Chapter 12: SaturnSpectacular Rings and Mysterious Moons

Saturn

Saturn: View from Earth

• Saturn reaches opposition every 378 days. • Saturn orbits the Sun at distance of ~ 9.5 AU.• Saturn’s solar year is ~ 29.5 years long.

– It moves very slowly through the Zodiac constellations, taking about two years to cross each constellation.

• Saturn rotates on its axis once every 10.2 hours.– The rapid rotation flattens Saturn at the poles by

about10%, making it the most oblate planet.

Saturn’s Rings from Earth• From outside in, the three rings are known as A, B, and C rings.

• The Cassini Division lies between rings A and B.

• Much narrower Encke gap (some 300 km wide) is found in outer part of the A ring.

Saturn’s Rings

• Twice during each orbit the plane of Saturn's rings pass through the Earth's orbital plane.

• The Voyager spacecraft found that the rings are only 10-50 meters thick. – The rings are translucent, so stars can be

seen shining through them.

• Because the rings are so thin, they become invisible at these times, and Earth-based observers often look to discover small moons at this time.

Rings: Edge View

Saturn: Vital FactsCharacteristic Relative to Earth

Mass 95

Radius 9.5 Mean density 0.15

(0.69 g/cm3) Orbital distance 9.54 AU

Orbital period 29.46 Earth years Orbital eccentricity 0.054 Orbital inclination to ecliptic

2.50

Inclination to orbital plane

270

Atmospheric temperature

-3500C 97K cloud tops

Rotation period 10.2 Earth hours

Escape velocity 36 km/s

Satellites 28 known

Surface magnetic field

1000

Saturn’s Atmosphere

Atmospheric Composition

• Earth-based and Pioneer and Voyager spacecraft studies indicate that Saturn’s atmosphere consists of– hydrogen 92.4%– helium 7.4%– methane 0.2%– ammonia 0.02%

• Similar to Jupiter, except missing about half the helium found in Jupiter’s atmosphere.

Circulation in Saturn’s Atmosphere

•Zones, belts, and spots are similar to Jupiter's, but much less obvious, probably because

– the colder temperature produces a high level haze, – its weaker gravitational field allows the clouds to be

spread out over a much greater distance.

•Both effects tend to mute Saturn's cloud features.

•Strong east-west winds also occur in Saturn's atmosphere (~4 x stronger than Jupiter's).

•Because of the tilt of its axis (27o), Saturn has more pronounced seasonal changes than Jupiter.

Saturn’s Atmosphere: Clouds• Above clouds lies a layer of haze

formed by action of sunlight on upper atmosphere.

• Clouds are arranged in three distinct layers by composition:

ammonia, ammonium hydrosulfide,

water ice. • Total thickness of three cloud

layers is roughly 200 km. – 80 km on Jupiter

• Colors of cloud layers due to same basic cloud chemistry as on Jupiter.

• Saturn's clouds are thicker; fewer holes and gaps in top layer.

Saturn’s Jet Stream• Saturn’s zonal flow is considerably

faster than Jupiter’s and shows fewer east—west bands.

• Equatorial eastward jet stream moves at 1500 km/hr (~400 km/hr on Jupiter) and extends to much higher latitudes.

• Not until latitudes 40° N and S of equator are first westward flows found. This latitude also marks strongest bands and most obvious ovals and turbulent eddies.

• Reasons for differences between Jupiter's and Saturn's flow patterns not fully known.

Storms on Saturn• Saturn has atmospheric wind patterns similar to Jupiter’s.

• Similar overall east-west zonal flow, which is quite stable.

• Computer-enhanced images clearly show the existence of bands, oval storm systems, and turbulent flow patterns .

• Scientists believe that Saturn's bands and storms have essentially the same cause as does Jupiter's weather.

Earth-sized storm on Saturn

Storms: The Great White Spot• The Great White Spot

reoccurs on Saturn about once every 30 years (about the length of Saturn's orbital period).

• It was recorded in 1876, 1903, 1933, 1960, and 1990.

• Remains visible for a few months and then gradually fades.

• Appears to be a seasonal phenomenon.

Saturn’s Hydrosphere

• Just as with Jupiter, there is probably a layer below the cloud tops where liquid water is stable in the atmosphere of Saturn.– Water (mostly ice) is quite abundant

in the outer Solar System.

Saturn’s Biosphere

• None is suspected, but just as with Jupiter, some have speculated that layers in Saturn’s atmosphere may be hospitable to life.

Saturn's Internal Structure• Probably similar to Jupiter's.

It may have– a less dense rocky core, – more molecular hydrogen, and

– less liquid metallic hydrogen.

