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Saturn
Cassini Saturn & rings in 2013 The Sun is to the right, casting a long
shadow across the rings at left. The probe has also monitored seasonal
atmospheric changes on Saturn as the planet enters northern spring and
the north pole emerges from 15 years of darkness. Note the hexagonal
weather pattern around Saturn’s north pole, apparently defined by a polar
jet stream circling the planet.
Mass: 5.7 1026 kg (95 ME)
Radius: 60,000 km (11.2 RE)
Density: 700 kg/m3 (0.7 g/cc) less than water! [float in
bathtub]
Rotation: Rapid and differential, enough to flatten Saturn
considerably
Rings: Very prominent; wide but extremely thin
Orbital and Physical Properties
Orbital & Physical Properties
View of rings from Earth changes
as Saturn orbits the Sun
1996
1997
1998
1999
2000
-Saturn’s atmosphere also shows zone and band structure,
but coloration is much more subdued than Jupiter’s
-Mostly molecular hydrogen, helium, methane, and ammonia;
helium fraction is much less than on Jupiter
Atmosphere
-Similar to Jupiter’s, except
pressure is lower
-Three cloud layers
-Cloud layers are thicker than
Jupiter’s; see only top layer
Atmosphere
Saturn’s weaker gravity results in thicker clouds and a more uniform appearance.
Structure in Saturn’s clouds can be seen more
clearly false-color image
computer processing and artificial color to enhance the contrast of Voyager images the atmosphere
Wind patterns on Saturn
are similar to those on
Jupiter, with zonal flow
Winds on Saturn reach speeds even greater than those on Jupiter. As on Jupiter, the visible bands appear to be associated with variations in wind speed.
Storm (Cassini 2011) churning its way through the northern
hemisphere, leaving a “tail” wrapping around the planet.
Jupiter-style “spots” can turn into large storms on Saturn,
then dissipate relatively quickly
After the storm faded from view in visible light,
infrared observations continued to show its
ongoing activity.
Giant vortices exist at
both poles apparently due
to jet streams
The north polar vortex, in enhanced color,
with the rings in the background (blue).
The south polar
vortex, in the
infrared.
Interior structure similar to Jupiter’s
Saturn’s Interior and Magnetosphere
Saturn’s internal structure, as deduced here from Voyagerobservations and computer modeling
Constituents 92.4% H2 : 7.4% He : 2% all else
(Note: Primordial and Jupiter's composition ~80% H2 : ~20% He)
Differentiation with some of the He sinking into the interior.
Saturn, cooler and less turbulent than Jupiter
- He droplets "rain" condense out of H2/He mixture and sink into
the interior (releasing gravitational energy)
- this leaves the observed He "shortage" near surface
- this generates energy - Saturn radiates 3 X more energy than the
sun gives it
Saturn also has a strong
magnetic field, but only
5% as strong as Jupiter’s
Creates aurorae
Magnetosphere
Hubble UV
Saturn has an extraordinarily large
and complex ring system, which was
visible even to the first telescopes
Saturn’s Spectacular Ring’s
Galileo - 1610
Galileo - 1616
Huygens - 1655
-Ring particles range in size from fractions
of a millimeter to tens of meters
-Composition: Water ice—similar to
snowballs
Why rings?
• Too close to planet for moon to form-tidal
forces would tear it apart
Artist impression of the aggregates of
icy particles that form the 'solid' portions
of Saturn's rings. These elongated
clumps are continually forming and
dispersing. The largest particles are a
few m across.
Rings
Overview of the
ring system
Main ring features are marked and shown in false color to represent
information about particle sizes inferred from radio observations.
Ring fine structure in this Cassini 2005 image.
Closest distance that moon could survive
is called Roche limit; jovian ring systems
are almost all inside this limit
Jovian Ring Systems
Tidal distortion and breakup of moon approaching Roche limit.
Voyager probes showed Saturn’s rings to be much
more complex than originally thought
(Earth is shown on the same scale as the rings)
Saturn’s Rings, Up Close. Cassini took this true-color image of Saturns dazzling ring structure just before flying through the planet’s tenuous outer rings. The inset at left is an overhead view of a portion of the B ring, showing the ringlet structure in even more detail; in fact the resolution here is an incredible 4 km.
This backlit view shows the fainter F, G, and E rings
Back-Lit Rings. Cassini image of rings as it passed through Saturn’s shadow. The normally hard to see F, G, and E outer rings are clearly visible in this contrast-enhanced image. The inset shows the moon Enceladus orbiting within the E ring; its eruptions likely give rise to the ring’s icy particles. (NASA)
Spokes in the Rings.a series of dark temporary “spokes” B ring (Voyager 2 fly by). The spokes are caused by small particles suspended just above the ring plane. Cassini, recorded less prominent spokes
Voyager also found radial “spokes” that formed and
then dissipated; this probably happens frequently
• Other edges and divisions in rings are
also the result of resonances with moons
• “Shepherd” moon defines outer edge
of A ring through gravitational
interactions
resonances
Outermost, F ring; it appears to have braids and kinks
-narrow F ring appears to contain unique kinks & braids -thinness is caused by two shepherd satellites that orbit near the ring
Pandora
Prometheus
F ring
dark channels that it has carved into the
inner strands of the ring.
potato-shaped shepherd satellite (~100 km across)
Inner edge of the Cassini Division- strong orbital resonance-
2 orbits : 1 orbit of the moon Mimas. The resonant pulls on these
ring particles accumulate, destabilizing their orbits and leading to
a sharp cutoff in ring density.
