2500-15-large satellites

Worlds of Fire and Ice: 

The Large Satellites 

Chapter 15 

Family Portrait of All Four Large Moons of JUPITER 

Io, Europa, Ganymede and Callisto 

Shown to Scale in this Composite Galileo Photograph

Jupiter Has 40 Natural Satellites 

Most Have Been Studied by: 

Galileo 

Pioneer 

Spacecraft 

Cassini/Huygens 

Voyager Spacecraft

JUPITER 

JUPITER’S Four Largest Moons 

(Galilean Moons – Discovered by Galileo) 

These Moons of JUPITER, When First Observed by Galileo, Were 

Pointed To as Proof That Not All Celestial Bodies Orbited 

Around EARTH, and That EARTH Was Not the Center of the 

Universe 

NASA/NGS Image Collection

ZEUS and Io 

Io was a priestess of Hera in Argo, a nymph who 

was seduced by Zeus, who changed her into a heifer 

to escape detection. Her mistress Hera set everwatchful 

Argus Panoptes to guard her, but Hermes 

was sent to distract the guardian and slay him. 

Heifer Io was loosed to roam the world, stung by a 

maddening gadfly sent by Hera, and wandered to 

Egypt 

Zeus and Io 

c. 1530 

by 

Antonio Allegri da Correggio 

1489 - 1534 

Rococo

ZEUS and EUROPA 

Zeus was enamored of Europa and decided to seduce or ravish her. He 

transformed himself into a tame white bull and mixed in with her father's herds. 

While Europa and her female attendants were gathering flowers, she saw the bull, 

caressed his flanks, and eventually got onto his back. Zeus took that opportunity 

and ran to the sea and swam, with her on his back, to the island of Crete. He then 

revealed his true identity, and Europa became the first queen of Crete. 

The Abduction of Europa 

1632 

by 

Rembrandt 

1606-1669 

Dutch Golden Age

The Rape of Ganymede 

1611 

Peter Paul Rubens 

1577 – 1640 

Flemish Baroque 

Zeus Abducts Ganymede by 

Turning Into an Eagle, and 

Immediately Falls in Love with 

Him 

Also Celebrated by: 

Schubert in Music 

Goethe in Poetry

JUPITER and CALLISTO 

Callisto took a vow to remain a virgin, as 

did all the nymphs of Artemis. But to 

have her, Zeus disguised himself as 

Artemis/Diana herself, in order to lure 

her into his embrace and rape her. 

Afterwards, Callisto, when she was 

already with child, was seen bathing and 

so discovered. Upon this, the goddess 

Artemis was enraged and changed her 

into a beast. Thus she became a bear and 

gave birth to a son called Arcus. 

Either Artemis "slew Callisto with a shot 

of her silver bow,” or later, Arcas, the 

eponym of Arcadia, nearly killed his 

bear-mother, when she wandered into the 

forbidden precinct of Zeus. Zeus placed 

them both in the sky as the Constellations 

Ursa Major, called Arktos, the "Bear", by 

the Greeks, and Ursa Minor 

Jupiter and Callisto 

Francois Boucher 

1703 – 1770 

Rococo

JUPITER 

JUPITER’S Four Largest Moons 

(Galilean Moons – Discovered by Galileo) 

Io - Innermost moon, one of only three volcanically active bodies in the solar system 

Europa – Smallest of Galilean Moons, icy surface cris-crossed by numerous features 

Ganymede – Largest Jovian satellite, cratered & smooth areas, numerous parallel grooves 

Callisto – Outmost Galilean satellite, densely cratered much like EARTH’S Moon 

NASA/NGS Image Collection

The Large Moons 

Characteristics 

• Bulk Composition-Prime Indicator of Body’s Interior Composition & Structure 

• Uncompressed Densities Suggest That Many of These Large Outer Satellites Are 

Composed of ½ Water Ice and ½ Rock

The Jovian Moons 

Surface 

Compositions 

• Long Before 

Spacecraft Reached 

the JUPIER System, 

We Knew That 

Callisto, Ganymede 

and Europa Were 

Covered with Ice 

• This Deduction Is 

Made from Infrared 

Spectra Such as These 

Obtained at Kitt Peak, 

Arizona

Impacts and Cratering in the Outer Solar System 

Materials – Ice v.s. Rock 

• Ice Is Plastic and Deformable on EARTH 

Little Chance of Crater Preservation 

• Ice Is Strong and Stiff Like Rock at SATURN and Beyond 

• Impactors 

• 75 - 90% of Inner Solar System Impactors Are Asteroids 

• JUPITER – Impactors Are Primarily JUPITER-Family Comets 

(Short Period Comets) from the Kuiper Belt 

• Beyond JUPITER – Impactors Are Primarily Long Period Comets 

from the Oort Cloud 

• Impact Rate Is a Factor of Two Less for Outer Bodies 

• Big Deficiency in Comets (Craters) Smaller Than 1 Km

Variations in Cratering Rates 

• Large Planets with Large Gravity Fields 

More Easily Attract Impactors 

• The Closer the Impactor Comes – The Faster It Travels 

• Satellites Located Closer to The Planet Receive More 

Impacts at Higher Speeds 

Mimas, the Innermost Satellite of SATURN, Receives 20 Times the 

Imapacts as Iapetus, Far Out from SATURN 

Mimas 

Iapetus

Crater-Retention 

Ages on the 

Satellites 

• Larry Soderbloom 

(right) Shares a 

Humorous Moment with 

Brad Smith During the 

Voyager Encounters with 

the Saturn System 

• Larry Soderbloom 

(of the USGS) 