• Its low density may be explained by its smaller rocky/icy core with a correspondingly relative higher abundance of hydrogen and helium.

• Saturn also radiates more energy into space (2 x 1017 watts) than it receives from the Sun: about 3 x more.

Saturn: Internal Heating•Since Saturn radiates about 3 times more energy into space than it receives from the Sun, it must have an internal heat source. –Jupiter’s excess energy is thought to come from left-

over heat from formation and contraction.–Saturn is much smaller; should cool more rapidly.

•The source of Saturn’s excess energy may be linked to the observed helium deficiency its atmosphere.

•Lower T and P conditions allow helium to condense and “rain” into Saturn’s interior, releasing gravitational energy.

•Known as “helium precipitation”.

Saturn’s Interior• Same basic internal composition as Jupiter,

but different relative proportions:–Metallic hydrogen layer is thinner (~1/3 x Jupiter’s).–Core is larger than Jupiter’s.–Less extreme core T, density, and P than Jupiter.

Saturn’s Magnetosphere

• Similar to Jupiter's but not as strong. – Its radiation belts are more similar to Earth's.

• The magnetic axis of Saturn is almost exactly parallel to its rotation axis.

• Variations in the flow of the solar wind cause size of Saturn's magnetosphere to fluctuate. – Sometimes the moon Titan is within the

magnetosphere, and sometimes it orbits just outside the magnetic field.

Saturn’s Magnetic Field•Magnetic field strength: 1/20 x Jupiter’s, 1000 x Earth’s.

– Aligned with rotation axis.

•Extends ~1 million km– contains rings and 16

innermost moons,– no significant plasma torus,– Titan (orbit =1.2 million km)

•Produces AM radio waves– cannot be detected from

Earth-based telescopes

•Aurora, whistler, radio frequency ES discharge

Comparison of Saturn & JupiterProperty Saturn Jupiter

Mass 1 3.34

Diameter 1 1.2

Density 1 2

Atmospheric Structure Muted Very pronounced

Atmospheric Composition H, He H, He

Atmospheric CirculationVery fast jet

streamEquatorial jet

stream

Internal StructureLower , P, T

coreLarge core

Rings Extensive Small

Seasons Significant None

Magnetic field Strong Very strong

Saturn’s Rings

FAQ’s about Saturn’s Rings•What are the rings? Solid, liquid, gas?

–Great number of small particles, in independent orbits.•What is the composition of the particles?

–Primarily water ice, some ice coated rocky material. –Reflects >80% of incident sunlight.

•How big are the particles?–Fractions of mm to tens of meters.–Most are the size of large snowballs.–Spaced by ~2 m. –moving 37,000-50,000 miles/hr around Saturn.

•How thick are the rings?–Only a few meters in places (paper, 1 km or 8 blocks, 80-stories)

Why are there rings around planets?

• Increasing tidal field of planet first distorts, and then destroys, a moon that strays too close.

• This critical distance, inside of which the moon is destroyed, is known as the tidal stability limit, or the Roche limit.

• The Roche limit is 2.4 x radius of the planet.

• For Saturn, no moon can survive within a distance of 144,000 km of the planet's center.

Roche Limit

Roche Limit for Jovian Planets

The rings of Jupiter, Saturn, Uranus, and Neptune are shown above.

All distances are expressed in planetary radii.

The red line represents the Roche limit.

In all cases, the rings lie within the Roche limit of the parent planet.

Tilt of the Rings• Over time, Saturn's rings change their appearance to terrestrial

observers as the tilted ring plane orbits the Sun.

• At times during Saturn's 29.5-year orbital period, the rings seem to disappear altogether as Earth passes through their plane and we view them edge-on.

Ring Inclination versus Time (as seen from Earth)

Views of the Rings

HST images, captured from 1996 to 2000, show Saturn's rings open up from just past edge-on to nearly fully open as it moves from autumn towards winter in its Northern Hemisphere. (Space Telescope Science Institute)

Unusual View

of Rings

• Rare view of Saturn's rings seen just after the Sun has set below the ring plane, taken with the HST on Nov. 21, 1995. Unusual perspective because Earth is slightly above Saturn's rings and the Sun is below them. Photograph shows three bright ring features: the F Ring, the Cassini Division, and the C Ring (from the outer rings to inner). The low concentration of material in these rings allows light from the Sun to shine through them. The A and B rings are much denser, which limits the amount of light that penetrates through them. Instead, they are faintly visible because they reflect light from Saturn's disk.

• Credit: Phil Nicholson (Cornell University), Steve Larson (University of Arizona), and NASA April 26, 1996

How did Saturn get its Rings?• The rings may be the remains of a satellite that

wandered too close to Saturn or matter that was prevented from forming into a moon by tidal disruption.

