Motivations• 氷衛星を形作った物質はなにか？
• 氷衛星の水と地球の水の起源は同じか？違うか？

ガリレオ衛星の内部構造と表層物質
木村 淳 (Jun Kimura)
ELSI/TitechIo2013 08/07-09@定山渓

mass(1020kg)

radius(km)

density(kg/m3)

MoI(C/MR2)

semimajor
axis
(km)

eccentricity

inclination(deg, toJ’s eq.)

893.2

1822

3529.50.37685

421,700(6 RJ)

0.00410.05

1.77

670,900(10 RJ)

0.00900.47

3.55

1,070,400(15 RJ)

0.00130.20

7.15

1,882,700(27 RJ)

0.0074

0.19

16.69

+/-0.00035

Europa

478.0

1565

2989.50.346+/-0.005

Ganymede

1481.72634

1935.60.3105+/-0.0028

Callisto1075.92410

1834.40.359+/-0.005

Anderson+, 1996, 1998, 2001a,b

orbitalperiod(days)

Europa
More ocean water than Earth
H2 O
CO2
SO2
Mg-OH
CH?
(CO2, O2)

H2 O
CO2
SO2
(Mg, Na)SO4+xH2O(Mg, Na2)CO3+xH2O
S-HC≡N(O, O2, O3?)

Mean (uncompressed) density is related
to bulk chemistry composition

H2O

930 kg/m3

Titan

Jupiter
SaturnUranus
NeptuneCallistoGroup 3Europa

Group 2Triton

61x10

Group 155x10Mimas

Titania
OberonArielRhea
Dione
UmbrielEnceladus
Iapetus
Miranda Tethys

1x107

1x108

1x109

Pressure at the bottom of H O layer[Pa]3300

Huge ice, tectonics, and magnetic field

1.5x106

01x106Rock

Ganymede

Ganymede

2x106

Radius of rocky core [m]Io More volcanism than Earth

H2 O
CO2?
SO2?
(Mg, Na)SO4+xH2O(Mg, Na2)CO3+xH2O
H2SO4+xH2O(O2)
modified from Kimura(2008)Sx
SO2
NaK
Cl2SO2?
H2S?
H2O?
Pyroxene?
Sulfides?
Salts?
(SO2 CO, NaCl, S, O)

2

木村・栗田 (2006) を修正Callisto

Undifferentiated “fossil” with ocean

frost(white/grey)

Silicate pyroclastics(black)Sulfur (S2, S4, S8...),
SO2 , and perhaps
Cl2SO2 (red/orange)

1.98 um (SO2)

3.15 um (H2O?)

Sulfur(yellow/yellow-green)

and radiation
damaged sulfur (brown)

S, S2, S4, S8…, SO2,H2O, NaCl, KCl, ClSO2
Solar reflectancespectra of Io
Fe-containing silicates orsalts, sulfides, or polymer?
(due to the loss of 1-1.3um
NIMS detector)(dark caldera)

Volcanic plumes containing S & SO2 (& Na, K, Cl) are ejected at ~1 km/s.

S2 and S4 make red rings around volcano and silicate pyroclastics make black spots.

Sulfur (S2) ejected from vents  S4, S6, S8…

Sulfuric red  (UV)  yellow.
HSTDeformation  Gravity field  Spacecraft flyby(s) J2 and/or C22  Darwin-Radau relation  MoI factor

Bulk density = 3.53 g/cc
MoI factor = 0.377

5150 kg/m3

8000 kg/m3

Ground-basedEuropa

Reference model:AssumingCore density=5150 kg/m3
Crust thickness=50 km
Crust density=2750 kg/m3★

↓Core radius=870 km (0.48R)
Mantle density=3400 kg/m3
Mantle thickness=902 km-Core radius is 550-900km (Fe-FeS core), 350-650km (Fe core)

Anderson+ 2001

If crust density = mantle density (2-layer model),core radius = 950 km (0.52R) for Fe-FeS (5150 kg/m3)
core radius = 650 km (0.36R) for Fe core (8000 kg/m3)-Galileo's magnetometer failed to detect an intrinsic magnetic field.

Silicate layer
MetallicCore

 Few impact craters and activetectonics (geologic age ~10 Myr) Tectonics features and electro-magnetic signatures strongly imply
an existence of subsurface ocean.

