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Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn &...

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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.
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Page 1: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 2: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 3: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

Orbital & Physical Properties

View of rings from Earth changes

as Saturn orbits the Sun

1996

1997

1998

1999

2000

Page 4: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

-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

Page 5: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

-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.

Page 6: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 7: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 8: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 9: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 10: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

Interior structure similar to Jupiter’s

Saturn’s Interior and Magnetosphere

Saturn’s internal structure, as deduced here from Voyagerobservations and computer modeling

Page 11: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 12: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

Saturn also has a strong

magnetic field, but only

5% as strong as Jupiter’s

Creates aurorae

Magnetosphere

Hubble UV

Page 13: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 14: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 15: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at
Page 16: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

Closest distance that moon could survive

is called Roche limit; jovian ring systems

are almost all inside this limit

Jovian Ring Systems

Page 17: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

Tidal distortion and breakup of moon approaching Roche limit.

Page 18: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 19: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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)

Page 20: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 21: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

• 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

Page 22: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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)

Page 23: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 24: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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)

Page 25: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 26: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 27: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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)

Page 28: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 29: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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

Page 30: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 31: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

Huygens spacecraft landed on Titan and returned images

Huygens’s landing site, The

foreground icy “rocks” are

about 10 centimeters across.

Page 32: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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)

Page 33: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 34: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

Saturn’s 6 mid-sized moons (from Cassini) heavily cratered !

Earth’s Moon

Page 35: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

• 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

Page 36: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 37: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 38: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

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.

Page 39: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

• 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

Page 40: Saturn - Rutgers Physics & Astronomy › ... › lec23-Saturn-rings.pdf · Saturn Cassini Saturn & rings in 2013 The Sun is to the right, casting a long shadow across the rings at

The Cassini spacecraft

uses multiple

“gravitational slingshots”

to make multiple close

passes around Saturn’s

moons.


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