Was Deputy Team 

Leader and Senior 

Geologist on the Voyager 

Imaging Team


Ages on the 

Satellites 

Effects of the Local 

Gravity Field Increase 

the Cratering Rate, 

Which Compensates for 

Fewer Impactors in the 

Outer Solar System 

Brad 

Smith 

1976 

Viking 

Mission 

Team


Ages on the 

Satellites 

• He Played a Important Role 

in Defining the Standard 

Cratering History For the 

Inner Solar System with Its 

Heavy Bombardment Ending 

About 3.8 Billion Years Ago 

• He (and Colleagues) 

Concluded That the Heavy 

Bombardment Was 

Associated with the Entire 

Solar System, and Not 

Limited to Just the 

Terrestrial Planets of the 

Inner System

JUPITER 

Ganymede and Callisto 

Near Twins (Size, Density, Regions of Space)

Callisto: Basic Facts 

• Diameter - 4840 km 

• Density - 1.9 g/cm3 

Equal Water Ice and Rock 

• Only Partially 

Differentiated 

Froze Part-Way Through 

the Process 

• Most Heavily 

Cratered, and Hence 

the Oldest, Surface of 

the Galilean Satellites

Callisto: Basic Facts 

• Surface Temperature: 

•150 o K Noon 

•100 o K Night 

• Frigid Conditions 

Water Ice Very Stable 

For a Long Time 

• Ice Observed on 

Surface 

• Albedo – 18% (Low) 

Darkened by Meteoric 

Dust with Little New 

Ice Resurfacing

First Look at Callisto 

Voyager

Geology of Callisto 

• The Ancient Surface of 

Callisto as Imaged by 

Voyager in 1979 

• Impact Craters Nearly 

as Dense as the Lunar 

Highlands 

• Crater Density of 250 

10-km Craters Per 

1 Million Km 2 

• The Big Bulls-Eye 

Structure is Called 

Valhalla


• Very Little Geological 

Activity 

• Surface Is as Old as 

Lunar Maria 

• Higher Cratering 

Rates During the Early 

Part of Callisto’s 

History 

• Early History 

Equivalent to the Heavy 

Bombardment Period of 

the Inner Planets


• Close-up Look at an Area Near 

the Equator of Callisto 

• The Craters Have a Subdued 

Topography Relative to Those 

on the Rocky Terrestrial Planets 

• Subdued Look Due to Plastic 

Deformation of Ice – Unable to 

Retain Sharp Contours 

• No Big Basins – Instead, 

Bulls-Eye Pattern Is All That 

Remains 

Approximately 200 km Wide 

300 km Long 

(The Size of Connecticut)

• High-Resolution View 

of the Surface of Callisto 

Taken by the Galileo 

Cameras in 1996 


• The Surface Has Been 

Extensively Modified by 

Sublimation and 

Degradation of the 

Dirty-Ice Surface 

• Ice-Capped Peaks 

Appear Amid Smoother 

Plains of Darker 

Material Left Over 

When the Ice 

Evaporated

Ganymede: A 

Moon with a 

History 

• Distant View of 

Ganymede Obtained 

by Voyager 2 

• 

• The Surface Has 

Both Dark and Light 

Terrains 

Reminiscent of the 

Appearance of Our 

Own Moon to the 

Naked Eye 

• Many Bright Ray 

Craters Are Visible

Ganymede 

3rd Closest Moon to Jupiter 

Largest Moon in Solar System 

Larger Than Planets PLUTO and MERCURY

Ganymede 

3rd Closest Moon to Jupiter 

• Density - 1.9 g/cm3 (Same as Callisto) 

• Differentiated: 

• Large, Dense Silicate Core 

• Mantle and Crust of Water Ice

Ganymede: A 

Moon with a 

History 

• Old, Dark Terrains and 

Less Cratered, Light 

Terrains Modified by 

Internal Activities 

• Cater Densities on Light 

Terrains – 

100-200 10 Km Craters 

Per 1 Million Km 2 

Much Lower Than the Heavily 

Cratered Areas of Callisto or 

Ganymede 

• Crater Retention Ages of 

1-2 Billion Years

Ganymede - Indications of Internal Activity 

Several Diverse Terrains Appear in This Galileo Image 

• Oldest Area (right) Consists of Rolling Hills with Many Craters 

• Intermediate Age (Left) Displays a Dense Pattern of Folded 

Mountains (Not Tectonic Compression – Rather Uplift-Downdrop 

Along Faults - Valleys Flooded by Waters from the Interior) 

