Transcript Inti didn`t form in the X wind (and neither did most CAIs)

SWRI presents a KUIPER BELT DOUBLE FEATURE…
It Came from
the Inner
Nebula
and
The Ice
Volcanoes
of Charon
Starring STEVE DESCH, Arizona State University School of
Earth and Space Exploration. A NASA Origins of Solar
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
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
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...
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)
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 Jupiter’s 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)
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
Planetary Migration
2:1 resonance crossing occurs about 650 Myr
after solar system formation
r (AU)
5
10
15
20
25
30
New Minimum Mass Solar Nebula
Desch (2007)
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 < 10%!!
Consistent with
many new
constraints
Desch (2007)
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)
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 < 3/2
New Minimum Mass Solar Nebula
In fact, if  ~ r-p and T ~r-q and p+q > 2, mass must flow
outwards (Takeuchi & Lin 2002)
Desch (2007) solved steady-state equations for alpha disk (LyndenBell & 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)
New Minimum Mass Solar Nebula
Steady-state alpha
decretion disk fits
even better.
Applies in outer solar
system (> 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
Explains presence of CAIs in comets!
Comet 81P/Wild 2
Scattered 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’. It’s a CAI,
formed (by condensation)
at > 1700 K.
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).
•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)
Masses of Solids in Planets
Inside 15 AU, planets
limited by availability
of solids; they achieve
isolation masses
Desch (2007)
Outside 15 AU,
planets cannot grow
before gas dissipates;
no gas = no damping
of eccentricities
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 steadystate alpha decretion disk being photoevaporated 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
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
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.
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 < 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.
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 > an order of magnitude larger than
interplanetary dust flux, as is possible in planetary
environments. (2003 EL61 collisional family, too?)
•Real ices don’t conform to experiments of amorphization
Cryovolcanism?
Still, cryovolcanism does
exist. Ariel’s surface <
100 Myr old (Plescia
1989), Triton’s even
younger (Schenk & Moore
2007)
Are these objects tidally heated,
or are young surfaces common
on KBOs, too??
Cryovolcanism needs ammonia
X = NH3 / (H2O+NH3). 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 < 1.5%, but may be devolatilized.
Ammonia content of KBOs is unknown, but X = 5% is not
unreasonable
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
Ammonia
We use simplified phase diagram to include following
phases:
1. Solid water ice
2. Solid ammonia dihydrate (ADH)
3. Liquid water
4. Liquid ammonia
5. Rock (analogs being ordinary chondrites)
Ammonia
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
Ammonia
For example, in regime 1 (T< 176-dT K),
Similar (but much more complicated) expressions apply to
other regimes
Ammonia
For example, in regime 3 (176+dT < T < Tliq),
Ammonia
Hunten et al (1984)
Just a few %
ammonia
drastically
lowers the
viscosity,
especially
once ADH
melts.
Arakawa &
Maeno (1994)
Limit for metersized rocks to
slip ~ 10 km/Myr
Differentiation
If the ice contains a few % ammonia, differentiation
can occur wherever T > 176 K
Maximum radius at which T=176 K ever = “Rdiff”
Within Rdiff, we separate into rocky core, then ADH
+ammonia+water = “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).
Radiogenic Heating
We consider heating by long-lived radionuclides
232Th and 40K only.
235U, 238U,
Avg heating during first 1 Gyr =
5 x Avg heating during last 1 Gyr!
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
Convection
We check for convection in water ice layer, but Ra << 1000
in all models we ran: no convection.
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
Thermal conductivities
Results
Canonical case, a Charon-like body
R = 600 km
 = 1.7 g cm-3 (rock fraction 63%)
X = 5%
Differentiation starts at t=65 Myr, reaches fullest
extent by 100 Myr
Rdiff = 516 km... half the mass differentiates
t=2 Gyr
t=1 Gyr
t=3 Gyr
slush
layer
t=4 Gyr
water ice
layer
t=4.6 Gyr
t=0 Gyr
rocky
core
ice+rock
crust
rocky
core
slush
layer
water ice ice+rock
layer
crust
H2O(s)
rock
rock
H2O(l) +
NH3(l) +
H2O(s)
+ ADH
H2O(s)
ADH
All ammonia within Rdiff liquid;
additional water liquid created
as temperatures rise.
Melted ADH
Differentiation takes
place within ~ 70 Myr
Temperatures in
slush layer drop
below ~ 176 K;
freezing starts at
t = 4.5 Gyr
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 (0.961+0.063)5.435 exp(0.401+0.633) ~ 182 K.
Release of heat from core found to enhance flux by amount
≈0.42
Release of stored heat from core is significant!
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.
61
T
M?,
S?
C
Q?
O
P
E
Lots of uncertainties in freeze-out times:
•Thermal conductivity of rock: x2 variation changes
freeze-out time by ~ 0.3 Gyr
•Raising ammonia to X=5% extends time of liquid
by ~0.1 Gyr
•Inclusion of methanol would increase freeze-out
time by about 1 Gyr!
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.
How does subsurface liquid surface?
On Europa, ∆ = 1.00 g cm-3 - 0.92 g cm-3 > 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
< 0 (if liquid > 230 K), and buoyancy can drive the
crack all the way to the surface.
Trapped pockets of liquid also create huge overpressures when they freeze (Fagents et al. 2003)
Cracks will propagate at several m/s (Crawford &
Stevenson 1988), reaching the surface in ~ 1 day.
How does subsurface liquid surface?
Cracks as small as 1 km can become selfpropagating within Charon’s 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 Charon’s surface with waterammonia 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.
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
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 > 1024 g,  > 1.3 g cm-3, X > 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 that’s 32% ammonia, and near -100ºC ??