Angular Momentum in the Angular Momentum in the Kuiper BeltKuiper Belt

Scott S. Sheppard

Carnegie Institution of WashingtonDepartment of Terrestrial Magnetism

Main Asteroid Belt24 > 200 km

Trojan Asteroids2 ~ 200 km

Kuiper Belt10,000 > 200 km

Gravitational Self Compression > Material Strength

Primordial Distribution of Angular Momentum

For Diameters > 200 km:

- Early Collisional environment

Size Comparison of Rocky/Icy Bodies in the Solar System

Dynamical Classes in the Outer Solar System

Dynamically Disturbed and Collisionally Processed.

Sedna

Plan View of the Kuiper Belt

Brightest KBO is19th magnitude

Diameter > 200 kmMag < 22.5

Overview of DataSample of over 40 large KBOs

1) Light curves

2) Phase curves

UH 2.2m

-> Shapes-> Surface Characteristics-> Densities-> Binaries-> Angular Momenta-> Outgassing

du Pont 2.5m

Short and Long Term Variations

Short-term Variability

2000 GN171

1. Albedo2. Elongation3. Binary

2000 GN171 period = 7.9 hours

29% > 0.15 mags18% > 0.40 mags12% > 0.60 mags

KBOs (40 in sample)

1. Albedo effects are usually only 10 to 20% (Degewij et al. 1979)

2. Elongation

Rotational Triaxial Ellipsoids (Jacobi Ellipsoids)

Fast Rotations < 7 hours

For large objects (> 200 km)

Spherical Gravitational Compression > Material Strength

Triaxial elongation from rotational angular momentum

(Leone et al. 1984)

P = (3 Pi / G rho)1/2

crit

Centripetal acceleration = gravitational acceleration

As angular momentum increases an object will go frombeing a sphere to biaxial to

a/b = 100.4 x delta mag

Axis Ratio from rotationallight curve:

Period and amplitude can be related to an objects density

Varuna

Jewitt and Sheppard 2002

Varuna Density ~ 1100 kg/m 3

Assume Rotationally distortedStrengthless Rubble Pile

Cosmochemically PlausibleRock Fraction ~ 0.5

Porosity ~ 10 to 20%

Chandrasekhar 1987Leone et al. 1984

1999 TC36

3. Eclipsing Binaries

-Probability of eclipse events to our line of sight decreases as the separation increases

-Tidal interactions distort close components

Photometric Range Max ~ 0.75 mags

Photometric Range Max ~ 1.2 mags(Leone et al. 1984)

(Trujillo and Brown 2002)

2001 QG298 Period = 13.7744 hoursRange = 1.1 mags

Diameter ~ 250 km

2001 QG298 HektorKleopatra

2001 QG298 is only the 3rd known minor planet with diameter > 50 km and a photometric range > 1 magnitude

Trojan Asteroid 624 Hektor

Main Belt Asteroid 216 Kleopatra

KBO 2001 QG298

Merline, Dumas and Menard 1999

CFHT Adaptive Optics images of Kleopatra

Sheppard and Jewitt 2004

Comparison of Large Main Belt Asteroids and Kuiper Belt Objects

Margot 2002

Comparison of typical binary systems within the Solar System.

100 km

100 km

1 km

20,000 km

1000 km

2.5 km

Funato et al. 2004

KBO Binary Formation Mechanisms:

TidalDisruption

DirectCollisions

Three BodyInteractions

Known Binaries of Large Minor Planets in the Solar System

Does a largeangular momentumof the primarycorrespond tosatellite formation?

Current angularMomentum ofLarge objects hintsAt an earlier denserKuiper Belt.

Maybe 100 times more dense.

Noll et al. (2002) found about 4% of KBOs were binary with separations > 0.15”

We find 5 of 34 KBOs are in the close, similar component, eclipsing binary region (15%)

Consistent with Goldreich et al. (2002) model of binary formation but not with the Weidenschilling model (2002)

Collisionless interactionsIn a denser Kuiper BeltDuring the formation epoch.

- Dynamical Friction would create more close in binaries

(Because of projection effects, the fraction may be much larger)

Conclusions- Many Kuiper Belt Objects have large amplitude light curves

- Some may be rotationally deformed rubble piles- Many are probably contact or nearly contact binaries

- Kuiper Belt must have been about 100 times more dense in the distant past to explain current amount of angular momentum we see.

- Binary formation is still unclear, but direct collisions may have be an important factor.

Short and Long Term Variability

Consecutive Nights

Multiple Months

Absolute Photometry

Mag = Msun – 2.5 log(albedo x radius x phase / heliocentric x geocentric )2 2 2

Period = 8.08 hours

Period = 4.04 hours

1995 SM55

V-R=0.38

Single-peak

Double-peak

Binary or Cometary orComplex Rotation?

t = u Q / p K r w

Damping time scale

2 3

u is rigidityQ is ratio energy in oscillation to that lost p is the densityK is irregularity of bodyr is the radiusw is angular frequency

1. Nonuniform Surface Markings

Iapetus

Photometric Range ~ 0.3 mags

Photometric Range ~ 2 mags

B – V ~ 0.1 mags (Millis 1977)

-synchronous rotation

-atmosphere

20000 Varuna Rotational Lightcurve (diameter ~ 900 km)

Period = 6.3442 hours

20000 Varuna: Found No Color Variation with Rotation

Asteroid and KBO Limiting Densities

Sheppard and Jewitt 2002

5 KBOs can not be easily explained from albedo or rotational elongation