“Deep Habitat” in the icy moons:
structure and evolution of the internal ocean
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Planets and moons in our Solar System
Size, density, and MoI (bulk water content and its interior distribution)
Liquid ocean exists? (observational constraints)
Evolution/lifetime of the ocean (theoretical predictions)
Jun Kimura (ELSI, Tokyo Tech)
Astrobiology Workshop 2013/11/29@ISAS
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Solar System Moons
0 0 1 2 67
Earth-Moon
Jovian moons
62 27 13
Saturnian moons
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1737 km
Mercury
2440 km
2634 km
2576 km
2410 km
Major sized moons are picked up at their correct relative sizes to each other and to Earth.
Radius 6371 km
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Icy moon: “a class of natural satellites with surfaces composed mostly of ice.”
1737 km

2634 km
“Icy moons”
Beyond the snow line, water can
condensate and icy planetesimals, as
seed materials to form giant planets
and icy moons, can be formed in the
proto-solar nebula.
Mercury
2440 km
2576 km
2410 km
Radius 6371 km
Moons in the outer solar system are covered by ice.
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
Most of moons have a bulk density less than 2 g/cc.


We can estimate the fraction of constituents from the bulk density.
Icy moons have substantial amounts (more than half) of ice.
H2O
Rocky
Rocky H2O
H2O
Mean (uncompressed) density is related to bulk chemistry composition
modified from Hussmann (2006)
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How is interior distribution of constituents (mass)
(layered interior? or homogeneous?)


Moment of Inertia gives us more information than density alone.
MoI can be inferred from gravity field observations by spacecraft.
Measure of an object’s
resistance to being “spun
up” or “spun down”
Light
Same density
Mixture
Dense
r
dm
M
R
Different MoI
Uniform sphere:
I/MR2 = 0.4
mass
(1020kg)
radius
(km)
Europa
478.0
1565
2989.5
0.346+-0.005
Ganymede
1481.7
2634
1935.6
0.3105+-0.0028
Callisto
1075.9
2410
1834.4
0.359+-0.005
Anderson+,
1996, 1998, 2001a,b

Layered sphere:
I/MR2 < 0.4
density
(kg/m3)
MoI
(I/MR2)
But, MoI can’t tell the state of water layer (small density difference).
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
If the water layer can be melted (ocean!)
under the solid icy crust, it may be a
frontier to be capable of harboring life
(deep habitat).

Some indirect evidences for the presence
of subsurface ocean,
inferred by…
 Formation mechanism of surface tectonics
 Electric/magnetic signature
 Shape and gravity change
 Direct detection

Theoretical prediction
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Inferring the internal ocean #1:
Formation mechanism of surface tectonics

Extensional tectonics (cracks and bands) are ubiquitous on the icy surface.
Europa (Sullivan+, 1998)

Possible source of stress is tidal deformation
and/or ocean freezing (Kimura+ 2007).
Peak tidal radial deflection on Europa
- ocean + ice crust -> ~30m -> generating large stress
- no ocean -> less than 1 m -> insufficient stress
 Ocean freezing (from liquid to Ice I) results in expansion. -> large stress


To drive an icy extensional tectonics, the ocean is needed.
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Inferring the internal ocean #2: Electromagnetic
signature
Jovian magnetosphere
Eddy currents due to
Secondary magnetic field
E-M induction
Field line is away from Jupiter
(northern hemisphere of JMS)
Field line is towards Jupiter
(southern hemisphere of JMS)
Magnetic
flipping
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
Near the moon, Jupiter’s M.F. line periodically changes with moon’s
orbital motion.
Moon responses as a conductor and generates an inductive secondary
magnetic field. Galileo spacecraft has detected this field in E, G and C.
Implying global conductive layer under the icy crust (e.g., salty ocean like
as the Earth’s ocean).
Salty?
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Salts on icy moons
Europa@Jupiter
Galileo Spacecraft found non-icy materials on
the Europa’s surface by NIR spectroscopy. Salts
are associated with the surface tensile features.
Mg4Si6O15(OH)2 *H2O
Dalton+, 2005
Dalton, 2003
[(Na,Ca)0.33(Al,Mg)2Si
4O2 10(OH)2*nH20]
Distribution of the non-icy
materials (McCord+ 1998).
Possible non-icy materials are,
MgSO4, Na2SO4, MgCO3, Na2CO3 +xH2O
 Ocean’s brine was exposed??
 Hydrothermal activity as occurs on Earth?
Na2CO3*10H2O
MgSO4*6H2O
MgSO4*7H2O
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Inferring the internal ocean #3: Shape
and Gravity change
Enceladus@Saturn