Galileo NIMS

Carlson+ 2007

Surface composition on Europa: sulfate
Distribution of the non-ice component, with red
being high non-ice concentration. The non-ice
material is associated with the surface cracks andchaos. (McCord+ 1998)

Surface composition on Europa: sulfuric acid
Carlson+ 2005
(H5O2+) 2SO42−f(H5O2+) 2SO42− •4H2O

8:1 H2O:H2SO4

Dalton+, 2005

Mg4Si6O15(OH)2 *H2O

Dalton, 2003

H2SO4 is another possible contaminant.

anti-apex
Dalton+,f = 2005
[H2SO4•nH2O]/([H2SO4•nH2O] + [H2O])

Hydrate abundance & distribution is consistent
with Iogenic sulfur ion implantation.

[(Na,Ca)0.33(Al,Mg)2Si
4O2 10(OH)2*nH20]

Near IR reflectance spectra of Europa’s non-icy andicy areas, and possible surface minerals. (McCord+1998)- MgSO4, Na2SO4, MgCO3, Na2CO3 +xH2O-> exposed the subsurface ocean’s brine??
-> implying hydrothermal activity as occurs on Earth?

Interior of Europa
Bulk density = 2.99 g/cc
MoI factor = 0.346
Reference model:

Na2CO3*10H2O
MgSO4*6H2OMgSO4*7H2O

Blue=water
Red=hydrated material

f=0.2~0.5

Anderson+ 1998

Ice I shellOcean?

 Largest moon in the solar system(r=2634km, larger than Mercury).
Silicate layerAssumingCore density = 5150 kg/m3
Water layer thickness = 100 km
Water layer density = 1000 kg/m3

H2SO4 concentration is lower in the lineae.

- argues against an endogenic source ofsulfurous or hydrated material ?

MetallicCore↓
Core radius = 704 km (0.45R)
Mantle density = 3300 kg/m3

 Smallest MoI in the solar system(0.31), implying that outer waterlayer (~1000km thick), rocky
mantle, and metallic core(600~1100km radius).
 Clear evidence of geologicalactivity (surface dichotomy).
Older cratered terrain
Young tectonic terrain

Mantle thickness = 761 km

 Inductive response implying an-Core radius is 0-780 km (Fe-FeS core),
0-600km (Fe core) Unique moon having an intrinsic

existence subsurface water ocean
has been observed (but ambiguous).
magnetic field.
Originated by a dynamo motionin the metallic core.

Surface composition on Ganymede
Water

Hydrated Minerals

CO2 on Ganymede

Grooves,normal fault system- CO2 is enriched in
the older cratered
terrain & containedin the non-icematerials.

Galileo NIMS
Tectonic age ~2 Ga* Mainly H2O, especiallydominant in young brightterrain, non-H2O materials
are dominant in old dark
terrain.

Dalton+, 2005

Dalton, 2003
S-H SO2Dalton+, 2005

Dalton, 2003

C≡N
CO2

* Hydrated minerals (salts)- MgSO4, Na2SO4- MgCO3, Na2CO3+xH2O

Hibbits+, 2003

NIMS spectra ofGanymede
Pappalardo+ 2004

NIMS 1.5-um
water ice band
depth mapNIMS CO2band depthmap* CO2 and SO2

Interior of Ganymede

Ice is crystalline or amorphous?
Cosmic ray
UV
Crystalline
Amorphous
Resurfacing
Heating

Bulk density = 1.94 g/cc,
MoI factor = 0.311

Crystallinity map on Ganymede
CrReference model:

Ice I shell
Ocean?
High pressure ice mantle

Assuming
Core density = 5150 kg/m3
Water layer thickness = 1000 km
Water layer density = 1200 kg/m3
Am[Hansen and McCord, 2004]↓

Silicate layer

MetallicCore

Core radius = 920 km (0.35R)
Mantle density = 3400 kg/m3
Mantle thickness = 724 km

この波長の光は氷の中では約0.2mm 進むうちに吸収されてしまう．従って，我々が議論しているのは氷の表層から約0.2mm の薄い層の平均の性質である．上でのべたように，この深さの領域が非晶質化するには，約10億年必要である．

Europa: amorphous (Strong radiation from J, low surface-T.)
Ganymede: both
Callisto: crystalline (Weak radiation from J, high surface-T.)

