Jovian Planet Rings The Rings of Saturn From
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Jovian Planet Rings
The Rings of Saturn • From Earth, they look solid. • concentric rings & Cassini division • From within the rings, we would see many individual particles • size ranges from boulders to dust • reflective H 2 O ice (snowballs) • many collisions keep ring thin
From spacecraft flybys, we see thousands of individual rings separated by narrow gaps Cassini Division
● ● Saturn’s rings are very thin, in some cases less than 100 meters thick. The rings are not solid sheets but are made up of small particles of water ice or water-ice mixed with dust.
Three distinct rings are visible from Earth, and were named (outer to inner) A, B, and C. Additional rings were detected by spacecraft and named D, E, F, … H. Prominent gaps in the rings are also named, e. g. , Cassini Division, Encke Division, …
● ● ● The largest division between rings is known as the Cassini division. This space is caused largely by the gravity of Mimas acting synchronously (2: 1 resonance) on the orbital path of nearby ring particles. Some other ring features are explained by the presence of small shepherd moons. Mimas
The F ring: Confined by Shephard Satellites Prometheus and Pandora The A ring Cassini Voyager
The other ring systems: fewer particles, smaller in extent, darker particles
Jupiter’s Rings Voyager from “behind” Jupiter Ø Voyager I discovered a thin ring around Jupiter. Ø The ring is close to Jupiter, extending to only about 1. 8 planetary radii. Ø The ring is thought to be replenished from the small moonlets within or near it.
Rings of Uranus & Neptune Ø The rings of Uranus and Neptune and are made of particles which are darker and smaller than that of Saturn. Ø The Uranian rings are narrow, a few of which are clearly confined by shepherding moons. Ø The Neptunian rings vary in width and are confined by resonances of some of the moons.
Roche Limit ● ● The Roche limit is the minimum radius at which a satellite (held together by gravitational forces) may orbit without being broken apart by tidal forces. Saturn’s rings are inside Saturn’s Roche limit, so no moons can form from the particles.
Origin of Planetary Rings • Within 2 or 3 planetary radii of a planet (the Roche Limit), tidal forces will be greater than the gravity holding a moon together. A moon which wanders inside the Roche limit will be torn apart. Matter from the mini-nebula at this distance will not form moon. • Rings can not last the age of the Solar System. Particles will be ground to dust by micrometeorite collisions. Atmospheric drag will cause ring particles to fall into planet. ● There must be a source to replenish ring particles. – ● gradual dismantling of small moons by collisions, tidal forces, etc. The appearance of ring systems must change dramatically over millions or billions of years.
Jupiter’s Galilean Satellite’s
Jovian Planets have Numerous Moons • small moons (< 300 km across) • not spherical • probably captured asteroids • Medium/large moons formed like planets out of the “mini-Solar nebulae” surrounding the Jovian planets
Are the large moons are too small for active geology to occur? • No! • terrestrial planets made mostly of rock; Jovian moons mostly ice Ices melt at lower temperatures than rock. ● less heating is required to have molten cores • volcanism and tectonics can occur There is another heat source. • • tidal heating plays a more important role There is very little erosion due to lack of substantial atmospheres with the exception of Titan. • ●
The Jovian Moons • The moons of Jupiter become less dense as you get farther from Jupiter • • “mini Solar System” Gravitational tidal heating keeps the interiors of the inner moons hot.
Io’s Volcanoes Io The most volcanically active world in the solar system.
Io �� • Jupiter’s tidal forces flex Io like a ball of silly putty. • friction generates heat • interior of Io is molten • Volcanoes erupt frequently. �� • sulfur in lava accounts for yellow color • surface ice vaporizes and jets away • Evidence of tectonics & impact cratering erased.
• Io loses volcanic gases into space. Ø ions of Sulfur, Oxygen, Sodium form a donut-shaped belt of charged particles, called the Io torus Ø they follow Io’s orbit & are a source of charged particles for the auroras of Jupiter The Io Torus
Europa: An ice-covered world
– Icebergs
ice cliffs
Grooves & channels
Jupiter’s Europa – Has tidal heating, similar to IO but weaker – Has a young cracked water ice crust perhaps only a few kilometers thick – Thought to have a warm ocean of salty liquid water below its crust. – life?
