Saturn’s Moons

Lecture 24

Saturn’s Moons

62 Moons, 53 named (18 above). Mostly icy, some with rocky cores. Titan is the 2nd largest moon in our Solar System & only one with a “real”

atmosphere with N2, CH4, CO2 (1.5 bar!) 98% of N2 : (N2=77% at Earth) No appreciable O2

Titan, the Masked!Voyager 2 image of Titan

sunlight 1/100 of Earth -180°C

A lot of organic moleculesCH4, C2H2, C2H6, C3H8, …, argon, CO2, etc.

Always covered with thick haze/smog

Cassini/Huygens in 2004+

Cassini + Huygens (2004- )

Titan’s landscape from Huygens descending image…

looks like a dry streambed!Water ice as rocks…

Interior of Titan

Satellite gravity measurement…

Similar to Callisto, Titan’s interior is not differentiated!

It has a subsurface ocean at very low temperature mixture of water and ammonia

Controversial… : some believe that it should have a rocky core + icy mantle…

A lot of NH3 !

Titan’s Atmosphere

Multi-layer of haze Titan once was believed to be the largest moon in the solar system because of its extended

haze layer (~200 km). Titan’s solid surface is only 60km smaller than Ganymede… NH3 + CH4 + solar UV photons organic molecules… Drizzle of methane and ethane. Possible lakes/oceans of methane

Liquid FlowMethane river A feature most likely

formed by a liquid methane flow. Taken by Huygens probe.

Theoretical models predict that a single methane rainstorm can produce several inches of rain…

Methane World

Cassini pictures of Saturn's moon Titan taken in 2004 and 2005 show that a large methane lake suddenly appeared after what looked like a heavy rainstorm

Sea of Methane on Titan

A Cassini radar image juxtaposed with an image of the Lake Superior

Lots of Natural Gases, but no Oxygen to burn with! Temperature range for liquid:

water: 0 to 100C, methane: -182C to -164C, ethane: -183C to -89C

Possible ethane world?

Origin of Atmosphere Image of Titan taken from Cassini orbiter

10 times more extended than Earth’s

Key factor size (gravity)

How does Titan have an atmosphere when even a larger moon Ganymede doesn’t?

1. distance from the Sun2. effect of their host

planets

• Ganymede does not have an atmosphere at Jupiter’s distance, only water ice could condense…, but at Saturn’s distance, ices such as methane and ammonia could condense!

• Due to the stronger gravity of Jupiter, impacts were generally stronger at Jupiter’s moons than Saturn’s moon. Stronger impacts more easily blew away atmospheres…

More surface feature : Sand Dunes

Windblown dunes Namib desert from Space Shuttle

Titan : summary Very similar features with very different composition and temperature!

A lot of liquid hydrocarbons!about 200°C colder than liquid water much slower chemical reaction slower

metabolism A lot of organic material (e.g., organic sand dunes!) Possible life in the upper atmosphere (acetylene [C2H2] based) or in the subsurface

liquid ocean!

Earth Titan

liquid water liquid methane

silicate rocks water ice rocks

molten lava volcano ice/slush volcano

silicate sand dunes organic particulate dunes

Mimas

Enceladus

Active Enceladus Ice geysers subsurface liquid water + ammonia mixture

Although we expect some tidal heating, it is hard to explain all these activities.

possible subsurface habitable zone!

Tiger stripes = fresh ices cracks or grooves

6th largest moon of Saturn

Enceladus

Tethys

Dione

Rhea

Iapetus

Iapetus : An Intelligence Test for Earthlings?

3rd largest moon of Saturn

Strange Surface

Heavily terraformed?

Equatorial bulge (how???)

Iapetus = Alien’s Starship?

?

Iapetus Deathstar!

In summary…

Important ConceptsTitan:• thick atmosphere• origin of the atmosphere• methane world• similarities and dissimilarities

compared to EarthOther moons of Saturn: What’s

special about these?• Enceladus• Iapetus

Important TermsIce geysers

Chapter/sections covered in this lecture : sections 13-8 through 13-10