• Youngest (Center) Displays Smooth Terrain 

with Numerous Craters 

(Craters Suggest that Surface Is at Least 2 Billion Years Old) 

Note: This Is a Single Picture – Not a Montage of Three Separate Images

Ganymede 

• Old, Dark Terrains and 

Young, Light Terrains - 

Both Contain Water Ice 

Mixed with “Dirty 

Contaminants” 

• Young Areas Are 

Brighter (40% Albedo) 

- Less Contaminated 

- Just Reverse of Lunar 

Surface (25% Albedo) 

• A Few Bright Craters 

with Crater Rays (Pure 

Water Splash)

• Sublimation and 

Erosion Has Formed 

Patterns of Bright Ice 

and Drifts of Dark 

Materials 

Ganymede Has Mountains Just Like 

the Mountains of Callisto 

• Dark Material Is 

Warmer – Absorbs 

Sunlight – No Ice 

Condensation 

• Bright Areas Are 

Colder – Promotes 

Formation of Ice 

• Areas Are 

Self-Perpetuating 

Mountains of Callisto

Ganymede 

Indications of Internal 

Activity 

Area Is 15 km Per Side 

• This Ancient Heavily 

Cratered Terrain Has Been 

Cut by a Series of Faults, 

Associated with Uplift and 

Subsidence, Forming Long 

Parallel Ridges 

• Smooth Areas Filling the 

Valley Floors Might Have 

Been Flooded by Water 

“Lava”

Ganymede 

Expansion and 

Contraction Due to 

Changes in the Density 

and Structure as Interior 

Ice Altered Crystalline 

Form with Slow Cooling 

of the Core ? 

Spacecraft Galileo Discovered 

Ganymede Has a Magnetic Field 

(Electrically Conductive Layer – Viscous Icy Slush?) 

Does This Indicate a Salty,

Anatomy of a Torn 

Comet 

Crater Chains and Disrupted Comets 

• This Crater Chain 

(Called Enki Catena) 

Was Made by the 

Impact of a Disrupted 

JUPITER-Family 

Comet 

(Such as Comet 

Shoemaker-Levy 9) 

• The Fragmented 

Comet Crashed Into 

Ganymede Just After a 

Close Encounter with 

JUPITER

15.3 Europa, the Moon with an Ocean 

Europa Is the Smoothest Satellite Known 

Absence of Easily Visible Impact Craters Means It Is a 

Relatively Young Surface 

Images From Galileo: 

Left – Natural Color Right – Colors Enhanced to Bring Out Detail

• Smallest Galilean Satellite 

3,138 Km Diameter 

• Rocky Composition with 

Only 10% Water Ice 

Density – 3 g/cm 3 

• Brightest Galilean 

Satellite 

Albedo – 70% 

• Spectra Indicates Nearly 

Pure Water Ice Surface 

Europa 

2 nd Closest 

Moon of Jupiter 

• Very Active Satellite 

Despite “Rule of Thumb” Small 

Bodies Cool Off More Quickly 

Assuming a Dormant State

Europa 

Growing Evidence of Large Liquid Ocean Water Below Icy Surface

• Little Topographic Relief 

• Smoothest Body in the 

Solar System 

• Almost No Visible Impact 

Craters 

• Early Record of 

Bombardment Has Been 

Erased Despite High 

Impact Flux This Close to 

JUPITER 

Europa 

2 nd Closest 


• Inferred Age of the 

Surface Is Only a Few 

Million Years Old or 

Younger (Resurfacing?)

• Close-Up of One of 

the Linear Ridges on 

Europa 

Image Approximately 

14 km by 17 km 

Linear Markings 

Global Scale, Tectonic Cracks 

Caused by Crustal Tension; Then Either 

Filled With Slushy Material from Below, or 

Closed by Compression Forming the Ridges 

• Typically a Double 

Ridge 

• This One Is the 

Youngest Feature on 

the Image, Cutting 

Across a Variety of 

Older Terrains 

Ridge is About 3 km Across 

and Rises 300 m Above the 

Surrounding Plains

Europa 

• Scalloped Arc Shaped Pattern Caused by Tidal Forces Changing 

Tensional Direction 

• Tidal Forces Vary - Period of 1 Europa Day (3.5 EARTH Days) 

• Cracks Propagate through Crust at 10 km/hr

Chaos and the Global Ocean 

Example of “Chaotic” Terrain (Small Percentage of the Crust) 

Many Individual Blocks of Older Grooved Crust Can Be Observed 

Embedded in Younger Ice: Some of Them Have Been Partially 

Rotated or Tilted as if They Were Icebergs Floating in a Liquid Sea

Europa 

Close-Up of Chaotic Surface 

Icy Surface – Water Upwelling? 

