A COMPARISON OF INTERNAL STRUCTURE
OF GANYMEDE AND TITAN.
Dunaeva A.N., Kronrod V.A., Kuskov O.L.
Vernadsky Institute of Geochemistry and Analytical Chemistry,
Russian Academy of Sciences,
Moscow, Russia
Ganymede
Titan
Ganymede and Titan:
are the two largest satellites in the Solar System;
were formed in the outer zones of their central planets (Jupiter and Saturn);
are regular satellites (their orbits and rotation are the same as the rotation of
associated central planets);
 satellites rotation is synchronous with their orbits;
 low density of the satellites suggests that they could contain remarkable
amounts of H2O.
The main differences between Ganymede and Titan
"Galileo", "Voyager" and "Cassini-Huygens" spacecraft missions to Jupiter and Saturn showed
that Ganymede and Titan are different both external and internally:
Ganymede Titan
Atmosphere

Trace oxygen atmosphere
Dense nitrogen atmosphere (~400 km):
– N2 - 98.4%, CH4 and Ar - 1.6%,
– CO2 and other trace organics.
– Free oxygen is absent.
Magnetic field

A relatively strong intrinsic magnetic field and
magnetosphere.
Intrinsic magnetic field is absent.
Climate

Exogenous
climatic
processes
(evaporation,
condensation, precipitation, cycle of substances,
seasons) are not available.
The seasonal weather patterns are similar to Earth,
but governed by methane cycle (including winds,
rains, seasons change, etc.).
Surface features
Two types of terrain:
– Very old, highly cratered, dark regions;
– Younger (but still ancient), lighter regions marked
with an extensive array of grooves and ridges;
Criovolcanism insignificant but important in the
formation of the bright terrain.
The surface is "complex, fluid-processed, and
geologically young" (c):
– Ridges, valleys, riverbeds, dunes, stable lakes of
liquid hydrocarbons;
– Minor amounts of relatively young impact
craters;
– Clearly defined criovolcanism.
Models of Ganymede and Titan.
Titan:
Ganymede:
Mitri et al., 2009
Sohl F. et al., 2003
Grasset et al., 2005
Ganymede’s general
image from NASA, JPL
Sohl F., 2010
Titan’s general image from NASA
Phase diagram of water and the temperature distribution
in the Ganymede’s and Titan’s icy crust.
325
T, K
50 100 150
300
250
200
300
350
400
450
500
H, km
L
T,o C
0
250
200
175
150
125
100
75
550
25
275
225
50
III
Ih
V
II
VI
H Ih =80 km -25
H L =310 km
-50
H Ih =95 km
H L =230 km
H Ih =150-160 km
(models without
internal ocean)
-75
-100
-125
-150
Straight thin lines - conductive temperature profiles through the
Ganymede's
surface conditionss external (ice-Ih) crust.
-175
Titan's
Dashed lines – adiabatic convective heat transfer
in the water
subcrustal ocean and in high-pressure ices.
H, HIh, HL - the distance from the satellite's surface (depth), the
P, kb
2
4 thickness of the6external ice-Ih crust
8 and of the inner
10liquid
ocean respectively.
Calculation of the Ganymede's and Titan’s heat flux
F(mW/m2) = [o(RSat - НIh) /(Н Ih RSat)]ln[T2 /Т1]
F,mW /m 2
400
20
350
300 250 200 150 100 50
0
H w , km
o = 567 W/m - thermal conductivity
of ice Ih,
RSat – satellite’s radius,
15
НIh – thickness of the icy crust,
F=7 mW /m 2 [2]
5
F=3.3 mW /m 2
Т1 – satellite's surface temperature,
M elting points L-Ih
10
Triple point
L-Ih-III
251.15 K / 2.07 kbar
F=5 mW /m 2 [1]
Т2 – the temperature at the ice-Ih liquid phase boundary,
Hw - the thickness of internal ocean,
F – heat flux.
F=2.9 mW /m 2
0
20
40
60
80
100
120
140
160
Í Ih , km
The thickness of the icy crust and internal ocean of
Ganymede (blue) and Titan (black) via the heat
flow through the satellites ice-Ih crust.
[1] Bland, M.T., et al., 2009
[2] Mitri G., Showman A., 2008
Initinal data for modeling, problem setting and methods of solution
