Cryovolcanism on Charon and other Kuiper Belt Objects

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Cryovolcanism on Charon and other Kuiper Belt
Objects

• Steve Desch
• School of Earth and Space Exploration
• Arizona State University

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• Outline
• Observational Signatures of Cryovolcanism
• Correlation with KBO Size
• Thermal Evolution Models
• Astrobiological Implications

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Observational Signatures Imaging and Spectra

• We will make the claim that the spectral
  signature of crystalline water ice on KBOs is
  diagnostic of cryovolcanism.
• One (unnamed) researcher has scoffed at this,
  saying crystalline water ice is found on nearly
  all the icy satellites (e.g., Ariel), and we
  know those satellites are geologically dead.

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Observational Signatures Imaging

• Europa
• Young surface (few craters)
• Linear troughs

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Enceladus
Dione
Mix of heavily cratered and relatively uncratered
terrains
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Tethys
Rhea
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Titania
linear troughs
Miranda
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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
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Observational Signatures Imaging

• Imaging of icy satellites shows that resurfacing
  is very, very common they are not geologically
  dead.
• One common mode extensional stresses produce
  grabens, into which ice or liquid can flow
  (think mid-ocean ridge).
• Another observed mode lobate flows of liquid
  on the surface, or cryovolcanism.

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Observational Signatures Crystalline Water Ice
All water ice absorbs at 1.5 and 2.0
microns Crystalline water ice alone absorbs
strongly at 1.65 microns
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0.12 crystal-line
0.03 amorph -ous
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Why is crystalline water ice diagnostic of
cryovolcanism?

• Cosmic rays doses of 2 - 3 eV / molecule destroy
  the crystalline structure of water ice
  (Strazzulla et al. 1992 Mastrapa Brown 2006)
  this takes 1.5 Myr in the Kuiper Belt (Cooper et
  al. 2003).
• Heat can anneal the ice, but it takes gt 5 Gyr
  unless T gt 90 K (Kouchi et al. 1994 Schmitt et
  al. 1989).
• Solar UV photons also amorphize ice if they
  deliver the same dose (Leto Baratta 2003) .

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Why is crystalline water ice diagnostic of
cryovolcanism?
Ice amorphized in lt 105 years!
Cook et al. (2007), ApJ
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• Crystalline water ice on KBOs indicates a surface
  that has been replenished or transientally heated
  (to gt 100 K) in the last 105 years.
• Possible mechanisms (besides cryovolcanism)
• Impact gardening uncovering unirradiated
  crystalline ice
• Frost created by micrometeorites vaporizing ice
• Transient heating of ice by micrometeorite
  impacts
• Solid-state greenhouse effect
• Solid-state convection
• All were reviewed by Cook et al. (2007) and found
  not to work for KBOs.

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Only mechanism that comes close is transient
heating by kinetic energy of impact by
micrometeorites.
At average impact speed of 1.8 km/s (Zahnle et
al. 2004), each particle can anneal 10 times its
mass of ice (Cook et al. 2007). Mass flux
observed by Pioneer 10 (Humes 1980) , scaled to
1.8 km/s, yields flux 2.4 x 10-17 g cm-2 s-1.
Can anneal to depths probed by H and K on 1/e
timescales of 3.1 Myr.
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Observational Signatures Ammonia Hydrates
Ammonia hydrates have absorption feature at 2.21
microns. These should be decomposed by cosmic ray
doses of 100 eV / molecule (Strazzulla Palumbo
1998). This should take about 20 Myr in the
Kuiper Belt (Cooper et al. 2003). Their presence
requires a physical replenishment.
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Observational Signatures

• Ammonia hydrates on surface show surface material
  has been replenished in last 20 Myr.
• Crystalline water ice on surface shows that
  surface has been transientally heated or
  replenished in the last 105 years.
• Annealing by micrometeorite impacts might just
  barely be competitive with amorphization by
  cosmic rays, but it would then apply equally to
  KBOs of all sizes...

