Chapter 11: Planetary Moons and Rings

Cause of ring arcs in Neptune

+ Puzzling, because mutual collisions among their particles should cause the particles to be spread uniformly around their orbits. + This clumping is suggested to caused by orbital resonances with the moon Galatea that orbits just inside the Adams Ring. Adam Ring: much of the material in it is clumped together into several ring arcs.

Ring formation

+ Result of tidal stresses, in which a moon or planetesimal comes within the Roche limit and then is pulled apart, leaving many small pieces orbiting the planet.

Backlighting on Saturn's rings.

Diffuse rings contain tiny particles that show up best when the viewer is looking at these rings in the direction of the Sun - backlighting (the Sun is behind the tiny objects). This is because light falling on tiny objects tends to continue in the direction away from the source of illumination. Very little of the light is scattered off to the side, and almost none is scattered back toward the source. Help discover Saturn's G Ring.

Titan (Saturn) atmosphere

+ 30% denser than that of the Earth. + Photodissociation of ammonia is the source of atmospheric nitrogen. + Photodissociation of methane makes methane breaks into fragments and recombines to form organic compounds such as ethane. + Orange hue color -> organic smog from organic compounds.

Triton surface

+ Absence of craters. -> young surface + Terrain looks like the skin of a melon. + Irregular pits and hills that may be caused by slushy ice emerging onto the surface from the interior. + Veinlike features result from ice oozing out along fractures. + Smooth volcanic plain: irregularly shaped depression formed when mixture of water, methane, and nitrogen ice melted in the interior of Triton and erupted onto the surface.

Europa's surface features

+ An outer shell of ice with surface cracks and creases. + Few impact craters -> young surface. + Chaotic terrain: icy crust has been broken into slabs that have shifted into new positions. + Thin, brittle crust of ice floating atop a liquid or slushy ocean.

Callisto's surface

+ Darkest of the Galilean moons, yet it is still as twice as reflective as Earth's Moon => mixture of water ice and rocky materials. + Uniform, heavily cratered terrain. + Valhalla: multiringed structure of impact. + A liquid ocean of water mixed with ammonia could exist beneath the heavily cratered surface.

Ring evolution

+ Do not have long-term stability. + Ring particle are constantly colliding with one another in their tightly packed environment, either gaining or losing orbital energy. This redistribution of orbital energy can cause particles at the ring edges to leave the rings and drift away, aided by the pressure of sunlight. + Eventually disperse.

Saturn's diffuse rings

+ Examples: E and G rings + Particles are far apart, and rare collision between them can cause their individual orbits to become eccentric, inclined, or both. + The particles tend to remain in these disturbed orbits. + Spread out horizontally and thicken vertically, without any obvious boundaries.

Ganymede's filled impact scars

+ Form as viscous flow smooths out structures left by impact on icy surfaces overlying water or slush. + Bright, flat, circular patches found in the moon's dark terrain.

The sources of Ring Material

+ From past moon's pieces => ring material is similar to the composition of the moons (can be used to predict the composition of the past moon). + Contributed from the dust of the moons. + Contributed by the volcanism of the moons (sulfur particles from Io of Jupiter, or ice crystals from Enceladus of Saturn).

Ganymede's differentiation

+ Fully differentiated. + Ice crust - Water or slush - Ice mantle - Silicate mantle - Iron core. + Large, salty ocean underneath its icy surface (25x volume of the Earth)

Titan's surface features

+ Has terrain reminiscent of those on Earth, with networks of channels, ridges, hills, and flat areas that may be dry lake basins. + These terrains suggest a methane cycle (analogous to Earth's water cycle) + Lake contains liquid and is not frozen or dry. + May have ocean buried beneath its surface. + Rich with other organic compounds, such as cyanogen and ethane.

Leading/ Trailing hemisphere

+ Leading: The hemisphere that faces forward, into the direction of motion of a tidally locked satellite. + Trailing: The hemisphere that faces backwards, away from the direction of motion of a tidally locked satellite.

