Cryovolcanism on Charon and other Kuiper Belt Objects

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
0.12
Band Ratio
0.03
1.65 micron feature is diagnostic
Mastrapa Brown (2005)
4
Crystalline Water Ice Sign of Cryovolcanism

• Crystalline water ice is rapidly amorphized
• in 1 Myr by cosmic rays (Cooper et al. 2003)
• in lt 0.1 Myr by solar UV (Cook et al. 2007)
• Heating and annealing of ice by micrometeorites?
• Takes gt 3 Myr (Cook et al. 2007)
• Would work equally on KBOs of all sizes
• Cryovolcanism most likely source of crystalline
  H2O ice
• Corroborated by ammonia hydrates (Charon,Quaoar)
• Appears limited to KBOs with radii gt 400-500 km

5
Only Large KBOs clearly have Crystalline Water
Ice
Object Radius Band
Ratio
2003 EL61 Quaoar Charon Orcus 2002 TX300
725 km 630 /- 95 km 603.6 km 600 55/-90 km lt
555 km (3?)
0.06 0.12 0.13 0.12 0.1 ?
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Charon (Cook et al 2007)
Quaoar (Jewitt Luu 2004)
2002 TX300 (Licandro et al 2006)
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Cook et al. (2007), in prep.
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Small KBOs have Amorphous Water Ice
Object Radius
Band Ratio
1996 TO66 S/2005 (2003 EL61) 1 1997
CU26 Hale-Bopp C/2002 T7
325 km 160 km 118 km 30 km 10 km
0.04 0.03 0.03 0.03 0.03
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1997 CU26 (Brown et al. 1998)
1996 TO66 (Brown et al. 1999)
S/2005 (2003 EL61) 1 Barkume et al. (2006)
10
C/2002 T7 (Kawakita et al. 2004)
Hale-Bopp (Davies et al. 1997)
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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 gt 273 K (pure water
  ice) or T gt 176 K (with ammonia)
• Internal temperatures of KBOs modeled using a
  thermal evolution code we wrote.

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Thermal Evolution Models

• 1-D spherical geometry ( 100 zones)
• Radiogenic heating from 40K, 235U, 238U, 232Th

• Conductive fluxes in and out of each zone

• Conductivities k(T), T(E) depend on material

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Thermal Evolution Models

• Five phases rock H2O(s) ADH H2O(l) NH3(l)
• Given heat capacities, latent heats, rock / H2O /
  NH3 fractions, and total internal energy E, we
  find ice phases, temperature T(E)

(176 lt T lt Tliq)
E.g.,
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Thermal Evolution Models

• Rdiff maximum radius to ever reach T gt 174 K
  (ADH melts ice creeps).
• Inside Rdiff, 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)

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Thermal Evolution Models Results

• Charon (63 rock, R 604 km, Tsurf 60 K)
  differentiates in lt 70 Myr,
• Rdiff 480 km, Rcore 330 km (has half of
  Charons rock)
• All ADH inside Rdiff melts, yielding 4 x 1022 g
  (if X 0.05) of NH3-rich (32) liquid.
• Core temperature rises until t 2.0 Gyr, to
  1300 K, steadily declines thereafter

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Thermal Evolution Models Results

• 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
• NH3-rich liquid much less dense (? lt 0.9 g cm-3)
  than overlying rocky layers (? 1.7 g cm-3), and
  is positively buoyant.
• Temperatures outside core always lt 273 K liquid
  only possible with ammonia.

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Thermal Evolution Models Results
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Thermal Evolution Models Results
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Thermal Evolution Models Results

• 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.

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Linear troughs extensional stresses
Ariel
Some terrains on Ariel lt 100 million years old
(Plescia 1989)
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N2 frost
CH4 frost
geysers driven by solid-state greenhouse effect
Triton
geysers
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more linear troughs from extensional stresses
grabens
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lobate flows
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