Rings SATURN’S RINGS

Saturn’s Rings: not a solid disk! But made of individual particles.

- Size: extend from 7,000km to 80,000km from Saturn - Thickness: 10meter- 100m - Composition: mainly water ice - Size of the particles making the rings: 1cm to 10m ; there are also particles down to 1 micron! - Origin of the ring: possibly the fragments of a moon colliding a comet, or left over from the

formation of the planet. It is not known for sure. It is believed that the rings have been around for a very long time, possibly since the formation of Saturn.

- The Rings are composed of a large number of “ringlets”. - The inner ring has a period of 5h45m, the outer ring 14h20m

Saturn’s Rings as seen from “behind” with the Sun in front eclipsed by Saturn:

Tiny particles (1 micron across) are scattering the light of the Sun (eclipsed behind Saturn) and help reveal the presence of the outer rings (not visible from Earth). This is due to diffraction of light by small particles, when the size of the particles is similar to the wavelength of the light.

JUPITER’S RINGS

- Jupiter’s Rings are made of rocky particles rather than ice they are darker and more difficult to observe. They are 1 micron in size. Origin: meteorite impacting the moons, volcano ejecta.

Jupiter’s rings observed in the infrared with the Keck Telescope. Voyager 2 photo of Jupiter’s Rings.

Scattered light from the Rings of Jupiter

THE RINGS-MOONS INTERACTION Keplerian Motion: the centrifugal force balances the gravitational force Keplerian velocity increases with radius Increasing the Speed of an object in Keplerian orbit moving outwards Decreasing the Speed of an object in Keplerian orbit moving inwards Application and Example: two objects in orbit attached with a cable increasing tension in the cable Resonance

- 2:1 resonance between the moon Mimas (with a period around Saturn of 22.6hr) and material in the Cassini division (with a period around Saturn of 11.3hr).

- The orbital motion of material in the Cassini division is periodically disturbed by the pull of Mimas every 2 revolutions (corresponding to one revolution of Mimas).

- Because of this disturbance material is “cleared” out of that region this is how the gap is formed.

Shepherd Moons

Inner Moon – Ring interaction: The inner moon rotates faster and therefore pulls the material in the Ring forward increasing its speed and therefore moving it outwards to a larger radius. Outer Moon- Ring interaction: The outer moon rotates more slowly and therefore pulls the material in the Ring backward decreasing its speed and therefore moving it inward to a smaller radius. In each case the net effect is to keep the ring edge away from the Moon. The Moon acts like a shepherd.

For a Moon within a Ring, it opens a gap and pushes the material in the ring away from the Moon.

Relative motion in the frame of reference of the Moon, similar to a car in the middle lane of a 3 lane traffic. Uranus and Nepture also have Rings:

The Moons of Jupiter and Saturn - Mercury and Venus have no moon - Earth’s moon is the Moon… - Mars has two tiny moons captured asteroids

- The 4 moons of Jupiter (first observed by Galileo Galile) together with Jupiter are called the Galilean system…

Regular moons: - Moons that were formed together with their “parent” planet (formed at the same time) - Formed out of a (debris) disk around the parent planet - Same rotation as the planet - Rotating in the equatorial plane of the planet - Example: Titan (from Saturn), Galilean Moons (Jupiter), etc…

Irregular Moons: - Captured moons - Can be much smaller (of the order of km) - Can have “retrograde” (backwards) rotation - Distant orbit - Not especially aligned with the equatorial plane of the planet - Example: Triton (from Neptunre)

Jupiter’s Moons

Saturn’s Moons:

Jupiter has actually 63 known satellites with orbital periods ranging from a few hours (Metis, Adrastea,..)

to 100s of days (~yr) (Carme, Sinope,..). Saturn has more than 30 known moons (Telesto and Calypso are

Trojans of Tethys)

IN ADDITION TO CLASSIFYING MOONS AS REGULAR AND IRREGULAR, ONE CLASSIFIES THE MOONS BASED ON THEIR GEOLOGICAL ACTIVITY:

- Active moons young surfaces Io (Jupiter), Enceladus (Saturn), Triton (Neptune) - Possibly active moons young surfaces Europe (Jupiter) and Titan (Saturn) - Active in the past only old surfaces Ganymede (Jupiter), Tethys (Saturn; also for Saturn:

Dione, Rhea, Iapetus and Mimas “the death star” from star wars) - Moon that were never active old surfaces Callisto (Jupiter), Umbriel (Uranus)

GEOLOGICALLY ACTIVE: - Volcanoes - Few craters - Evidence of lava flows, magma surfacing - Ice: ice cracks, fresh ice, plumes of ice - Atmosphere lakes, rivers, …

Io – Jupiter’s active moon

- Diameter: 3,630km (as big as or Moon) - It has active volcanoes

With lava flow

DUE TO TIDAL COMPRESSION AND STRESS, THE CRUST WARMS UP AND MELTS VOLCANOES Tidal forces are large because of the proximity to Jupiter, large mass of Jupiter and relatively large size of Io. For example: bending a spoon many times until it breaks the broken pieces are hot

IO IS THE MOST ACTIVE BODY IN THESOLAR SYSTEM! - Volcanoes - Lava flows - Volcanic craters - NO impact craters (unique in the solar system) - 300 volcanic vents - 60 active volcanoes - Volcanoes are spread evenly on its surface (unlike on Earth)

The colors of Io are due mainly to S, SO2, S-salts with Na (sodium) and K (potassium) [SO2 ice is bright] H2O and CO2 escape to space and form part of Io’s plasma torus (ionized atoms, source of magnetospheric particles for Jupiter).

Europa: a possibly active moon of Jupiter

- It has a surface of frozen water - And a global liquid water ocean beneath the ice. It is liquid beneath the ice because of TIDAL

HEATING. - It has few impact craters - Chaotic terrain due to broken slabs - Ice cracks with new ice filling the cracks [the fresh material is dark ice due to Sulfur S and

magnesium sulfate MgSO4 – a salt; or possibly sulfuric acid hydrates]. - it has a very thin atmosphere of oxygen (H2O O + H2 H2 escapes to space, left with

Oxygen). ITS SALTY OCEAN BELOW THE SURFACE IS A PLACE TO LOOK FOR POTENTIAL EXTRA-TERRESTRIAL LIFE The ice crust is somewhere between about 2km and 50km thick.

High resolution images show a network of overlapping ridges,

As the ices cracks, it forms ice rafts, which can also move around

The existence of rafts indicates that there is either liquid water or soft ice beneath the surface. Measurement of the magnetic field of Io indicates it originates in a subsurface electrically conducting fluid: the salty ocean beneath the surface. The ocean could be 100-200km deep. The liquid water emerges from beneath the icy crust to regenerate the surface with fresh ice. The liquid water underneath and the water ice on the surface are in a way taking the place of lava and rocks on Earth. POSSIBLE LIFE FORMS IN EUROPA’S OCEAN…

Ganymede, the largest of all The orbital period of Io, Europa and Ganymede are in the ratio: 1:2:4, due to the gravitational mutual

attraction of the 3 moons, as they are a few 100s thousands km from each other.

The 4 Galilean moons are each locked in synchronous rotation (spin) around Jupiter they always

show their same side to Jupiter.

-Ganymede’s diameter: 5260km larger than Mercury (but only 45% percent of Mercury’s mass!) - the dark terrain is heavily cratered ancient - the bright terrain has ridges and grooves newer

Ganymede average density is about twice that of water composed of rocky material and water (ice) Water ice is on the surface, especially in the brighter regions. Other materials detected on the surface: CO2, SO2, … slats Mg SO4 (magnesium sulfate), Na2SO4 (sodium sulfate).. these salts possibly originated from a subsurface OCEAN.