Physical characteristics of the satellites
Ganymede
Pressure at the surface, P[bar]
1.0e-06
1.467
Temperature at the surface, T [K]
110.0
93.0
Gravity acceleration,
1.428
1.35486
Radius, R [km]
2634.0
2575.0
Average density, g/cm3
1.936
1.88202
Mass, M [kg]
0.14819e24
0.1346e24
Normalized moment of inertia, I/MR2
0.3105
0.3419
g R  [m/s2];
Titan
Models of the satellites internal structure described by the system of following equations:
 Equations of hydrostatic equilibrium:
dg
 4  G   R   2 g R  R
dR
dP
   R   g R  ,
dR
 The equations of the satellites mass and moment of inertia:
n
8
I   
15 i 0 i
R  R 
5
5
i
i 1
,
4 n
M   
3 i 0 i
R  R 
3
3
i
i 1
 R   density of the
water-ice shell,
 ice,m  average density of
ice in mantle,
 Fe Si  density of the
rock–iron
component,
 The equation for calculating ice component concentration in mantle:
ice,m  Fe Si   m  
Cice 
Fe Si = 3.15 - 3.62 g/cm3 (LL-chondrites)
 m  Fe Si  ice,m 
High-pressure water ices equations of state.
m
 average density of
mantle
Ganymede's surface
2500
liquid ocean
~465 km
Ih (~95 km)
~230 km
V (~55 km)
~530 km VI
1500
 core= 5.15 g/cm3
 core= 5.7 g/cm3
 core= 6.5 g/cm3
Fe-Si mantle
500
Fe-FeS core
820
840
860
880
900
0.388
0.392
0.396
I/MR 2 for rock–iron core
3.5
3.55
3.6
3.65
Mantle density, g/cm 3
Titan's surface
2500
1500
Ih (80 km)
liquid ocean (~310 km)
V+VI (~120 km)
rock-ice mantle
500
rock-iron core
400
440
480
520
Thickness of the water-icy shell, km
Thickness of the w ater-icy shell, km
0.384
Distance from Titan's center, km
Distance from Ganimede's center, km
The internal structure of Ganymede and Titan.
0.4
In general three-layer models of satellites including the outer
water-ice shell, mantle (rock or rock-ice) and the inner core (Fe-Si
or Fe-FeS) can be made.
Moreover, two-layer models (without inner core) could be realized.
In this case satellite has significant large outer water-icy shell, but
a) its inner core not forms.
On this model the maximum possible thickness of the water-ice
shell is about 900 km and 500 km for Ganymede and Titan
respectively.
Water content and density gradients in large icy satellites of
Jupiter and Saturn.
3.6
60
Io
3.2
Ganymede
Europa
Callisto
H2O, wt %
density, g/cm3
Titan
40
20
2.8
2.4
Titan
Ganymede
2
Callisto
Europa
Io
1.6
0
5
10
15
20
25
Orbital distance (in Rplanet)
30
5
10
15
20
25
Orbital distance (in Rplanet)
The total water content in Ganymede is 46-48% , in Titan - 45-52% .
30
Conclusion.
Ganymede and Titan are the similar in size and chemical composition: the density of
the satellites’ rock material is typical for the hydrated L/LL chondrites.
The satellites do not differ in terms of bulk water content which in average is about
50 wt.% (water/rock ratio is close to 1).
Ganymede and Titan both may have subsurface oceans.
– Internal ocean in the satellites not forms when the heat fluxes less than 3.3
mW/m2 and 2.9 mW/m2 for Titan and Ganymede respectively.
Internal structure of the satellites can differ fundamentally:
– Ganymede is a completely differentiated body, with the inner region formed by
separating of the original L/LL-chondritic substance into the silicate mantle
and metallic core.
– Titan is differentiated only partially: its inner areas are represented by a
mixture of rock and ice components.
 Equal content of bulk H2O and the same density of the satellites’ rock material allow
to have assumption that Ganymede and Titan could have been formed from the
planetesimals with similar composition corresponding to the ordinary L/LLchondrites. Different conditions of the satellites’ formation from accretion disks led to
major differences in their internal structure.