1. Structure and Evolution of Large Kuiper Belt Objects and Dwarf Planets
William B. McKinnon1, S. Alan Stern2, E. Asphaug3
1Dept. EPSC and McDonnell Center for the Space Sci.,Washington Univ., St. Louis, MO USA
2Southwest Research Inst., Boulder, CO USA
3UC Santa Cruz, CA USA
TNO 2006, Catania, 5 July 2006

2. The nature of the beast…
Jewitt & Sheppard 2002
Stern, McKinnon & Lunine 1997
r min
r max
Axial ratio ≥ 1.5:1:0.7 –>
1090 kg/m3 ≤ r ≤ 1270 kg/m3
The nature of the beast…

3. r min
r max
Sheppard & Jewitt 2002
≤ 1270 kg/m3
740
530
375

4. porosity vs. solid density 1999 TC36 Stansberry et al. 2006
1998 SM Spencer et al., this meeting
Dprimary ≈ 250–330 km; r = 0.35 –0.85 g cm-3
porosity
vs.
solid density

5. Porosity Pressures in large KBOs (r ≥ 100 km) non-negigible:
Pc = 30 MPa for spherical Varuna of ice density
Real geological rubble, regoliths have f ≈ 0.4
Cometary porosities (f ≥ 0.5) are possible ... for comets! R r
Carrier et al. 1991
Real geological rubble, regoliths have f ≈ 0.4
Cometary porosities (f ≥ 0.5) are possible ... for comets!

6. Phydro
Durham, McKinnon & Stern, 2005

7. Ice Compaction
DMS2005

8. EOS Comparison

9. ACM 2005

10. ACM 2005

11. Radius (km)
rrock = 2.63 g cm-3
ACM 2005

12. Phoebe Thermal Evolution

13. 2003 EL61 1999 TC36 Brown, Rabinowitz and coworkers Yes, Gerard,
there is a
Santa KBO…

14. 1999 TC36

15. Discussion
Range of believable densities outside the Pluto/Triton + porosity paradigm
Moderate to large KBOs have a range of densities/compositions-- classes?
Asteroid compositional classes – S, M, C, P, D, etc due to different factors:
Distance from Sun --> ice content
Internal differentiation and collisional evolution
Large ice-rich and rock-rich KBOs require strong collisional evolution
Pluto and EL61 imply different collisional environments
More constraints on formation models (Viva Niza!)
Much more to come on internal evolution of big KBOs!

16. rock/ice = 70/30

17. Accretion
tacc ~10 to 100 Myr for R ~100 to 1000-km (Pluto) radius KBOs (Kenyon & Luu 1999a,b)
Accretional heating, after Safronov and Kaula: r ≈ 103 kg/m3; rock/ice ≈ 60/40 & 40% porosity
Thermal conduction length
R ≥ 300 km KBOs
thermally& chemically
evolve during accretion

18. Composition I
Pluto-Charon and Triton have r ≈ 2000 kg/m3 & rock/ice ≈ 70/30
McKinnon et al. 1997

19. Composition II
High rock/ice (partly) explainable by CO sequestration of solar nebula O
McKinnon et al. 1997
Grevesse & Sauval 1998

20. Pluto Wars

21. Viscosity Strikes Back!

22. Revenge of the Rheology

23. Pluto: A New Beginning