Structure and Evolution of Large Kuiper Belt Objects
Structure and Evolution of Large Kuiper Belt Objects and Dwarf Planets William B. Mc. Kinnon 1, S. Alan Stern 2, E. Asphaug 3 1 Dept. EPSC and Mc. Donnell Center for the Space Sci. , Washington Univ. , St. Louis, MO USA 2 Southwest Research Inst. , Boulder, CO USA 3 UC Santa Cruz, CA USA TNO 2006, Catania, 5 July 2006
r min r max Axial ratio ≥ 1. 5: 1: 0. 7 –> 1090 kg/m 3 ≤ r ≤ 1270 kg/m 3 Stern, Mc. Kinnon & Lunine 1997 The nature of the beast… Jewitt & Sheppard 2002
r min r max r ≤ 1270 kg/m 3 740 530 375 Sheppard & Jewitt 2002
Stansberry et al. 2006 1998 SM 165 1999 TC 36 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: R r • 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! Carrier et al. 1991
Phydro Durham, Mc. Kinnon & Stern, 2005
Ice Compaction DMS 2005
EOS Comparison
ACM 2005
ACM 2005
rrock = 2. 63 g cm-3 ACM 2005 Radius (km)
Phoebe Thermal Evolution
2003 EL 61 1999 TC 36 Brown, Rabinowitz and coworkers Yes, Gerard, there is a Santa KBO…
1999 TC 36
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 EL 61 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 1999 a, b) • Accretional heating, after Safronov and Kaula: r ≈ 103 kg/m 3; 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/m 3 & rock/ice ≈ 70/30 Mc. Kinnon et al. 1997
Composition II • High rock/ice (partly) explainable by CO sequestration of solar nebula O Grevesse & Sauval 1998 Mc. Kinnon et al. 1997
Pluto Wars
Viscosity Strikes Back!
Revenge of the Rheology
Pluto: A New Beginning
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