Inti didnt form in the X wind and neither did most CAIs

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SWRI presents a KUIPER BELT DOUBLE FEATURE
The Ice Volcanoes of Charon
It Came from the Inner Nebula
and
Starring STEVE DESCH, Arizona State University
School of Earth and Space Exploration. A NASA
Origins of Solar Systems production. Opens July
29, 2008.
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Mass Distribution and Planet Formation in the
Solar Nebula
Steve Desch School of Earth and Space
Exploration Arizona State University Lunar and
Planetary Science Conference March 12, 2008
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Outline

• Minimum Mass Solar Nebula
• Nice Model of Planet Migration
• Updated MMSN Model of Desch (2007)
• Implications for Disk Evolution, Particle
  Transport, and Planetary Growth
• Summary

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Minimum Mass Solar Nebula
It is essential to constrain the distribution of
mass in the solar nebula. Pressures in region
where meteoritic components formed (e.g.,
conditions of chondrule formation) Densities of
solids and gas in outer solar system (e.g.,
formation of giant planets) Distribution of
transport of mass (what caused the disk to
evolve?) Many authors developed Minimum Mass
Solar Nebula (Edgeworth 1949 Kuiper 1956
Safronov 1967 Alfven Arrhenius 1970
Weidenschilling 1977 Hayashi 1981 Hayashi et
al. 1985) Model of Weidenschilling (1977) is well
developed...
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MMSN H and He are added to planet masses until
they have solar composition, The augmented mass
then spread out over the annuli in which they
orbit. Surface density roughly ?(r)
r-1.5 Hayashi et al. (1985) widely used ?(r)
1700 (r / 1AU)-1.5 g cm-2 54 (r / 10
AU)-1.5 g cm-2
Weidenschilling (1977)
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A Few Problems with the MMSN
Densities in MMSN model are not consistent with
pressures expected for chondrule formation (Desch
Connolly 2002) Models of formation of Jupiters
core routinely have to increase solids densities
from canonical value (1 - 2 g cm-2) to 10 g
cm-2 (e.g., Pollack et al. 1996) Not possible to
explain formation of Uranus and Neptune cores
within lifetime of disk, while H and He gas are
available to accrete (Lissauer Stewart
1993). Underlying assumption of MMSN - that
planets current orbits reflect where mass was in
the solar nebula - is wrong! Planets migrated!
(Fernandez Ip 1984 Malhotra 1993). Turns out,
planets migrated a lot!! (Tsiganis et al. 2005)
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Planetary Migration

• The Nice Model (Tsiganis et al. 2005 Gomes et
  al. 2005 Morbidelli et al. 2005 Levison et al.
  2007, 2008) explains
• The timing and magnitude of Late Heavy
  Bombardment
• Giant planets' semi-major axes, eccentricities
  and inclinations
• Numbers of Trojan asteroids and irregular
  satellites
• Structure of Kuiper Belt, etc.
• IF
• Planets formed at 5.45 AU (Jupiter), 8.18 AU
  (Saturn), 11.5 AU (Neptune / Uranus) and 14.2 AU
  (Uranus / Neptune)
• A 35 M? Disk of Planetesimals extended from 15 -
  30 AU
• Best fits involve encounter between Uranus and
  Neptune in 50 of simulations they switch places

