Summerschool on Geophysics of Terrestrial Planets July 21, 2014, Alpbach, Austria
Geophysics of Outer Planet Satellites
Hauke Hussmann DLR Institute of Planetary Research Berlin, Germany Overview Module
Outer Planet Satellite Systems Missions to the Outer Solar System Geophysics of the Moons Specific Applications Major Open Questions Future Exploration Overview Module
Outer Planet Satellite Systems Missions to the Outer Solar System Geophysics of the Moons Specific Applications Major Open Questions Future Exploration Outer Solar System Satellites
Jupiter
Saturn
Uranus
Neptune
Pluto-Charon Sizes of Planets and Satellites
Large Satellites
Ice is a major component in the interior of outer planet satellites
- Rocky Moons: Io, Earth's Moon, (Europa) - Large Icy Satellites: Ganymede Callisto, Titan - mid-sized satellites: Saturn's moons (without Titan), the moons of Uranus - Trans-Neptunian Objects and Triton Temperature gradient in the Solar Nebula
Volatile components can condense in the outer solar system. Outer planet satellites contain large fractions of water-ice. Comparative Study of the Galilean Satellites
Greeley, 2004 Density Gradient in the Jovian System
Density gradient of the Galilean satellites
Io: 0% ice Europa: 10% ice Ganymede: 50% ice Callisto: 50% ice The Saturn System
Titan Enceladus
Dione
Rhea Iapetus Mimas
Tethys
distance, r/RSaturn
Lack of density gradient in the Saturnian system. Oceans
ice-I ocean
high-pressure ices
ice/rock-mixture rock
core
Models for heat transport and thermal evolution suggest that Europa, Ganymede and Callisto possess subsurface oceans of more than 100 km of thickness. Experimental evidence comes from the induced magnetic fields detected by the Galileo mission. Planetary Satellites Satellite systems, compared
Satellite Count (status March 2014): http://home.dtm.ciw.edu/users/sheppard/satellites/
Syste m total R > 190 km Jupiter 67 4 Saturn 62 7 Uranus 27 5 Neptune 14 2 Pluto 5 1 Overview Module
Outer Planet Satellite Systems Missions to the Outer Solar System Geophysics of the Moons Specific Applications Major Open Questions Future Exploration 400 Years of Exploration
Galileo‘s observations in 1610 400 Years of Exploration
Grooved terrain on Ganymede
Galileo‘s observations in 1610 Mountains on Io Galileo‘s observations in 1995-2003 400 Years of Exploration
Medium- and high-resolution Galileo‘s observations in 1610 images of Europa‘s ice shell Galileo‘s observations in 1995 - 2003 Missions to the Jupitersystem
Pioneer 10 (1973) Pioneer 11 (1974)
First missions to the outer solar system, past the asteroid main belt.
Main objective: investigation of the radiation belts of Jupiter.
Missions to the Jupitersystem
Voyager 1 (1979) Voyager 2 (1979)
First investigation of the Galilean moons.
Missions to the Jupitersystem
Galileo (1995-2003) Io
- geologically most active body in the Solar System
- continous surface alteration
- silicate volcanism
- enormous surface heat flux (several watts per m²)
- subject to extreme tidal forces Europa
- surface of water ice
- lots of evidence for past (maybe ongoing) geologic activity
- complex geology
- young surface age (~ 30 – 150 Ma)
- subsurface ocean
Ganymede
- largest satellite
- highly differentiated
- intrinsic magnetic field
- past intense geologic activity
- surface modified by tectonism
- two different types of terrain (dark, heavily cratered, bright grooved terrain)
- subsurface ocean
Callisto
- similar in size, mass and bulk composition to Ganymede
- old cratered surface
- almost no geologic activity
- not fully differentiated
- subsurface ocean
Missions to the Jupitersystem
Cassini (2000) Missions to the Jupitersystem
Cassini (2004) Missions to the Jupitersystem
New Horizons (Feb 28, 2007) Missions to the Jupitersystem
Ulysses:
● flyby Oct 6 1990. ● Orbiter around the Sun, ● Mainly magnetospheric measurements ● No specific satellite science
Next Mission to Jupiter:
Juno:
● arrival in 2015 ● Near-polar orbit around Jupiter on a highly elliptical orbit ● Magnetic field and gravity field of Jupiter ● No dedicated satellite science Missions to the Saturn System
Voyager 1 and 2 flybys in 1981
Missions to the Saturn System
Voyager 1 and 2 flybys in 1981
Missions to the Saturn System
Cassini/Huygens: NASA/ESA Cassini is in orbit around Saturn since 2004 (possibly until 2017) Huygens touchdown on Titan’s surface in Jan 2015.
