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Chapter 12: Saturn Spectacular Rings and Mysterious Moons

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Title: Chapter 12: Saturn Spectacular Rings and Mysterious Moons


1
Chapter 12 SaturnSpectacular Rings and
Mysterious Moons
2
Saturn
3
Saturn View from Earth
  • 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 reaches opposition every 378 days.
  • 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.

4
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.

5
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.

6
Rings Edge View
7
Saturn Vital Facts
8
Saturns Atmosphere
9
Atmospheric Composition
  • Earth-based studies with 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.

10
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.

11
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.

12
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.

13
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
14
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).
  • Recorded in 1876, 1903, 1933, 1960, and 1990.
  • Remains visible for a few months
    and then gradually fades.
  • Appears to be a seasonal
    phenomenon.

15
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.

16
Saturns Biosphere
  • None is suspected, but just as with Jupiter, some
    have speculated that layers in Saturns
    atmosphere may be hospitable to life.

17
Saturn's Internal Structure
  • Probably similar to Jupiter's.
    It may have
  • a less dense rocky core,
  • more molecular hydrogen,
  • 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
    than it receives from the Sun about 3 x
    more.

18
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.

19
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.

20
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.

21
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

22
Comparison of Saturn Jupiter
23
Saturns Rings
24
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 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)

25
Why are there rings around planets?
Roche Limit
  • Increasing tidal field of planet first distorts,
    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.

26
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.

27
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.

28
Ring Inclination versus Time (as seen from Earth)
29
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)

30
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), NASA April 26, 1996

31
How did Saturn get its Rings?
  • The rings may be
  • the remains of a satellite that wandered
    too close to Saturn,
  • matter that was prevented from forming
    into a moon by
    tidal disruption, or
  • the gradual accretion of particles 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.

32
Saturns Famous Rings from Voyager
33
Saturns A-Ring
34
Spokes within Saturns B-ring
35
Saturns C-Ring
36
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.

37
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 (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.

38
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.

39
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)
40
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.
41
Cassini Mission Goals
42
The Moons of Saturn
43
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
    ring material
    into narrow ringlets.
  • The innermost satellites actually orbit
    within the
    outermost rings.

44
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45
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

46
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.

47
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.

48
Moon Comparison
  • Titan is similar in size to the other large moons
    in the Solar system, but the only one that
    possesses an atmosphere.

49
Titan Saturns Largest Satellite
50
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.
51
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.

52
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.

53
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).

54
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.

55
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

56
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.

57
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.

58
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.

59
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.

60
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.

61
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.

62
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.

63
Impact Craters on Dione
  • Most cratering on side facing orbital direction
  • Largest crater on Dione
  • shows a well-developed central peak.
  • Maria-like features.
  • Sinuous valleys observed on surface may have
    formed when faults broke moon's icy crust.

64
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.

65
Hyperion
  • Irregular shape, unknown density.
  • Tumbles in orbit with chaotic rotation.
  • constantly changes rotation axis
    and rotation speed

66
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.

67
Saturns Small Moons
68
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).

69
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.

70
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

71
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
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