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Correlation with KBO Size
Strength of the absorption feature of crystalline
water ice at 1.65 microns correlates with KBO
size. The largest KBOs with water ice features
have crystalline water ice. No smaller objects
(KBOs, comets and Centaurs) have crystalline
water ice.
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Charon
Radius 603.6 km (Sicardy et al. 2006) Surface
has crystalline (band ratio 0.13) water ice and
ammonia hydrates (Brown Calvin 2000 Dumas et
al. 2001 Cook et al. 2007)
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Cook et al. (2007), ApJ, in press
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Water ice clearly crystalline Hapke models
including amorphous water ice call for gt 95
crystalline
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Feature of ammonia hydrates at 2.21 microns
present. Variations in hydration state?
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Quaoar
Radius 630 /- 95 km (Brown Trujillo
2004) Surface has crystalline water ice (we
compute band ratio 0.12), and ammonia hydrates
(Jewitt Luu 2004)
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Quaoar
Jewitt Luu (2004)
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2003 EL61
Radius 980 x 759 x 498 km
(Rabinowitz et al. 2006) Triaxial ellipsoid
because of rapid rotation consistent with mean
density 2.6 g cm-3 (!) 2003 EL61 is largest
member of a collisional family (Brown et al.
2007) collision probably the cause of the rapid
rotation.
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2003 EL61
Mean density consistent with mass fraction of
rock 0.90(!). Water and other volatiles
probably lost during collision? Surface has water
ice that is apparently weakly (band ratio 0.06)
crystalline (Barkume et al. 2006)
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Barkume et al. (2006)
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Orcus
Radius 600 55/-90 km (Lykawka Mukai
2005) Surface has water ice, apparently
crystalline (de Bergh et al. 2005)
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de Bergh et al. (2005)
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2002 TX300
Radius upper limit 555 km (3 sigma), or 455 km
(2 sigma) (Ortiz et al. 2004) Surface has water
ice, apparently crystalline (Licandro et al.
2001 Cook et al. 2007, in prep.) If confirmed,
2002 TX300 would be the smallest KBO with
crystalline water ice on its surface.
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Licandro et al. (2006)
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Cook et al. (2007), in prep.
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1996 TO66
Radius 325 km (Hainaut et al. 2000) Surface has
water ice, but probably amorphous (we find band
ratio 0.04). Brown et al. (1999) say the
weakness or absence of this the 1.65 micron
band in our data is consistent with amorphous
water ice rather than crystalline water
ice... 1996 TO66 is the largest KBO not to have
crystalline water ice.
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1.65 micron feature very weak.
Brown et al. (1999)
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S/2005 (2003 EL61) 1
Radius about 160 km (Barkume et al. 2006) Surface
has water ice, but not obviously crystalline we
estimate band ratio 0.03
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Barkume et al. (2006)
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Centaur 1997 CU26 (Chariklo)
Radius 118 /- 6 km (Groussin et al.
2004) Surface has water ice that is clearly
amorphous (we estimate band ratio 0.03). Brown
et al. (1998) say within the precision of the
full-resolution data... the weakness of this
1.65 micron band is consistent with mostly
amorphous rather than mostly crystalline water
ice on 1997 CU26.
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Brown et al. (1998)
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1.65 micron feature very weak
Brown et al. (1998)
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C/1995 O1 Hale-Bopp
Radius 30 km (Fernandez 2000) Water ice
detected while comet at 7 AU, but not
crystalline (we estimate band ratio 0.03).
Davies et al. (1997) say The absence of the
1.65 micron absorption feature is therefore
suggestive of the presence of amorphous ice.
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models including crystalline water ice do not
match at 1.65 microns
1.4 1.6 1.8 2.0 2.2 2.4
Davies et al. (1997)
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C/2002 T7
Radius 10 km? Water ice detected while comet at
3.5 AU, but not crystalline (we estimate a band
ratio 0.03). Kawakita et al. (2004) say The
absence of the 1.65 micron absorption feature of
crystalline ice may indicate that the cometary
ice was in an amorphous state.
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Kawakita et al. (2004)
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Kawakita et al. (2004)
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Correlation with KBO Size
Only the largest KBOs (R gt 600 km) have
crystalline water ice (Quaoar, Charon, Orcus,