Saturn: Enceladus
Active Cryovolcanism Young and old surface terrain Thermal Activity Tidal heating as a driver? Source of Saturn’s E-ring
Saturn: Titan
Dense Atmosphere
Geologic Activity
Methane Cycle
Saturn: Titan
Huygens Landing
Oceans in the Outer Solar System
Europa: warm salty H2O, mantle contact Ganymede & Callisto: Titan: Ocean + open perched salty H2O(-NH3?) Methane seas
Enceladus: Global or locally confined ocean? Large KBO‘s and mid-size satellites Saturnsystem im Überblick Saturn‘s Moons – Impact Basins
DioneEinschlagskrater Rhea
Tethys
Mimas Iapetus Saturnsystem im Überblick Mimas‘ Herschel
Mimas‘ diameter: 396 km Herschels diameter: 130 km Saturnmonde: Tektonik (II) Tethys Saturnsystem im Überblick Chasmata auf Dione
Dione (1118 km)
50 km Saturnsystem im Überblick Iapetus
Leading hemisphere Trailing hemisphere
Iapetus: 1436 km; ridge: > 1300 km; ~ 20 km wide, ~ 20 km high Saturn: Iapetus
- Two hemispheres with different albedo - Shape consistent with hydrostatic euilibrium of an early rotation state - shape was ‘frozen’ during de-spinning
-- huge equatorial ridge
Uranus System
Voyager 2 flyby in 1986
Miranda Ariel Umbriel Titania Oberon Uranus System
Miranda, the innermost and smallest (199 km radius) of the large satellites.
Neptune: Triton
-- Voyager 2 flyby in 1989 -- Captured satellite
-- former KBO
-- Retrograde rotation
-- Active surface
Pluto-Charon System
Not yet, but watch out for New Horizons flyby! Closest approach to Pluto will be on July 14, 2015.
Artist concept of New Horizons spacecraft. Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI) Overview Module
Outer Planet Satellite Systems Missions to the Outer Solar System Geophysics of the Moons Specific Applications Major Open Questions Future Exploration How to infer the internal structure
Determination of C (Principal moment of Inertia). C is a function of the density distribution.
Homogeneous sphere with mass M and radius R (rotational axis through center): MoI = C/(MR2) = 2/5 = 0.4
Mass concentration towards the center: → MoI < 0.4
Measuring C: a) Precise determination of the gravity field(flybys + Doppler tracking)… b) Precession constant (surface element)…
… combined with model assumptions.
The Jupiter System
Density gradient in the Jovian system
Io: 0% ice Europa: 10% ice Ganymede: 50% ice Callisto: 50% ice
MoI < 0.4 => density increase towards the center Simple Models
Example: Jupiter's moon Europa
Low-order gravitational field.
-Line of evidence: -- acceleration of a spacecraft (here: Galileo) during close flybys (Doppler tracking) → Gravitational potential. Here up to degree 2 (J2 und C22) -- assumption of hydrostatic equilibrium taking into account the rotation and the gravitational perturbation by Jupiter -→ relation between moment of inertia and internal density distribution for a synchronously rotating satellite → two equations: 1. mass (mean density) 2. moment of inertia - assumptions on the density of each layer of a layered planet. Example: Europa
Europa (Anderson et al. 1998, Science 281) 4 close Galileo flybys (distance: 697, 591, 2048, 205 km). Example: Europa
Europa (Anderson et al. 1998, Science 281) 4 close Galileo flybys (distance: 697, 591, 2048, 205 km). Example: Europa Europa Flyby E12: Accelerations Compared Example: Europa Europa Flyby E12: Accelerations Compared Simple Models
Example: Jupiter’s moon Europa (Anderson et al. 1998, Science 281) 4 close flybys of Galileo (distance: 697, 591, 2048, 205 km)
C00 = 1 -6 C20 = -J2 = - 435.5 x 10 -6 C21 = -1.4 x 10 -6 S21 = 14.0 x 10 -6 C22 = 131.0 x 10 -6 S22 = -11.9 x 10
→ apart from the 2-body potential (C00-Term), Europa’s potential is mainly determined from the oblateness due to rotation (J2-term) and from the gravitational perturbation of Jupiter (C22-term).
