Transcript News from the Kuiper Belt Hermann Boehnhardt Max-Planck Institute for Solar System Research (Katlenburg-Lindau/Germany)

News from the Kuiper Belt
Hermann Boehnhardt
Max-Planck Institute for Solar System Research
(Katlenburg-Lindau/Germany)
Gloves & Moonboots On
Program of the Talk
– Intro: Kuiper Belt dynamics
– Physical Properties of TNOs (size & albedo &
surface structure, chemistry: colours, spectra)
– Kuiper Belt Evolution
– Binaries & Large KBOs
– Lessons from Pluto
Notation:
TNO = Transneptunian Object (Europe)
KBO = Kuiper Belt Object (USA)
for the talk: TNO = KBO
2
Important Notes
(not further explained)
• Kuiper Belt = remnant bodies from formation
period of solar system
• orbit dynamics controlled by Neptune
• KBO population is large in number, but small in
total mass
3
Outer Solar System:
Current Situation
Kuiper Belt:
Cubiwanos
Plutinos
Uranus
Neptune
Escaped from
Kuiper Belt:
ShortP. Comets
Centaurs
Scattered
4
Global Structure of the
Kuiper-Belt
Example from ESO TNO Survey: 1999 HU11
• KB: d ~ 30 – 55 AU
• Orbit: a = 30 – 48 AU
(… 80 …>100)
• Incl.: Ecliptic oriented
Peculiarities:
• Sharp outer edge (~50 AU)
• High inclination population
5
Plutinos, CDOs/Cubewanos,
SDOs
The KBO Zoo
– Resonant Population
Plutinos: a ~ 39 AU
e ~ 0.1 - 0.3
 2:3 Neptune resonance
– Classical Disk (CDOs) or
Cubewanos: a ~ 40-46 AU
e < 0.1
 outside of resonance
– Scattered Disk (SDOs):
a > 50 AU
q ~ 32 AU
 main populations in Kuiper Belt
6
The Extremists:
Centaurs & Detached Objects
– Centaurs: a ~ 5 - 32 AU
 inward scattered KBOs, gravitationally cascading orbits in
giant planet region
• Jupiter = great selector (either Jupiter family comet or
outward scattering)
• orbit lifetime ~ few million years
• JFCs = only comet family in solar system
– Detached Objects: a > 50 AU & q > 32 AU
• original SDOs got “detached” from Kuiper Belt by
gravitational interaction with passing object (star,
planetary embryo)
 larger (in number and in size) population expected
7
Who Has Stolen The Ice Cream?
“Missing” Mass & Extension of EKB
strawman model: SS mass density distribution
scaled with  Pic disk
 KB is (too) light/small
(0.2 Earth masses, but 40 needed for Pluto formation)
Scenarios: KB beyond 50 AU
 ‘wall’ of KBOs
 truncated disk
 ‘cold’ disk
Deep surveys: no classical disk objects
(Cubewano) beyond 50 AU (>30mag)

(a)
(b)
The important message:
solar system formation disk < 50 AU
change in physical properties > 50AU
8
Size & Albedo: Simple Principles
 FTNO ~ a R2/r4
reflected sunlight
steep r dependence
FTNO = Fo π R2 a p(φ) / (r2 Δ2)
 T ~ (1-a)1/4 r-1/2
thermal flux
Fo π R2 (1-a) / Δ2 = σ T4 4(2) π R2
weak a dependence
FTNO
= flux of TNO
independent of R
Fo
= solar flux
R
= radius
T
= temperature
a
= albedo
p(φ)
= phase function
r
= heliocentric distance
Δ
= distance to Earth
4(2)
= fast(slow) rotator
1998 SF36 Radius - Albedo
Radius (km)
0.30
0.20
0.10
0.00
0.05
0.15
0.25
0.35
0.45
0.55
Geom etric Albedo
visible_mean
visible_max
visible_min
MIR_mean
MIR_max
MIR_min