Cassini Division
A-ringB-ring
Mimas
Small moon Daphnis wanders through the Keeler gap- note
induced vertical waves on opposing sides of the moon due to the
differential rotation of the A ring. (Cassini)
Encke Gap- 325-km-wide gap caused by the
small moon Pan orbiting within it. (Cassini probe)
3 thin, knotted
ringlets within
the gap.
Spiral density
waves visible on
both sides of it
are induced by
resonances with
nearby moons
exterior to the
rings
Rings
Details of formation are unknown:
• Probably too active to have lasted since birth of solar system
• Not all rings may be the same age
• Either must be continually replenished, or are the result of a
catastrophic event
Saturn’s many moons appear to be made of water ice
In addition to the small moons, Saturn has
• Six medium-sized moons (Mimas, Enceladus, Tethys,
Dione, Rhea, and Iapetus)
• One large moon (Titan), almost as large as Jupiter’s
Ganymede
Moons
Titan -atmosphere thicker
& denser than Earth’s;
mostly nitrogen and argon
Surface cant be seen
Titan. Larger than Mercury & ~1/2 size of Earth.
Voyager 1 (1980) from 4000 km.
Infrared adaptive-optics image (Mauna Kea), showing large-scale surface features. Bright regions - highlands, frozen methane?
upper atmosphere haze (Cassini 2005)
Trace chemicals in Titan’s
atmosphere make it chemically
complex
Structure of atmosphere (Voyager 1) observations. The solid blue line temperature vs. altitude. The inset shows the haze layers in Titan’s upper atmosphere, depicted in false-color green above Titan’s orange surface.
Some surface features on Titan
visible Cassini infrared images
Cassini’s infrared, false-color view of Titan’s surface (2004).-semicircular area near the center old impact basin?-dark linear feature to its NW mountain ranges, ancient tectonic activity?
- geological activity suggested on icy moon’s surface
-circular surface feature icy volcano?
(NASA/ESA)
resolution~25 km
Huygens spacecraft landed on Titan and returned images
Enlargement showing a network of
dark channels reminiscent of
streams/rivers draining from the
light-shaded uplifted terrain into
darker, low-lying regions (at
bottom).
Titan’s surface from ~15 km.
Huygens spacecraft landed on Titan and returned images
Huygens’s landing site, The
foreground icy “rocks” are
about 10 centimeters across.
Radar aboard Cassini detected smooth regions on Titan
thought to be lakes
Titan’s Lakes. Many smooth regions
(colored dark blue in this false-
colored radio image), thought to be
lakes of liquid ethane & methane,
near Titan’s north pole (marked). The
largest features are larger than the
Great Lakes on Earth. More than 95%
of all the liquid on Titan’s surface lies
within the region shown here, which
spans some 1300 km across, about
one-quarter of Titan’s diameter. The
white regions are areas for which no
data are available. (NASA/JPL-
Caltech/ASI/USGS)
Based on gravitational field measurements made by Cassini
and Huygens, this is the current best understanding as to
what the interior of Titan looks like.
Titan’s Interior. Interior appears to be largely a rock–ice mixture. Most intriguing is the subsurface layer of liquid water, similar to that hypothesized on Jupiter’s Europa and Ganymede.
Saturn’s 6 mid-sized moons (from Cassini) heavily cratered !
Earth’s Moon
• Mimas, Enceladus, Tethys, Dione, and Rhea all
orbit between 3 and 9 planetary radii from Saturn,
and all are tidally locked—this means they have
“leading” and “trailing” surfaces
• Iapetus orbits 59 radii away and is also tidally
locked
Enceladus- surface seems oddly youthful; inset shows icy
jets. A large ocean is believed to exist under the ice.
-youthful terrain in the south where
craters are mostly absent.
-long blue “tiger stripe” streaks (about 1
km wide) are fractures in the ice through
which gas escapes to form a thin but real
atmosphere.
-Some of the jets emerging from cracks
near the south pole can be clearly seen
in backlit image.
interior model,
-note an saltwater
ocean below the south
pole.
Masses of small moons not well known: 2 share a single orbit
Co-orbital satellites Janus and Epimetheus play perpetual game of tag while moving around the planet in their orbits. Their peculiar motions are depicted here by the labeled points that represent the locations of the two moons at a few successive times.
Two more moons are at the Lagrangian points of
Tethys
Synchronous Orbits of the moons Telesto and Calypso are tied to the motion of the moon Tethys. The combined gravitational pulls of Saturn and Tethys keep the small moons exactly 60° ahead and behind the larger moon at all times, so all three moons share an orbit and never change their relative positions.
• Saturn, like Jupiter, rotates differentially and is significantly
flattened
• Saturn’s weather patterns are in some ways similar to
Jupiter’s, but there are far fewer storms
• Saturn generates its own heat through “helium raindrops”
sinking into interior
• Saturn has a large magnetic field and extensive
magnetosphere
• Saturn’s most prominent feature is its rings, which are in its
equatorial plane
• The rings have considerable gross and fine structure, with
segments and gaps; their particles are icy and grain- to
boulder-sized
• Interactions with medium and small moons determine the
ring structure
• The rings are entirely within the Roche limit, where larger
bodies would be torn apart by tidal forces
Summary
The Cassini spacecraft
uses multiple
“gravitational slingshots”
to make multiple close
passes around Saturn’s
moons.