For Europa only one coefficient has been determined independently.
In hydrostatic equilibrium (fully relaxed state) Europa assumes the shape of a tri- axial ellipsoid. In this case: J2 = 10/3 C22.
Simple Structure Models
To obtain an internal structure model from the gravity field, the following assumptions are used:
Europa rotates synchronously in Jupiter's equatorial plane. Europa is in hydrostatic equilibrium Europa consists of constant-density layers (incompressible)
f For a synchronously rotating satellite C22 = k2 qr/4 f k2 is the fluid Love number k that depends on the density profile. qr is the rotational parameter, the ratio of centrifugal force and gravitational force at the surface.
Omega is the rotation frequency, and T the rotation period. -4 For Europa: qr= 5 x 10 <<1 f k2 = 3/2 for a homogeneous sphere and <3/2 for density increase towards the center. For Europa: Radau-Darwin Relation
The Radau Darwin Equation provides the relation between fluid Love number and the principal moment of inertia of a body in hydrostatic equilibrium. Thus, by determining C22 (or in hydrostatic state alternatively J2) we obtain the dimensionless Moment of inertia factor MoI.
f For Europa MoI=0.346 using k2 =1.048.
The determined MoI yields an additional equation to compute the internal structure of a layered planet. It has to be solved simultaneously with the mass balance equation.
1. mass (or mean density ρ) 2. Moment of inertia
Simultaneously solving these equations determines 2 unknowns, e.g., core size and core density. The other unknown values must be assumed in a reasonable range. Therefore such models do not have unique solutions. However, variation of parameters, e.g. the density range of the individual layers can rule out several classes of models.
Europa is best described by a three-layer model consisting of an iron core, a silicate mantle, and an outer H2O-layer.
Europa’s H2O layer has a thickness of 70-170 km.
Next step: Calculate pressure and gravity from mass distribution using the hydrostatic equation:
Jupiter System The Galileian Satellites
During the Galileo mission (1995-2003 in Jupiter orbit) the moments of inertia Io Europa of all four Galilean satellites were determined from flybys.
Ganymede Callisto
Io, Europa, and Ganymede are highly differentiated. Callisto, however, is only partially differentiated.
However, these models are non-unique solutions. The Jupiter System
Model planets that are consistent with moment of inertia
Because of the small density contrast between water and solid ice it is not possible to determine the aggregation state of the H2O-layers. Other evidence is required to determine whether internal oceans exist in these moons.
The here shown structure models are however the basis for more complex models. E.g. the simple Europa model constrains the H2O-layer thickness to 70- 170 km.
Tides
In an external gravity field planetary bodies can be deformed. Part of the deformation is inelastic. This leads to tidal friction, which can be an internal heat source. A consequence of the dissipation of energy is the decrease of rotation periods.
During this phase of de-spinning stable couplings between rotational and orbital period can occur. Most common for satellites is the 1:1 spin-orbit coupling. Synchronous Rotation
The synchronous rotation state is a 1:1 coupling between rotation period and orbital period.
All major satellites and all the inner small satellites are locked in a synchronous state.
The central planet decreases the rotation rate of the satellite until the torques reach a minimum. In case of a circular orbit the torques vanish completely.
The time-scale on which satellites get into the synchronous state is significantly smaller than the age of the solar system.
Moon 20 Ma dΩ/dt = 3k2/(2αs Qs) mp/ms (Rs/a)³ n² Io 2 ka Europa 40 ka Ω rotational angular velocity Hyperion 1 Ga k2 the satellite’s Love number Triton 40 ka α axial moment of inertia Charon 600 ka Q dissipation factor m mass Pluto 10 Ma a semi-major axis n mean motion Tides in synchronous rotation In non-circular orbits gravitational forces are varying with time deforming the planet periodically. The friction due to the tidal flexing is an internal heat source.
