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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 6: Life Elsewhere in the Solar System

Key Ideas

Titan has a thick atmosphere
Titan's atmosphere is composed of Nitrogen, Methane, Argon, Ethane, Acetylene, and other hydrocarbons
Titan's Methane is converted to more complex hydrocarbons
Requires a Methane supply - hydrocarbon lakes
Cassini orbiter and Huygens probe provided important information
While Titan may not have life, it provides an enormous 'laboratory' for organic processes
Enceladus shows evidence for cryovolcanism and may have a subterranean liquid water ocean
Triton is very cold but shows evidence for activity

Saturn and its Moons

Properties of Saturn
a = 9.5 AU
P = 29.5 years

Equilibrium Temperature at Saturn

a-9.5AU --> T~90K, -180 C, -298 F

Spacecraft Visits to Saturn
--Pioneer 11 (Sept 1979)
--Voyager 1 (Nov 1980)
--Voyager 2 (Aug 1981)
Cassini Orbiter:
--Launched Oct 1997
--Arrived July 2004
--Huygens Titan probe,
--landed January 2005

The Moons of Saturn
Saturn has 56 moons:
--1 Giant moon: Titan

55 smaller moons:
--Sizes: 20 - 1500 km
--Density: 0.3 - 1.5 g/cc
--Rock/ice or mostly icy
--All have heavily cratered, ancient surfaces.


Saturn's giant moon:
--Radius: 2575 km
--Density: ~1.9 g/cc
--Icy mantle over a rocky core.

Only moon with a heavy atmosphere
--Cold enough to retain a heavy atmosphere

Titan's Atmosphere
--~80% N2 (nitrogen)
--~3% CH4 (methane)
--Hydrocarbons like:
--Ethane = C2H6
--Acetylene = C2H2
--Ethylene = C2H6
--Propane = C3H8

Cold, dense atmosphere:
--Temperature: 94 K (-179 C, -290 F)
--~1.6 Earth atmospheres
--Clouds of methane & N2 ices
--Hydrocarbon "smog"

Hydrocarbon Production
UV radiation from Sun breaks up Methane
Products for more complex hydrocarbons (ethane, acetylene, etc)

Liquids on Titan

Methane and Ethane are liquids on Titan
Constant breakdown of methane requires a source
Standing liquid methane provides a reservoir

Methane Cycle on Titan
Methane (CH4) plays the same role on Titan that water does on the Earth:
--94 K is between the boiling & freezing points of Methane
--Get gaseous methane in the atmosphere.
Ethane and methane condense, fall as rain
Also more complex hydrocarbons, HCN, etc.
Should be signs of liquid alteration and oceans, seas, lakes of ethane/methane

Evidence for Liquid on Titan

Huygens Probe
Landed on Jan 14, 2005
Designed to float!
Touched down on solid land
No evidence for liquids
Some evidence for liquid alteration

Cassini imaging of poles found liquid lakes and seas

Life on Titan?
Probably not
--Too cold
--No liquid water

But who knows?
--Impacts may temporarily provide better conditions
--Plenty of energy
--Alternative forms of life

Still very interesting
Complex hydrocarbon chemistry over billions of years
Vast 'laboratory' for prebiotic chemistry


Brightest body in the Solar System
--Fresh, clean ice on surface layers

Thin water-vapor atmosphere & fresh surface ices are fed by fountains at surface cracks

Too small for appreciable latent heat

Enceladus is one of 3 known active moons in the Solar System:
--Others are Io (Jupiter) & Triton (Neptune)
Activity is powered by Tidal Heating
--Enceladus is in a 2:1 orbital resonance with Dione (2 orbits for every 1 of Dione)
--Melts subsurface water, which geysers through cracks in the ice
Ice also feeds the faint E-ring of Saturn

Life on Enceladus?
Enceladus may harbor a subsurface liquid water ocean, similar to Europa

Thus Enceladus may harbor life, for similar reasons.

Cassini detected simple organics:
--Carbon Dioxide

Uranus and Neptune and their Moons

Uranus & Neptune at a Glance

a = 19.2 AU
P = 84 years
a = 30.1 AU
P = 165 years

Equilibrium Temperatures
Uranus: a=19.2AU, T~60K~-350F
Neptune: a=30.1AU, T~50K~-370F

Spacecraft studies
Voyager 2 flew past Uranus & Neptune:
--Uranus: January 1986
--Neptune: August 1989
Both have been extensively studied using the Hubble Space Telescope:
--Long-term monitoring of weather patterns.
--Infrared imaging studies of their atmospheres, rings, and moons.

Moons of Uranus
Uranus has 27 moons:
None are large enough to be "giant moons".
--5 icy, spherical moons: Miranda, Ariel, Umbriel, Titania, & Oberon
--22 tiny, irregular, icy moons

Moons of Neptune
Neptune has 13 moons:
1 Giant Moon: Triton, a very cold, icy moon
Triton orbits retrograde
12 tiny, irregular icy moons


Giant moon of Neptune:
--Diameter: 2710 km (21% RE)
Mean density: ~2.05 g/cc
--Icy mantle over a rocky core.
Cold, icy surface:
--Temperature 34 K (-398 F)
--Ices of N2, CH4, CO2, H2O, and CO
--Thin N2 Atmosphere
Young surface with few craters

Cryovolcanoes & Geysers
Smooth plains paved by cryovolcanic flows:
--Liquid material oozes through cracks in the crust
--Interior heated by tides.
Geysers of N2 gas:
--Plumes of ices & dark dusty particles.
--Swept downwind, making dark streaks.
Feed Triton's thin atmosphere

Life on Triton?
Seems hard to imagine, but?
Internal heat may provide enough energy to keep water liquid

Trans-Neptunian Objects

Class of icy bodies that orbit beyond Neptune:
--Range from 30 AU outward
--Maybe further than 100 AU

Divided into various sub-classes:
--Kuiper Belt Objects
--Plutinos ("little Plutos")
--Scattered Disk Objects
--Distinguished by their orbits

Frozen Pluto

Ice Planet:
--Diameter 2306 km (19% the Radius of the Earth)
--Mass: 0.21% the Mass of the Earth
--Density is ~2 g/cc
--Rocky core & ice mantle
Thin Atmosphere:
--Cold: T=35-45 K (-378 to -396 F)
--Surface covered with frozen
--N2 mixed with CH4 & traces of CO and H2O.
--Thin nitrogen atmosphere.

Not promising places for life

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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