Download Lecture18

Life in the Terrestrial Planet Region:
X
Terrible Extremes of Temperature, No Atmosphere,
UV, Cosmic Rays, Little or No Volatiles, No Liquids
X
Terrible Extremes of Temperature, No Atmosphere, UV,
Cosmic Rays, Little or No Volatiles, No Liquids
X
Very High Temperatures, No or Little Water
Young Surface  No Fossil Record
– MERCURY
– MOON
– VENUS
– MARS
?
Evidence for Liquid Water in Past
Possible Environments for Life to Survive?
Volatiles and Water Present Now
Moving
Outwards
Gas and Ice Giant
Planets:
Prospects for Life?
– Ingredients for organic chemistry
– Atmospheric layers with roughly Earth-like Temperature &
Pressure;
– But no solid surfaces (except ice crystals) and no liquid water
(except very deep in Uranus and Neptune),
– Violent winds and convective turbulence would mix any lifebearing gas quickly over extremes of temperasture & pressure.
– Although the sunlight is very weak, internal heat is available.
– They are very difficult to explore!
Life on Jupiter?
•All the basic molecular ingredients are
present in the atmospheres
•But convection will ultimately (and quickly)
bring any organic molecules down to regions
where T~ 700 C (over 1200 F!) where they will
be destroyed
Sagan & Salpeter
“speculations”
“floaters”
“hunters”
Life on Moons of Planets?
For instance, the Galilean
Satellites of Jupiter
Io
Europa
Ganymede
Callisto
Medium &
large moons
• Enough self-gravity to be
spherical
• Are, or were, geologically active.
• Have substantial amounts of ice.
• Roughly circular, equatorial orbits
in same direction as planet
rotation.
Small moons
•
•
•
•
•
Far more numerous than the medium and large moons.
Not enough gravity to be spherical: “potato-shaped”
Captured asteroids, so orbits do not follow patterns.
Orbits can be tilted, elliptical, and even backwards!
No atmospheres or liquid water – not suitable for life
Focus on the large &
medium sized moons
Sizes and orbits
Earth’s Moon
Diameter Orbit Period
(km)
(days)
comments
3,474
28
orbit stabilizes Earth’s tilt & seasons
Io (Jupiter)
3,640
1.77
orbit perturbed by Ganymede,
Europa, eccentric irregular orbit,
extremely variable tidal force
Europa (Jupiter)
3,130
3.55
eccentric orbit
Ganymede (Jupiter)
5,268
7.15
Callisto (Jupiter)
4,806
16.69
Enceladus (Saturn)
494
1.37
Titan (Saturn)
5,150
15.94
Largest in solar system, substantial
atmosphere
What about…
- a source of energy (Sun is too faint)?
- presence of liquid water?
Tidal Forces
Because the gravitational force decreases with (distance)2,
the attractive force experienced by one object (e.g., the
Earth) due to the gravitational field of a second object (e.g.,
the Moon) varies with position (closest parts attracted most
strongly).
Tidal forces are
difference forces.
Synchronous Rotation
• …is when the rotation period of a moon, planet, or star
equals its orbital period about another object.
• Tidal friction on the Moon (caused by Earth) has slowed its
rotation down to a period of one month.
• The Moon now rotates synchronously.
– We always see the same side of the Moon.
• Tidal friction on the Moon has ceased since its tidal bulges
are always aligned with Earth.
• All of the large moons in the solar system are
in synchronous rotation.
Tidal Heating in Jovian Moons
The four inner moons of Jupiter - Io, Europa, and Ganymede - all
show evidence of geological activity - indicators of molten interiors.
The heat source is tidal heating.
Moons have elliptical orbit and synchronous rotation - one side always faces Jupiter
- as Ganymede completes one orbit, Europa completes exactly two orbits, and Io completes
exactly four orbits - moons periodically line up - causes orbital ellipticity.
- tidal bulges are constantly being flexed in different directions - generates friction inside
Effects of Tidal Interactions
• Rotation
– Rotation of moons become synchronized with their orbits.
– They keep the same face toward the planet.
– The rotation of the planet is slowed down.
• Orbits
– Orbits of moons mostly evolve outward.
