1. Cryovolcanism on Charon and other Kuiper Belt Objects Steve Desch Jason Cook, Wendy Hawley, Thomas Doggett School of Earth and Space Exploration Arizona State University

2. Outline Crystalline water ice as a signature of cryovolcanism Correlation of crystalline water ice with KBO Size Thermal evolution models

3. Some KBOs have Crystalline Water Ice on their Surfaces; Some have Amorphous Ice Mastrapa & Brown (2005) 0.03 0.12 Band Ratio 1.65 micron feature is diagnostic

4. Crystalline Water Ice = Sign of Cryovolcanism  Crystalline water ice is rapidly amorphized in ~ 1 Myr by cosmic rays (Cooper et al. 2003) in < 0.1 Myr by solar UV (Cook et al. 2007) Heating and annealing of ice by micrometeorites? Takes > 3 Myr (Cook et al. 2007) Would work equally on KBOs of all sizes Cryovolcanism most likely source of crystalline H 2 O ice Corroborated by ammonia hydrates (Charon,Quaoar) Appears limited to KBOs with radii > 400-500 km

5. Only Large KBOs clearly have Crystalline Water Ice 2003 EL 61 Quaoar Charon Orcus 2002 TX 300 ~ 725 km 630 +/- 95 km 603.6 km 600 +55/-90 km < 555 km (3  ) 0.06 0.12 0.13 ~ 0.12 ~ 0.1 ? Object Radius Band Ratio

6. 2002 TX 300 (Licandro et al 2006) Charon (Cook et al 2007) Quaoar (Jewitt & Luu 2004)

7. Cook et al. (2007), in prep.

8. Small KBOs have Amorphous Water Ice 1996 TO 66 S/2005 (2003 EL 61 ) 1 1997 CU26 Hale-Bopp C/2002 T7 ~ 325 km ~ 160 km 118 km ~ 30 km ~ 10 km 0.04 0.03 Object Radius Band Ratio

9. 1996 TO 66 (Brown et al. 1999) S/2005 (2003 EL 61 ) 1 Barkume et al. (2006) 1997 CU26 (Brown et al. 1998)

10. Hale-Bopp (Davies et al. 1997) C/2002 T7 (Kawakita et al. 2004)

11. Cryovolcanism on KBOs If less dense than overlying layers, subsurface liquid easily rises to surface via self-propagating cracks (Crawford & Stevenson 1988). Subsurface liquid requires T > 273 K (pure water ice) or T > 176 K (with ammonia) Internal temperatures of KBOs modeled using a thermal evolution code we wrote.

12. Thermal Evolution Models 1-D spherical geometry (~ 100 zones) Radiogenic heating from 40 K, 235 U, 238 U, 232 Th Conductive fluxes in and out of each zone Conductivities k(T), T(E) depend on material

13. Thermal Evolution Models Five phases: rock; H 2 O(s); ADH; H 2 O(l); NH 3 (l) Given heat capacities, latent heats, rock / H 2 O / NH 3 fractions, and total internal energy E, we find ice phases, temperature T(E) (176 < T < Tliq)E.g.,

14. Thermal Evolution Models R diff =maximum radius to ever reach T > 174 K (ADH melts; ice creeps). Inside R diff, rock sinks to core, water ice floats, liquid/ADH slush in between. Thermal conductivities of ordinary chondrites (Yomogida & Matsui 1983), ADH (Lorenz & Shandera 2001) used; combined using Sirono &Yamamoto (2001)

15. Charon (63% rock, R = 604 km, T surf = 60 K) differentiates in < 70 Myr, R diff = 480 km, R core = 330 km (has half of Charon’s rock) All ADH inside R diff melts, yielding 4 x 10 22 g (if X = 0.05) of NH 3 -rich (32%) liquid. Core temperature rises until t = 2.0 Gyr, to 1300 K, steadily declines thereafter Thermal Evolution Models: Results

16. Release of heat from core over next 2.5 Gyr increases average flux from 1.0 to 1.5 erg cm -2 s -1 Temperature just outside core maintained above 176 K until about 4.8 Gyr: has liquid today NH 3 -rich liquid much less dense (  < 0.9 g cm -3 ) than overlying rocky layers (  ~ 1.7 g cm -3 ), and is positively buoyant. Temperatures outside core always < 273 K: liquid only possible with ammonia. Thermal Evolution Models: Results

19. Calculation repeated for smaller bodies: R = 600 km (Charon): liquid until 4.8 Gyr R = 500 km maintains liquid until 4.4 Gyr R = 400 km maintains liquid until 3.2 Gyr Cryovolcanism possible, today, on Charon, Quaoar & Orcus Minimum radius needed close to 500 km, consistent with observations of crystalline ice. Thermal Evolution Models: Results

20. Ariel Linear troughs = extensional stresses Some terrains on Ariel < 100 million years old (Plescia 1989)

21. N 2 frost geysers geysers driven by solid-state greenhouse effect Triton CH 4 frost

22. more linear troughs from extensional stresses = “grabens”

23. From NASA Photojournal. Original caption says: two depressions (impact basins?) extensively modified by flooding, melting, faulting and collapse, several episodes of filling and partial removal of material. Hardly any craters. 500 km “lobate flows”