Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.cosmos.ru/conf/2009elw/presentations/presentations_pdf/session4/Kuskov_ELW.pdf Äàòà èçìåíåíèÿ: Mon Mar 2 19:59:13 2009 Äàòà èíäåêñèðîâàíèÿ: Mon Apr 6 23:51:02 2009 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: phoenix

Internal Structure of Icy Satellites of Jupiter
Oleg Kuskov, Victor Kronrod
Vernadsky Institute of Geochemistry, Moscow

Europa Lander: science goals and experiments February 9­13, 2009 IKI, Moscow


The Galilean satellites and large asteroids

1821 km

1565

2634

2410

Ganymede and Callisto are the size of Mercury, and Io and Europa are the size of the Earth's Moon. Io, Europa and Ganymede are differentiated satellites and Callisto is a partially differentiated satellite.


Outline
Evidence for Ice/Water shell Methods of estimating interiors Gravity and magnetic measurements, moment-of-inertia Heat flow

Ganymede

Titan

Callisto

Europa

Triton Pluto Enceladus

The probable presence of liquid water beneath the Meteorites/chondrites outer ice crust is expected on these bodies. Internal structure and thermochemical evolution of Thermodynamics, the icy satellites are poorly known. The data Equations of state, obtained by the Galileo mission on the gravitational H2O phase diagram fields of the satellites made it possible to determine the radii, density and moments of inertia values, and Summary to estimate the thickness of outer water-ice shells, core sizes and chemical composition.


The main questions

· · · · · ·

Is there a water ocean beneath the iced surface? How deep is the ocean? How thick is the ice shell? Is Callisto differentiated / partially differentiated /undifferentiated satellite ? What is the thermal and chemical structure of the Galilean moons? Is there life???


Europa Ganymede

Models of the Satellites
ice shell (water-ice) ter

crust mantle
Fe-FeS FeS core

Io
crust asthenosphere mantle Fe-FeS core

· Leaving aside the details, we de consider models of the internal structure of the struct satellites, including an ice (water-ice) layer and a rockiron core. Rock-iron cores of Europa and Ganymede consist of crust, mantle and Fe-FeS core. · In contrast, Io consists of crust, asthenosphere, silicate mantle and Fe-FeS core.


Major Aims
We present the results for determining (1) the thickness of an icewater shell, (2) the satellite chemistry (Fe/Si) ratio and metallic iron content, (3) the Fe-S core sizes and masses. Mass and moment-of-inertia values are used to model the internal structure of the satellites. Ordinary and carbonaceous chondrites are taken as representatives of nebula matter


Approach
·

We assume a three-layer model: water-ice shell, silicate mantle, and metallic core consistent with the values of mass and moment of inertia Core models: Fe-10 wt%Score for ordinary (H, L, LL) chondrites, and Fe-FeS and FeS (troilite)core for carbonaceous (CI, CM, CV) chondrites.

For phase equilibrium calculations in the system Na2O-TiO2-CaO-FeO-MgOAl2O3-SiO2, the Mie-Gruneisen equation of state of minerals and free-energy-minimization technique were used. Phase diagram of H2O The core radii and masses are found by the Monte-Carlo method.


Mathematical model
The known parameters moment of inertia and mass + chemistry of chondrites + thermodynamics
The density of the mantle mineral assemblages is calculated from the silicate fraction of ordinary and carbonaceous chondrites.

The unknown parameters The distribution of density The radius of Fe-FeS core The thickness of the crust The thickness of the water-ice shell The Fe(tot)/Si ratio in the rock-iron core The distribution of temperature


H2O Phase Diagram
0.01 2500 0.1 1 10 100

T,0C
2250

T, K
2250

0.2 300
T, K

0.4

0.6

0.8

1
T,0C

20

2000

275

2000
III

0
1750 -20

250
Ih
1750

V

VI

-40
225

1500

II
-60

1500 200 2
1250

X

1250

4

6

8

10 1000

1000

750

750

500

500

250

III
250

V VI

VII
0

Ih
0 0.1

II XIII
1

VIII XIV
100 1000
-250

XI

10

These figures illustrate the stability fields of high-pressure ices and possibility of the existence of a liquid-water ocean on the icy satellites, because the melting temperature of ice-I decreases with pressure up to the triple point.


Europa - Second major satellite from Jupiter, Smallest of the Galileans (a little smaller than Earth's Moon) =2.989 g/cm3, R= 1565 km, I/MR2 =0.346±0.005
The main facts
· · ·

Ice-cracks on Europa's surface consistent with either "warm-ice" or water beneath the surface Moment-of-inertia values (Anderson et al., 1998) Magnetometer data from Galileo mission confirm presence of water (Kivelson et al., 2000) Geophysical and geochemical constraints are used to model the internal structure of Europa for three firstorder parameters (none of which is known): (1) the thickness of an outer water-ice shell; (2) the chemical composition - the ratio of total iron to silicon (Fetot/Si), FeO content, and metallic iron content; (3) the core sizes and masses.


