A COMPARISON OF INTERNAL STRUCTURE A COMPARISON OF INTERNAL STRUCTURE

OF GANYMEDE AND TITAN.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 and TitanGanymede 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.

Jupiter Saturn

Ganymede Titan

The main differences between Ganymede andThe main differences between Ganymede and TitanTitan"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 Models of Ganymede Ganymede and Titan.and Titan.

Ganymede:

Ganymede’s general image from NASA, JPL

Titan:

Sohl F. et al., 2003

Mitri et al., 2009

Sohl F., 2010 Titan’s general image from NASA

Grasset et al., 2005

2 4 6 8 10

100

150

200

250

300

7 5

1 2 5

1 7 5

2 2 5

2 7 5

3 2 55 0 100 150 200 250

P , k b

T, K

L

IhII

VIVIII

T ita n 's

H Ih = 1 5 0 -1 6 0 k m(m o d e ls w ith o u t in tern a l o cea n )

300 350 400 450 500 550

-150

-100

-50

0

50

-175

-125

-75

-25

25

T ,o C

H , k m

H Ih = 8 0 k mH L = 3 1 0 k m

G an ym ed e's su rfa ce co n d itio n ss

H Ih = 9 5 k mH L = 2 3 0 k m

Phase diagram of water and the temperature distribution Phase diagram of water and the temperature distribution

in the Ganymede’s and Titan’s icy crust.in the Ganymede’s and Titan’s icy crust.

Straight thin lines - conductive temperature profiles through the external (ice-Ih) crust. 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 thickness of the external ice-Ih crust and of the inner liquid ocean respectively.

Calculation of the Ganymede's and Titan’s heat fluxCalculation of the Ganymede's and Titan’s heat flux

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.

F(mW/m2) = [o(RSat - НIh) /(Н Ih RSat)]ln[T2 /Т1]

o = 567 W/m - thermal conductivity of ice Ih,

RSat – satellite’s radius,

НIh – thickness of the icy crust,

Т1 – satellite's surface temperature,

Т2 – the temperature at the ice-Ih - liquid phase boundary,

Hw - the thickness of internal ocean,

F – heat flux.

[1] Bland, M.T., et al., 2009

[2] Mitri G., Showman A., 2008

2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0Í Ih , k m

0

5

10

15

20 400 300 250 100H w , k m

F = 5 m W /m 2 [1 ]

F = 2 .9 m W /m 2

F ,m W /m 205 0200 150350

T rip le p o in t

L -Ih -III2 5 1 .1 5 K / 2 .0 7 k b a r

F = 3 .3 m W /m 2

F = 7 m W /m 2 [2 ]

M eltin g p o in ts L -Ih

Physical characteristics of the satellites Ganymede Titan

Pressure at the surface, P[bar] 1.0e-06 1.467

Temperature at the surface, T [K] 110.0 93.0

Gravity acceleration, [m/s2]; 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

Models of the satellites internal structure described by the system of following equations: Equations of hydrostatic equilibrium:

,

The equations of the satellites mass and moment of inertia:

,

The equation for calculating ice component concentration in mantle:

High-pressure water ices equations of state.

RR iii

n

i

I5

1

5

015

8

Initinal data for modeling, problem setting and methods of solutionInitinal data for modeling, problem setting and methods of solution

RgRdRdP RRgRG

dR

dg24

RR iii

n

i

M3

1

3

03

4

miceSiFem

mSiFemiceiceC

,

,

R

Rg

m

density of the water-ice shell,

average density of ice in mantle,

density of the rock–iron component,

average density of mantle

mice,

SiFe

= 3.15 - 3.62 g/cm3 (LL-chondrites)SiFe

The internal structure of The internal structure of Ganymede and Ganymede and Titan.Titan.

820 840 860 880 900T h ick n ess o f th e w a ter-icy sh e ll, k m

500

1500

2500

Dis

tanc

e fr

om G

anim

ede'

s ce

nte

r, k

m

0.384 0.388 0.392 0.396 0.4

I/M R 2 fo r ro ck – iro n co re

G a n y m ed e 's su r fa ce

F e-S i m a n tle

Ih (~95 km)

F e-F eS co re

~ 23 0 k m

3 . 5 3 . 5 5 3 . 6 3 . 6 5

M a n tle d en sity , g /cm 3

~ 4 6 5 k m

core= 5.15 g/cm 3

core= 5.7 g/cm 3

core= 6.5 g/cm 3

V (~55 km)

VI ~ 5 3 0 k m

a)

liq u id o cea n

400 440 480 520

T h ick n ess o f th e w a ter-icy sh e ll, k m

500

1500

2500

Dis

tan

ce f

rom

Tit

an's

cen

ter,

km T ita n 's su rfa ce

ro ck -ice m a n tle

Ih (80 km)liq u id o cea n (~ 3 1 0 k m )

V + V I (~ 1 2 0 k m )

ro ck -iro n co re

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 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.

5 10 15 20 25 30Orbital distance (in Rplanet)

0

20

40

60

H2O

, w

t %

IoEuropa

GanymedeCallisto

Titan

Water content and density gradients in large icy satellites of Water content and density gradients in large icy satellites of Jupiter and Saturn.Jupiter and Saturn.

5 10 15 20 25 30Orbital distance (in Rplanet)

1 . 6

2

2 . 4

2 . 8

3 . 2

3 . 6

de

ns

ity,

g/c

m3

Io

Europa

Ganymede CallistoTitan

The total water content in Ganymede is 46-48% , in Titan - 45-52% .

Conclusion.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/LL-chondrites. Different conditions of the satellites’ formation from accretion disks led to major differences in their internal structure.