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

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…

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

porosity vs. solid density 1999 TC36 Stansberry et al. 2006 1998 SM165 Spencer et al., this meeting Dprimary ≈ 250–330 km; r = 0.35 –0.85 g cm-3 porosity vs. solid density

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!

Phydro Durham, McKinnon & Stern, 2005

Ice Compaction DMS2005

EOS Comparison

ACM 2005

ACM 2005

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

Phoebe Thermal Evolution

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

1999 TC36

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!

rock/ice = 70/30

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

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

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

Pluto Wars

Viscosity Strikes Back!

Revenge of the Rheology

Pluto: A New Beginning