Homework 9 is due

Tuesday, Nov. 23, 5:00 pm

TitanEnceladus

Titan

Titan Huygens spacecraft landed on surface

Cassini spacecraft has made several close flybys

2nd largest moon

Only moon with a substantial atmosphere

Saturn’s TitanSaturn’s Titan

atmosphere denser than Earth’sdenser than Earth’s but very cold (100K100K) and composed mostly of N2 and methane (CH4)

Completely enshrouded in smog-smog-like cloudslike clouds

Methane Methane acts like water (liquid).

Few craters on the surface.

Surface eroded by liquids

Methane/Ethane lakes

View from Cassini Spacecraft during Flyby

Huygens Probe

On the surface! “Rocks” of ice?

View from Huygens Spacecraft during descent to surface

Sunlit side

Looking through the atmosphere

Hubble view at wavelengths that penetrate atmosphere

Thick atmosphere with photochemical hydrocarbon “smog”

Physical Characteristics

• Size– Among moons, second only to Ganymede (measured

by surface, not atmosphere)

• Mass– Almost double that of our Moon– Density: 1.9 gm/cm3 equal mixture of rock and ice– Thought to be differentiated: rocky core of silicates

with a crust of water ice

Surface• Gross features:

– Few impact craters surface 130-300 Myr old– Tectonics: thin features for hundreds of miles

• Cryo-volcano:– 30 km volcano observed on Titan, including caldera

inside– Magma would be mainly CH4 & H2O– Energy?: tidal heating or radioactivity

• Erosion:– Huygens saw round ice pebbles– Sinuous channels: liquids– East-west dunes near equator with sharp western

boundaries: super-rotating winds

Dunes

Earth

Titan

Possible Earthlike Processes

• Tectonics• Weather, including rain (methane)• Erosion by winds and liquids• Formation of complex organic compounds• Greenhouse effect• Volcanism (molten water, not rock)• But: all at a much lower temperature

Atmosphere • Pressure: 1.5 bar• Surface temperature: -

180C (-290F)• Composition: 9298% N2

+ 26% methane (CH4)• Constantly smoggy: UV

breaking up CH4 into radicals

• Radicals combine to form complex hydrocarbons: C2H6, C2H2, HCN, C6H6

Why does Titan have an atmosphere while the larger Ganymede does not?

• At Saturn’s distance from the Sun, the protosolar nebula was much colder that at Jupiter.

• Ammonia (NH3), methane (CH4) & ethane (C2H6) ices could condense at Saturn’s distance, but not at Jupiter, where only water ices condensed.

• Moons formed at Saturn could have significant amount of methane, ethane, ammonia - this provided molecules for UV interactions to form atmosphere

• Comets and asteroids hit at a smaller velocity (~ half the energy), so collisional losses were smaller.

• NH3 (ammonia) broken up by UV radiation into N2 and H2. H2 escaped, and N2 stayed behind

• Expect similar process for CH4 (methane)

• CH and CH2 products of CH4 breakup help form larger organic molecules, e.g., C2H6

• Why are we still seeing CH4?

– Possible reservoir of CH4 and/or C2H6.

– Rain or drizzle of CH4 and/or C2H6

CH4C2H6 Ocean?

• Cassini: no global ocean, but many (relatively) small lakes, and observed a number of clouds

Hydrocarbon lakes

River gully?

Coastline?

Cold Life?• Liquid methane - ethane (CH4C2H6):

– Chemical reaction rates orders of magnitude slower– Poorer solvents than water– No density anomaly: liquids freeze completely– But it’s a liquid

• Saving graces:– UV forms organic molecules in the upper atmosphere,

which sink to the CH4C2H6 lakes and the surface– Comet or asteroid impacts can create pockets of

water lasting thousands of years?– Underground water ocean heated by radioactivity?

?

Enceladus

Enceladus Enceladus is smallis small

It was not It was not considereconsidered a moon d a moon

of of particular particular interest, interest, until this until this image image was was

obtained obtained by Cassiniby Cassini

Saturn’s EnceladusSaturn’s EnceladusSmall icy moon (500 km500 km) in diameterYoung, crater-free surface regions with like those on EuropaOrbit resonance with DioneSouth polar hot spot and ice plumes Thin “atmosphere” of water vapor Subsurface ocean!?

“tiger stripes”

Ice Plumes from Enceladus

Area of plumes is much warmer than surroundings -

evidence of subsurface reservoir of liquid water

Liquid water + energy source + chemicals life?

NeptuneUranus

anything of interest?

ArielUmbrielTitaniaOberonCalibanSycoraxProsperoSetebosStephanoTrinculo

CordeliaOpheliaBiancaCressidaDesdemonaJulietPortiaRosalindBelindaPuckMiranda

Named moons of Uranus

Moons of Uranus

No large moons, nothing of particular interest as far as

the search for life

Moons of Neptune

One location of interest

• Neptune’s TritonNeptune’s Triton

– Extremely cold (< 40K< 40K) objects made from volatile materials produce icy volcanism.icy volcanism.

– Huge geysers of nitrogen!

– Pluto and the Kuiper Belt Objects may look and act similarly.

Very unlikely location for life

Solar system beyond Saturn

• Decline of probability of life– Main factor is temperature– Europa Ganymede Callisto Titan Enceladus ?

• Triton– Retrograde rotation capture– Uneven surface:

• Cantaloupe terrain, Smooth parts, Frost deposits?, Wind streaks

– Few impact craters recent geological activity (10100 Myr)• Pluto and remaining moons

– Too cold and too small– But, amino acids seen in meteorites

Time to reach for the stars!