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Observing TNOs
Distance: > 32AU (Neptune)
Size: < 1000km
Brightness: > 20mag & < 3”/h
 reflected light: faint & slow
Temperature: 50 -- 70 K
 thermal: far IR & submm
Maximum R Filter Brightness of Solar System Bodies
10kmlow
100kmlow
1000kmlow
1kmhigh
10kmhigh
100kmhigh
1000kmhigh
T (fast)
T (slow)
Lambda (fast)
Lambda (slow)
1000
35
Radius
1km
30
10km
25
100km
20
1000km
Temperature
slow
fast
Temperature T [K]
R Filter Brightness [mag]
40
15
10
Searches&orbits: 2-4m
5
100
100
slow
10
fast
10
Wavelength of
Thermal Continuum
Peak
ISO, Spitzer,
Herschel, ALMA
Parameter = Albedo: low = 0.04 high = 0.50
Physical studies: 8-10m+HST
0
1
0.1
-5
0
10
20
30
40
50
Sun Distance [AU]
60
70
80
90
1.0
10.0
Thermal Continuum Peak
Lambda [micron]
1kmlow
Solar System Thermal Environment
1
100.0
Sun Distance [AU]
10
Like Dark Satellites
Sizes & Albedo
– HST direct imaging
Pluto & Charon, Sedna
– Visible & thermal-IR/submm
fluxes (see above)
 “normal” TNOs
~ dark planetesimals
(not quite as dark as comets)
 “big ones” ~ very high
reflectivity (ice surface)
 no clear trends found so far
11
BVRI Colour-Colour Plots
-10 Reddening [%/100nm]
+50
- spectral slope change towards red end of visible spectrum
- bi-modality in B-V vs V-R (Tegler&Romanishin 1997): no
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Between Blue And Red
Spectral Gradient Statistics
All ESO Data
Visible Wavelength
– Cubewanos: red population
with blue tail
b
Cu
ano
ew
18
16
14
12
10
8
6
4
2
0
s
os
t in
D.
ed
ter
t
a
Sc
85/95
75/85
65/75
55/65
45/55
35/45
25/35
5/15
15/25
-5/+5
<-25
-15/-5
– Plutinos&SDOs: moderately
red (comparable to comets)
rs
tau
n
Ce
-25/-15
Plu
Number of Objects
– diversity by dynamical type
Gradient Range [%/100nm]
– Centaurs: 2 colour groups
-5/+5
5/15
15/25
25/35
35/45
45/55
55/65
65/75
75/85
85/95
Centaurs
<-25
0
-25/-15 -15/-5
0
0
1
8
0
2
3
0
1
0
0
0
Scattered D.
0
0
0
1
11
7
5
0
0
0
0
0
0
Plutinos
0
0
0
3
9
9
4
0
0
0
0
0
0
Cubewanos
0
0
0
5
5
13
17
3
0
0
0
0
0
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What makes red cheeks
and gray faces?
Red: high-energy radiation
time scales: ~ 106 - 107 y
complications for high doses
Gray: impact resurfacing
time scales: ~ 106 - 107 y
ejecta coma: 10 - 100 d
(escape, impact)
Gray: intrinsic activity &
recondensation on surface
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Visible & Near-IR Spectroscopy
- spectra confirm photometric gradient determinations
15
Looking for Ice Cream
Oct, 2001
Surface Chemistry
– featureless vis. spectra
Sept, 2001
 reddening = wide absorpt.
- Tholins & amorphous carbons
for continuum
– H2O absorptions in IR  few
% in ~ ¼ of all objects
– heterogeneous surface
- big TNOs: CH4, N2, SO2
ices
16
Hot/Cold Cubewanos:
Compositional & Size Diversity
best explanation: population shift by planet migration
(not so good: scattering by proto-planet embryos / passing stars)
Hot
B-R vs. vrms : 3.3s
Pholus
Different at 99%
5°
Cold
600 Km
400 Km
200 Km
D-type
Asteroids
SPC
Sun
17
Liquid Water in the Kuiper Belt?
