Chapter 12: Saturn Spectacular Rings and Mysterious Moons

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Chapter 12 SaturnSpectacular Rings and
Mysterious Moons
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Saturn
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Saturn View from Earth

• Saturn reaches opposition every 378 days.
• Saturn orbits the Sun at distance of 9.5 AU.
• Saturns 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.

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Saturns 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.

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Saturns 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.

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Rings Edge View
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Saturn Vital Facts
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Saturns Atmosphere
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Atmospheric Composition

• Earth-based and Pioneer and Voyager spacecraft
  studies indicate that Saturns 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 Jupiters atmosphere.

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Circulation in Saturns 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.

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Saturns 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.

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Saturns Jet Stream

• Saturns zonal flow is considerably faster than
  Jupiters and shows fewer eastwest 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.

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Storms on Saturn

• Saturn has atmospheric wind patterns similar to
  Jupiters.
• 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
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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.

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Saturns 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.

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Saturns Biosphere

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

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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.

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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.
• Jupiters 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 Saturns excess energy may be
  linked to the observed helium deficiency its
  atmosphere.
• Lower T and P conditions allow helium to condense
  and rain into Saturns interior, releasing
  gravitational energy.
• Known as helium precipitation.

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Saturns Interior

• Same basic internal composition as Jupiter,
  but
  different relative proportions
• Metallic hydrogen layer is thinner (1/3 x
  Jupiters).
• Core is larger than Jupiters.
• Less extreme core T, density, and P than Jupiter.

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Saturns 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.

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Saturns Magnetic Field

• Magnetic field strength 1/20 x Jupiters,
  1000 x
  Earths.
• 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

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Comparison of Saturn Jupiter
Property 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 Circulation Very fast jet stream Equatorial jet stream
Internal Structure Lower ?, P, T core Large core
Rings Extensive Small
Seasons Significant None
Magnetic field Strong Very strong
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Saturns Rings
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FAQs about Saturns 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 gt80 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)

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Why are there rings around planets?
Roche Limit

• 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.

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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.

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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.

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Ring Inclination versus Time (as seen from Earth)
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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)

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Unusual Viewof 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

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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.

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Saturns Famous Rings from Voyager
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Saturns A-Ring
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Spokes within Saturns B-ring
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Saturns C-Ring
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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.

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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.

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Saturns F-ring

• Outside the A ring lies strangest ring of all,
  Saturns F-ring.
• Just inside Saturn's Roche limit, and, unlike the
  inner major rings, the F ring is narrow (lt 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.

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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.

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Saturns Ring Structure and Shepherd Moons
Cassini division Mimas - 21 (orbital
resonance) F-ring Pandora and Prometheus
(shepherd satellites) Enke division Pan (gap
produced by embedded satellite)
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Cassini Mission
Joint effort of USA, ESA, and Italy scheduled
arrival July, 2004 to study Saturns
atmosphere, magnetosphere, rings, moons
probe to parachute through Titans atmosphere.
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Cassini Mission Goals
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The Moons of Saturn
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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.

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The Moons of Saturn

• Satellite Orbit(1000 km) Radius(km)
  Mass(kg) Discoverer Date
• Pan 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

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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.

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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.

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Moon Comparison

• Titan is similar in size to the other large moons
  in the Solar system, but the only one that
  possesses an atmosphere.

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Titan Saturns Largest Satellite
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Titan

• The 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.
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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.

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Titans 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 Earths
• extends 10 x further from surface than Earths
• has surface pressure of 1.6 x Earths.

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Titans Interior

• Internal composition probably similar to
  Jupiters 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).

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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.

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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. Lunine
• Taken from a press briefing, 3 September 1997,
  Washington DC

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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.

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Understanding the Origins of Life

• Titans 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.

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Mid-sized Icy Moons of SaturnMimas, 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.

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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.

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Enceladus

• 1/3 size of Earths 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.

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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.

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Dione

• One-half size of Rhea
• Density 1.4 gm/cm3
• 21 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.

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Impact Craters on Dione

• Most cratering on side facing orbital direction
• Largest crater on Dione
• lt 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.

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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.

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Hyperion

• Irregular shape, unknown density.
• Tumbles in orbit with chaotic rotation.
• constantly changes rotation axis
  and rotation speed

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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.

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Jupiters Small Moons
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Saturns 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).

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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.

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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 Jupiters.
• Excess internal heat result of helium
  precipitation.
• Interior structure similar to Jupiters, 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

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Saturns 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 moons
  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