Chapter 11 & Chapter 12 Jupiter & Saturn

Home , Atmosphere of Jupiter, Moons of Jupiter, Moons of Saturn, Shepherd moon

11.1 Orbital and Physical Properties 11.2 The Atmosphere of Jupiter A Cometary Impact 11.3 Internal Structure Almost a Star? 11.4 Jupiter’s Magnetosphere 11.5 The Moons of Jupiter 11.6 Jupiter’s Ring

© 2011 Pearson Education, Inc. 11.1 Orbital and Physical Properties This figure shows the solar system from a vantage point that emphasizes the relationship of the jovian planets to the rest of the system

Three views of Jupiter: From a small telescope on Earth; from the Hubble Space Telescope; and from the Cassini spacecraft

© 2011 Pearson Education, Inc. 11.1 Orbital and Physical Properties • Mass: 1.9 × 1027 kg (twice as much as all other planets put together) • Radius: 71,500 km (11.2 times Earth’s) • Density: 1300 kg/m3—cannot be rocky or metallic as inner planets are • Rotation rate: Problematic, as Jupiter has no solid surface; different parts of atmosphere rotate at different rates – Equator spins faster • From magnetic field, rotation period is 9 hr, 55 min (9h 50min at equator)

© 2011 Pearson Education, Inc. 11.2 The Atmosphere of Jupiter • Atmosphere has bright zones and dark belts • Zones are warmer, and are higher than belts • Stable flow, called zonal flow, underlies zones and bands • Simplified model

Here: Wind speed with respect to internal rotation rate

Composition of atmosphere: mostly molecular hydrogen and helium; small amounts of methane, ammonia, and water vapor These cannot account for color; probably due to complex chemical interactions

No solid surface; take top of troposphere to be at 0 km Lowest cloud layer cannot be seen by optical telescopes Measurements by Galileo probe show high wind speeds even at great depth—probably due to heating from planet, not from Sun

Lightning-like flashes have been seen; also shorter- lived rotating storms One example: Brown Oval, really a large gap in clouds

© 2011 Pearson Education, Inc. 11.2 The Atmosphere of Jupiter Recently, three white storms were observed to merge into a single storm, which then turned red. This may provide some clues to the dynamics behind Jupiter’s cloud movements.

Find that Jupiter radiates more energy than it receives from the Sun: • Core is still cooling off from heating during gravitational compression

Could Jupiter have been a star?

• No; it is far too cool and too small for that. It would need to be about 80 times more massive to be even a very faint star.

© 2011 Pearson Education, Inc. 11.3 Internal Structure No direct information is available about Jupiter’s interior, but its main components, hydrogen and helium, are quite well understood. The central portion is a rocky core.

Jupiter is much too small to have become a star—needs 80 times more mass! But its energy output was larger in the past; could have been 100 times brighter than the Moon as seen from Earth Dwarf star in Jupiter’s place probably would have made stable planetary orbits impossible Jupiter played invaluable role in sweeping solar system clear of debris before too much reached Earth—otherwise life on Earth might not have been possible

Intrinsic field strength is 20,000 times that of Earth Magnetosphere can extend beyond the orbit of Saturn

63 moons have now been found orbiting Jupiter, but most are very small The four largest are the Galilean moons, so called because they were first observed by Galileo: • Io, Europa, Ganymede, Callisto Galilean moons have similarities to terrestrial planets: orbits have low eccentricity, largest is somewhat larger than Mercury, and density decreases as distance from Jupiter increases

Jupiter with Io and Europa. Note the relative sizes!

Interiors of the Galilean moons

Io is the densest of Jupiter’s moons, and the most geologically active object in the solar system:

• Many active volcanoes, some quite large

• Can change surface features in a few weeks

•No craters; they fill in too fast—Io has the youngest surface of any solar system object

Cause of volcanism: Gravity!

Io is very close to Jupiter and also experiences gravitational forces from Europa. The tidal forces are huge and provide the energy for the volcanoes.

