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