Europa
Morphology features are the best evidence that Europa has or had a subsurface ocean. The lines represent tectonic features, such as cracks and rifts, and ridges. The characteristic feature of Europa is the probable presence of liquid water beneath the outer ice crust, which gave rise to attractive hypothesis on the existence of extraterrestrial forms of primitive life. The existence of a water ocean on Europa was supported by magnetic measurements obtained with Galileo.


Moment of inertia values for the rock-iron core of Europa vs the rock-iron core density
The models of the rock-iron core (chondritic mantle + central Fe-10% S core for ordinary chondrites or FeS core for carbonaceous chondrites) are consistent with the total mass and moment of inertia for Europa (M = 47.991021 kg, I°/MR2=0.346±0.005) at varying thickness of the outer water-ice shell, H(ice). The cross denotes the mean density and moment of inertia for Io (Anderson et al., 2001) and shows that H/L/LL chondritic models of Europa's rock-iron core with H(H2O) = 100-140 km are closer to the position of anhydrous Io than C-chondritic models.


Europa: Thickness of an outer shell and core sizes
Water-ice shell and core radius, km
Thickness of Europa's water-ice shell, km
800
80

Chondritic models of Europa
700

Radius of Fe-FeS core

100

600

120

500
H L LL CM

140

400 180 140 100 3.30

Thickness of an outer shell

160 3.00

3.20

3.40

3.60

-3 Upper mantle density, g cm

3.40

3.50

3.60
3

3.70

A geophysically permissible thickness of the Europa's shell lies between 80 and 160 km. Any thickness of the outer shell beyond the range obtained is incompatible with the constraints on the mass and moment of Europa.

Mantle density, g/cm

RE, km

R(Fe-S) core, km

H(ice), km

Mice,%

Thickness of an outer water-ice shell depends on the presence of a crust (dashed lines) and density of a chondritic mantle: 100-140 km ­ H/L/LL; 130-160 km ­ C chondrites. Europa contains 6-8% of H2O and has a core (R=450-670 km).

1565

450670

100160

6-8


Element ratios for the chondritic models of Europa

Element ratios for Europa (empty boxes) and chondrites (shaded boxes). The boxes outline an allowed Fe/Si ratio and content of metallic iron in chondrites and satellite. Europa's major element composition matches the bulk composition of L/LL chondrites. H and C (probably) chondritic materials may be excluded for Europa's bulk compositions.


GANYMEDE ­ Biggest of the Solar system satellites
Ganymede's Fe-FeS core radius, km
1000 800 600 400 200 0 700

A

B

Thickness of water-ice shell, km

800

900

1000

Two alternative models of an outer shell are considered. Model (A) - an outer shell is completely composed of the high-pressure ice phases (no water is present) - 890-920 km thick. Model (B) - below a shell of ice-I (30-120 km thick), a liquid layer of 230-140 km thick may exist. The water-ice thickness in model (B) is 780-850 km. The content of H2O in Ganymede's outer envelope is 46-48% of the total mass.

R(Fe-FeS-core) = 800-950 km


Element ratios for the chondritic models of GANYMEDE
Element weight ratios Fetot/Si vs Fem+Fem(FeS) for chondritic models of Ganymede's rock-iron core (empty boxes) derived from the geophysical constraints in comparison with those in chondrites (shaded boxes). Bulk composition of the rock-iron core of Ganymede may be described by the composition close to the L/LL chondrites. Geochemical constraints show that H and C chondritic matter was not the building material of the rock­iron core.


Callisto
R=2410 km, I*=0.3549, =1.834 g/cm3
We consider a six-layer model of Callisto consisting of an ice layer, a water ocean, a three-layer ice-rock mantle, and a rock-iron core. The major uncertainty in the internal structure of Callisto is related to the equation of state of rock-iron material, because its density can vary from low (hydrous silicates + Fe-FeS alloy) to high values (anhydrous silicates + Fe-FeS alloy). Taking into account the considerations from the constitution of Europa and Ganymede, we accept that the composition of the rockiron material of Callisto is similar to the bulk composition of L/LL type chondritic material containing up to 10-15% of iron and iron sulfide.

ice

water

rock-iron core

ice-rock mantle

Callisto


Model of Callisto with a continuous icy shell ­ ocean is completely frozen
In general, Callisto can consist of three chemically different layers: an icy lithosphere, a rock­ice mantle, and an ice-free rock­iron core. The maximal thickness of the icy lithosphere is 320 km. Two-layer models of Callisto are possible: (1) an outer icy lithosphere and a rock­ ice mantle (without a central rock­iron core), (2) an outer rock­ice shell and a central ice free rock­iron core (without an icy lithosphere). One-layer model of Callisto with constant ice concentration from its surface to the center is not realistic. A two-layer model composed of an icy lithosphere and a dry rock­iron core contradicts the constraints on the moment of inertia.