Crust 10 – 20 km Thick ?

Chaos and the Global Ocean 

Europan Icebergs Up Close 

Berg “Conamura Chaos” Displays Cliffs Along the Edges of High- 

Standing Ice Plates, Presumably Parts of Older Crust That Were 

Floating Rather Recently in Liquid Water or Partially Melted Slush 

Silhouette of 

the Ocean 

Liner Titanic 

Indicates the 

Scale of the 

Image

• Magnetic Field Indicates a 

Coupling Between the Intense 

Jovian Magnetosphere and an 

Electrically Conducting Layer in 

the Upper 100 Km of Europa 

• Liquid Water with Dissolved Salts 

or Other Chemicals Works 

• Internal Heat Source? and Tidal 

Heating Keeps Water Liquid 

Europa 

2 nd Closest 


• Europa Could Have the Largest 

Volume of Water of Any Body in 

the Solar System 

• A Major Target to Find Life Due 

to Chemosynthesis

Io 

Closest Moon 

of JUPITER

Io 

Density – 3.3 g/cm3 

• Higher than Europa (3 g/cm3) 

• About the Same as EARTH’S Moon 

Must Be: 

Rocky with Little Water

Io 

• High Albedo 

Not White or Gay Like 

Europa Ganymede or Callisto 

• Yellow, Red, and Brown 

Due to Volcanic Activity

15.4 Io 

The Volcanic Moon 

• Surface Looks 

Completely 

Different From 

Other Members of 

the Solar System 

Because of High 

Level of Volcanic 

Activity 

• Subtle Shades of 

Color Caused by 

Sulfur From 

Volcanoes 

Image from Galileo: 

1999 - First Close Fly-by

Io 

Innermost of 

JUPITER’S 

Galilean Moons 

Images from 

Voyager and 

Galileo 

Spacecrafts 

- No Impact Craters 

- More Than 100 Active Volcanoes 

- Sulfurous Gas and Ash from These 

Volcanoes Bury Any Newly 

Formed Meteorite Impacts 

- Tidal Heating (Due to Its Proximity 

to JUPITER) Partially Melts Interior 

and Drives Volcanism

Io 

• About Same Distance 

as Moon from EARTH 

• JUPITER 300 Times 

Larger Than EARTH 

Voyager 

• Major Tidal Heating 

• JUPITER Pulls Io Into an Elongate Shape 

Several Kilometer Bulge Toward JUPITER 

• Gravitational Pulls of Europa and Ganymede Keep Io from 

Settling Into a Circular Orbit 

Perihelion - Io Moving Fastest (Rotation Rate Falls Behind Revolution Rate) 

Aphelion – Io Moving Slowest (Rotation Rate Exceeds Revolution Rate)

Io 

• Lower Limit of Tidal Energy Source Can Be Estimated 

By Measuring Heat from Volcanoes 

• At Least 100 Million Megawatts 

(More Than 10 Times All the Energy Consumed by Humans on EARTH)

Io 

• Water, Other Ices, Carbon, 

Nitrogen Compounds 

All Driven Off 

• Mostly Sulfur Compounds 

Remaining 

• Silicate Interior 

Completely Melted 

• Crust Only 25 Km Thick 

• Crust Constantly Recycled 

by Volcanic Activity

Geology of Io 

Mountains, Volcanic Calderas, Extensive Silicate Lava Flows and 

White Sulfur Dioxide “Snow” Make for a Colorful Surface 

Image from: Galileo 

Mountain at Far Right Is 8 km High 

Features as Small as 1 km Are Visible

Geology of Io 

High Resolution View of an Eroding Plain or Mesa 

Subsurface Sifting of Magma Can Leave Isolated Highlands 

That Subsequently Erode by Collapse and Debris Flow 

(Rather Like the Erosion Along the Walls of the Valles Marineris on MARS)

Volcanic Eruptions 

Discovery Image of 

Io’s Erupting 

Volcanoes 

In This Distant and 

Overexposed 

Voyager Photo, 

Two Huge Plumes 

Can Be Observed: 

1. One Silhouetted 

Against Space 

2. Another Shining 

Brightly Near the 

Edge of the 

Lighted Crescent

Volcanic Eruption on 

JUPITERR’S Moon Io 

Composite Image of Pele Volcano 

Plume of Volcanic Gases and 

Debris Is Rising Over 

300 km 

Above Io’s Surface 

Discovered 

by 

Linda Hyder 

Data Gathered by NASA’s Voyager 1 probe 

Computer, Color Enhanced Photo

• Volcanoes on Io Are 

Named after Volcano 

Fire Gods from Various 

Cultures 

IO 

Plume Eruptions 

• Hot, Fluid Sulfur and 

Sulfur Dioxide Drives 

the Plume 

• Emits About 100,000 

Tons/Sec. 