• Another view states that the particles gradually accreted from the solar nebula.

• More recent studies based on the dynamics of the ring particles favor the idea that the rings are relatively young and are constantly being replenished from the debris of impacts constantly occurring within the rings and moon system of Saturn.

• In any case, the mass of the rings is only one millionth the mass of the Earth's Moon.

Saturn’s Famous Rings from Voyager

Saturn’s A-Ring

Spokes within Saturn’s B-ring

Saturn’s C-Ring

Rings of Saturn: Dimensions

• RING INNER RADIUS(km) OUTER RADIUS(km) WIDTH(km)

• D 67,000 74,700 7,700

• C 74,700 92,000 17,300

• B 92,000 117,500 25,500

• Cassini 117,500 122,300 4,800 Division

• A 122,300 136,800 14,500

• Encke gap* 133,400 133,700 300

• F 140,300 140,400 100• E 180,000 480,000 300,000

*The Encke gap lies within the A ring.

Ring Structures

• RINGLETS – The rings are composed of thousands of individual ringlets that

look like the grooves on a phonograph record.

– Shepherd satellites control the shape of some of the ringlets.

• BRAIDED STRUCTURE – This structure is very difficult to explain by gravitational

forces alone.

– Possibly an optical illusion caused by differing viewing angles.

• SPOKES – These features resemble the spokes on a wagon wheel. They

are probably caused by electromagnetic forces that suspend the very find ring particles.

Saturn’s F-ring• Outside the A ring lies strangest

ring of all, Saturn’s F-ring.

• Just inside Saturn's Roche limit, and, unlike the inner major rings, the F ring is narrow (< 100 km wide).

• Its oddest feature is that it looks as though it is made up of several separate strands braided together.

• The ring's intricate structure, as well as its thinness, arise from the influence of two small moons, known as shepherd satellites, that orbit on either side of it.

Shepherd Satellites• The F-ring's thinness, and possibly its other peculiarities too,

can be explained by the effects of two shepherd satellites that orbit a few hundred kilometers inside and outside the ring.

• The F-ring shepherd satellites operate by forcing the F-ring particles back into the main ring.

• As a consequence of Newton's third law of motion, the satellites themselves slowly drift away from the ring.

Saturn’s Ring Structure and Shepherd Moons

Cassini division: Mimas - 2:1 (orbital resonance)

F-ring: Pandora and Prometheus (shepherd satellites)

Enke division: Pan (gap produced by embedded satellite)

Cassini Mission

Joint effort of USA, ESA, and Italy scheduled arrival July, 2004; to study Saturn’s atmosphere, magnetosphere, rings, moons;

probe to parachute through Titan’s atmosphere.

Cassini Mission Goals

The Moons of Saturn

Moon Facts• The satellite system is dominated by large moon Titan. • In addition there are at least 27 more small to

moderate sized icy moons. • The moons are predominantly icy and some have

curious dark and light hemispheres. • Some satellites actually share the same orbit

(co-orbital moons). • Small shepherd satellites confine the ring material

into narrow ringlets. • The innermost satellites actually orbit within the

outermost rings.

The Moons of SaturnSatellite Orbit(1000 km) Radius(km) Mass(kg) Discoverer DatePan 134 10 ? Showalter 1990

Atlas 138 14 ? Terrile 1980

Prometheus 139 46 2.70e17 Collins 1980

Pandora 142 46 2.20e17 Collins 1980

Epimetheus 151 57 5.60e17 Walker 1980

Janus 151 89 2.01e18 Dollfus 1966

Mimas 186 196 3.80e19 Herschel 1789

Enceladus 238 260 8.40e19 Herschel 1789

Tethys 295 530 7.55e20 Cassini 1684

Telesto 295 15 ? Reitsema 1980

Calypso 295 13 ? Pascu 1980

Dione 377 560 1.05e21 Cassini 1684

Helene 377 16 ? Laques 1980

Rhea 527 765 2.49e21 Cassini 1672

Titan 1222 2575 1.35e23 Huygens 1655

Hyperion 1481 143 1.77e19 Bond 1848

Iapetus 3561 730 1.88e21 Cassini 1671

Phoebe 12952 110 4.00e18 Pickering 1898

Four New Moons for Saturn

• Four new outer moons have been discovered orbiting Saturn at a distance of at least 15 million km.

• The new moons are – irregular in shape,– between 10 and 50 km across, – in eccentric orbits, and– probably captured after formation.