Fe-FeS core
5150 kg/m3-Core radius is 0-1300 km (Fe-FeS core),
0-1050 km (Fe core)

Fe core8000 kg/m3

Anderson+ 1996

Reference model:
Rc=0.4-0.5R (c=5150 kg/m3)
Rc=0.15-0.2R (c=8000 kg/m3)

 Similar to Ganymede’s dark terrain


Globally old surface.

 Water ice•

heavily cratered, dirty ice•

cratering reveals clean, white ice
underneath•no evidence of tectonics

No tidal heating, no orbital
resonances (in current orbital state).



But it has (inductive) magnetic field !!
•Could it have a subsurface ocean
anyway?

- Crystalline (Callisto/Ganymede)- Amorphous (Ganymede) Dark materials- Organics? (CN, CH)- Hydrate salts (sulfates and carbonates) Other Volatiles- CO2- SO2

Moore+ (2004)

Interior of Callisto• CO2 is often concentrated in and around morphologically fresh and bright impact
craters, but not in degraded craters.
• CO2-bearing material is being excavated by impacts?
• CO2 Source is possibly not exogenic because known comet crater chains are
not CO2-rich.
• If CO2 is being exhumed, water-ice clathrates would act as a stable subsurface
reservoir of CO2.

Anderson+ (2001)

Fe-FeS core
5150 kg/m3

Hibbits+ (2002)

Bulk density = 1.83 g/cc, MoI factor = 0.359
Reference model: AssumingCore density = 5150 kg/m3
Outer layer thickness = 500 kmOuter layer density = 1300 kg/m3

Density [kg/m3]
1000

Fe core8000 kg/m3 1500

22003800

Anderson+ 1996

↓depths of the 4.26-um
absorption bandCore radius = 480 km (0.2R)
Mid-layer density = 2400 kg/m3

Bagenal+ 2004

JUICE

JUpiterICy moon
Explorer(JUICE)JUICE (JUpiter ICy moons Explorer)

JUICE Mission

JUpiter ICy moons Explorer
Exploring the emergence of habitable worlds around gas giants

JUICE Science Themes

2022年6月

打ち上げ• Emergence of habitable worlds
around gas giants• Jupiter system as an archetype forgas giants

2030年1月

木星周回軌道投入
木星ツアー

2032年9月

ガニメデ周回軌道投入
ガニメデツアーCosmic Vision Themes• What are the conditions for planet
formation and emergence of life?
• How does the Solar System work?

JUICE concept• European-led mission to the Jovian system• JGO/Laplace scenario upgraded with two Europa flybys andhigh-inclination phase at Jupiter• Model payload is the same as it was on JGO/Laplace

2033年6月

ノミナルミッション終了

地球－金星－地球－地球重力アシスト- エウロパフライバイ2回- カリストフライバイ3回
高軌道傾斜角フェイズ- カリストフライバイ9回

高高度楕円軌道周回フェイズ（1か月）
高高度（5000km）円軌道周回フェイズ（3か月）
高高度楕円軌道周回フェイズ（1か月）
中高度（500km）円軌道周回フェイズ（3か月）
低高度（200km）円軌道周回フェイズ（1か月）

SELECTED INSTRUMENTS
観測装置
JANUS（Jovis, Amorum ac Natorum Undique Scrutator）
MAJIS（Moons And Jupiter Imaging Spectrometer）
UVS（UV Imaging Spectrograph）
SWI（Submillimetre Wave Instrument）
GALA（GAnymede Laser Altimeter）
RIME（Radar for Icy Moons Exploration）
J-MAG（Magnetometer for JUICE）
PEP（Particle Environment Package）
RPWI（Radio and Plasma Wave Investigations）
3GM（Gravity and Geophysics of Jupiter and Galilean Moons）
PRIDE（Planetary Radio Interferometer and Doppler Experiment）

可視分光撮像カメラ
可視・近赤外撮像分光計
紫外撮像分光計
サブミリ波観測器
レーザ高度計レーダーサウンダ
磁力計
非熱的中性粒子観測器
電波・プラズマ波動観測器
重力観測器
惑星間電波干渉・ドップラー実験

JUICE Mission

JUpiter ICy moons Explorer
Exploring the emergence of habitable worlds around gas giants