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Planetary Migration
21 resonance crossing occurs about 650 Myr after
solar system formation
r (AU) 5 10 15
20 25 30
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New Minimum Mass Solar Nebula
Desch (2007)
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New Minimum Mass Solar Nebula
Disk much denser! Disk much more massive 0.092
M? from 1-30 AU vs. 0.011 M? Density falls
steeply (as r-2.2) but very smoothly and
monotonically! Matches to lt 10!! Consistent
with many new constraints
Desch (2007)
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New Minimum Mass Solar Nebula
Mass distribution is not smooth and monotonic if
Uranus and Neptune did not switch orbits. Very
strong circumstantial evidence that Neptune
formed closer to the Sun
Desch (2007)
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New Minimum Mass Solar Nebula
Steep profile ?(r) 343 (r / 10 AU)-2.17 g cm-2
is not consistent with steady-state alpha
accretion disk (Lynden-Bell Pringle 1974)
Implies p 3/2 - q lt 3/2
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New Minimum Mass Solar Nebula
In fact, if ? r-p and T r-q and pq gt 2, mass
must flow outwards (Takeuchi Lin 2002)
Desch (2007) solved steady-state equations for
alpha disk (Lynden-Bell Pringle 1974) with an
outer boundary condition due to photoevaporation.
Found a steady-state alpha disk solution if
solar nebula was a decretion disk
Two parameters ? ( 3 x 10-4), and disk outer
edge rd ( 50 AU)
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New Minimum Mass Solar Nebula
Steady-state alpha decretion disk fits even
better. Applies in outer solar system (gt few
AU) Applies when large planetesimals formed and
dynamically decoupled from gas (a few x 105 yrs)
Small particles will trace the gas and move
outward in a few Myr
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Explains presence of CAIs in comets!
Comet 81P/Wild 2Scattered into present orbit in
1974 was previously a member of the Kuiper Belt
Scattered Disk Probably formed at 10-30 AU
Zolensky et al (2006)
Stardust Sample Track 25 called Inti. Its a
CAI, formed (by condensation) at gt 1700 K.
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New Model Explains Rapid Growth of Planet Cores

• Planets form closer to Sun in Nice model orbital
  timescales faster
• Density of solids higher than in traditional MMSN
• Higher gas densities damp eccentricities of
  planetesimals, facilitating accretion
• Desch (2007) calculated growth rate of planetary
  cores using formulism of Kokubo Ida (2002).
• Tidal disruption considered assumed mass of
  planetesimals 3 x 1012 g (R 0.1 km,
  i.e., comets).

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• Cores grow in 0.5 Myr (J), 2 Myr (S), 5-6 Myr (N)
  and 9-11 Myr (U)
• Even Uranus and Neptune reach 10 M? before H, He
  gas gone

Desch (2007)
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Masses of Solids in Planets
Inside 15 AU, planets limited by availability of
solids they achieve isolation masses
Outside 15 AU, planets cannot grow before gas
dissipates no gas no damping of eccentricities
Desch (2007)
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Summary
Past planet migration implies solar nebula was
more massive and concentrated than thought. Using
Nice model positions, Desch (2007) found new MMSN
model. Mass 0.1 M?, ?(r) r-2.2. Strongly
implies Uranus and Neptune switched
orbits. Cannot be in steady-state accretion but
?(r) is consistent with outer solar system as a
steady-state alpha decretion disk being
photo-evaporated at about 60 AU (like in
Orion) Dust (read Inti) would have moved from a
few AU to comet-forming zone in a few Myr All the
giant planet cores could reach 10 M? and accrete
H, He gas in lifetime of the nebula
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Cryovolcanism on Charon and other Kuiper Belt
Objects
Steve Desch Jason Cook now at SwRI, Thomas
Doggett, Simon Porter School of Earth and Space
Exploration Arizona State University
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Can KBOs experience cryovolcanism?

• A few words about cryovolcanism.
• A description of our model to calculate the
  thermal evolution of KBOs
• Results for Charon, including analysis of the
  physics
• Likelihood of subsurface liquid on other KBOs.
• Outline of a process for bringing liquid to the
  surface.

KBOs the size of Charon or larger can retain
subsurface liquid to the present day, and may
even be experiencing cryovolcanism, provided they
formed with moderate amounts of ammonia.
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Crystalline Water Ice Cryovolcanism?
Crystalline water ice observed on many large
KBOs Crystalline water ice is expected to be
amorphized by cosmic rays doses of 2-3
eV/molecule (Strazzulla et al. 1992 Mastrapa
Brown 2006), which takes lt 3 Myr in Kuiper Belt
(Cooper et al. 2003). Once amorphized, KBO
surfaces stay amorphous because of low
temperatures. Cook et al. (2007) reviewed
annealing mechanisms. Most favorable was
micrometeorite impacts, but all of them were
found unable to compete with cosmic-ray
amorphization.
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Crystalline Water Ice Cryovolcanism?

