Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 37: Giant Moons

Key Ideas:

4 Galilean Moons:: Io, Europa, Ganymede, and Callisto; Large (>3000 km), spherical, and differentiated; Io is volcanically active; Europa may have an ocean below the ice
Titan, Saturn's Giant Moon: Only moon with a thick atmosphere
Prospects for life?

The Galilean Moons

In circular orbits in the same direction around Jupiter
Orbital Periods:: Io: 1.8 days; Europa: 3.6 days (2:1 orbital resonance with Io); Ganymede: 7.2 days (4:1 orbital resonance with Io); Callisto: 16.7 days

Volcanically Active Io

Hot, molten interior:: Tidal heating by Jupiter and Europa; Interior is molten silicates and sulfur
Active volcanoes:: Active eruptions and pools of molten sulfur; Most volcanically active world in the Solar System
Constant stretching and squeezing keeps Io molten
The closest moon to Jupiter shows the most geological activity

Smooth Europa

Icy surface covering a large, rocky core: Surface is very smooth and young; Fractured into ice rafts and floes a few kilometers across; Repaved by water geysering through the cracks in the ice
Does Europa have liquid water? Two ideas:: 100-200 km of ice above a rocky core; Thin ice crust over a 150 km deep water ocean
If there is liquid water, life may be present

Groovy Ganymede

Solar System's largest moon: Density of 1.9 g/cc; Thick ice mantle over rocky core
Grooved terrain:: 10 km wide and 300 meters deep; < 2 Gyr old based on the number of impact craters seen on top of them

Cratered Callisto

Outermost Galilean moon
Heavily cratered, dirty-ice surface: Inactive for ~4 Gyr; Craters are bright, with clean ice
Density is 1.8 g/cc: Ice layered on a rocky core

Galilean Moons in Comparison

Io and Europa are mostly rock:: Mean densities of 3.5 and 3.0 g/cc, respectively; Io: rocky crust, molten mantle, and active volcanoes; Europa: icy lithosphere and rocky core
Ganymede and Callisto are mixed ice and rock: Mean densities of 1.9 and 1.8 g/cc, respectively; Deep ice mantles over rocky/icy cores; Less geologically active

Interior Heat

In the terrestrial planets, interior heat is determined by the planet's size:: Large Earth and Venus have hot interiors; Smaller Mercury and Mars have cold interiors
In the Galilean moons, interior heat is determined by proximity to Jupiter: Io is hottest, outermost Callisto is coldest; Energy source: tidal heating by Jupiter

Titan: Saturn's Giant Moon

Huygens Lander

Part of the Cassini mission to Saturn
The Huygens probed separated from Cassini and parachuted to the surface of Titan on January 14, 2005
Goal to measure surface composition and conditions

Titan's Atmosphere

Composition:: 98% N₂ (Nitrogen); 2% CH₄ (Methane); Argon, hydrocarbons like Ethane
Cold, dense atmosphere:: Temperature: 94 K (-290 F); Pressure: ~1.6 Earth atmospheres; Clouds of Nitrogen and methane ices; Hydrocarbon "smog"

Titan's Surface

Young surface with very few impact craters
Varied terrain:: Smooth, dark plains (methane mud flats?); Rugged highlands; Drainage channels; Impact basins

Is Methane Titan's "Water?"

Methane (CH₄) may play the same role on Titan that water does on the Earth:: 94 K is between the boiling and freezing points of Methane; Get gaseous methane in the atmosphere
Methane condenses into clouds that rain liquid methane: Signs of drainage flows; Huygens landed in soft methane/water ice mud; Water ice is like sand on Titan

Life in the Outer Solar System?

Europa and Titan are considered the most promising sites to search for life in the Solar System after Mars
Jupiter's Giant Moon Europa: Liquid ocean under the ice?; Life could be present
Saturn's Giant Moon Titan: Methane-based, rather than water-based, life could be present

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