The Unexpected Surprise
– most KBOs with featureless vis.
spectra
– 3 Plutinos with weak dips in red
part of vis. spectrum
 wide absorption similar to
C asteroids?
 water alteration of
silicates!
2000 EB173
590 nm
740 nm
- liquid/gaseous water in KBOs?
- 26AL radioactivity from SN
explosion close to formation disk?
- dust mixing in protoplanet.
nebula
(Boehnhardt & de Bergh et al.)
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The Kuiper Belt Evolution
-
Sharp Edge at 50 AU: remnant from formation
 no stellar encounter < 100 AU since end of migration
- Evolution modeling:
Properties to be explained:
- dynamical families
- dyn. populations of CDOs (hot & cold) incl. orbital
parameter distribution
- outer edge of the Kuiper Belt at 50AU
- mass deficit of the Kuiper Belt (40 mEarth  0.2 mEarth)
- correlation of dynamical and physical properties (colors,
sizes)
- possible consequences for the inner solar system (late
heavy bombardment)
19
Disk Clean-Up & Heavy
Bombardment
giant planet disk  Oort Cloud
inner disk  craters on moon
early bombardment
late bombardment
(KBOs?)
20
The Nice Model
– planet migration due to scattering
of remnant disk bodies
 Jupiter inward
 others outward
- resonance and hot population
forms
- cold population remains untouched
- stop of migration when edge of
remnant disk is reached (@
32 AU)
- Jupiter/Saturn 2:1 resonance may
have produced late heavy
bombardment
21
The New Kuiper Belt Paradigm
early period
today
early period: hot Cubewanos (& Plutinos?) migrated to Kuiper Belt
 cold Cubewanos = original population
until today: hot & cold Cubewanos & Plutinos scattered inward
 two Centaurs color populations
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The TNO Binaries
From the observations:
– More than 50 double TNOs
(2 multiple systems
included)
13 with orbits measured
– Bound orbits within several
1000km distance (0.1-2”
separation, most close)
– Similar brightness (sizes)
of components
– Origin: formation (unlikely)
capture (favoured)
impact (likely for
small satellites of
large TNOs)
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The TNO Binaries
higher than
exponential
growth
First trends:
- Small objects “overabundant”
- cold CDOs have more
binaries
– large bodies seem to
have more binaries (?)
hot CDOs
– similar colors
 similar composition??
cold CDOs
24
The TNO Binaries
Physical properties:
mass determination through
Kepler’s law
Msys = 4π2a3/γT2
Msys with known albedo/size
 bulk density of objects or
system
- dense & light objects ??
(low statistics!)
 evolutionary effect ??
25
The Large TNOs
- large TNO ~ 1000km diam.
(Pluto, Sedna et al.)
- Sedna outside of Neptune
grav. influence
 larger (detached)
population still awaiting
discovery
- detachment processes:
- star encounter
- planet embryo
- large TNOs in all dynamical
classes except in cold CDOs
 cold CDOs and other TNOs
must have formed in different
environment
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The Large TNOs
– CH4, N2, CO dominated
spectra
cumulative number
 resurfacing due to
recondensation of (less
organic) volatiles from
temporary atmosphere
(gravity/temperature
balance)
 higher albedo (observed)
 deviation from expecting
power law
- large TNO = high bulk density (!?)
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The Degraded Planet And The Early Example
Pluto (since 1930) &
Charon (since 1978) &
Satellites (since 2005)
– Orbit:
Plutino-like
– Size:
large TNO
– Type:
multiple system
– Density:
~1.9 g/cm3
(not only ices)
– Albedo:
0.5/0,3
very high
(resurfacing)
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The Degraded Planet And The Early Example
- Surface:
non-uniform
– Chemistry:
Pluto: N2 ice
Charon: H2O ice
– Environment: temp.atmosphere
produced by
intrinsic activity
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New Horizons
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