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 MimasMimas orbits once for every two times a ring particle orbits

ring particle is strongly perturbed by Mimas at same place in each orbitcauses 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 MimasMimas orbits once for every two times a ring particle orbits

ring particle is strongly perturbed by Mimas at same place in each orbitcauses 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

Enceladus

Tethys

Dione

Rhea

Titan

Iapetus

Phoebe

e i(°) Diam. (km)M 0.0202 1.566 396E 0.0047 0.010 504T(L4) 0.0000 1.1558 25T 0.0001 0.168 1062T(L5) 0.0000 1.474 21D(L4) 0.0022 0.212 35D 0.0022 0.002 1123D(L5) 0.0192 0.177 3R 0.0013 0.327 1527Titan 0.0288 0.3485 5151I 0.0286 15.7 1469P 0.1562 173.0 213*within Saturn’s E ring

Increasin

g distan

ce from

Saturn

Satellites mostly water ice (low densities of around 1 gram/cm3, high albedo surfaces [albedo near 1])

albedo: fraction of incident light (e.g. sunlight) reflected from surface

Satellite images not to scale

For scale:Radius of the Moon: 1737 km Mercury: 2440 km

(A few) Saturnian Satellites

Mimas

Enceladus

Tethys

Dione

Rhea

Titan

Iapetus

Phoebe

e i(°) Diam. (km)M 0.0202 1.566 396E 0.0047 0.010 504T(L4) 0.0000 1.1558 25T 0.0001 0.168 1062T(L5) 0.0000 1.474 21D(L4) 0.0022 0.212 35D 0.0022 0.002 1123D(L5) 0.0192 0.177 3R 0.0013 0.327 1527Titan 0.0288 0.3485 5151I 0.0286 15.7 1469P 0.1562 173.0 213*within Saturn’s E ring

Closest to main rings,Herschel crater (~1/3 Mimas’ diameter)

Cryovolcanic south pole

Thick atmosphere (see next lecture)

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

False color image of Enceladus

Cryovolcanic jets back-illuminated by sunlightJets originate from “tiger stripes”

South pole of EnceladusTiger stripes

Tiger stripes appear warmer than surrounding areas

One model for observed Enceladus’ cryolcanism

(A few) Saturnian Satellites

Mimas

Enceladus

Tethys

Dione

Rhea

Titan

Iapetus

Phoebe

Telesto (L4)

Calypso (L5)

Helene (L4)

Polydeuces (L5)

e i(°) Diam. (km)M 0.0202 1.566 396E 0.0047 0.010 504T(L4) 0.0000 1.1558 25T 0.0001 0.168 1062T(L5) 0.0000 1.474 21D(L4) 0.0022 0.212 35D 0.0022 0.002 1123D(L5) 0.0192 0.177 3R 0.0013 0.327 1527Titan 0.0288 0.3485 5151I 0.0286 15.7 1469P 0.1562 173.0 213*within Saturn’s E ring(surfaces “painted” with E ring material)

Closest to main rings,Herschel crater (~1/3 Mimas’ diameter)

Cryovolcanic south pole

(A few) Saturnian Satellites

Mimas

Enceladus

Tethys

Dione

Rhea

Titan

Iapetus

Phoebe

e i(°) Diam. (km)M 0.0202 1.566 396E 0.0047 0.010 504T(L4) 0.0000 1.1558 25T 0.0001 0.168 1062T(L5) 0.0000 1.474 21D(L4) 0.0022 0.212 35D 0.0022 0.002 1123D(L5) 0.0192 0.177 3R 0.0013 0.327 1527Titan 0.0288 0.3485 5151I 0.0286 15.7 1469P 0.1562 173.0 213*within Saturn’s E ring(surfaces “painted” with E ring material)

Closest to main rings,Herschel crater (~1/3 Mimas’ diameter)

Cryovolcanic south pole

Source of Phoebe ring (debris from impacts)

Roughly half light, half dark material

Dark material from Phoebe ring?

Telesto (L4)

Calypso (L5)

Helene (L4)

Polydeuces (L5)

The largest Jovian Satellites

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)

Increasin

g distan

ce from

Jup

iter

Io

Europa

Ganymede

Callisto

The largest Jovian Satellites

e i(°) Diam. (km) Dens. (g/cm3)I 0.0041 0.05 3660 3.528E 0.0094 0.471 3122 3.014G 0.0011 0.204 5262 1.942C 0.0074 0.205 4821 1.834

Increasin

g distan

ce from

Jup

iter

Io

Europa

Ganymede

Callisto

Rocky

Icy

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 increasesSimilar 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 resonancekeeps 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 cratersyoung surface

Europa

Icy surface covered in dark cracks (lineae)Few impact craters

Chaotic terrain formed via near surface warm ice or liquid watericy shell few km thick

Cycloidal ridges(chain of arcing ridges) indicate thin icy shell atop liquid ocean

But impact craters point to thicker shell (about 20 km)

Europa

Icy surface covered in dark cracks (lineae)Few impact craters

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 location superimposed (inferred from Hubble UV data)

Chaotic terrain formed via near surface warm ice or liquid water

Future mission to Europa in planning stages

The largest Jovian Satellites: Interiors

Increasin

g distan

ce from

Jup

iter

Io

Europa

Ganymede

Callisto

Jupiter’s magnetic field induces a magnetic field in the moonssome 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 rotationEnough interior heat for conductive liquid layer to convect

The largest Jovian Satellites: Interiors

Europa

Ganymede

Callisto

Jupiter’s magnetic field induces a magnetic field in the moonssome 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)Induced magnetic fieldsWater phase diagram

Clausius-Clapeyron relation: slope of liquid-solid transition depends on density differencenegative slopesolid less dense than liquidWater’s unusual behavior: solid floats on liquidWater ice shell with subsurface ocean

The largest Jovian Satellites: Interiors

Europa

Ganymede

Callisto

Jupiter’s magnetic field induces a magnetic field in the moonssome 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)Induced magnetic fieldsWater 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*hrho=density of overlying mat. (H2O ice=1000kg/m3)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

Europa

Ganymede

Callisto

Jupiter’s magnetic field induces a magnetic field in the moonssome 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)Induced magnetic fieldsWater 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*hrho=density of overlying mat. (H2O ice=1000kg/m3)g=gravitational acceleration h=depthFor 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 Enceladusto 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%)