ω = n
ω > n ω < n
ω = n ω spin n orbital angular velocity => librational tides => radial tides Tides
Io
Moon
Earth Io Jupiter Europa Ganymede
Callisto
Mass and size of Jupiter and the small distance of the satellites cause strong tidal forces. This is an important energy source in the Jupiter system. Solid Body Tide Love Numbers
Measures for tidally-induced global distortion of a solid planet or satellite
hn … radial displacement kn … potential variation ln … tangential displacement
complex numbers (amplitude, phase) depending on the interior structure (density profile), tidally-effective rheology, and period of tidal forcing.
n … degree of spherical harmonic representation of tidal potential Heat sources: tidal heating
Tidal deformation is characterized by ... the potential Love number k2 = Φi /Φe ... the radial displacement Love number h2 = g ur /Φe
Tidal dissipation (tidal heating rate) is proportional to the imaginary part of the potential Love number Im(k), and depends on the orbital elements.
Im(k) is a measure for the tidal phase lag depending on
structure, Rheology (=> temperature), orbital state (forcing period) Heat sources
Tidal heating of Io ...
... is about two orders of magnitude larger than radiogenic heating.
... drives silicate volcanism.
Europa's Ocean
- tidal dissipation in the ice shell and/or the silicate mantle - an ocean decouples the ice shell from the deep interior - ocean enhances tidal deformation and dissipation in the ice shell - heat budget ~70% tidal heating, ~30% radiogenic heating - conditions may be conducive to biological evolution 140 x 100 km Oceans in Icy Satellites
Oceans in Jupiter’s Moons
Europa: Ocean between ice-I layer and silicate mantle Ganymede: Ocean between ice-I and high-pressure ice Callisto: Partially differentiated, ocean between ice-I and ice-rock mixture
Induced Magnetic Fields
The axis of Jupiter's magnetic field is tilted by 9.6 degrees with respect to the rotational axis.
=> the satellites feel a time varying field
=> the field induces an electrical current (Faraday's law)
=> the current produces a secondary magnetic field (Ampere's law), the induced field Induced Magnetic Fields
... were detected at Callisto and Europa (Kivelson et al. 1998; Zimmer et al., 2000)
=> very good electrical conductor close to the surface
Interpretation: ocean + salts (electrolyte)
Interpretation of data at Ganymede is difficult because of the intrinsic field ( Kivelson et al., 2003) Detecting Oceans
Internal structure can be cold brittle ice determined by measuring the tidal thin ice layer deformation.
Indirect evidence for oceans can be inferred from k2 (tidal potential) or h2 (tidal amplitudes)
completely frozen ocean
The tidal amplitudes and phase lag strongly depend on the presence of an ocean. Amplitude at Europa: with ocean ~ 30 m, without ocean < 1 m Example: Titan Detecting Oceans
maximum amplitude h2 k2
Europa ~ 20 – 30 m (~60 cm) ~1.16 – 1.26 (~0.03) ~0.1 – 0.3 (~0.01) Ganymede ~ 3 – 4 m (~20 cm) ~1.0 – 1.5 (~0.2) ~0.5 (~0.08) Callisto ~ 2 – 3 m (~10 cm) ~0.9 – 1.5 (~0.2) ~0.3 (~0.08) in red: without ocean reference: Moore and Schubert 2000, 2003
cold brittle ice thin ice layer
completely frozen ocean Overview Module
Outer Planet Satellite Systems Missions to the Outer Solar System Geophysics of the Moons Specific Applications Major Open Questions Future Exploration Titan Module Gravity field from Cassini flybys: Iess et al., 2010 MoI of 0.34 suggests incomplete differentiation.
Determination of the tidal Love number k2 from the time variability of the quadrupole moments suggests a subsurface ocean (Iess et al., 2012) Two independent determinations: k2 = 0.589 +/- 0.150 and k2 = 0.637 +/- 0.224 (2σ). Titan
Complex evolution scenario of Titan providing a link between Methane production and thermal evolution (Tobie et al., 2006) Ganymede Internal Structure
Complex models take the compressibility into account (Sohl et al, Icarus, 2002). Interior Structure
Anderson et al., 1996
Sohl et al., 2002 Mass Anomalies
Evidence for local mass anomalies (Palguta et al., 2006, point-mass model; and Palguta et al., 2009, spherical cap disks model). One positive anomaly about 100 km thick, two negative anomalies less than 1000km thick. Lateral extension on the order of 1000 km. Interpretation of rock in Galileo regio and rock-free ice in two bright terrain regions. Ganymede‘s Intrinsic Magnetic Field
• Apart from the giant planets only three planetary bodies at present generate magnetic fields in their metal cores: the Earth, Mercury, and Ganymede
• Those three are very diverse with respect to size, composition and evolution
Which conditions are required for dynamos to operate?