• Enough to Cover the 

Entire Surface of Io 

10’s of Meters Thick in 

1 Million Years 

Data Gathered by NASA’s Voyager 1 

Probe, Computer Enhanced Photo

IO 

Plume Eruptions 

• 10 Tons/Sec of Suflur 

and Sulfur Compounds 

Escapes from Io 

• Quickly Broken Down 

and Ionized by UV 

Sunlight 

• Provides Most of the 

Charged Sulfur and 

Oxygen Ions in the 

Inner Jovian 

Magnetosphere 

Data Gathered by NASA’s Voyager 1 

Probe, Computer Enhanced Photo

Io 

The Great 

Eruption of 

Tvashtar 

Volcano 

Lava Flows with 

Bright Fresh Lava at 

Their Toes 

• First Direct 

Images of Large- 

Scale Silicate 

Volcanism on 

Io 

• Silica 

NOT Plumes 

Are Emitted from 

Volcanic Vents 

“Curtin of Fire” 

Fresh Lava Flow 

60 km Long 

Edge of Caldera 

(Depression) 

Galileo Image – February 22, 2000 

Colors Are Enhanced - Red Added 

Image Width 250 km Across

Volcanic Eruption on 

JUPITERR’S Moon Io 

Composite Image of Pele Volcano 

Plume of Volcanic Gases and 

Debris Is Rising Over 

300 km 

Above Io’s Surface 

Plumes Form When 

Silicate Lava Flows 

Encounter Thick Deposits 

of Frozen Sulfur Dioxide 

That Cover Most of the 

Surface of Io 

With Almost No Atmosphere, 

Plumes Can Shoot –Up To 

Tremendous Heights 

Data Gathered by NASA’s Voyager 1 probe 

Computer, Color Enhanced Photo

Io 

Plume Eruptions From Pele and Pillan Volcanoes 

Changes During Three Years 

April, 1997 September, 1997 July, 1999 

Image Width 500 km Across 

Colors Are Enhanced

Io 

Infrared Emissions 

from Hot Spots 

Visible Image Showing 

Several Active Lava 

Flows from the 

Amirani Volcano 

• 100’s of 

•“Hot Spots” 

Dot Io Forming 

“Lava Lakes” 

on the Surface 

• Much Internal 

Heat Escapes 

Through These 

Hot Spots 

Thermal Infrared Data at 

a Wavelength of 5 um 

Image Resolution Is 6 km for Infrared Image (left) 

Image Resolution Is 1 km for Visible Image (right)

Galileo 

JUPITER and Io

Dione – Foreground, Tethys and Mimas – Lower Right; 

Enceladus and Rhea – Upper Left; 

Titan – Upper Right 

Saturn’s Moon Titan 

Montage Courtesy of NASA

Titan 

Density – 1.9 g/cm 3 

• Same as Ganymede and 

Callisto 

• But Unlike These Other 

Two Essentially Colorless 

Satellites 

Titan is Reddish Colored 

Titan Has an Atmosphere 

• First Documented by Gerald Kuiper in 1944 

• Spectrograph Attached to the McDonald Observatory 

82 inch (2.1 m) Telescope 

Montage Courtesy of NASA

• Infrared 

Spectrum of 

Titan (top) 

Showing 

Emission Bands 

from Several 

Gases 

15.5 Titan, The Atmospheric Moon 

Besides the Expected Solar Bands – Titan 

Shows Gas Absorption Bands of Methane Gas 

• 2 Laboratory 

Spectra of 

Individual Gases 

Show How These 

Substances Can 

Be Identified in 

Titan’s 

Atmosphere

Voyager 1 Results 

Photos of Titan Showing Only Its Ubiquitous 

Haze-Filled (Smog) Atmosphere 

Fully Illuminated 

Hemisphere 

Occultation of Voyager Behind 

Titan Allowed for a 

Determination of the Density of 

Titan’s Atmosphere 

Surface Pressure – 1.5 Bars 

Back-Lit Image 

Revealing Extended 

Atmosphere More Clearly

Titan 

Composition of the Atmosphere 

• Gravity Is Less on Titan 

Than on EARTH – SO… 

• It Takes Nearly 10 Times the Gas on 

Titan to Exert the Same Pressure on an 

Equal Area of the Surface as on 

EARTH 

• The Atmosphere on Titan Also 

Extends 10 Times Farther Into Space 

Than Above EARTH 

• Kuiper’s Methane Is Only a Small 

Constituent of the Overall Composition

Titan 

Composition of the Atmosphere 

Implications for Prebiotic 

Chemistry 

Compounds Necessary for the Origins 

of Life 

• Hydrogen Cyanide (HCN) 

Starting Point for the Formation of 

Some Components of DNA 

• Carbon Monoxide (CO) 