Why does the Io's surface have many color?

+ Mixtures of sulfur, sulfur dioxide, and sulfurous salts of sodium and potassium cause a wide variety of colors on Io's surface - red, yellow, orange, and brown. + Bright patches: fields of SO2 snow.

Speed of Ring particles

+ Orbital speed: closer -> faster + Relative speed to one another: low because they are all orbiting in the same direction. Upward and downward motion will be cancelled, and inward and outward motion will be cancelled.

Triton (Neptune) orbit

+ Retrograde orbit -> Triton was captured by Neptune after the planet's formation. + Experiences extreme tidal stresses from Neptune, generating thermal energy. -> The interior may be melted, allowing Triton to become chemically differentiated.

Uranus' rings

+ The Epsilon Rings is the most prominent ring. + The innermost ring is wide and diffuse, with an undefined inner edge.

Thickness of Saturn's rings

+ The diameter of Saturn's bright ring system is 10 million times the thickness of the rings. + For comparison, if Saturn were shrunk down to the size of a basketball, then the thickness of the rings would be about 1/250 the thickness of a human hair.

Surface features providing clues to a moon's geological history

+ The freshness of the water ice on the surface indicates the age of those surface. Meteorite dusk darkens the icy surfaces of moons just as dirt darkens snow. A bright surface often means a fresh surface. + Older surfaces have more craters.

Cassini Division

+ The lower-density gap between Saturn's A and B rings. + Caused by the gravitational tug of the moon Mimas. + So wide that a planet Mercury would almost fit within it. + Filled with materials, although it is less dense than the material in the bright rings.

Why is Titan a target for extra terrestrial life exploration?

+ The only moon with a significant atmosphere and the only Solar System body besides Earth that has standing liquid on the surface and a cycle of liquid rain and evaporation. + The presence of liquid and of organic compounds that could be biological precursors for life.

Iapetus's surface

+ The trailing hemisphere is bright, reflecting half the light. + Much of the leading hemisphere is as black as tar. These dark deposits appear only in the leading hemisphere of Iapetus, suggesting that they might be debris that was blasted off small retrograde moons of Saturn by micrometeoritic impacts and swept up by Iapetus as it moved along in its prograde orbit around Saturn.

Neptune's rings

+ Thought to be uncomplete but rather several arclike ring segments, until pictures from Voyager 2 in 1989, it was determined that Neptune's rings are complete. + All of Neptune's rings are faint and, with the exception of the ring arcs, it was difficult to detect them.

Europa's interior features

+ Variable magnetic field -> has an internal electrically conducting fluid. + Global ocean 100 km deep, which might be salty with dissolved materials. + Young surface -> there is energy exchange between the icy crust and liquid water. + Not yet know if Europa has volcanic activity at the seafloor.

Chemical differentiation of Io

+ Volatiles such as water and CO2 escaped into space long ago. + Most of the heavier materials sank to the interior to form a core. + Sulfur and sulfurous compounds are constantly being recycled to form the complex surface.

Surface features of Io

+ Volcanic features, including vast lava flows, volcanoes, etc + No craters. + Vigorous eruption spray sulfurous gases and solids hundreds of kilometers above the surface. + Many color

Enceladus (Saturn) surface features

+ Wide variety of ridges, faults, and smooth plains. + Undergo cryovolcanism. + Some impact craters appear softened, by the viscous flow of ice. + Some parts of the moon have no craters, indicating recent resurfacing. + Terrain near the south pole is cracked and twisted. The cracks are warmer than the surrounding, suggesting tidal heating and radioactive decay within the rocky core. + Active cryovolcanism plumes expel water vapor, tiny ice crystal, and salts. Some of the crystals fall back onto the surface as an extremely fine, powdery snow.