The internal structure of Ganymede is differentiated (made of layers) with an Iron Sulfide Core, silicate mantel and an outer water ice mantle.

There is evidence for past geologic activity on the surface of Ganymede

- The dark terrain is on the left, the bright terrain is on the right - The left area is crippled with craters and is possibly several billions years old - The right area has many less craters and is (only) one billion yr old. - In the past Ganymede was geologically active with possible liquid water beneath the surface. - Today, It could still have some salty liquid water 170km below the surface to explain its magnetic

field and the variation of its magnetic field.

Ganymede has tenuous atmosphere made of O2 (oxygen) and O3 (ozone), due to the photo dissociation of H2O into H2 (escaping) and O (remaining).

Callisto: a dead moon of Jupiter, or not so dead after all… ? As seen from Galileo satellite Callisto exhibits a dirty ice surface with many craters. While there are many large craters, very few are found with a diameter less than 1km.

- It is not known how small craters have been eroded away - It is not known where does the dark dusty material blanketing the surface came from.

Additional high resolution (close up) view of the surface showing “material” covering craters and small features being eroded:

In addition callisto has a magnetic field like Ganymede which indicates a liquid conducting layer (salty water ?

Some Galileo research scientists even suggested the layer might be some organic material (a kind of vegetation, in such a frigid temperature?) and even indicate the slightly “greenish” color observed here

Callisto has a tenuous atmosphere of CO2.

Saturn’s Moon Titan – with a methane cycle - Titan is larger than Mercury with a diameter of 5150km. - It has a thick atmosphere (30 percent more than Earth’s).

The atmosphere is made mainly of - Nitrogen (from ammonia NH3, where H has escaped due to photo dissociation) 95% - Methane CH4 (methane must be renewed constantly to be present) - Ethane C2H6 - Complex hydrocarbons (“organic compounds”) such as C2H2 (acetylene), C2H4 (ethylene), C2H6

(ethane), C3H8 (propane) …

- It is the largest moon of Saturn and the only moon known to have a dense atmosphere. - Composed of water ice and rocky materials.

Titan was extensively studied with the Cassini-Huygens mission in 2004. - Discovery of liquid hydrocarbon lakes on its surface young surface - Active cryovolcanism (volcanoes of ice ejecting water, ammonia or methane)

Titan has a climate with winds and rains, creating surface features such as sand dunes, rivers, lakes and seas (of liquid methane or ethane) and shorelines. Titan is viewed as an “early Earth”, but at a much lower temperature. Titan is differentiated with several layers:

- A 3,400km rocky core - Several layers composed of different crystal forms of ice - A possible liquid layer composed of water and ammonia between the ice and crust (the

ammonia allows the water to remain liquid at these temperatures). - The crust is decoupled from the interior, evidence for a liquid layer beneath it.

Huygens Probe landed on Titan (2004) The findings indicate in the upper atmosphere:

- Winds of up to 120m/s - T=88Kelvin - Methane clouds

On the surface: - No wind - T=112Kelvin - Methane rivers and methane lakes, methane rain

The atmosphere of Titan :

- Denser than earth’s - The atmosphere is blocking most of the sunlight to reach the surface which is obscure.

The composition of the atmosphere is mainly nitrogen (see beginning)

Clouds in the atmosphere if Titan:

The cloud is possibly made of Ethane rather than methane.

The surface of Titan is complex, and geologically young. There are volcanoes releasing water and ammonia (cryo-volcanoes). Some impact craters have been filled with raining hydrocarbons. No surface feature elevated above 1000m.

No ocean, but rather many lakes:

Sand Dunes on Earth (top) and on Titan (bottom)

Pictures from the Huygens probe:

There are signs of erosion at the bottom of some of the rocks, indicating fluvial activity (running liquid on the surface).

Rivers and other features on Titan:

Additional moons of Saturn…

Enceladus shows some evidence of tectonic (ice) processes, cryovolcanism …

Mimas: the death star

Enceladus

Mimas: the Death Star…