Nine “Classical” Moons of Saturn

• Observed and identified before 1900.• In order of distance from Saturn (mnemonic:

MET DR THIP) • Mimas, Enceladus, Tethys, Dione, Rhea,

Titan, Hyperion, Iapetus, and Pheobe• Of group, only Titan considered to be a large moon.

Moon ComparisonTitan is similar in size to the other large moons in the Solar system, but

the only one that possesses an atmosphere.

Titan: Saturn’s Largest Satellite

TitanThe second largest satellite in the Solar System. Has a very dense atmosphere composed of nitrogen, methane, and "smoggy" hydrocarbons.

–Photochemical reactions in upper atmosphere produce dense smoggy and cloudy layer, preventing direct observations of surface.

–May have oceans of methane and ethane on surface.

The Cassini spacecraft will orbit Saturn and send a probe through the atmosphere of Titan in 2004.

Titan• Similar in diameter and composition

to Ganymede and Callisto.• Formed and retained a very thick

atmosphere.– from Earth: methane and ethane– from Voyager 1: mostly nitrogen

• Origin of atmosphere:– Lower T at Titan allowed more gas

(methane, ammonia, nitrogen) to be trapped in freezing water.

– Internal heating and impacts released gases.

Titan’s Atmosphere• Composition

–Predominately nitrogen (80-90%)

• Atmosphere–has clouds layers of

methane and perhaps ethane.

–includes several layers of haze

–contains 10 x more gas than Earth’s

–extends 10 x further from surface than Earth’s

–has surface pressure of 1.6 x Earth’s.

Titan’s Interior• Internal composition probably

similar to Jupiter’s Ganymede and Callisto.

– rocky core

– thick water ice mantle

• Degree of differentiation unknown.

• Average density = 1.89 g/cm3

• Surface temperature is 94K (-180oC or -288oF), so methane could exist as a gas, liquid, or solid on its surface (like water on Earth).

Hot Spots on Titan

• Titan is the only moon known to have a thick atmosphere.

• Picture shows places below the clouds of Titan which are hot.

• Such “hot spots” allow a means for determining what is happening near the surface.

Why study Titan?• Imagine a world somewhat smaller than Mars and bigger than

Mercury, where the air is denser than that in your living room, and the pressure is about the same as at the bottom of a swimming pool.

• The distant Sun is never seen, and high noon is no brighter than twilight on Earth. The cold is so great that water is always frozen out of the atmosphere; yet the simplest organic molecule methane takes its place as cloud-former and rain maker - perhaps even the stuff of lakes or seas.

• Methane, wafted hundreds of miles above the surface of this world, is cracked open by sunlight and cosmic rays; a menagerie of more complicated organics are produced, and these float down to the surface to accumulate over time.

• Courtesy Jonathan I. LunineTaken from a press briefing, 3 September 1997, Washington DC

Atmosphere and Climate • Greenhouse-warmed climate, powered by sunlight, like Earth's,

but sustained by different gases.– methane, hydrogen, nitrogen

• These gases are part of the cycle of organic chemistry, and the stability of Titan's climate is tied to this chemistry.

– Methane is being steadily depleted over time. If it is not replenished, or replenished irregularly, Titan's atmosphere may occasionally thin and cool down as methane's greenhouse contribution is lost.

• Cassini/Huygens will look for evidence of past episodes of climate collapse in the surface geology,

– e.g., by finding small impact craters which could not have formed under the current very thick atmosphere.

• The response of Titan's atmosphere to methane depletion may have been much stronger early in its history, IF the Sun was fainter back then than it is today

– So-called 'faint early sun' seems discordant with geological evidence for liquid water on Mars and Earth early in their histories, and so anything Titan can tell us of this ancient time is potentially quite exciting.

Understanding the Origins of Life• Titan’s surface is so cold that liquid water is only a

transient product of volcanism or impacts. • Almost certainly not the home of life today, but its organic

chemical cycles may constitute a natural laboratory for replaying some of the steps leading to life. – Know that life is abundant on Earth, and has played

key roles in our planet's evolution. – In some ways, Titan is the closest analogue

to Earth's environment before life began. • Suspect that the outermost solar system probably retains

the original inventory of organics from the beginning. • Speculate that three objects - Mars, Europa, Titan -

may have undergone some amount of organic chemical evolution, perhaps almost to the threshold of life.

Mid-sized Icy Moons of Saturn:Mimas, Enceladus, Tethys,

Dione, Rhea, Iapetus•Density form 1.0-1.4 gm/cm3 implies water ice interiors.•Studies indicate water ice surfaces. •All have synchronous rotation in orbit around Saturn.•Each has one side more heavily cratered than other side.•Vary greatly in surface evidence of past internal activity.

