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 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 (A few) Saturnian Satellites Mimas Increasingdistance from Saturn e i(°) Diam. (km) M 0.0202 1.566 396 E 0.0047 0.010 504 Enceladus Satellites mostly T(L4) 0.0000 1.1558 25 water ice (low T 0.0001 0.168 1062 densities of T(L5) 0.0000 1.474 21 around 1 D(L4) 0.0022 0.212 35 3 Tethys gram/cm , high D 0.0022 0.002 1123 albedo surfaces D(L5) 0.0192 0.177 3 [albedo near 1]) R 0.0013 0.327 1527 Dione Titan 0.0288 0.3485 5151 albedo: fraction of I 0.0286 15.7 1469 incident light (e.g. P 0.1562 173.0 213 Rhea sunlight) reflected *within Saturn’s E ring from surface

Titan For scale: Radius of the Moon: 1737 km Iapetus Mercury: 2440 km Phoebe Satellite images not to scale (A few) Saturnian Satellites Mimas Closest to main rings, Herschel crater (~1/3 Mimas’ e i(°) Diam. (km) diameter) M 0.0202 1.566 396 E 0.0047 0.010 504 Enceladus Cryovolcanic south pole T(L4) 0.0000 1.1558 25 T 0.0001 0.168 1062 T(L5) 0.0000 1.474 21 D(L4) 0.0022 0.212 35 Tethys D 0.0022 0.002 1123 D(L5) 0.0192 0.177 3 R 0.0013 0.327 1527 Dione Titan 0.0288 0.3485 5151 I 0.0286 15.7 1469 P 0.1562 173.0 213 Rhea *within Saturn’s E ring

Titan Thick atmosphere (see next lecture)

Iapetus Phoebe Enceladus: Cryovolcanism and the source of Saturn’s E ring

Cryovolcanic jets South pole of Enceladus Tiger stripes back-illuminated by sunlight False color image of Jets originate from Enceladus “tiger stripes”

Tiger stripes appear warmer than surrounding areas

One model for observed Enceladus’ cryolcanism (A few) Saturnian Satellites Mimas Closest to main rings, Herschel crater (~1/3 Mimas’ e i(°) Diam. (km) diameter) M 0.0202 1.566 396 E 0.0047 0.010 504 Enceladus Cryovolcanic south pole T(L4) 0.0000 1.1558 25 T 0.0001 0.168 1062 T(L5) 0.0000 1.474 21 Telesto (L4) D(L4) 0.0022 0.212 35 Tethys D 0.0022 0.002 1123 Calypso (L ) 5 D(L5) 0.0192 0.177 3

Helene (L4) R 0.0013 0.327 1527 Dione Titan 0.0288 0.3485 5151 Polydeuces (L5) I 0.0286 15.7 1469 P 0.1562 173.0 213 Rhea *within Saturn’s E ring (surfaces “painted” with E ring material)

Titan

Iapetus Phoebe (A few) Saturnian Satellites Mimas Closest to main rings, Herschel crater (~1/3 Mimas’ e i(°) Diam. (km) diameter) M 0.0202 1.566 396 E 0.0047 0.010 504 Enceladus Cryovolcanic south pole T(L4) 0.0000 1.1558 25 T 0.0001 0.168 1062 T(L5) 0.0000 1.474 21 Telesto (L4) D(L4) 0.0022 0.212 35 Tethys D 0.0022 0.002 1123 Calypso (L ) 5 D(L5) 0.0192 0.177 3

Helene (L4) R 0.0013 0.327 1527 Dione Titan 0.0288 0.3485 5151 Polydeuces (L5) I 0.0286 15.7 1469 P 0.1562 173.0 213 Rhea *within Saturn’s E ring (surfaces “painted” with E ring material)

Titan

Roughly half light, half dark Dark material Iapetus material from Phoebe Phoebe ring? Source of Phoebe ring (debris from impacts) The largest Jovian Satellites These four satellites are also known as the Galilean

Increasingdistance from Jupiter satellites because they were identified by Galileo Galilei (early 1600s) with his improved telescope of the time (20x Io magnification).

The fact that these objects were orbiting Jupiter cast Europa doubt on the belief at the time that everything including the Sun orbited the Earth (Ptolemaic system)

Ganymede

Callisto The largest Jovian Satellites e i(°) Diam. (km) Dens. (g/cm3)

Increasingdistance from Jupiter I 0.0041 0.05 3660 3.528 Rocky E 0.0094 0.471 3122 3.014 Io G 0.0011 0.204 5262 1.942 Icy C 0.0074 0.205 4821 1.834

Europa 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) Ganymede 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, Callisto volatiles further out) All 4 rotate synchronously Io, Europa, Ganymede in resonance with each other 4:2:1 resonancekeeps orbits eccentric 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 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

Cycloidal ridges But impact (chain of arcing craters point ridges) indicate to thicker shell thin icy shell (about 20 km) atop liquid ocean Europa Icy surface covered in dark cracks (lineae) Few impact craters

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 Visible Europa image with estimated plume Future mission to Europa in planning stages location superimposed (inferred from Hubble UV data) The largest Jovian Satellites: Interiors Jupiter’s magnetic field induces a magnetic field in

Increasingdistance from Jupiter the moonssome of interior is conductive (molten silicate for Io, liquid salty water for others) Io

Europa

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

How do we know about the subsurface oceans? Surface features (for Europa) Ganymede Induced magnetic fields Water phase diagram

Callisto

Clausius-Clapeyron relation: slope of liquid- solid 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 The largest Jovian Satellites: Interiors Jupiter’s magnetic field induces a magnetic field in  Europa the moons some of interior is conductive (molten silicate for Io, liquid salty water for others)

How do we know about the subsurface oceans? Surface features (for Europa) Ganymede Induced magnetic fields Water phase diagram (high pressures have different ice phases, Earth has only Ice I) Minimum melting temp of water: about 250K Callisto

Pressure at depth: P=rho*g*h 3 rho=density of overlying mat. (H2O ice=1000kg/m ) g=gravitational acceleration (9.8 m/s2 for Earth, 1.31 m/s2 for Europa) h=depth, Assuming 100-200km thick ice layer: Pressure below this layer: 1300-2700 bars The largest Jovian Satellites: Interiors Jupiter’s magnetic field induces a magnetic field in  Europa the moons some of interior is conductive (molten silicate for Io, liquid salty water for others)

How do we know about the subsurface oceans? Surface features (for Europa) Ganymede Induced magnetic fields Water phase diagram (high pressures have different ice phases, Earth has only Ice I) Minimum melting temp of water: about 250K Callisto

Pressure at depth: P=rho*g*h 3 rho=density of overlying mat. (H2O ice=1000kg/m ) g=gravitational acceleration h=depth For Ganymede (1000km thick ice layer): Higher pressures, can get unusual Ice II, III, IV 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 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%) 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%)