By studying Ganymede’s magnetic field generation, we shed light on the questions why Europa and Io are lacking such fields or why dynamo activity has ceased on Mars.
Important objective: Clearly separate the induced signal from the self-induced field
Magnetic Field Magnetic dipole field confirmed by Galileo (Kivelson et al., 1996; Schubert et al., 1996)
Basic Conditions for a thermally-driven dynamo: (1) Temperature at the core-mantle boundary must exceed the melting temperature (2) Heat flux through the core-mantle boundary must exceed the adiabatic heat flux. -> Convective motion in the liquid outer core; freezing of inner core (Hauck et al., 2006; Bland et al., 2008)
Dynamo regime (Kimura et al. 2009).
Left: initial CMB temperature 1250 K; right: initial CMB temperature 1500K
Alternative concepts: Fe-snow (Rückriemen et al. 2013); Fe-snow, FeS flotation (Zhan and Schubert, 2012)
Tidal Distortion: Does Ganymede have a subsurface ocean?
Tidal amplitudes at the sub-jovian point with and without an ocean. Periodic variation of the tidal potential during one tidal cycle (one orbit of 7.15 days around Jupiter). Tidal deformation on Ganymede
Deformation pattern (maximum amplitudes for one tidal cycle) Combining k2 and h2 Measurements (example: Europa):
Measuring h2, k2, and Δ = 1+k2-h2
→ Combined measurements of k2 and h2 can significantly reduce the uncertainty of ice thickness due to the not well-constrained rigidity. See: Wahr et al., 2006, Journal of Geophys. Res. Io
• Sulfur-rich surface lacking impact craters. • Intense resurfacing due to exceptionally strong volcanic activity. • Surface heat flow in the order of ~ 1014 W, primarily due to the dissipation of tidal energy. • MoI factor, shape, density and surface composition suggest differentiated interior and hydrostaticity. Io Io
Moon More than 99% of Io‘s heating rate Earth is caused by tidal heating. Jupiter
Io, Global Coverage and Monitoring
• Spatial temperature distribution due to internal activity and not due Distribution of hot spots and active regions to solar insolation. • Temperatures above 1500 K confirmed by Galileo suggest silicate volcanism. • Io's heat flux not well constrained (~ several W/m2, < 13 W/m2).
Tupan Caldera: Imaging and thermal measurements suggest episodic volcanic activity. Enceladus Enceladus diameter (~ 504 km) Enceladus and Europa Enceladus‘ „Ridges“ Enceladus‘ „Grooves“ Enceladus - Geysers
Enceladus‘ geysers at the South-pole discovered in 2005 Enceladus Geysers – Saturn‘s E Ring
Enceladus‘ geysers are feeding Saturn‘s E-Ring Enceladus: Thermal Anomaly
(July 2005) Enceladus: Thermal Anomaly Enceladus: Thermal Anomaly Enceladus: Tidal Heating Rates (for global ocean models) Enceladus (fully Enceladus (without ocean) Enceladusdifferentiated) (with ocean)
viscosity range: 1013 – 5 x 1015 Pa s Europa Europa
• H2O dominated surface • Complex geology/young surface age • Sub-surface ocean • Ocean in contact with the silicate layer • Heat sources available due to radiogenic heating and tidal heating • Conditions might be conducive for life to evolve • Two-layer model (no metal core) is still possible based on galileo data
Is Europa Active?
Recently, evidence for plumes on Europa were found from HST observations (Roth et al., 2013). The source region has been constrained from modelling the tidal stresses at the surface. Constraints for Ice/Ocean Thickness at Europa
µice= 10 GPa 1 GPa Measurement Technique:
150 Hypothetical Static gravity range of (density structure) allowable values Tidal deformation 100 (Love number) A=0.85 Magnetic induction
50 Ocean thickness, km ? A=0.75 Radar penetration (lower bound)
1 10 100 Ice shell thickness, km
Europa; Gravity and Topography
• Topography, gravity, imaging, and subsurface sounding will tell us how internal dynamics are linked to surface features.