–From heavily cratered with little evidence of resurfacing to lightly cratered with smooth regions that appear to have been recently resurfaced.

•No obvious pattern relating internal activity to mass, diameters, or distances from Saturn.

Mimas•Smallest of mid-sized (390 km)

•Density = 1.2 gm/cm3 (water ice?)•Pockmarked with craters.

•Largest crater Herschel gives Mimas its unique shape similar to

“Death Star”.•Perhaps represents largest impact

small body could sustain without shattering.

• ~135 km (90 miles) across (~ width of Lake Michigan)

covering 1/3 diameter of Mimas with central peak 6 km high.

•Possible that similar collision caused older moon to break apart,

forming Epimetheus and Janus.

Enceladus• 1/3 size of Earth’s moon.• Surface reflects 90% of

incident sunlight. • Shows greatest evidence

of internal activity.– Abundance of impact

craters in some areas. – Flows near center of disk

contain many fewer craters and cut some craters in half.

• Suggests that multiple stages or episodes of volcanism formed and reformed the icy body's surface.

• Possible source of E-ring material.

Global mosaic of Enceladus assembled from Voyager 2 images.

Tethys• Similar to Dione• Surface heavily cratered• Extensive regions of

smooth plains• Wispy, white streaks• Ithaca Chasm

– trench extending for 3/4 of circumference

– 100 km wide with walls several km high

• Shares orbit with two small moons, Telesto and Calypso.

Dione

• One-half size of Rhea• Density = 1.4 gm/cm3

• 2:1 orbit resonance with Enceladus.• Shares orbit with small moon Helene.• Surface cratered with evidence

of resurfacing. • Wispy, white streaks

– extend for many km – visible over entire surface.– indicate that Dione may

have had active internal processes in distant past.

Impact Craters on Dione

• Most cratering on side facing orbital direction

• Largest crater on Dione– < 100 km (62 mi) in

diameter – shows a well-developed

central peak. • Maria-like features.• Sinuous valleys observed on

surface may have formed when faults broke moon's icy crust.

Rhea

•Largest of mid-sized moons.

•Density suggests predominately water ice with some rocky material.

•Forward facing hemisphere has two sections: – one has large craters, few small craters and

– the other has small craters without large ones.

•Trailing side has wispy features.

Hyperion

• Irregular shape, unknown density.

• Tumbles in orbit with chaotic rotation.– constantly changes rotation axis

and rotation speed

Iapetus

•Leading hemisphere of Iapetus is covered by dark material; trailing hemisphere is covered with bright material.

•Two models proposed:

– Dark material from Phoebe (dark exterior moon) falls onto Iapetus from orbit.

– Dark material erupted from the interior of Iapetus into a low area in the leading hemisphere.

Jupiter’s Small Moons

Saturn’s Co-orbital Moons

• Saturn's co-orbital satellites, Janus and Epimetheus, play a never-ending game of tag as they move in their orbits around planet.

• From point A to C, satellite 2 gains on satellite 1. • However, before 2 overtakes 1, the two moons swap orbits, and

satellite 1 starts to pull ahead of satellite 2 again (points D to E).

Lagrange Points• Several other small moons

also share orbits.

• Telesto and Calypso have orbits that are synchronized with the orbit of Tethys, always remaining fixed relative to the larger moon.

• The small moons are precisely 60° ahead of and 60° behind Tethys as it travels around Saturn.

• These 60° points are known as Lagrange points.

Saturn• Outermost planet known to ancients.• Rings and moons discovered by telescope.• Large size • Rapid, differential rotation

w/ pronounced flattening.• Atmosphere, weather systems similar to Jupiter’s.• Excess internal heat result of helium precipitation.• Interior structure similar to Jupiter’s, but with thinner metallic hydrogen

layer and larger core.• Strong magnetic field and extensive magnetosphere.• Ring system

– in equatorial plane that is tilted to ecliptic; seasons and viewing – composition, origin, location, interaction with moons

• Moons– Large: Titan, second largest in solar system; thick atmosphere– Medium: rock and water ice, tidally locked to planet– Small: complex, often shared orbits

Saturn’s Classical Moons• Mimas

– old, heavily cratered surface– one crater ~1/3 moon diameter

• Enceladus– bright surface with geologically young region, possible continuous resurfacing

• Tethys– heavily cratered with gouge covering 3/4 moon’s circumference

• Dione and Rhea– cratered with regions containing wisps of relatively freshly produced ice

• Titan– second largest moon in solar system– dense nitrogen atmosphere divided into observable layers

• Hyperion– chaotic rotation

• Iapetus– one side highly reflective, one side black

• Phoebe– irregular shape, retrograde orbit