Transcript Document 7107704

Chapter 12: Saturn
Spectacular 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 Facts
Characteristic
Mass
Radius
Mean density
Orbital distance
Orbital period
Orbital eccentricity
Orbital inclination
to ecliptic
Inclination to
orbital plane
Atmospheric
temperature
Rotation period
Escape velocity
Satellites
Surface magnetic
field
Relative to Earth
95
9.5
0.15
(0.69 g/cm3)
9.54 AU
29.46 Earth years
0.054
2.50
270
-3500C
97K cloud tops
10.2 Earth hours
36 km/s
28 known
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 leftover 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 & Jupiter
Property
Saturn
Jupiter
Mass
Diameter
1
1
3.34
1.2
Density
1
2
Atmospheric Structure
Muted
Very pronounced
Atmospheric Composition
H, He
Very fast jet
stream
H, He
Equatorial jet
stream
Internal Structure
Lower , P, T
core
Large core
Rings
Extensive
Small
Seasons
Significant
None
Magnetic field
Strong
Very strong
Atmospheric Circulation
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?
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.
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)
D
67,000
C
74,700
B
92,000
Cassini
117,500
Division
A
122,300
Encke gap* 133,400
F
140,300
E
180,000
*The Encke gap lies within the A ring.
WIDTH(km)
74,700
92,000
117,500
122,300
7,700
17,300
25,500
4,800
136,800
133,700
140,400
480,000
14,500
300
100
300,000
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 Saturn
Satellite
Pan
Atlas
Prometheus
Pandora
Epimetheus
Janus
Mimas
Enceladus
Tethys
Telesto
Calypso
Dione
Helene
Rhea
Titan
Hyperion
Iapetus
Phoebe
Orbit(1000 km) Radius(km) Mass(kg)
134
138
139
142
151
151
186
238
295
295
295
377
377
527
1222
1481
3561
12952
10
14
46
46
57
89
196
260
530
15
13
560
16
765
2575
143
730
110
?
?
2.70e17
2.20e17
5.60e17
2.01e18
3.80e19
8.40e19
7.55e20
?
?
1.05e21
?
2.49e21
1.35e23
1.77e19
1.88e21
4.00e18
Discoverer
Date
Showalter
Terrile
Collins
Collins
Walker
Dollfus
Herschel
Herschel
Cassini
Reitsema
Pascu
Cassini
Laques
Cassini
Huygens
Bond
Cassini
Pickering
1990
1980
1980
1980
1980
1966
1789
1789
1684
1980
1980
1684
1980
1672
1655
1848
1671
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 Comparison
Titan 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
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.
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. Lunine
Taken 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.
• 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.
Tethys
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 neverending 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