Observed south polar depression is possibly
formed by melting of ice and existing
“localized ocean” (Collins and Goodman 2007).
Inferring the internal ocean #4: Direct
detection
Enceladus@Saturn
Water is erupting from cracks at SP.
NASA/JPL/Space Science Institute
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Tectonics
Electric/Mag
netic signature
Global shape
/Gravity
Direct
detection
Ocean
distribution
Europa
Ganymede
X
X?
Cracks and
chaos
Normal faults
and grabens
Callisto
Enceladus
Titan
-
X
X?
Cracks
Cryo-volcano?
X
X
X
E-M induction
E-M induction
E-M induction
-
-
-
-
-
-
X
-
Electric
(Schumann)
resonance
X
X
Depression at
SP
Tidal Love
number
X
-
Water erupting
from SP
Moon having the subsurface ocean might not be special.
Global
Global
Global
Local
Global
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Inferring the internal ocean #5: Theoretical

prediction
Numerical calculation for the interior thermal state and its evolution
Heat sources
1-D Model for heat transfer
·
Fcond  kc T
0

kv   c p gl 4
 18

Q
Fconv  kv T   ad T 
 T   T  
 


 r   r  ad 
 T   T 



 r   r  ad
 T   T 



 r   r  ad
(Sasaki & Nakazawa 1986, Abe 1997, Kimura+ 2009)
- Radiogenic heating (Mason 1971)
U = 12.0 ppb, Th/U = 4.0, K/U = 8.4x104
- w/wo tidal heating
Rheology
(Hobbs 1974, Karato+ 1986)
- Liq. water
- Water ice
10
10
- Rock
4.9 10 exp 23.25
,
Pas
exp 25
1
/
Water ocean
- Surface T = 100 K (fixed)
Rocky mantle
- Initial Condition:
- H2O shell is whole liq. with
adiabatic T-grad.
- Rocky mantle has conductive
T-profile with eutectic for FeFeS system at CMB.
Ice crust
ocean
Rocky mantle
Thermal properties
(Kirk & Stevenson 1987)
(Hobbs, 1974)
Specific heats (J/kg/K)
- H2O : 4200 (lq.), 7.0T+185 (sl.)
- Rock: 900
Thermal conductivities (W/m/K)
- H2O : 0.6 (lq.), 488/T+0.5(sl.)
- Rock: 3.0
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1600
Radius (km)
1550
Surface
“Hard ice” case:
0ice=1015 Pas
Ice crust
Initial T
1500
1450
Liquid ocean
1400
1350
Rocky layer
“Soft ice” case:
0ice=1014
Ice-rock boundary
Primordial ocean
Water ocean
Rocky mantle
Rocky mantle
1300
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (Gyr)
Kimura+, in prep.
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Initially the ocean begins to freeze, then the rocky mantle warms up due
to radiogenic heat and the ice crust remelts.
As the radiogenic heat source depletes, the crust turns to thicken again.
The ocean can survive till present in the harder ice crust case, but the
ocean is almost disappearing if the crust is softer.
Ice crust
ocean
Rocky mantle
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w/ tidal heating)
1600
Radius (km)
1550
Surface
Ice crust
0ice=1015 Pas
Initial T
1500
1450
Liquid ocean
1400
1350
0ice=1014
w/ tidal heating
0ice=1014
Ice-rock boundary
Rocky layer
Primordial ocean
Rocky mantle
1300
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (Gyr)
Kimura+, in prep.

“Tidal heating” is a frictional heat due to tidal deformation (of mainly ice crust) interacting
with Jupiter.

The tidal heating can prevent to freeze the ocean, especially in case of the softer ice crust.
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Longevity of the internal ocean (for Ganymede, pure H2O case)
Primordial ocean
Radius (km)
2600
2400
2200
2000
Surface
Ice crust
Initial T
Liquid ocean
0ice=1015 Pas
0ice=1014
High pressure ice mantle
Water ocean
1800
1600
Ice-rock boundary
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (Gyr)
Kimura+, in prep.



Rocky layer
Entirely different evolution compared to Europa’s ocean because
of the “High-Pressure ice mantle”.
Ocean solidifies due to simultaneous growth of the HP icy mantle
and the ice crust.
Finally, the ocean disappears within 1 Gyr.
Rocky mantle
Ice crust
ocean
HP ice mantle
Rocky mantle
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Longevity of the internal ocean (for Ganymede, MgSO4-H2O case)
Primordial ocean
2400
2200
2000
Surface
Ice crust
Liquid
ocean
Initial T
0ice=1015 Pas
High pressure ice mantle
1800
1600
Ice-rock boundary
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (Gyr)
Kimura+, in prep.

Even this case finally the ocean disappears within 1 Gyr.

If Ganymede has the ocean at present, large tidal heat and/or stronger
antifreeze (e.g., NH3) are needed.
Salts are there.
Radius (km)
2600
Rocky layer
This document is provided by JAXA.
Inferring the internal ocean #5: Theoretical
prediction
Key factors for longevity of ocean in the icy moons are…

Icy fraction of the body (pressure range for the water layer)
- 2 schemes of evolution for subsurface ocean:
1. Only LP ice (icy crust) appears: ocean can survive for a long time.
2. Both LP ice & HP ice (HP ice mantle) appear: ocean will disappear in short time.
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Tidal heating (and orbital evolution)
Anti-freeze materials (e.g., salts and/or NH3)
1550
Radius (km)

1600
1500
1450
1400
1350
Ice rheology
1300
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (Gyr)
Radius (km)
Future observation to determine more precisely
a depth and thickness of the “current” ocean
might be able to resolve above issues (e.g.,
JUICE mission).
2600
2400
2200
2000
1800
1600
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (Gyr)
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 Icy Moons are ubiquitous in our Solar System, and several
icy moons have a potential to have the internal water ocean
inferred from the previous spacecraft observations.
• Analyses of surface tectonics
• Electric/magnetic signature
• Shape and gravity
 Theoretical calculations can also predict the existence of
the ocean within a range of possible material properties.
 Future exploration mission should confirm that the ocean
is really there and should contribute to clarify above
unknowns and to improve the evolutional model.
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