Europa has no craters; surface is water ice, possibly with liquid water below Tidal forces stress and crack ice; water flows, keeping surface relatively flat

Ganymede is the largest moon in the solar system— larger than Pluto and Mercury History similar to Earth’s Moon, but water ice instead of lunar rock

Jupiter has been found to have a small, thin ring

12.1 Orbital and Physical Properties

12.2 Saturn’s Atmosphere

12.3 Saturn’s Interior and Magnetosphere

12.4 Saturn’s Spectacular Ring System

12.5 The Moons of Saturn

Dancing Among Saturn’s Moons

© 2011 Pearson Education, Inc. 12.1 Orbital and Physical Properties View of rings from Earth changes as Saturn orbits the Sun - Why is Saturn the flattest planet? 10% difference between Equatorial and polar diameters Saturn Orbits Differentially, similar to Jupiter. Equator – 10 hrs 14 min Higher Latitudes – 10 hrs 40 min

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

This true-color image shows the delicate coloration of the cloud patterns on Saturn

Similar to Jupiter’s, except pressure is lower Three cloud layers Cloud layers are thicker than Jupiter’s; see only top layer

Structure in Saturn’s clouds can be seen more clearly in this false-color image

Wind patterns on Saturn are similar to those on Jupiter, with zonal flow

This image shows what is thought to be a vast thunderstorm on Saturn, as well as the polar vortex at Saturn’s south pole.

Saturn also radiates more energy than it gets from the Sun, but not because of cooling:

• Helium and hydrogen are not well mixed; helium tends to condense into droplets and then fall

• Gravitational field compresses helium and heats it up

Saturn also has a strong magnetic field, but only 5% as strong as Jupiter’s

Creates aurorae

Saturn has an extraordinarily large and complex ring system, which was visible even to the first telescopes

Overview of the ring system

A, B, C Rings - B is brightest - C is almost translucent - A is in between in visibility

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

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 12.4 Saturn’s Spectacular Ring System Closest distance that moon could survive is called Roche limit; ring systems are all inside this limit

© 2011 Pearson Education, Inc. 12.4 Saturn’s Spectacular Ring System Voyager probes showed Saturn’s rings to be much more complex than originally thought (Earth is shown on the same scale as the rings)

Small gaps due to moonlets – (only 2 confirmed)

This backlit view shows the fainter F, G, and E rings

Voyager also found radial “spikes” that formed and then dissipated; this probably happens frequently

• Other edges and divisions in rings are also the result of resonance

• “Shepherd” moon defines outer edge of A ring through gravitational interactions

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

Titan has been known for many years to have an atmosphere thicker and denser than Earth’s; mostly nitrogen and argon Makes surface impossible to see; the upper picture at right was taken from only 4000 km away

Trace chemicals in Titan’s atmosphere make it chemically complex

Some surface features on Titan are visible in this Cassini infrared image

The Huygens spacecraft has landed on Titan and returned images directly from the surface

Based on measurements made by Cassini and Huygens, this is the current best guess as to what the interior of Titan looks like

The Cassini spacecraft uses multiple “gravitational slingshots” to make multiple close passes around Saturn’s moons. Precise orbits are decided on the fly.

This image shows Saturn’s mid-sized moons

• 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

Surface of Enceladus seems oddly youthful

Masses of small moons not well known Two of them share a single orbit

Two more moons are at the Lagrangian points of Tethys

• Jupiter is the largest planet in the solar system • Rotates rapidly • Cloud cover has three main layers, forms zone and band pattern • Great Red Spot is a very stable storm • Pressure and density of atmosphere increase with depth; atmosphere becomes liquid and then “metallic”

• Relatively small rocky core (but still about 10x size of Earth) • Still radiating energy from original formation • 63 moons, four very large • Io: active volcanoes, due to tidal forces • Europa: cracked, icy surface; may be liquid water underneath • Ganymede and Callisto: similar; rock and ice

• 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 the compression of “helium raindrops” • Saturn has a large magnetic field and extensive magnetosphere

© 2011 Pearson Education, Inc. Summary of Chapter 12 (cont.) • 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

• Titan is the second-largest moon in the solar system • Titan has an extremely thick atmosphere, and little is known about its surface or interior • Medium-sized moons are rock and water ice; their terrains vary • These moons are tidally locked to Saturn • Several of the small moons share orbits, either with each other or with larger moons