Life around Saturn,and beyond

ASTR 1420

Lecture 14

Sections 9.3

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 molecules

CH4, 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 dried streambed!Water ice as rocks…

Taken at an altitude of ~8 km.

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 55km smaller than Ganymede…• NH3 + CH4 + solar UV photons organic molecules…• Drizzle of methane and ethane. Possible lakes/oceans of methane

Ganymede : 2631 km radiusTitan : 2576 km radius 55km

Liquid Flow

Methane 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

-183C

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

made of hydrocarbon sediments.

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!• interesting to see if we can find right- and left-handed amino acids in life!

Earth Titan

liquid water liquid methane

silicate rocks water ice rocks

molten lava volcano ice/slush volcano

silicate sand dunes organic particulate dunes

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

Cryovolcanism

feeding a ring of Saturn

Iapetus : An Intelligence Test for Earthlings?

3rd largest moon of Saturn

a large brightness change (10+ times) over one rotation period!

Strange Surface• Heavily terraformed?

• Equatorial bulge (how???)

Iapetus = Alien’s Starship?

?

No, Iapetus is in fact Deathstar!

Triton: Surprising possibility of potential habitability

• Cryovolcanism…

largest moon of Neptune

Triton’s cantalope skin

Possibly formed by diapirism (i.e., slow boiling pattern)

• Retrograde motion = Triton orbits Neptune “backward” captured moon!

• Crater count Triton’s surface is 10-100 million years old.

• Active ice geysers!!

• Remnant internal heat from the capture may drive the geological activity…possible subsurface liquid oceaneven at -230°C, possible habitable world!

Triton: Surprising possibility of potential habitability

Cosmic Messengers

Signal from Pioneer

• A signal from the Pioneer 10 spacecraft, sent from a distance of more than 6 billion kilometers. The spacecraft transmitted the signal with a power of only one watt (about the power of X-mas tree light)!

In summary…

Important Concepts• At least six potentially habitable

jovian moons! Europa, Ganymede, Callisto,

Titan, Enceladus, and Triton

• Origin of Titan’s atmosphere

• Prominent characteristics of Titan, Enceladus, Iapetus, and Triton.

Important Terms• icy volcanism (cryo-volcanism)

Chapter/sections covered in this lecture : 9.3Next lecture : Exo-planets!