' thermal emission

55 K

70 K40 K

30 K

← Eris

Temperature ~1/√Dh

Brightness~1/√Dh/Dg

2

KBOs' thermal emission

Brightness temperature Tb = ε Tsurface

ε : emissivity = departure from black-body

Frequency (GHz)

← Solar reflected

Rayleigh-Jeans regime

30-100 μm peak

Radiative effects

Snell-Fresnel laws at surface/air interface :- reflection- non isotropic refraction- polarisation

Emissivity depends on refraction index, surface roughness

Thermal emission

n>1

n=1

Radiative effects

Surfaces not transparent at thermal wavelenghts :effectively sounding subsurfaces down to ~10λ

Emissivity depends on absorption coefficient, vertical thermal profile

The total emission combines contributions from different depths

Moullet et al., 2008b :Variation of Io's Tb with wavelength

Temperature distribution

Mueller et al., 2008 : temperature distribution model for Haumea

Temperature depends on

geometric properties : shape, rotation rate

orbital properties ::hel. Distance, pole direction

surface properties :albedo, thermal inertia

The radiometric method

Optical magnitude

~ albedo . D2

Thermal emission

~ B(ν,T((1-a)0.25)) . D2

Assuming thermal model

Morrison et al., 1977

Independant estimate of albedo and effective size

If mass known (binaries) : density estimate

The radiometric method

Thermals models, defined through beaming parameter η

Low inertia High inertia

Slow Rotator model Quick Rotator model

Varying η η=1 η=2

η constrained by multi-wavelengths thermal photometryAverage value for KBOs : 1.2

Lightcurve interpretation

Brightness variation during rotation (average 8h period)

Time-resolved radiometric method can distinguish albedo distribution/ shape (apparent size variation)

Lellouch et al, 2010 :Haumea's optical and thermal lightcurves with Herschel

Lacerda et al, 2006

Results obtained

Frequency (GHz)

ALMA

Herschel

Spitzer

JVLA

IRAM

Before 2010...

~4 sizes with ISO 90 μm

~45 sizes with Spitzer-MIPS (Centaurs) – 24 and 70 μm

~8 sizes with IRAM-30m MAMBO bolometer – 1.2 mm

Sensitivity very limiting !

The Herschel Large Program : ''TNOs are cool''

370 hours awarded (PIs Mueller and Lellouch)

- 140 (40) targets at 60, 100 and 160 μm (PACS), 17 targets at 250 μm (SPIRE) : sizes (>200 km, error. 25%) / albedos

- 25 (10) binaries : densities

- 25 (1) lightcurves : shapes

Vilenius et al., 2012 :KBO's albedo and inclination relation

Jansky Very Large Array

Best sensitivity with band Ka(1cm )

Detection of Quaoar and MakeMake (very cold : Tb~27K)

Imaging of Pluto/Charon :Tb~40 / 55 K(different albedo)

Makemake detection at EVLA(B. Butler)

ALMA : sensitivity AND imaging

- Mm-interferometer : less sky-confusion in galactic plane than IR

- In bands 7/6, sensitivity better than Herschel

- Possibility to investigate atmosphere through CO lines - Spatial resolution down to ~0.01'' (high frequency) Pluto : 0.1'', large KBO : 0.05'', most KBOs <0.015''

Thermal detection : Cycle 1

1 hour on source, 16 GHzBand 6 : 18 μJyBand 7 : 31 μJy

Typical KBO thermal model assumed Albedo assumed (if necessary) : 0.08

Diameter threshold for 5 σ detection

Thermal detection : Full science

1 hour on source, 16 GHzBand 6 : 9 μJyBand 7 : 15 μJy

Typical KBO thermal model assumed Albedo assumed (if necessary) : 0.08

Moullet et al., 2011

Diameter threshold for 5 σ detection

Radiometric measurements

Can be applied to ~ 500 bodies (>35% of total) for 1 h. obs each ~ 600 bodies (>40%) for 2 h. obs each