Saturn's main rings

1. A Ring: outermost bright ring, narrowest 3 brightest rings. It has several narrow gaps. 2. B Ring: + Width is roughly twice the Earth's diameter. + Brightest ring + No internal gaps on the scale of those seen in the other bright rings. 3. C Ring: + Much fainter than the neighboring rings. + No known gap between the C Ring and either of the adjacent rings; only an abrupt change in brightness marks the boundary between them. 4. D Ring: + Innermost, has no definable inner edge. + Extend all the way down to the top of the Saturn's atmosphere, where its ring particles would burn up as meteors. Saturn's bring rings are not uniform

Strange orbital characteristics of regular moons

1. Phobos (Mars) + Orbits Mars faster than Mars rotates 2. Hyperion (Saturn) + Has chaotic rotation, tumbles in its orbit with a rotation period and a spin-axis orientation that are constantly and unpredictably changing. 3. Charon (Pluto) + Is half as big as Pluto + Pluto and Charon are tidally locked to each other. + May be created by a massive collision in the past.

Moon clasification

1. Regular moon: + Lies in its planet's equatorial plane + Is close to its planet + Has a nearly circular orbit in the same direction in which its planet rotates. + Was formed from an accretion disk. + Most is tidally locked with its planet. 2. Irregular moon: + Has elliptical and more inclined orbit. + Is farther away from its planet. + Orbits in a direction that is opposite to the rotation of the parent planet. + Was formed elsewhere and was later captured by the planet.

The Composition of Ring Material

1. Saturn: + Brightest ring + Only one we know is composed of water ice 2. Uranus: + Among the darkest objects in the Solar System + Are likely composed of organic materials and ices that have been radiation darkened by high-energy, charged particles in the magnetospheres of the planet. (because no rocky materials can be this dark) 3. Jupiter: + Intermediate brightness => rich in silicate materials, like the innermost of Jupiter's small moons.

Shepherd Moons

A pair of small moons (one orbiting just inside and one just outside a ring) can force particles into a narrow ring, by a gravitational tug. The F Ring is created by Prometheus and Pandora moon. These two moons cause the bizarre structure in the F ring.

Orbital resonance

A situation in which one object's orbital period is a simple ratio of another object's period, such as 1/2, 1/4, or 5/3. In such cases, the two objects periodically line up with each other, and the extra gravitational attractions at these times can affect the objects' orbits. For example: Jupiter's moons Ganymede, Europa, and Io are in a resonance 1:2:4.

Why don't Earth and other giant planets have stable ring?

Because they lack shepherd moons to provide orbital stability to rings.

Which planetary moons are larger than Mercury?

Ganymede (Jupiter) Titan (Saturn)

Titan's geological activity

Heat supplied by radioactive decay may cause cryovolcanism that release "new" methane from underground (helps keeping the methane atmosphere under the photodissociation of sunlight)

Geologically Dead Moons

Moons for which there is little or no evidence of internal activity having occurred at any time since their formation. Ex: Callisto (Jupiter), Hyperion (Saturn), Umbriel (Uranus)

Jupiter's rings

Most of the material in Jupiter's ring is made up of fine dust dislodged by meteoritic impacts on the surfaces of Jupiter's small inner moons.

Triton cryovolcanism

Nitrogen gas is formed when nitrogen ices vaporize by solar energy => The expanding vapor exerts very high pressures beneath the ice cap. => Eventually, the ice ruptures and vents the gas explosively into the low-density atmosphere. => Plumes of gas and dust

Does Io have plate techtonics?

No, because volcanoes on Io are spread much more randomly than those on Earth.

Cryovolcanism

The activity that is similar to terrestrial volcanism but is driven by subsurface low-temperature liquids such as water and hydrogen rather than molten rock.

Why is Io so volcanically active?

The continual flexing of Io created by the Jupiter gravity generates enough energy to melt parts of its mantle.

Source of heat of Enceladus

Tidal flexing

Ganymede's surface

Two prominent terrains: 1. A dark, heavily cratered, ancient terrain. + Period of intense bombardment + Furrowlike depressions 2. A bright terrain characterized by ridges and grooves. + Created when the fracturing and faulting completely deformed the icy crust, destroying all signs of older features, such as impact craters.