Download Life in the Universe

Life on
Jovian
Moons
ASTR 1420
Lecture 14
Sections 9.1 & 9.2
Background…
• Jovian planets themselves are unlikely habitable (Do you remember why?)
• There are many moons orbiting these planets  Jovian Moons.
• Some jovian moons are potentially habitable (i.e., with liquid water).
• If any one of them is habitable, this will greatly increase the chance of
finding life in the Universe
 many more jovian moons than jovian planets!
• Let’s explore the mechanisms that make some jovian moons habitable.
Galileo, the inventor of telescope?
• Did Galileo invent the
telescope?
Discovering the
Moons
Soon after the news on the invention of
telescope (1608), Galileo built his own one in
1609.
A page from Gelileo’s
notebook written in 1610.
‘*’ are Galilean Moons!
Wood strips, leather cover, objective & eyepiece lenses
Four Galilean Moons
• On January 7th 1610, gazing at Jupiter, Galileo for the first time saw three
moons of Jupiter
• March 1610, Galileo published his results in a pamphlet called The Starry
Messenger claiming to have found four bodies moving around the giant planet
“as Venus and Mercury around the Sun”
• The four moons are now called the “Galilean Moons” - Io, Europa, Ganymede
and Callisto
• Currently, 63 moons were discovered orbiting Jupiter.
Galileo Galilei
Galileo’s Finger
Galileo Telescope’s Objective Lens
In 1677, lens mounted in an ebony
frame to preserve it.
On March 12, 1737
Galileo's remains were
transferred to the
church of Santa Croce.
During the transfer, a
devotee cut off
Galileo's middle finger
of his right hand.
Today, Galileo's finger
can be found on
display in the Florence
Institute and Museum
of the History of
Science.
Discovering Moons
• In 1656, Christiaan Huygens found the largest of Saturn’s moon – Titan.
Huygens was also the first one to realize that the rings around Saturn do
not touch the planet.
• Cassini discovered four more moons of Saturn and showed that the ring of
Saturn is not solid but instead of a collection of smaller rocks with a division
(“Cassini division”)
• Saturn has about 62 moons that we currently know of.
HST near infrared light image
Jupiter has a ring, too!
• The ring of Jupiter was discovered by Voyager 1 in March
of 1979.
• All four Jovian planets have rings!!
Larger Moons
• Ganymede and Titan are
larger than Mercury
• Big2 + Io, Europa,
Callisto, Triton, and our
Moon are larger than
Pluto
• smallest moons are
about the size of a single
mountain on Earth
• Moderate—large moons
= miniature solar system
• spherical shape
• orbit in the same plane
• Orbit in the same
direction to their
planets
Schematics of Jovian Moons
Synchronous Rotation
keeping the same face turned toward the Earth  the Moon completes exactly one
rotation around its axis while it makes one orbit.
Synchronous Rotation
• Nearly all jovian moons show synchronous rotation similar to our Moon.
keep the same face turned toward their host planets  a moon completes exactly
one rotation around its axis while it makes one orbit.
• This is due to the same gravitational effect for the tides on Earth!
Tides on Earth
• Tides on Earth are due to the differential gravity.
Two tidal bulges  two high tides per day.
• Why two bulges - one facing the Moon and the other opposite??
Tidal Friction
• Moon’s orbital period = ~29 days
• Earth’s rotation period = 1 day
Earth needs to rotate through two tidal bulges  tidal friction!
Bulges are slightly pulled forward w.r.t. the line connecting Earth—Moon
Earth rotation gets slower  while Moon’s orbit gets larger (due to angular
momentum conservation)
Tidal Friction  Synchronous Rotation
• A larger tidal effect on the Moon than on the Planet
• Adjusting rotational+orbital motions until “bulges” on the Moon
always face to the Earth  Synchronous Rotation = “Tidal Locking”
 the orbit will get ‘circularized’!
• Tidal locking takes a short time (a few Myrs!)
• All close-in objects to a larger object are tidally locked-in!
Tidal Friction  Synchronous Rotation
When a satellite rotates too slowly
A lot of words with ‘tidal’ adjective
tidal locking
tidal friction
tidal heating
…
When a satellite rotates too rapidly
Io : Excellent Example of Tidal Heating
• We know that Mercury and Moon had lost their internal energy…
• 1979 Voyager 1 image of Io  active volcano!! But Io is smaller than
Mercury!
• Io is the most volcanically active place in the solar system.
• If Io was completely “tidally locked”, there should be no more on-going
heating because of no more tidal friction.
Io : Continuing tidal
heating…
• Due to its elliptical orbit,
bulges are “misaligned”
w.r.t. Jupiter!
continuing tidal friction
 increased internal
heat!