Thickness of the icy crust versus the heat flow from the surface of Callisto
The maximal pressure at which the water layer can exist corresponds to a depth of ~176 km. Depending on the surface temperature, the heat flux through the crust of such a thickness varies from ~2 (T = 130 K) to 2.7 (T = 100 K) mW/m2 . If the heat flux from the satellite exceeds these values, there should be water beneath the ice layer. At present, the heat flows expected from the satellite are estimated as 3.3­3.9 mW/m2 (Mueller and McKinnon, 1988), indicating that there can be an ocean beneath the ice-I layer.

400

T, K

Ice I ð2
20 0

adiabat

T 3 adiabat
3- 3

2 + 2

Water
ð1

Ice + rock

0

100

2 00

300

4 00

Z, km


The permissible thickness of Callisto's water-ice shell
F = 3.3-3.7
275
10 0

mW/m2
Z, km
30 0 40 0

20 0

H shell max=320 km
270

The distribution of temperature was calculated from the condition of conductive heat transfer in the ice-I layer, and the T-profile in the stability fields of water and highpressure ices goes along the adiabat.

Liquid
26 5

T, K

V

H ice min
26 0

I
255

Adiabat

The heat flow values are in the range of 3.3-3.7 mW/ m2. In this case the geophysically permissible thickness of ice-I crust is 135-150 km, and the thickness of the underlying internal ocean is between 120 and 180 km.

H ice max
250 0 1 2

III

P, kbar

3

4

5

6


A model of Callisto with an internal ocean
Callisto consists of an ice layer, a water ocean, a rockice mantle, and a rock-iron core free of ice. Because Callisto is only partially differentiated, a layer of a mixture of highpressure ices and rock-iron material (ice-rock mantle) must exist between the outer shell and the rock-iron core. The total thickness of the outer shell is 270-315 km, the thicknesses of the ice crust is ~150 km and that of the ocean is ~120-180 km.
2500
water ice-I

2000

Radius, km

ice-rock mantle

1500
1

1000

500

rock-iron core
2

0 135

145

155

165

175

Ice-I thickness, km

(Fe-Si) g/cm3 3.15 3.62

H(iceI) km 135150 135150

H(wat) km

H(ice+ wat) 270315 270300

120 -180 120-165


Models of Icy Satellites
ice

water

rock-iron core

ice-rock mantle

Callisto

Titan Callisto Titan
(Sohl et al., 2003)

Ganymede Ganymede
(Kuskov, Kronrod, 2001)

Water-ice shell, km

(Kuskov, Kronrod, 2005)

Ice crust Ocean Water+ices-V-VI Total thickness of 2 shell

150 150 0 300

70 220 600 900

120 140 600 800-900


Amount of H2O in the satellites
50
Callisto

H2O, wt.%

40 30 20 10 0

Ganymede

Io 0 5 10

Europa 15 20 25 30

Distance from Jupiter in RJ
Ice-free Io Europa ~6-8% of H2O. The water and ice content in Ganymede is between 46-48%. The total amount of H2O in Callisto is found to be 49­55 wt%.


Fe/Si ratio and chemical composition in the protojovian nebula
2

Mercury

Fe/Si, at

1

Earth

EH H CM

Planetesimals composed of the L/LL types of ordinary chondrites could be considered as analogues of building material for the rock­iron cores of the Galilean satellites.
CI CV EL L/LL

Mars Moon

Galilean satellites

Isochemical composition of Io, Europa, Ganymede and Callisto
6

0

0

1

2

3

4

5

Fetot/Si~0.5; (FeO)mantle~16 wt.%
Similarity of bulk composition of the rock­iron cores of the inner and outer satellites implies low temperatures in the protojovian nebula and the absence of iron­ silicon fractionation.

Distance from the Sun, au
The Fe/Si ratio varies from 4.5 for Mercury to 0.5 for the Galilean satellites. Geophysical and geochemical constraints show that the bulk compositions of the rockiron cores of the Galilean satellites are similar and may be described by the composition close to the L/LL chondrites. Fe/Si ratio for Cchondrites is ~0.85. These results indicate that the jovian satellites did not form from Cchondritic material.


Conclusions
(1) We have constructed models of Europa, Ganymede and Callisto based on Galileo measurements and cosmochemical constraints on the composition of chondrites. (2) Chemistry of the satellites and the amount of H2O are estimated. Fe/Si ratio is close to that of the L/LL chondrites. The results indicate that the jovian satellites did not form from Cchondritic material. (3) H2O-content: Europa - 6-8%, Ganymede and Callisto ­ 46-55%. (4) The results of modelling support the hypothesis that Callisto may have an internal liquid-water ocean. (5) Rock-iron material forming the Galilean satellites may reflect the chemical composition of the solar nebula at the radial distance of Jupiter.


Thank you for your attention!