The Moons of the Gas Giants

Astronomy 311Professor Lee

CarknerLecture 20

The Group of Seven There are seven large (diameter >2000

km) satellites in the solar system

Each is a distinct world of its own

Jupiter’s Lovers

Io, Europa and Callisto were Jupiter’s lovers in Greek mythology

Saturn was the

king of the Titans

The Galilean Moons

Discovered by Galileo in 1610 Studied by Voyager 1 and 2, HST

and Galileo (the spacecraft)

Orbits of the Galilean Moons

All are tidally locked to Jupiter in a 1-to-1 ratio 1

The periods of the orbits of the 3 inner moons are in a 1:2:4 ratio

Formation of the Galilean Moons

The inner parts of the nebula were hotter than the outer

The inner 3 satellites experienced tidal heating and differentiated into a rocky core and an icy crust

The Interiors of the Galilean

Moons

Galileo Visits the Galilean Moons

The Galileo spacecraft arrived at Jupiter in 1995 and has been studying the moons from Jupiter orbit ever since

Galileo at Io

Surfaces of the Galilean Moons

Io Io is the most volcanically active world in the

solar system These changing forces squeeze and flex Io

producing heat The interior heat has also produced a

differentiated interior

Volcanism on Io Io has no impact craters

Volcanoes produce plumes of

material that extend up to 280 km above the surface

Volcanoes can be very long lived

Io’s Plasma Torus Io’s volcanoes put lots of ions into

its orbit

The ions are effected by Jupiter’s magnetic field producing a plasma torus

Europa Europa’s surface is covered with a

layer of ice Water flows up to the top continually

resurfacing Europa

Tidal flex may also crack the surface

Evidence for Warm Oceans on Europa

Europa has ice rafts where the surface has been broken up and reassembled

Europa also has smooth areas where water has

flowed up and re-frozen Galileo magnetometer measurements indicate

that Europa has a variable magnetic field

Models for the Interior of Europa

Ganymede Ganymede is the largest moon in the solar

system

Ganymede should have little tidal heating due to its distance from Jupiter

Ganymede must have had more geological activity in the past

The Surface of Ganymede

Ganymede shows 2 types of surface features Old dark terrain

New bright terrain

Callisto Callisto is the most distant Galilean moon

from Jupiter It has experienced the least tidal heating

Callisto shows few signs of interior or

surface activity Large impact basin Valhalla

Titan The second largest moon in the

solar system

Only moon with an atmosphere Why does Titan have an

atmosphere?

Titan’s Atmosphere Titan has a thicker atmosphere than the

Earth

Titan’s atmosphere may have originally been composed of ammonia (NH3) and methane (CH4)

90% of the atmosphere is N2

Chemicals in Titan’s Atmosphere Titan’s atmosphere also contains hydrocarbons

(composed of H and C) and polymers (long chains of H, N and C)

Titan may have the necessary organic material to form the building blocks of life

Triton Triton is in a decaying, highly inclined (23

degrees), retrograde orbit

Triton shows evidence of geologic activity

When Triton was first captured it was probably in a highly eccentric orbit which resulted in tidal heating

Triton’s Atmosphere

Triton has a very thin nitrogen atmosphere (1.6 X 10-5 atmospheres of pressure)

A little bit of nitrogen evaporates to

produce the atmosphere

Summary The six large moons of the gas giants resemble

the terrestrial planets of the inner solar system They can have volcanoes, atmospheres, and

evidence of resurfacing In general they are cold and have rocky

interiors and icy exteriors Some produce internal energy through tidal

heating Europa and Titan may possibly have the

conditions for life to exist

Summary: Io and Europa

Io Strong tidal heating produces massive volcanism Volcanism produces powerful outgassed plumes,

sulfurous surface and plasma torus of ions Europa

Icy surface shows evidence for water flowing up from interior

May have a warm subsurface ocean due to tidal heating

Summary: Ganymede and Callisto

Ganymede Shows both old dark terrain and bright new

terrain Must have had more internal heat to drive

geologic activity in the past Callisto

No tidal heating results in no differentiation Fairly uniform mixture of icy and rock with

many craters

Summary: Titan and Triton

Titan Large size and low temperatures results in

an thick atmosphere Atmosphere composed of nitrogen,

methane, hydrocarbons and polymers Triton

Has a decaying, inclined retrograde orbit Thin atmosphere and surface activity