• This internal heat is ~200
times more than Earth’s
radioactive decay heat
energy!
But, tidal locking forces the
orbit to be circular.
? Why Io’s orbit is elliptical
still?
Orbital resonance
• Three inner most Galilean moons orbit
Jupiter in a resonant way…
• Periodic alignments exert force on each
other  smallest, shortest orbit got
influenced the most  Io’s orbit is
“distorted” to be elliptical!
• Initially, Io’s rotation was faster and it was
located closer to Jupiter
being tidally locked
moving outward
“meet” Europa and formed 1:2 resonance.
Io+Europa being tidally locked together
and moving outward
They “meet” Ganymede and formed a
resonance 1:2:4
Three of them are being moving outward
to meet Callisto now…
Io
Europa
Life on Galilean Moons?
Ganymede
Callisto
Too strong tidal heating
 lack of water + extreme volcanism
“Just right” tidal heating?
??
??
Europa
• Existence of water was
already known from the
ground-based spectrum
of Europa
• Diameter – 3138 km (slightly smaller than
Earth’s Moon)
• Mass – less than Earth’s moon ( 1/125th of
Earth’s mass, 65% of lunar mass)
• Distance from Jupiter ~ 410,000 miles ( more
than Earth - Moon), period=3.55 days
• Has a very weak magnetic field
• Has a very tenuous atmosphere – about 10-11
bar of mostly oxygen gas.
• Surface is exceedingly smooth with highest
elevations of a few hundred meters high.
• The smooth surface has few craters but lots
of cracks
Europa
Galileo orbiter image =
very few impact craters
 young surface (<~ 100
Myr)…
• Galileo spacecraft measured gravitational
field of Europa  dense core + low density
(~1 g/cm3; some sorts of water) material
near the surface
 central metallic core, thick rocky interior, 80170km thick water layer, very cold (-150C)
surface ice crust.
Severe lacking of impact craters (only a few) 
surface younger than 100 Myr  resurfacing by
occasional breakthrough of subsurface water
Surface feature of Europa
• Chaotic terrain :
“…a surface that looked as if it had been clawed by a tiger with talons several
kilometers wide,…”
• Closer views resolved each line into a groove flanked by ridges.
• The larger channels travel thousands of kilometers along great circles
without being diverted by the terrain. Whatever mechanism formed them
must explain this tendency.
• Repeated tidal cracking and compression of ice is too chaotic a process to
explain
Fissures, cracks, domes, and pits…
Broken Ice and Refrozen Water
• chaotic terrain (top left); an enigmatic dark spot nicknamed “The puddle”
(bottom left); cycloidal ridges (right); and a shallow impact crater (bottom
right).
Europa Craters
• 140 km wide crater formed from a mountain size asteroid or comet…
• similar to a gunshot glass structure…
Tracking Evolution
Youngest
E
D
C
B
A
Oldest
Enigma…
• Conventional explantion 
If true, we should be able to find
some single ridges. But, all Europan
ridges are double ridges!
Ganymede
• the largest moon in the
solar system
• surface of hard ice
• old + young surfaces
• grooves  tectonic
stresses
• underground ocean 
weak induced magnetic
field (+ its own) & surface
salt.
• lesser tidal heating. But
with its larger size,
enough to maintain an
ocean
• Very thick ice crust
(>150km)  life less likely
or harder to detect!
Ganymede
• white craters and rims = impact exposes
mantle ice
• thin oxygen atmosphere,
possible aurora at its
poles
• ghost craters =
smoothed by ice flow
very diversified surface with dark regions,
valleys, mountains, evidence of past tectonic
activity and lots of vertical relief
Callisto
• pockmarked surface
 as old as late heavy
bombardment
• dark powder at the lowlying areas  dusts after
sublimated ice?
• Gravity  a ball of mixed
rock and ice + hundred
km of water ice…
• Undifferentiated interior
 interior was never
warm enough!
• Induced magnetic field 
subsurface salty ocean!!
Galilean Moons
Too hot
Io
recent
Maybe
Europa
< 60 Myr
less likely
Ganymede
2-3 Gyr?
Too cold, no?
Callisto
4+ Gyr
In summary…
Important Concepts
Important Terms
• Tidal Effects
•
•
•
•
•
• Evidence of subsurface ocean in
Europa
• Relative age-dating of surface
features
Jovian Moons
Synchronous rotation
Tidal friction
Tidal heating
Orbital resonance
• Induced magnetic field
Chapter/sections covered in this lecture : 9.1 & 9.2
Life on Titan and other places in SS : next class!!