2003 EL61?) or ammonia hydrates (Quaoar,
Charon) No large KBOs are clearly amorphous. Only
small objects (R lt 350 km) are clearly amorphous
1996 TO66, S/2005 (2003 EL61) 1, Chariklo,
Hale-Bopp, C/2002 T7. No small objects are
clearly crystalline.
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Correlation with KBO Size
Correlation of crystalline water ice 1.65 micron
feature with KBO size is not predicted if ice is
annealed by micrometeorite impacts. Correlation
with KBO size is predicted if ice is replenished
by cryovolcanism, which requires a minimum KBO
size. Observations suggest the minimum size is gt
350 km but lt 600 km 2002 TX300 (radius 455 - 555
km) may be near minimum.
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Thermal Evolution Models
Cryovolcanism requires presence of subsurface
liquid water (273 K), or water-ammonia (176 K),
and a means of rapid delivery to the
surface. Delivery is possible via
self-propagating cracks (Crawford Stevenson
1988). Presence of subsurface liquid harder to
constrain there are no adequate thermal
evolution models of KBOs.
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Delivery of Liquid to Surface
According to Crawford Stevenson (1988), using
linear elastic fracture mechanics, the stress
intensity at the tip of a fluid-filled crack
extending from a subsurface ocean up a distance l
is
When this exceeds Kc 6 x 108 dyne cm-3/2, the
crack is self-propagating.
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Delivery of Liquid to Surface
On Europa, ?? 1.00 g cm-3 - 0.92 g cm-3 gt 0,
and tension T is needed to initiate crack, but
crack is self-limiting. On KBOs, ?? lt 0 likely,
and cracks gt 1 km long will propagate all the way
to the surface.
Cracks 1 m wide likely, propagating at a few
m/s, reaching the surface in tens of hours.
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Thermal Evolution Models
Does subsurface liquid exist? We need
time-dependent thermal evolution models that
include differentiation. Thermal evolution models
exist for the saturnian icy satellites, without
differentiation (Ellsworth Schubert 1983
Multhaup Spohn 2007). Differentiated but
steady-state models exist for KBOs (Hussmann et
al. 2006) and the Galilean satellites (Spohn
Schubert 2003).
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Thermal Evolution Models
Hussmann et al. (2006) consider a rocky core
overlain by an icy shell. Radiogenic heating in
core, Q, creates flux F conducted through shell
Integration inward from surface, with known
temperature, yields T(r). They find R gt 800 km
needed for T gt 176 K outside of rocky core.
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Thermal Evolution Models
Actually, Hussmann et al. (2006) should have
found R gt 1050 km is needed, but they assumed k
3.3 W m-1 K-1 without reference. Ice
conductivity is closer to k 567/T W m-1 K-1
(Klinger 1980) and would climb to 11 W m-1 K-1
near surface.
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Thermal Evolution Models
All steady-state models (even those with correct
conductivities) will fail to include stores of
energy thermal inertia, phase transitions. Radiog
enic heating (esp. by 40K, with t1/2 1.3 Gyr)
was 10 times more intense 4.56 Gyr ago than
today Most of Earths heat flux is from release
of latent heat of crystallization---using
radiogenic heating alone is wrong!
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Thermal Evolution Models
Thermal evolution models exist, but none include
differentiation of rock and ice. In an
undifferentiated body, Q r3 throughout. In a
differentiated body, Q r3 in core, but is
constant outside the core
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Thermal Evolution Models
My models Planet subdivided into about 100
radial zones. Within each zone, 5 phases are
possible rock, H2O(s), ADH, H2O(l), NH3(l)
(ammonia concentrations few - 15 assumed). As
ammonia-water-rock mixture is heated from 0 K,
the systems gains energy measured by integrating
heat capacities, and including latent heats.
Ammonia-water phase diagram used.
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Thermal Evolution Models
My models For a given energy E and ammonia
concentration and rock fraction in each zone, we
find the temperature T consistent with the
mixture, and the equilibrium abundances. That
is, T T(E).
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Thermal Evolution Models
My models Solid-state creep assumed to be
important when T gt 174 K. ADH assumed to melt
174 lt T lt 178 K. Rdiff largest r inside which T
gt 174 K ever. Inside Rdiff, rock sinks to form
core H2O(s) floats to form icy shell
ammonia-rich slush in between. Components carry
their internal energies while differentiating.
Gravitational potential energy included.
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Thermal Evolution Models
My models Slush layer will convect vigorously
(but I gave it finite conductivity for now). Ice
layer could convect but does not for the bodies
considered today (R lt 600 km) Ra lt 10.
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Thermal Evolution Models

