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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 5: Life on the Moons of Jupiter

Key Ideas

Tides result from a differential gravitational force
Io, Europa, and Ganymede are locked in orbital resonances
These resonances result in a time-variable tidal force
The time variable tidal force results in internal heating
On Io, internal heating leads to strong volcanic activity
On Europa, strong tides may result in a liquid ocean
Induced magnetic fields may indicate a salty liquid ocean
Europan oceans may have requirements for habitability
Ganymede and Callisto may also have liquid oceans

Jupiter's Moons

Equilibrium temperature at Jupiter's distance = 120K
Irregular Moons
Jupiter has many, small moons
Irregularly shaped
--Completely cool
--No atmosphere
--No liquid water

Galilean Moons

Io, Europa, Ganymede, Callisto

Large and spherical
Can support atmospheres: water vapor, nitrogen, carbon dioxide
Too small for any residual heat

Yet Io is the most volcanically active body in the solar system!!


Differential Gravity leads to tides

The net front-to-back differential force
--Stretches Earth along the Moon-Earth line
--Squeezes Earth at right angles to the this line.

How big is the Tidal Bulge of the Earth?
Rock is stiff & resists tidal deformation
--"Body Tides" on Earth are only 30 cm high

Water is fluid & flows easily with gravity:
--Ocean Tides are 1 meter high in the open sea

Tidal Locking of the Moon
The Earth raises body tides on the Moon
--Earth is more massive, so the tides are stronger
Constant squeezing & stretching in a rapidly rotating Moon would generate heat:
--Energy gets taken from the Moon's rotation
--The Moon slows down until its rotation & orbit periods are the same, stopping the squeezing.
Result: Moon is Tidally Locked to the Earth.
--Always keeps the same face toward the Earth.

Tidal Braking of the Earth
The Earth rotates faster than the Moon orbits.
Friction between the oceans and seafloor drag the ocean tidal bulges in the direction of rotation (eastward).
--Ocean Tides lead the Moon by about 10
Friction also robs energy from Earth's rotation
--Slows the Earth down a tiny bit
--Day is getting longer by 0.0023 sec / century
This effect is called Tidal Braking.

Lunar Recession
The Moon feels a slight forward gravitational tug from the ocean tidal bulge
--Results in a net acceleration of the Moon
--Moves it outward into a slightly larger orbit
Lunar Recession
--Increase in average Moon-Earth distance by about 3.8 cm per year.
--Measured by Doppler Laser Ranging

Tidal Heating of the Galilean Moons

The Galilean Moons
In circular orbits in the same direction around Jupiter:
Orbital Periods:
Io: 1.8 days
Europa: 3.6 days
--(2 times Io's period)
Ganymede: 7.2 days
--(4 times Io's period)
Callisto: 16.7 days

Orbits of Io, Europa, and Ganymede are in resonance
--this leads to significant eccentricities
--Eccentricities lead to tidal heating


Europa's surface is smooth and young
Icy surface covering a large rocky core:
Fractured into ice rafts & floes a few kilometers across,
Repaved by water or slush geysering through the cracks in the ice.

Does Europa have liquid water?
What lies beneath Europa's surface?
Two ideas:
--100-200 km of ice above a rocky core
--Thin ice crust over a 150 km deep water ocean.
Evidence for liquid water
Induced magnetic field from Jupiter is evidence of a conducting liquid -- salty water!
Life on Europa?
Europa may have the three requirements for life:
--Liquid Water
--Energy Source
--Elements of Life
Searching for Life on Europa
Two step process:
--Determine if there is an ocean
--Drill into ice to look for life

Ganymede and Callisto

Less active

Ganymede further from Jupiter (less tidal heating)

Callisto not in resonance (no tidal heating)

Yet both have evidence for induced magnetic fields!
Thus both may have subsurface oceans.
Galilean moons may be the best places to look for life in the solar system.

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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