• Cook et al. (2007) intepreted crystalline water
  ice as diagnostic of cryovolcanism on KBOs. This
  would be incorrect IF
• Dust fluxes were gt an order of magnitude larger
  than interplanetary dust flux, as is possible in
  planetary environments. (2003 EL61 collisional
  family, too?)
• Real ices dont conform to experiments of
  amorphization

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Cryovolcanism?
Still, cryovolcanism does exist. Ariels surface
lt 100 Myr old (Plescia 1989), Tritons even
younger (Schenk Moore 2007)
Are these objects tidally heated, or are young
surfaces common on KBOs, too??
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Cryovolcanism needs ammonia
X NH3 / (H2ONH3). Maximum cosmochemical value
is X 15 (Lodders 2003). Models of
molecular cloud chemistry predict N2 is
efficiently dissociated, converted into NH3
(Charnley Rodgers 2002). Depletion of N2
recently confirmed observationally (Maret et al.
2007). Models predict 25 of all N in NH3
ices, for X 5 Observations of 9.3 micron band
of ammonia ice suggest X 5 - 10 (Gibb
et al. 2001, Gurtler et al. 2002), but are
disputed (Taban et al. 2003). Comets show X lt
1.5, but may be devolatilized. Ammonia content
of KBOs is unknown, but X 5 is not unreasonable
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Description of Model
Model updates internal energy in zone i
Qi(t) rate of heating by long-lived
radionuclides Fluxes into zone i (Fi-1) and out
of zone i (Fi) found assuming thermal conduction
Equation of state is used to convert E back
into temperature
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• Ammonia
• We use simplified phase diagram to include
  following phases
• Solid water ice
• Solid ammonia dihydrate (ADH)
• Liquid water
• Liquid ammonia
• Rock (analogs being ordinary chondrites)

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Ammonia
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Ammonia Energy added to each zone goes into
heating components via heat capacity, or into
latent heats due to phase transitions. Each
shell with mass M has energy E at the end of each
timestep. We then find temperature T and fraction
of mass in each (non-rock) phase that is
consistent with this E
k refers to regime in phase diagram
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Ammonia For example, in regime 1 (Tlt 176-dT K),
Similar (but much more complicated) expressions
apply to other regimes
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Ammonia For example, in regime 3 (176dT lt T lt
Tliq),
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Ammonia
Hunten et al (1984)
Just a few ammonia drastically lowers the
viscosity, especially once ADH melts.
Limit for meter-sized rocks to slip 10 km/Myr
Arakawa Maeno (1994)
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Differentiation
If the ice contains a few ammonia,
differentiation can occur wherever T gt 176
K Maximum radius at which T176 K ever
Rdiff Within Rdiff, we separate into rocky
core, then ADH ammoniawater slush layer,
then water ice on top. Undifferentiated rock-ice
crust lies outside Rdiff. ADH denser than its
melt, so slush layer well mixed we mix
compositions and internal energies after each
timestep (this mimics convection).
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Radiogenic Heating We consider heating by
long-lived radionuclides 235U, 238U, 232Th and
40K only.
Avg heating during first 1 Gyr 5 x Avg
heating during last 1 Gyr!
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Thermal conductivities Rock We use values
measured for ordinary chondrites at low
temperatures (100 - 500 K) by Yomogida Matsui
(1983) k 1.0 W/m/K, independent of
temperature Water Ice k 567 / T W/m/K (Klinger
1980) Ammonia Dihydrate (ADH)k 1.2 W/m/K
(based on Lorenz Shandera 2001) Water /
Ammonia Liquids assumed to be convecting k set
to high value k 40 W/m/K ConvectionWe check
for convection in water ice layer, but Ra ltlt 1000
in all models we ran no convection.
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Thermal conductivities Conductivities of non-rock
components combined using geometric mean, using
volume fractions Conductivities of rock and ice
components combined using percolation theory
formula of Sirono Yamamoto (2001)
Conductivity of undifferentiated rock-ice mixture
on Charon well described by k(T) 3.21 (T/100
K)-0.73 W/m/K
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Thermal conductivities
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• Results
• Canonical case, a Charon-like body
• R 600 km
• ? 1.7 g cm-3 (rock fraction 63)
• X 5
• Differentiation starts at t65 Myr, reaches
  fullest extent by 100 Myr
• Rdiff 516 km... half the mass differentiates