• Internal “Tidal Heating”
– Eccentric orbits lead to periodic flexing of the moon’s shape which
heats the interior.
– Orbital resonances with other moons can maintain eccentric orbits
and tidal heating.
Jupiter’s Galilean Satellite’s
Io
Europa
Ganymede
Callisto
Io
Io’s Volcanoes
Io is the most volcanically active world in the solar
system.
Io
Jupiter’s tidal forces flex Io like a ball of silly
putty.
- friction generates heat
- interior of Io is molten
Volcanoes erupt frequently.
- sulfur in the lava accounts for yellow
color
- surface ice vaporizes and jets away
-Thin atmosphere made up mainly of sulfur
dioxide, produced by volcanic activity and
temporarily retained by the moon’s gravity.
Evidence of tectonics and impact cratering is
covered.
Volcanic Plumes
Lava fountain - active lava hot enough
to cause "bleeding" in Galileo's
camera - overloading of camera by the
brightness of the target
Newly erupted hot lava flow. Dark,
"L"-shaped lava flow marks the
location of the November 1999
eruption.
Gas and Dust Plume
A broad plume of gas and dust about 80 km high above a lava flow
Europa
Jupiter’s Europa
– Has similar but weaker tidal
heating,
– Has a young cracked water ice
crust perhaps only a few
kilometers thick, and
– May have a warm ocean of
liquid water below the crust.
– Could there be life?
Europa
Icy surface - “fresh” almost no craters
Ocean under the ice?
• Evidence
– Gravity measurements: central metallic core surrounded by
80170 km of water/ice
– Lack of craters  ice tectonics  liquid below (but could be
“fluid” ice, like glaciers)
– Chaotic terrain: like arctic ice pack, with separating pieces
– Magnetic field: conducting liquid for internal dynamo & metallic
core too cold  brine ocean
– Tidal heating: computations show it can do the job
• Estimated size
– Crust depth: 525 km, based on flooded impact crater
– Ocean 50150 km deep (< 11 km on Earth)
Surface of Jupiter’s Moon: :
Europa
Fractures in
Floating Ice
Icebergs
Evidence of a Subsurface ocean
Jumbled crust with icebergs and surface cracks with double-ridged pattern caused by tidal flexing of thick layer of ice on top of liquid ocean of water.
Europa’s interior also warmed by tidal heating
Salty - Europa has a magnetic field
Sub-Crust
Ocean
..
.
Hydrogen-Carbon
compounds likely:
Amino acids
Life in the
Ocean?
First New Ocean
Since Balboa
Missions to
Europa
http://www.jpl.nasa.gov/europaorbiter/
Ganymede
Ganymede
• Largest moon in the solar system
• Clear evidence of geological activity
• Tidal heating expected - but is it enough?
Ganymede
Wrinkles due to tectonic movement
in ice crust in (distant) past possible water deep below?
Ganymede
• Cratering
– Dark areas: cratering upon cratering  several byr old
– Bright areas: far fewer craters and grooves
– Explanation: “lava” (i.e., water) eruptions followed by freezing
• Ocean?
– Magnetic field  convecting core
– Part of magnetic field varies with Jupiter’s rotation  electrically
conducting interior (brine?)
– Salts found on the surface
• Heat source
– Less tidal heating than Europa (larger distance from Jupiter)
– Large mass  more radioactivity
– Much less heat than in Europa  thick crust (>150 km?)
 Much harder to prove the existence of life never mind finding it
Callisto
• “Classic” cratered
iceball.
• No tidal heating no orbital
resonances.
• But it has
magnetic field !?
Callisto
Callisto
Scarp close up
Possible water deep?
Callisto
• Cratering
– Heavily cratered everywhere  no water gushing to
the surface
• Gravity
– Undifferentiated: mix of ice and rock throughout
• Induced magnetic field
– Exists  underground ocean?
• Heat source?
– Does not participate in the tidal resonance
– Radioactive decay: only possibility
Life on Galilean Moons?
• Io
X
• Europa
Very active volcanically.
Hostile environment
?
• Ganymede
Subsurface saline
ocean, hydrothermal
vents?
? Subsurface saline
ocean? hydrothermal
vents?
• Callisto
saline
?? Subsurface
ocean?