AST 111 Lecture 21 Jovian Worlds II

The Jovian Moons Numerous! Galilean Moons

The Jovian Moons Jupiter:63 Moons Saturn: 62 Moons Uranus: 27 Moons Neptune:13 Moons

The Jovian Moons They resemble terrestrial planets – Unique geology – Solid surface – Hot interiors – Magnetic Fields But they differ in composition – These are much more icy

The Jovian Moons Medium and large moons: – Formed by accretion in gas disks – “Mini solar systems” Small moons mostly captured asteroids and comets

The Galilean Moons Galilean Moons geologically active Io is the most volcanically active world in our solar system – Impact craters totally erased – Low gravity, thin atmosphere  good plumes!

The Galilean Moons Why is Io active whereas our Moon is not? A planet’s gravity gets weaker farther away Results in tidal forces – Stretches bodies in orbit – Farther away, weaker

The Galilean Moons Io has a very elliptical orbit – Tidal forces change significantly So Io is constantly being stretched and contracted! This gives it the heat that produces its geological activity.

The Galilean Moons Europa’s surface is covered by water ice Europa farther from Jupiter – Weaker tidal forces Few impact craters – Many erased… how? Liquid water below crust or convection of ice

The Galilean Moons Tidal forces on Europa not as strong (it’s farther from Jupiter) – Cause sliding and fracturing of icy crust – Warms and melts

The Galilean Moons

Could Europa have a warm liquid ocean? – Computer models suggest mile thick region that could have liquid water – Warmth provided by tidal heating – Could also be convecting ice, but: Studies of magnetic field suggest liquid salt water Europa’s magnetic field responds to Jupiter’s Can’t do that if it’s ice

The Galilean Moons Ganymede – Surface of water ice – Could also have liquid ocean, but deeper – Impacts exposed fresh ice around craters Callisto – Not much internal heat – Not much tidal heating – Cooled quickly; captured dust

Saturn’s Moon Titan Titan has a thick atmosphere – Hides most colors of surface 90% Nitrogen, 10% H compounds Color from complex chemistry – Organic molecules! – Too cold for life, but gives clues about life on Earth

Prebiotic Molecules Researchers created the pressure and composition of Titan’s upper atmosphere Prebiotic molecules were found!

Saturn’s Moon Titan

Methane evaporation / condensation cycle Like water cycle on Earth Rivers and lakes of methane

Saturn’s Moon Enceladus 500 km across Geologically active – Heat provided by tidal forces Venting plumes of water Surface of water ice Sub-surface salt water?

Saturn’s Moon Enceladus

Saturn’s Moon Mimas Impact that created Hershel crater almost energetic enough to fracture Mimas

Uranus’ Moon Miranda A collision may have shattered it, but only barely It “reassembled”, but everything didn’t quite fit! Rock tried to settle, ice tried to float

Neptune’s Moon Triton Orbits Neptune “backwards” – Captured, not formed in Neptune’s disc Signs of “ice geology” – “Cantaloupe terrain”: blobs of ice rise and fall Colder than Pluto – Reflects more of the (very weak) sunlight

Saturn’s Rings

Rings look continuous They’re actually many particles – Size ranges from “grains of dust to boulders” Made of reflective water ice

Saturn’s Rings Many distinct rings with narrow gaps – Specifics not well understood Gap moons keep gaps clear and create ripples in rings These gap moons can line up periodically – Creates large gaps like the Cassini division

Saturn’s Rings The moons can also act like “shepherds” and keep particles in line

Saturn’s Rings Saturn’s F-ring looks sharp farther out Closer in, not sharp at all

Rings on the Other Jovian Worlds Which of the other Jovian worlds have rings?

Rings on the Other Jovian Worlds Rings on Uranus and Neptune found by stellar occultation A star blinked nine times before going behind Uranus Have been imaged with Voyager and telescopes

Rings on the Other Jovian Worlds Saturn’s rings are made of reflective water ice – Dustier (darker) farther inward Rings on other Jovian worlds are made of dark dust

Why do Jovian worlds have rings? Tidal forces can rip a moon to shreds – Must get close to planet – Roche Tidal Zone – BUT… large moons don’t just wander close to a planet A moon may have tried to form close to Saturn, but broke up into smaller pieces These small pieces constantly grind against each other – This continually supplies the rings with material

Why do Jovian worlds have rings?

Saturn’s E-Ring E-ring material supplied by: Plumes from Enceladus Meteorite impacts on all moons