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.