• Information on topography is only Double ridges on Europa. available at very few places on Europa from previous missions
• High-order gravity sounding may detect features at the silicate boundary (?)
Crater Cilix on Europa Europa: Double Ridges Europa: Chaos Overview Module
Outer Planet Satellite Systems Missions to the Outer Solar System Geophysics of the Moons Specific Applications Major Open Questions Future Exploration Major Questions Module
Do oceans exist on Europa, Ganymede, Callisto, Titan, Enceladus and other satellites? How thick are the ice shells? What is the composition of the oceans and how are they connected to the deep interior? What drives Enceladus’ activity? Are Europa and Ganymede currently active? What role does tidal heating play in the outer solar system? What is the history of differentiation of the satellites? How are orbital and thermal evolution connected?
How can we understand the diversity of outer planet satellites? Are these worlds habitable?
Overview Module
Outer Planet Satellite Systems Missions to the Outer Solar System Geophysics of the Moons Specific Applications Major Open Questions Future Exploration ESA’s Jupiter Icy Moon Explorer (JUICE)
Mission design JUICE SpacecraftJUICE Design Mission Model Profile instruments – phaseMission 2 – phases in Launch the JJuneupiter 2022 system (August 2023) Interplanetary transfer 7.6 years (Earth-Venus-Earth_Earth) (8 years) Jupiter orbit insertion and 11 months apocentre reduction with Ganymede gravity assists Mission design JUICE Spacecraft Design Model instruments Mission phases
Launch June 2022 Interplanetary transfer 7.6 years (Earth-Venus-Earth_Earth) Jupiter orbit insertion and 11 months apocentre reduction with Ganymede gravity assists 2 Europa flybys 36 days
Europa
East longitude 90 180 270 Altitude 1000 2000 3000 4000 5000 6000 7000 8000 Liquid water Explore Europa recently active zones
JUICE will tell us: • If liquid reservoirs exist • If the salinity is comparable to our oceans • How thick the crust is in chaos regions • If the moon is still active • Potentially where we could land in the future Mission design JUICE SpacecraftJUICE Design MissionModel Profile instruments – phaseMission 4 phases–
Launch CallistoJune + 2022 high latitudesCallisto Interplanetary transfer 7.6 years (Earth-Venus-Earth_Earth) (8 years) Jupiter orbit insertion and 11 months apocentre reduction with Ganymede gravity assists 2 Europa flybys 36 days
Reduction of vinf (Ganymede, 60 days Callisto) Increase inclination with 10 200 days Callisto gravity assists Liquid water Study Callisto as a remnant of the early Jovian system
JUICE OBJECTIVES • Characterise the outer shells, including the ocean • Determine the composition of the non-water ice material • Study the past activity including the differentiation processes Mission design JUICE SpacecraftJUICE Design Mission ModelProfile instruments – phasesMission 5-10 phases -
Launch GanymedeJune 2022 Interplanetary transfer (Earth- 7.6 years Venus-Earth_Earth) Jupiter orbit insertion and 11 months apocentre reduction with Ganymede gravity assists 2 Europa flybys 36 days
Reduction of vinf (Ganymede, 60 days Callisto) 7 Increase inclination with 10 200 days Callisto gravity assists Callisto to Ganymede 11 months Ganymede (polar) 10,000x200 km & 5000 km 150 days 9-10 500 km circular 102 days 6 8 200 km circular 30 days Total mission at Jupiter 3 years JUICE Payload
Selected by ESA in Feb. 2013
Liquid water Characterise Ganymede as a planetary object and possible habitat
JUICE OBJECTIVES • Characterise the ice shell, the extent of the ocean and its relation to the deeper interior • Determine global composition, distribution and evolution of surface materials • Understand the formation of surface features and search for past and present activity • Characterise the local environment and its interaction with the Jovian magnetosphere Europa Clipper Mission (NASA)
Are these Habitable Worlds ?
Figure adopted from S. Vance