• Carbon Dioxide (CO 2 ) 

Makes the Formation of Amino Acids 

Possible

Photochemical 

Smog 

• Detached Layers of 

Haze in Titan’s 

Atmosphere 

• Featureless Haze Is 

Even More Uniform 

Than the Clouds of 

VENUS 

• Only When Viewed 

from the Side Does It 

Show a Distinct 

Layer Hundreds of 

Kilometers Above 

the Surface 

This Voyager Close-up 

Image Has Been Enhanced 

and the Color Intensified

Photochemical 

Smog 

• Primitive Chemical 

Environment 

• Reactions May 

Resemble Those 

That Preceded the 

Evolution of Life on 

EARTH 

• Is Titan a “Natural 

Laboratory” to Test 

Chemical Evolution 

and Preferred 

Pathways Toward 

Complexity (Life) 




Voyager Ultraviolet 

Spectrometer 

Revealed Additional 

Absorbing Layers 

Between 

300 - 500 Km 

Above the Surface 

• HCN in the 

Atmosphere Forms 

Long Polymer 

Chains 

• Polymer Chains 

and Condensed 

Organic Compounds 

Produce the Reddish 

Haze 




• Haze Particles Grow 

Larger and Drop Out 

of Atmosphere onto 

the Surface 

• Lower Atmosphere 

and Surface Colder 

Then Upper 

Atmosphere – So 

No Evaporation and 

Re-Mixing 

• Only Ethane and 

Methane Might Be 

Able to Re-Mix Much 

Like EARTH’S 

Terrestrial Water 

Cycle 




Evolution of the Atmosphere 

• Methane (CH 4 ) Is Broken Down by Electrons and Ultraviolet 

Photons 

• Some Hydrogen Escapes to Space Enriching Deuterium on Titan 

• Evidence Shows There Is a High Abundance of CH 3 D Methane 

• “Left-Over” Carbon Must Be Removed from the System to Remain 

in Balance 

• Ethane (C 2 H 6 ) Is the Most Abundant End Product 

• Titan Is Cold Enough That This Gas Would Condense Forming 

Ethane Lakes on the Surface of Titan 

• Titan is 94 o K (-179 o C) 

• It Is So Cold That H 2 0 Vapor Would Be Completely Lacking 

• Therefore: Lacking Any Appreciable Amount of Oxygen – Methane 

Remains Un-Oxidized, Adding to the Condensed Hydrocarbon Lake 

on the Surface of Titan

Surface of Titan 

The Surface of Titan 

May Be 

Predominantly: 

1. - Ice with a 

Covering of 

Organic Matter 

2. - Lakes 

and 

3. - Sea of 

Hydrocarbons 

Thus, It Could Be an 

Interesting Place to 

Explore by Boat

SATURN 

Cassini Orbiter 

Remained in Orbit 

to Photograph Saturn 

and Moons 

Cassini / Huygens Spacecraft 

September 2004 (USA) 

Huygens Lander 

Sampled Atmosphere 

During Descent and 

Landed on Titan

TITAN 

Cassini / Huygens Mission 

September 2004 

(USA) 


Sampled Atmosphere 

During Descent

TITAN 


September 2004 

(USA) 

Photographed Surface of Titan 

Dendritic Drainage River Pattern

TITAN 

Cassini/Huygens Mission 

September 2004 (USA) 


Landed on Mushy 

“Wet-Sand-Like” Surface 

Surface of Titan Strewn 

with Rocks Composed 

of Frozen Methane 

and Other 

Carbon-Containing 

Compounds 

NASA/JPL/SSI/ESA/University of Arizona

NEPTUNE 

by Itself - 

Proved to Be 

“Rather 

Boring” 

15.6 

Triton 

NEPTUNE’S 

Maverick Moon 

Courtesy of JPL

Triton - 

Son of Neptune

Triton 

(in Foreground) 

Large Moon 

• Diameter – 2,705 Km 

• Half the Size of Titan 

• The Size of Pluto 

Orbits a Planet So It Is Considered a Moon 

But: if It Orbited a Sun, It Would Be a Planet

Triton 


Retrograde Orbit 

• Revolves Around Neptune 

Opposite to the Direction the Planet 

Rotates, and to the Direction All the 

Other Planets Orbit Around the SUN 

• Rotation Is Still Synchronous with Its 

Period of Revolution 

• Always Keeps One Face Toward 

NEPTUNE

Triton 


Retrograde Orbit 

Indicates that Triton Was Captured 

into Neptune’s Orbit

Triton 


• Density – 21. g/cm3 

Calculated by: 

• Volume Based Upon the 

Diameter of Triton 

• Mass Determined by the Effect 

Upon the Trajectory of Voyager 

Spacecraft 

• Rock Dominant in a Rock-Ice 

Mixture 

• Based Upon Diameter & Density 

PLUTO Will Be Very 

Similar to Triton

Triton’s Surface 

Unique Surface Appearance Due to Extremely Low Temperatures 

• Most Gases Freeze Onto the Surface 

(Methane, Carbon Dioxide, Carbon Monoxide, Molecular Nitrogen, Minor Water) 