• Results
• For Charon (rock fraction 0.63), R 604 km,
  differentiation takes place in 70 Myr, yielding
  Rdiff 480 km and a core with half of Charons
  rock and a radius of 330 km.
• All ADH in system melts, yielding a peritectic
  melt. Assuming an ammonia concentration of 5, 4
  x 1022 g of liquid is produced.

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Thermal Evolution Models

• Core temperatures reach maximum (gt 1400 K) at
  about 2.0 Gyr, and decline thereafter.
• Thermal inertia of core significantly enhances
  fluxes (by about 50) thereafter.
• Subsurface ammonia-rich liquid maintained to
  present day.
• Rock-rich, undifferentiated layer (? 1.7 g
  cm-3) over NH3-rich liquid (? 0.9 g cm-3)
  yields ?? lt 0, aiding crack development and
  propagation.

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Thermal Evolution Models

• Results
• Temperature outside core never reaches 273 K
  amount of liquid produced is directly
  proportional to ammonia inside Charon.

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Thermal Evolution Models
Results Calculations repeated for same rock/ice
fraction but smaller radiiAll liquid freezes on
400 km KBO after 3.2 Gyr. Liquid on 500 km KBO
freezes after 4.4 Gyr. Full exploration of
parameter space to come, but 500 km radius is the
likely lower limit to have liquid (and
cryovolcanism) today.
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Astrobiological Implications
Subsurface liquid probably exists on Charon,
Quaoar, Orcus, and many other KBOs. (About 1023
g of liquid on these 3 alone!) The liquid is
very ammonia-rich, near eutectic composition of
32 NH3. Biology in this regime is unexplored.
Liquid lies over a very hot, porous rock
core and hydrothermal circulation is expected.
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Astrobiological Implications
New Horizons will not see Charons water
directly, but we expect it to find relatively
uncratered terrains, and lobate flows like on
those on Triton.
Charon is probably the prototype of similarly
sized KBOs. If New Horizons observes
cryovolcanism on Charon, it means liquid is very
common in the Kuiper Belt, today.
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Conclusions
Crystalline water ice indicates a surface lt 105
years old its presence indicates a resurfacing
process. All processes besides cryovolcanism
shown not to work the process is correlated with
KBO size, with a cutoff between 325 and 555 km,
strongly suggesting cryovolcanism. Cryovolcanism
requires liquid. We have constructed thermal
evolution models to see if subsurface liquid can
exist on KBOs.
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Conclusions
Our models are the first time evolution models to
include differentiation, among other significant
improvements. On small KBOs, no liquid is
produced without ammonia. For Charon-like rock
fraction and 5 ammonia concentration, we find a
minimum radius of 500 km to have liquid today,
consistent with observations.
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