Mimas, moon of Saturn and Death Star impersonator responsible for several gaps in Saturn s ring system

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1 Last time: Gravitational signs of large outer moons in the rings Ring shepherding/gap maintenance Longer lived structures due to mean motion resonances with large satellites Example: 2:1 resonance with Mimas Mimas orbits once for every two times a ring particle orbits ring particle is strongly perturbed by Mimas at same place in each orbit causes perturbation to be stronger than if not in resonance increases eccentricity of ring particle orbits, creates a gap Cassini Division Mimas, moon of Saturn and Death Star impersonator responsible for several gaps in Saturn s ring system

2 Last time: Gravitational signs of large outer moons in the rings Ring shepherding/gap maintenance Longer lived structures due to mean motion resonances with large satellites Example: 2:1 resonance with Mimas Mimas orbits once for every two times a ring particle orbits ring particle is strongly perturbed by Mimas at same place in each orbit causes perturbation to be stronger than if not in resonance increases eccentricity of ring particle orbits, creates a gap Now: Icy Satellites (particularly those of Saturn and Jupiter) The Saturnian System

3 Mimas Enceladus Tethys Dione Rhea Increasing distance from Saturn (A few) Saturnian Satellites Satellites mostly water ice (low densities of around 1 gram/cm 3, high albedo surfaces [albedo near 1]) albedo: fraction of incident light (e.g. sunlight) reflected from surface e i( ) Diam. (km) M E T(L4) T T(L5) D(L4) D D(L5) R Titan I P *within Saturn s E ring Titan Iapetus Phoebe For scale: Radius of the Moon: 1737 km Mercury: 2440 km Satellite images not to scale

4 Mimas Enceladus Tethys Dione Rhea (A few) Saturnian Satellites Closest to main rings, Herschel crater (~1/3 Mimas diameter) Cryovolcanic south pole e i( ) Diam. (km) M E T(L4) T T(L5) D(L4) D D(L5) R Titan I P *within Saturn s E ring Titan Thick atmosphere (see next lecture) Iapetus Phoebe

5 Enceladus: Cryovolcanism and the source of Saturn s E ring False color image of Enceladus Tiger stripes South pole of Enceladus Cryovolcanic jets back-illuminated by sunlight Jets originate from tiger stripes Tiger stripes appear warmer than surrounding areas One model for observed Enceladus cryolcanism

6 Mimas Enceladus Tethys Dione Rhea (A few) Saturnian Satellites Closest to main rings, Herschel crater (~1/3 Mimas diameter) Cryovolcanic south pole Telesto (L 4 ) Calypso (L 5 ) Helene (L 4 ) Polydeuces (L 5 ) e i( ) Diam. (km) M E T(L4) T T(L5) D(L4) D D(L5) R Titan I P *within Saturn s E ring (surfaces painted with E ring material) Titan Iapetus Phoebe

7 Mimas Enceladus Tethys Dione Rhea (A few) Saturnian Satellites Closest to main rings, Herschel crater (~1/3 Mimas diameter) Cryovolcanic south pole Telesto (L 4 ) Calypso (L 5 ) Helene (L 4 ) Polydeuces (L 5 ) e i( ) Diam. (km) M E T(L4) T T(L5) D(L4) D D(L5) R Titan I P *within Saturn s E ring (surfaces painted with E ring material) Titan Iapetus Phoebe Roughly half light, half dark material Source of Phoebe ring (debris from impacts) Dark material from Phoebe ring?

8 Io Europa Ganymede The largest Jovian Satellites Increasing distance from Jupiter These four satellites are also known as the Galilean satellites because they were identified by Galileo Galilei (early 1600s) with his improved telescope of the time (20x magnification). The fact that these objects were orbiting Jupiter cast doubt on the belief at the time that everything including the Sun orbited the Earth (Ptolemaic system) Callisto

9 Io Europa Ganymede Callisto The largest Jovian Satellites Increasing distance from Jupiter e i( ) Diam. (km) Dens. (g/cm 3 ) I E G C Rocky Missions to Jupiter that imaged moons at high-res: Voyagers 1&2 (flyby, 1979) Galileo (orbiter, 1995) Recent flyby: New Horizons (2007, on the way to Pluto in 2015) Future orbiter: Juno (2016) Decrease in density as distance from Jupiter increases Similar pattern to planets with distance from Sun formation similar to Solar System formation (rocky/refractory material condenses closer in, volatiles further out) All 4 rotate synchronously Io, Europa, Ganymede in resonance with each other 4:2:1 resonance keeps orbits eccentric Icy