Europa’s interior warmed by tidal heating. Internal structure derived from measurements taken by spacecraft in the Jovian system. Salty - Europa has a magnetic
Sub-Crust Ocean . . . First New Ocean Since Balboa
Life in the Ocean? Conditions are consistent with the. presence of volcanic. vents. (black smokers) at bottoms of ocean. Life could have developed there.
Missions to Europa http: //www. jpl. nasa. gov/europaorbiter/
Ganymede
Ice-covered moon
Ganymede ● Largest moon in the solar system ● Clear evidence of geological activity ● Tidal heating expected - but is it enough?
Ganymede Wrinkles due to tectonic movement in ice crust in (distant) past possible water deep below?
● Cratering – – – ● Dark areas: cratering upon cratering several byr old Bright areas: far fewer craters and grooves Explanation: “lava” (i. e. , water) eruptions followed by freezing Ocean? – – – ● Ganymede Magnetic field convecting core Part of magnetic field varies with Jupiter’s rotation electrically conducting interior (brine? ) Salts found on the surface Heat source – – – Less tidal heating than Europa (larger distance from Jupiter) Large mass more radioactivity Much less heat than in Europa thick crust (>150 km? )
Callisto
● ● ● “Classic” cratered iceball. No tidal heating no orbital resonances. But it has magnetic field! This is not understood. Callisto
Scarp close up Callisto Possible water deep?
Callisto ● Cratering ● Heavily cratered everywhere no water gushing to the surface Gravity ● Undifferentiated: mix of ice and rock throughout Induced magnetic field ● Exists underground ocean? Not clear. Heat source? – – – Does not participate in the tidal resonance Radioactive decay: only possibility for heating of interior
Given our discussion of tidal synchronization of the rotation and orbital periods, what might this say about planets and stars? (red) (yellow) (green) (blue) nothing planets close to stars will have synchronized rotations planets far from stars will have synchronized rotations it will depend upon the composition of the planet
Given our discussion of tidal synchronization of the rotation and orbital periods, what might this say about planets and stars? (yellow) (red) nothing planets close to stars will have synchronized rotations
Enceladus Titan
Titan
Titan Ø Huygens spacecraft landed on surface Ø Cassini spacecraft has made several close flybys Ø 2 nd largest moon Ø Only moon with a substantial atmosphere
Saturn’s Titan Ø atmosphere denser than Earth’s but very cold (100 K) 100 K and composed mostly of N 2 and methane (CH 4) Ø Completely enshrouded in smog -like clouds Ø Methane acts like water (liquid). Ø Few craters on the surface. Ø Surface eroded by liquids Ø Methane/Ethane lakes
Huygens Probe
On the surface! “Rocks” of ice?
View from Huygens Spacecraft during descent to surface
River gully? Coastline? Hydrocarbon lakes
Cold, windy, surface like wet clay, ice “rocks”
Dunes Earth Titan
Physical Characteristics ● Size – ● Among moons, second only to Ganymede (measured by surface, not atmosphere) Mass – Almost double that of our Moon – Density: 1. 9 gm/cm 3 equal mixture of rock and ice – Thought to be differentiated: rocky core of silicates with a crust of water ice
Surface ● ● ● Gross features: – Few impact craters surface 130 -300 Myr old – Tectonics: thin features for hundreds of miles Cryo-volcano: – 30 km volcano observed on Titan, including caldera inside – Magma would be mainly CH 4 & H 2 O – Energy? : tidal heating or radioactivity Erosion: – Huygens saw round ice pebbles – Sinuous channels: liquids – East-west dunes near equator with sharp western boundaries: super-rotating winds
Atmosphere ● ● ● Pressure: 1. 5 bar Surface temperature: 180 C (-290 F) Composition: 92 98% N 2 + 2 6% methane (CH 4) Constantly smoggy: UV breaking up CH 4 into radicals Radicals combine to form complex hydrocarbons: C 2 H 6, C 2 H 2, HCN, C 6 H 6
Possible Earthlike Processes ● Tectonics ● Weather, including rain (methane) ● Erosion by winds and liquids ● Formation of complex organic compounds ● Greenhouse effect ● Volcanism (molten water, not rock) ● But: all at a much lower temperature
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