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

Outline

•Crystalline water ice as a

signature of cryovolcanism

•Correlation of crystalline water

ice with KBO Size

•Thermal evolution models

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

Crystalline Water Ice = Sign of CryovolcanismCrystalline 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 H2O ice

•Corroborated by ammonia hydrates (Charon,Quaoar)

•Appears limited to KBOs with radii > 400-500 km

Only Large KBOs clearly have Crystalline Water Ice

2003 EL61

Quaoar

Charon

Orcus

2002 TX300

~ 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

2002 TX300

(Licandro et al 2006)

Charon (Cook et al 2007) Quaoar (Jewitt & Luu 2004)

Cook et al. (2007), in prep.

Small KBOs have Amorphous Water Ice

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

Object Radius Band Ratio

1996 TO66 (Brown et al. 1999)

S/2005 (2003 EL61) 1

Barkume et al. (2006)

1997 CU26

(Brown et al. 1998)

Hale-Bopp

(Davies et al. 1997)

C/2002 T7 (Kawakita et al. 2004)

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.

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

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 < T < Tliq)E.g.,

Thermal Evolution Models

•Rdiff =maximum radius to ever reach T > 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)

•Charon (63% rock, R = 604 km, Tsurf = 60 K) differentiates in < 70 Myr,

• Rdiff = 480 km, Rcore = 330 km (has half of Charon’s 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

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 ( < 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

Thermal Evolution Models: Results

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are needed to see this picture.

Thermal Evolution Models: Results

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are needed to see this picture.

•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

ArielLinear troughs = extensional stresses

Some terrains on Ariel < 100 million years old (Plescia 1989)

N2 frost

geysersgeysers driven by solid-state greenhouse effect

Triton

CH4 frost

more linear troughs from extensional stresses = “grabens”

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”