Surface in Upper Part of Image is Called “Cantaloupe Terrain” 

• Surface Is Very 

Young 

No Large Impact Craters 

• High Albedo 

Little Energy Absorbed 

• Surface Temperature 

37 o K 

Image from: Voyager 2 

Colors Are Enhanced

• This Seasonal Transport May 

Be How Triton Maintains Such 

a High Albedo 

Cantaloupe 

Terrain 

Triton’ Surface 

• Relatively Flat Plain 

• Mottled with Darker Material 

South Pole 

Ice Cap 

• Dark Material Blown About 

by Wind (Triangular Wind Streaks) 

• Wind Streaks May Be Due to 

Surface Ice Sublimation During 

Summer 

• This Leads to Flow of Nitrogen 

Gas Towards the Poles

Triton 

Nitrogen Atmosphere

Triton 

(Foreground) 

• Wind 

(Streaks) 

• Haze and Clouds 

(Photographs) 

• Thin Atmosphere of Nitrogen (N 2 ) 

(Ultraviolet Spectrometer on Spacecraft) 

• Planet Covered with Bright Material 

Very Cold (37 o Above Absolute 0 o ) 

Nitrogen Freezes Every Winter 

• Random Dark Areas Cover Bright Surface 

Thought to Be of Recent Origin

• Calculations Based Upon the 

Apparent Escape of Hydrogen 

Indicates a 6-m Layer of 

Hydrocarbons Could Have 

Been Produced on the Surface 

of TRITON 

• If Nitrogen (N 2 ) Is the 

Dominant Gas on PLUTO 

• Then PLUTO Will Be Very 

Similar to Triton 

• 16 Microbars of Pressure 

(16 Millionths of the Sea-Level 

Pressure on EARTH) 

• 35 – 40 o K

Triton 

Atmospheric Chemistry 

• Dark Splotches Must Be 

Relatively Young Because They 

Subside Into the Ice With Time 

• Much of Triton’s Atmospheric 

Chemical Products May 

Actually Be Buried Under the 

Icy Geological Surface

Evolution of the Atmosphere 

Geyser on Triton 

• Voyager 2 Image Shows What Appears to Be an Eruption Rising 

to an Altitude of About 8 km 

• There, It Encounters a Transverse Wind in a Very Thin 

Atmosphere 

• Winds Carry the Materials 100 – 150 Km Downwind 

• Streamers Are About 10 Km Wide

Triton 

(Foreground) 

NEPTUNE 

(Background) 

Nitrogen Geyser 

• There Must Be a 

Warm Source 

Still Less Than -200 o C 

to Produce Buoyant 

Gas 

• Geysers Parallel 

Nitrogen Evaporation 

of Cold Comet Nuclei

Triton 

Moon of NEPTUNE 


Discovered by 

Larry Soderbloom 

2006

Triton 

Moon of NEPTUNE 


Discovered by 

Larry Soderbloom 

2006 

1970’s 

Deputy Team Leader 

and Senior Geologist on 

the Voyager Imaging 

Team

15.7 Comparing the Large Satellites 


Three High-Resolution Images Show How Different the Surface 

Geology Is Even for the Three Satellites of JUPITER with Ice Crusts 

Europa Ganymede Callisto



Why Did Ganymede Differentiate and Maintain a 

Substantial Level of Geological Activity for Millions of 

Years While Callisto Did Not ? 



Ganymede Is: 

• Closer to JUPITER – Subject to Larger Tidal Stress (Heating) 

• Subject to More Impacts Due to Attraction of JUPITER’S Gravity 

• Larger and Slightly Denser Than Callisto 

- Contains More Radioactive Materials (More Internal Heating) 

- Retains Heat Slightly Better 

Which Means Higher Internal Temperature and Slower Cooling 



Ganymede 

• These Factors Are Small – But Even When Taken Together - They 

Would Not Produce Differentiation on Their Own 

• Rather - They Triggered a Major Change in Another Internal 

Process – Possibly Upper Mantle Convection 

• Sufficient to Drive Early Plate Tectonics for 1 Billion Years 

• Finally - Cooling Produced a Phase Change of Interior Liquid 

Water to Solid Ice - Crust Cracked with Minor Mountain Building 



Europa and Io 

Why Are Europa and Io So Depleted in 

Water and Other Volatiles Relative to 

Callisto and Ganymede ? 



