Composition and Surface
Diversity
of the
Kuiper Belt objects
Audrey Delsanti
IFA - University of Hawai`i - NAI
An historical overview…
With naked eyes:
• Venus and Mercury
• Mars, Jupiter, Saturn
With telescopes:
• Uranus discovered in 1781 by William Herschell
• 1801: discovery of Ceres by Piazzi
• 1851: 15 objects known as the “Asteroid Belt”
• 1846: discovery of Neptune
• “Planet X” ?
Pluto
• discovered in 1930
• by Clyde Tombaugh
• at the Lowell Observatory
33cm telescope
• Tombaugh looked for other
objects for 13 years
The outer solar system: First ideas
• 1930, Leonard see Pluto as the first member of
a swarm of distant objects
• Edgeworth 1943, 1949
• Kuiper 1951
Independently described the
existence of a disk of a large number of small
(kilometer sized) objects beyond Neptune
The discovery of 1992 QB1
• August 1992 - Hawai`i
• UH 2.2m telescope
• Jewitt and Luu discovered
The first Kuiper Belt Object
Mauna Kea
The Kuiper Belt Objects
• Now, about 1,000 objects
~ 340
objects
!!!
have
beenlost
discovered
~ 230
objects
in of
critical
situation
(bright
end
the distribution)
-> strong need for follow up
and
!!! D>100km
~ 70recovery
000 objects
~ 10 objects D>1000 km
1999 KR16, D. Jewitt Website
• They might retain the most pristine material of the
Solar System
Current Outer
Solar System
view
Classical belt
Saturn
Plutinos
Scattered disk
Jupiter
Uranus
Neptune
Pluto
Centaurs
Comets
The giant Sedna
November 2003
Sedna’s orbit
The outer Solar System
?
Surface density profile
Ecliptic surveys
• Hainaut & Delsanti, survey 1999-2001
ESO 2.2m + 8x8K, 20 deg2 on sky, mR ~23, 40 new objects
• Trujillo et al. (2001)
CFHT 4m + 12K×8K, 73 deg2 on sky, mR ~ 23.7, 86 new objects
• Allen et al. (2001)
CTIO 1.5m + BTC, 1.5 deg2 on sky, mR~24.9-25.9, 24 new objects
No objects with
Perihelion > 50 UA
A truncature at 50 AU ?
• No objects
•
•
•
•
Truncature of proto-solar nebula by a passing star
Existence of a Martian-mass body, a~60 AU, 1Gy
Initial truncature at 30 AU + further migration
Other
• Objects
• “cold disk” ?
• Change of regime in albedo/size distribution ?
Studying Kuiper Belt Objects properties
• Faint (mV~18-26)
• distant objects
• spatially not resolved
• Difficult to observe
 4 to 8m class telescopes
needed
The bulk of physical information
comes from
• Broadband photometry
• (Spectroscopy)
HST image of 50000 Quaoar
(Brown & Trujillo, 2004)
In the visible & near IR domain
The surface color diversity of KBOs
ESO Large Program
Visible
Near-IR
Reflectivities
Meech & Jewitt (1986)
Normalization at 1.
In V band
Spectral slope
(%/100nm)
Visible near-infrared reflectivity spectra
The reddening curve
The surface color diversity
• Intrinsic different composition
• Same initial composition but different evolution
- Surface irradiation by high energy particles
(solar UV, cosmic rays, …)
- Non disruptive collisions between KBOs
- Cometary activity ?
Spectroscopic study of bright KBOs & Centaurs
KBOs
Centaurs
Constraints for KBO spectra modeling
1) The presence or absence of absorption bands
arising from
- minerals
- ices (H2O, CO, CO2, CH4, NH3, …)
- organic solids
2) The spectral range
3) The spectral gradient (ex: V-J color)
4) The surface albedo
KBO spectra modeling
Radiative transfert model (Douté & Schmitt, 1998)
Synthetic spectra of several geographical (spatial) mixtures
= linear combination of the spectra of the components
= juxtaposition of regions covered by a single component
 Collisions between KBOs
LIMITATIONS : the component grain size used should be
greater than the wavelength of the spectrum
THE SOLUTION IS NOT UNIQUE !!!
Organics signatures in Solar System objects
Ex: carbonaceous chondrites contain
• amino acids
• hydrocarbons
• insoluble polymers close to terrestrial kerogen
• nitrogen compounds
Ex: comets contain
• Methanol + more complex organics
(Bockelée-Morvan et al. 1995)
• Ethylene glycol in Hale-Bopp (Crovisier et al. 2004)
HOCH2CH2OH
Organics in Solar System objects
Organic compounds may be
- primordial
- or the result of on-going chemical reactions
Ex: KBO surface irradiation by high energy
particles (solar UV, cosmic rays, …)
Minerals and silicates
• Abundant on asteroids surfaces
• Enter in cometary grains composition
Ex: fosferite Mg2SiO4 (magnesium-rich olivine)
on comet Hale-Bopp
also crystalline pyroxenes, amorphous silicates
(Crovisier et al. 2000)
• centaur Pholus (Cruikshank et al. 1998)
(26181) 1996 GQ21
15 % Titan tholin
35 % Ice tholin
50 % Amorphous carbon
Visible albedo: 5%
Doressoundiram et al. 2003
Tholins: example of composition
Name
Initial Mixture
References
Titan tholin
90% N2 – 10% CH4 (gas)
Khare et al. (1984)
McDonald et al. (1994)
Triton tholin
99.9% N2 – 0.1% CH4 (gas) McDonald et al. (1994)
Ice tholin I
86% H2O – 14% C2H6
Khare et al. (1993)
McDonald et al. (1996)
Ice tholin II
80% H2O – 16% CH3OH
3.2% CO2 – 0.8% C2H6
McDonald et al. (1996)
(26375) 1999 DE9
… 99% kerogen
1% tremolite
pV = 2%
Scattered-disk object
Red visible colors
Neutral IR colors
__ 24% titan tholin
15% ice tholin
54% amorphous carbon
7% water ice
pV = 10%
Doressoundiram et al. 2003
Jewitt et al (2001):
• Water ice
• Hydroxyl group with
possible interaction with
an Al or Mg compound
Kerogen and water ice
2000 QC243 - Centaur
Suggestion of interpretation
for both objects
- 96-97 % kerogen
- 1% olivine
- 3-2% water ice
Dotto et al. 2003
1998 SG35 - Centaur
Other results
Water ice on 1999 TC36
(Plutino)
(90482) 2004 DW
VLT + FORS2
- 38% kerogen
- 7% water ice
- 55% amorphous carbon
Albedo 0.07 at 0.55 µm
VLT + ISAAC
April 11, 2004
Summary of the current situation
• Lack of surface albedos
• Geographical mixture: spectra modeling does not drive
to unique solutions
 intimate mixture models
• Lack of optical constants n & k for most components
 Laboratory experiments
THE END