10 Io- most volcanically active body in the Solar System Over 400 active volcanoes! Tvashtar plume on Io as seen by New Horizons Virtually no impact craters young surface

11 Europa Icy surface covered in dark cracks (lineae) Few impact craters Chaotic terrain formed via near surface warm ice or liquid water icy shell few km thick But impact craters point to thicker shell (about 20 km) Cycloidal ridges (chain of arcing ridges) indicate thin icy shell atop liquid ocean

12 Europa Icy surface covered in dark cracks (lineae) Few impact craters Visible Europa image with estimated plume location superimposed (inferred from Hubble UV data) Chaotic terrain formed via near surface warm ice or liquid water Discovery announced this past December: Plumes (likely made of water) Discovered via interaction of plumes with radiation in the Jovian magnetosphere (ionizes plume material and emits light at ultraviolet wavelengths) outer ice shell may be thinner in some places Future mission to Europa in planning stages

13 Io Europa Ganymede Callisto The largest Jovian Satellites: Interiors Increasing distance from Jupiter Jupiter s magnetic field induces a magnetic field in the moons some of interior is conductive (molten silicate for Io, liquid salty water for others) Ganymede also has its own magnetosphere (a magnetosphere within a magnetosphere!) Requires: Rapid rotation Enough interior heat for conductive liquid layer to convect

14 Europa The largest Jovian Satellites: Interiors Jupiter s magnetic field induces a magnetic field in the moons some of interior is conductive (molten silicate for Io, liquid salty water for others) Ganymede How do we know about the subsurface oceans? Surface features (for Europa) Induced magnetic fields Water phase diagram Callisto Clausius-Clapeyron relation: slope of liquidsolid transition depends on density difference negative slope solid less dense than liquid Water s unusual behavior: solid floats on liquid Water ice shell with subsurface ocean

15 Europa The largest Jovian Satellites: Interiors Jupiter s magnetic field induces a magnetic field in the moons some of interior is conductive (molten silicate for Io, liquid salty water for others) Ganymede Callisto How do we know about the subsurface oceans? Surface features (for Europa) Induced magnetic fields Water phase diagram (high pressures have different ice phases, Earth has only Ice I) Minimum melting temp of water: about 250K Pressure at depth: P=rho*g*h rho=density of overlying mat. (H 2 O ice=1000kg/m 3 ) g=gravitational acceleration (9.8 m/s 2 for Earth, 1.31 m/s 2 for Europa) h=depth, Assuming km thick ice layer: Pressure below this layer: bars

16 Europa The largest Jovian Satellites: Interiors Jupiter s magnetic field induces a magnetic field in the moons some of interior is conductive (molten silicate for Io, liquid salty water for others) Ganymede Callisto How do we know about the subsurface oceans? Surface features (for Europa) Induced magnetic fields Water phase diagram (high pressures have different ice phases, Earth has only Ice I) Minimum melting temp of water: about 250K Pressure at depth: P=rho*g*h rho=density of overlying mat. (H 2 O ice=1000kg/m 3 ) g=gravitational acceleration h=depth For Ganymede (1000km thick ice layer): Higher pressures, can get unusual Ice II, III, IV

17 Summary: Degree of internal heating Surface activity and interior properties indicate sources of heat needed in addition to radiogenic heating Recall: Enceladus very small icy moon of Saturn with cryovolcanism Size comparison of Enceladus to the British Isles

18 Summary: Degree of internal heating from tidal flexure Surface activity and interior properties indicate sources of heat needed in addition to radiogenic heating Tidal flexure more effective for -Closely orbiting bodies (e.g. Io) and -Eccentric orbits (like those caused by resonances, Io s distance from Jupiter varies by 1%)

19 Summary: Degree of internal heating from tidal flexure Surface activity and interior properties indicate sources of heat needed in addition to radiogenic heating Tidal flexure more effective for -Closely orbiting bodies (e.g. Io) and -Eccentric orbits (like those caused by resonances, Io s distance from Jupiter varies by 1%)