>60 km diameter @ 20 AU>110 km @ 30 AU>160 km @ 40 AU>210 km @ 50 AU

Errors on diameter 15-25%(dominated by model uncertainty)

Mueller et al., 2008

Sizes and albedos surveys : science output

- Significant increase of the size/albedo database for establishing correlations → retrieving physical, dynamical surface processes

- Albedo necessary to interpret optical/IR spectra→ surface chemical composition

- Filling of the size distribution in the 100-200 km range→ constrain formation and collisional history

- Density measurements (binaries)→ primitive disk composition/structure

Size and shape : direct determination

- Direct analysis of visibilities (~ imaging) - SNR/beam >20 - spatial resolution 0.6-1.2 x source size

- Possible on ~ 30 bodies,Accuracy <15%, non-model dependant

- Possible to identify ellipticity in the plane of sky on few bodies

- Thermal lightcurves on ~ 30 objects

Moullet et al., 2011 : simulated Charon visibilities @345 GHz

Size and shape : science output

- Independent size measurements→ refinement of thermal models, albedo

- Precise size determination for large sources :→ compare to atmospheric/volatiles models

- Shape determination on pole-on geometries

- 3D shape combining lightcurves / imaging :→ constraints on internal strength, density, formation

Surface mapping

- First KBOs thermal mapping possible with very extended configurations, resolution ~15mas

- 10% temperature variations on 6 large bodies (4 h integration in bands 7,9 or 10)

- Horizontal variations of albedo/ thermal inertia reflect surface collisions / resurfacing processes

Surface mapping

Pluto, Band 7, very extended configuration

Pluto, Band 9, very extended configuration

Pluto : variegated surface in albedo/composition.Expected Tsurf variations

Young et al., 1998

Multiple system mapping

Grundy et al., 2011 : improved orbits of large KBO binary systems

- Large fraction of multiple systems: ~10%. Many ~equally-sized

- Separation 2'' → contact binaries

- Orbit determines mass

- First resolved thermal imaging *

→ individual size/albedo → constraint on system formation (capture, disruption,...)

- Better resolution than Hubble : binary searches, astrometry

* except for Pluto/Charon system

Multiple system mapping

Haumea system, Band 7, Cycle1-3 Orcus system, Band 7, Cycle1-6

Beam ~ 0.5'' Beam ~ 0.2''

Hi'iaka' Vanth

- Cycle 1 : handful of very separated systems

- Full science : large-scale binary search, contact binaries imaging

Atmospheres : Pluto

Lellouch et al., 2011 : detection of the CO atmospheric lines near 2.3 μm

Greaves et al., 2011 : detection of the CO(2-1) line at JCMT

- N2-based atmosphere, ~10-40 μbar pressure.

- CO-ice detected : expected to maintain 1-10.e-4 abundance

- Contradictory detections :

q=0.5e-3 q~1e-1, optically thick

Atmospheric detection Atmospheric detection : Pluto

Expected CO(3-2) disk-integrated lines on Pluto (from M. Gurwell)

- CO detection expected in ~1 hour Cycle 1.

- Constraints on sublimation mechanism : horizontal/vertical ice segregation

- Pluto is backing : atmosphere may freeze outsoon !

- In full science : Haumea, Makemake, Eris, Sedna

Summary : ALMA has an important and unique role for KBOs studies

- Radiometric measurements of sizes and albedos and hundreds of bodies

- Direct size and shape estimation om large bodies

- Detection and imaging of binaries down to 10mas close

- First thermal maps of KBOs

Possible wealth of physical information

Invaluable tool complementing Herschel

Observation Challenges

- track moving sources (~1''/ hour)

- handling of ephemeris objects (data reduction, array operation)

- confusion (non-uniform background)

- tight scheduling constraints (lightcurves, multiple system orbits)

Future perspectives for KBO studies

- detection : GAIA. LSST, Pan-Starrs

- occultation search : TAOS - optical/near-IR spectroscopy : E-ELT, JWST

- New Horizons: Pluto flyby in 2015. Comprehensive caracterization, no thermal instrument. Possible other KBO target