Download Life on Jovian Moons

Life on Jovian Moons
Chapter 9
Details of the Jovian (Jovian planet) Moons
Habitability – life requirements
! 
Life requires a source of energy
◦  Closest system, Jupiter, is at 5.2 AU so 1/5.22=1/27 less solar energy
◦  Surprisingly, in the Jovian moon systems the prospects for life are
pretty good!
The frost line
When the Solar System was formed, there was a boundary beyond which
it was sufficiently cold to form and retain water ice
!  Jovian planets rich in water ice
!  Terrestrial planets very dry, some water retained by early comets
The Jovian Moon System
• 
Jupiter is outside the ‘frost line’, where water ice can form
•  The ice grains contained H, so a large amount of mass came together
•  When large enough, gravity took over from collisions in building a
bigger body (‘direct gravitational accumulation’)
•  4 large moons, the ‘Galilean Moons’
Formation:
! 
Many of the larger Jovian moons orbit nearly in the equatorial plane of Jupiter,
and in the same direction
! 
=> formed from a rotating disk of gas and dust like a miniature solar system
Formation contd.
! 
Most smaller Jovian moons are
most likely captured asteroids
and comets.
! 
The largest Jovian moon that
appears to have been captured
is Triton, the moon of Neptune
– it orbits backwards relative
to its planet’s rotation
Composition
Planetary objects fall into
several groups:
◦  Giant planets with radii
greater than 10,000 km and
low densities
◦  Terrestrial planets and
terrestrial-like bodies
◦  Icy satellites
◦  Small asteroids
! 
Jovian moons typically made of ice and rock. Outer Solar System cold
enough to allow ices to condense along with metal and rock.
! 
Average densities of Jovian moons << lower than Earth (water ice).
Variations as a function of temperature within Solar System
! 
Water ice condensed easily at temperatures near Jupiter.
Methane and other ices condensed only at colder temperatures – farther away
from the Sun.
! 
Jupiter’s moon contains significant quantities of water ice, but no other ices.
! 
◦  Jupiter’s moons show a decrease in density with distance from Jupiter – formed of a cloud of gas that
was hotter in the center than in its outer regions.
! 
Moons of more distant planets contain higher proportion of water ice compared
to rocks, and contain other ices in addition to water ice (methane).
Orbital periods
! 
! 
! 
Io
Europa
Ganymede
1.77 days
3.55 days
7.15 days
1:2:4 ratio – not a coincidence.
Reason why Europa and perhaps Ganymede are good prospects for life.
This induces tidal heating!
Tides - Earth
! 
! 
! 
A feature of oceans (but solid material has small tides too).
Two high and two low tides per day.
Tides are due to the gravitational pull being stronger on side of Earth
closest to it (Sun causes smaller tides)
Earth-Moon gravity keeps them orbiting
each other. But side of Earth closest to
Moon has slightly stronger pull to Moon =>
bulges towards it.
Other side has weaker pull => bulges away
compared to rest of Earth.
The Earth spins once a day while the bulge
always points towards and away from the
Moon => high and low tides.
! 
We always see the same face of the Moon.
◦  This means: period of orbit = period of spin = tidal locking
! 
Tidal bulge of Earth is ‘leading’ because its rotates fast.
◦  The Moon will pull it back wards, slowing rotation of the Earth.
◦  Bulges of Earth will pull on the Moon, into a larger orbit!
! 
Similarly Earth causes bulges on the Moon
! 
Once the Moon is slowed
down sufficiently, it will be
locked into position
Tides – Jupiter
! 
Jupiter rotates in 9.8 hours.
◦  => Tidal bulges raised by its moons always lead the moons.
◦  Just as with our Moon, Galilean satellites are moving outwards with
time.
◦  The solid material in the bulges of the satellites will be ‘squeezed’ and
‘kneeded’ => source of heat
Suppose that there were only one big moon around Jupiter.
! 
Tidal effects would push moon away, and quickly make that moon’s
rotation period equal to its orbit
◦  it would always put the same face to Jupiter
! 
Then, there would be no ‘squeezing and kneading’ of the moon, hence no
heat produced.
! 
Io, Europa, and Ganymede cold and dead because they are too small to
hold onto any significant amount of their heat of formation?
! 
We have several Moons, not only one
◦  Orbital resonances between Io,
Europa and Ganymede
! 
Sequence of orbits for 1 period for
Ganymede, 2 periods for Europa and
4 periods of Io.
◦  Regular gravitational kicks at the
same phase – resonance
◦  Orbits get eccentric, so can’t keep
the same face to Jupiter all the
time
◦  Tidal forces ‘squeeze’ the planets
and heat them up (mostly Io and
Europa)
Tidal heating of the Jovian Moons
! 
! 
Normally the moons would loose their internal heat as they cool off, but
the tidal forces creates internal heat
Io is the most volcanically active body in the solar system!
! 
More than 80 currently active volcanoes lasting months-years
! 
Ejecta speed 1000 km/s
Each volcano ejects about 10,000 tons/s
! 
! 
! 
! 
! 
Io’s red colors from different sulfur compounds and molten silicon rock
Yellow colors are from final stable configuration of S in a ring (S8, ordinary
sulfur)
Frozen SO2 snowflakes are white
Time between photos a few months
! 
Plume on left rises to 260 km (150 mi) above surface. Blue due
to scattered sunlight.
Voyager 1 & Galileo
Geyser function (Earth)
!  Surface water trickle down until
heated by rocks (heated by magma,
geothermally)
! 
Convection then brings water
upwards, in one or more 'tubes' in
rocks
•  The tubes are narrow: cooling by convection cannot occur
•  Upper cooler water pressure downwards on warmer water (pressurecooker), causing a rapid heating process
•  When boiling occurs, the water expands and flashes out
Geysers on Io
! 
! 
! 
! 
Surface gravity lower than on Earth, no atmospheric pressure
‘Old faithful’ on Io: 37km plumes
No water: sulfur and sulfur dioxide
Fast transitions from liquid => gas => sulfurous snowflakes
! 
Activity causes surface to slowly change over the years
! 
Surface completely lacks impact craters
Io lacks water and extreme volcanic activity makes is improbable for
hosting life.
! 
Voyager 2 (1979)
Galileo (1996)
Europa
! 
! 
Surface and structure very
different from Io
Spectroscopic observations
from Earth indicated Europa’s
surface is almost pure frozen
water, probably covers an
ocean
! 
Europa is the smoothest body in the Solar System
◦  Almost no craters, no mountains
◦  Young surface reprocessed by geological activity