Europa and Io 

• Both Are Closer to JUPITER Than Ganymede and Callisto 

• Cool Enough for Ice to Condense at Ganymede and Callisto – But 

Too Hot Closer To JUPITER Where Europa and Io Formed 

• Water and Volatiles Driven Off 



• BUT – Europa Contains 10%Water 

• Water Supplied by Cometary Impacts to Io and Europa 

• BUT - Io Was in the Grip of Europa and Ganymede Preventing Io 

From Being Able to Circularize Its Orbit Around JUPITER 

• Additional Tidal Heating of Io Drove Off All Water and Volatiles 



Titan and Ganymede 

Both Ganymede and Titan Are 

About the Same Size and 

Composed of 

Half Ice and Half Rock 

SO - 

Why Does Ganymede 

Not Have an Atmosphere 

While Titan Has an Atmosphere 

Denser Than EARTH’S


Titan Point #1 

• Did Titan Capture Its Atmosphere from 

the Primary Nebula 

or 

• Was the Atmosphere Formed by the 

Release of Gases from the Solid Bulk 

Material of Titan 

• The Primary Nebula Contained Nearly Equal 

Amounts of Nitrogen and Neon 

(Determined by the Present Composition of the SUN) 

• Ultraviolet Spectrometer on Voyager 1 Showed 

Almost No Evidence of Neon (Upper Limit of 0.1 %)



• Did Titan Capture Its Atmosphere from 

the Primary Nebula 

or 

• Was the Atmosphere Formed by the 

Release of Gases from the Solid Bulk 

Material of Titan 

• The Primary Nebula Contained Nearly Equal 

Amounts of Nitrogen and Neon 

(Determined by the Present Composition of the SUN) 

• Ultraviolet Spectrometer on Voyager 1 Showed 

Almost No Evidence of Neon (Upper Limit of 0.1 %)



Like the Inner Planets, Titan Must 

Also Have Accumulated Some Gases 

as a Result of Cometary Impacts 

• Approximately 50% of Titan’s Initial Mass Was 

Water Ice That Now Forms a Crust and Mantle 

Around a Silicate Core 

• Titan Is Partially Made-Up of Comets That 

Added to the Volatile Mixture Accreted by the 

Original Icy Planetesimals



Titan Must Have Lost Most of Its 

Original Atmosphere 

• Light Isotopes of Nitrogen Are Strongly 

Depleted in Titan’s HCN 

(Atmosphere Has Changed Throughout Time) 

• BUT - Like MARS 

Carbon Shows No Signs of 

Isotopic Fractionation 

(Always Being Replaced)



Titan Must Have A Large Reservoir 

of Methane (CH 4 ) To Replenish Any 

Carbon Lost from the Atmosphere 

• The Rate of Conversion of Methane (CH 4 ) in 

the Atmosphere into C 2 H 2 and Other 

Hydrocarbons Would Use-Up the Available 

Methane in 20 Million Years 

• Subsurface Seas of Methane Have Been 

Suggested to Supply the Necessary Methane



The Ability of Water Ice to Trap Gases 

Depends Upon the Temperature of the Ice 

(and Therefore the Structure), Along With 

the Size and Electrical Properties of the 

Gas Molecules 

• Hydrogen (H), Helium (He) and Neon (Ne) Are 

Only Trapped at Temperatures Below 25 o K 

• Argon (Ar), Methane (CH 4 ), Nitrogen (N 2 ), and 

Carbon Monoxide (CO) Are Trapped in Varying 

Amounts Depending on the Ice Temperature



The Jovian Satellites Were Simply 

Too Warm for Ice to Trap Any of 

These Gases 

(Closer to the Sun and Tidal Heating by JUPITER) 

• Titan, SATURN’S Largest Satellite, Is Farther 

Away from the SUN, and Does Not Experience 

Tidal Heating to the Degree of the Jovian Satellites 

• Titan’s Gas-Rich Ice Was Able to Condense and 

Serve as the Carrier of Nitrogen and Other 

Volatiles Found on the Satellite



JUPITER Has a Much Larger Mass 

Than SATURN, and Therefore, a 

Much Larger Gravitational 

Attraction 

• Comets Colliding with Ganymede and Callisto 

Would Crash with Much Higher Energy Than 

Those Hitting Titan 

• Ganymede and Callisto Were Unable to Retain 

Gases After Impact; Whereas Titan, with Lower 

Energy Impacts, in the Colder URANUS- 

NEPTUNE Region, Was Able to Retain Gases

1994 

Galileo Spacecraft 

Launched 

To Explore JUPITER and Its Moons 

• Io Displayed Fresh Sulfuric 

Eruptions 

• Europa Is Covered by Ridges and 

Canyons 

Not as “Smooth” as First Thought 

• Ganymede Was Observed in More 

Detail 

Galileo, JUPITER and Io 

• Callisto Showed More 

“Destruction”

Worlds of Fire and Ice: 

The Large Satellites 

Chapter 15 

Family Portrait of All Four Large Moons of JUPITER 

Io, Europa, Ganymede and Callisto

2500-15-large satellites

Create successful ePaper yourself

Delete template?

Save as template?