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t2 Gyr
t1 Gyr
slush layer
t3 Gyr
t4 Gyr
water ice layer
t4.6 Gyr
icerock crust
rocky core
t0 Gyr
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water ice layer
icerock crust
rocky core
slush layer
H2O(s)
rock
rock
H2O(l) NH3(l)
H2O(s)
H2O(s) ADH
ADH
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All ammonia within Rdiff liquid additional water
liquid created as temperatures rise.
Melted ADH
Temperatures in slush layer drop below 176 K
freezing starts at t 4.5 Gyr
Differentiation takes place within 70 Myr
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Present-day steady-state radiogenic heat flux at
surface would be F 1.216 erg cm-2 s-1.
Analytical estimate of temperature at base of
ice shell would be T 100 (0.961)5.435
exp(0.401) 129 K. Flux is enhanced over
steady-state radiogenic heat flux by amount ? F
by release of heat from rocky core. Temperatures
in ice shell and in undifferentiated crust
explained to within 1 by model with ?0.42.
Temperature at base of ice layer now predicted to
be T 100
(0.9610.063?)5.435 exp(0.4010.633?) 182
K. Release of heat from core found to enhance
flux by amount ?0.42 Release of stored heat from
core is significant!
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• Our model is highly favorable to maintenance of
  subsurface liquid
• Undifferentiated crust containing half the rock
  (as well as ADH) is thermally insulating
  (compared to pure water ice).
• Core containing the other half of the rock---and
  its radionuclides---concentrates and stores heat
• Release of stored heat and latent heat of
  freezing is significant, and demands a time
  evolution model.
• These physical effects would not be captured in a
  steady-state, fully differentiated model.

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61
T
P
M?, S?
E
C
Q?
O
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• Lots of uncertainties in freeze-out times
• Thermal conductivity of rock x2 variation
  changes freeze-out time by 0.3 Gyr
• Raising ammonia to X5 extends time of liquid by
  0.1 Gyr
• Inclusion of methanol would increase freeze-out
  time by about 1 Gyr!

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How does subsurface liquid surface?
Crawford Stevenson (1988) use linear elastic
fracture mechanics to show that the stress
intensity at the tip of a fluid-filled crack of
length l, extending from base of ice layer (top
of subsrface ocean), is
If this exceeds Kc 6 x 108 dyne cm-3/2, the
crack will self-propagate.
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How does subsurface liquid surface?
On Europa, ?? 1.00 g cm-3 - 0.92 g cm-3 gt 0,
and tension T is needed to initiate a crack. The
crack has a maximum possible length. In our
models, ?? 0.88-0.95 g cm-3 - 0.935 g cm-3 lt 0
(if liquid gt 230 K), and buoyancy can drive the
crack all the way to the surface. Trapped pockets
of liquid also create huge over-pressures when
they freeze (Fagents et al. 2003) Cracks will
propagate at several m/s (Crawford Stevenson
1988), reaching the surface in 1 day.
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How does subsurface liquid surface?
Cracks as small as 1 km can become
self-propagating within Charons ice
layer. Cracks are likely to be initiated during
freezing of slush layer, when its volume must
increase by 7. Displacement of 7 of 1022 g
of liquid over 2.5 Gyr would coat Charons
surface with water-ammonia ices to depth 5 cm /
Myr 350 um in only 7 kyr. Total depth 0.1 km
total). Heat flux carried to surface only 0.001
erg cm-2 s-1, too small to affect thermal
evolution.
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cracks form here
Conclusions
Basic structure of KBOs 400-800 km in radius
thermally insulating, undifferentiated rock-ice
crust
pure water ice layer, convects early on
ADH - ammonia - water layer
hot rocky core
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Conclusions

• Our models include time evolution, ammonia and
  differentiation. These are significant factors
  for thermal evolution of KBOs, and their effects
  are favorable for maintaining subsurface liquid.
• Rule-of-thumb for subsurface liquid today
• M gt 1024 g, ? gt 1.3 g cm-3, X gt 1-2
• Charon likely to have subsurface liquid.
• Liquid could be brought to surface via cracks,
  especially as bodies freeze (which is now for
  Charon)
• Obvious astrobiological implications can
  bacteria live in water thats 32 ammonia, and
  near -100ºC ??