Life on Giant Planets
& Their Moons
Stephen Eikenberry
27 March 2013
AST 2037
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The Jovian Planets
•much larger than terrestrial planets
•not solid - gaseous
•all have rings
•all have many moons
JupiterJupiter, Saturn, Uranus, Neptune
Jupiter
•Named after the most
powerful Roman god
• third-brightest object in the
night sky (after the Moon and
Venus)
•Atmospheric bands are very
different than inner planets
•Many moons – four largest
called Galilean Moons
Saturn
• Named after the father of Jupiter in Greco-Roman mythology
• Almost twice Jupiter’s distance from the Sun
• Similar banded atmosphere
• Uniform butterscotch hue
• Many moons
• Spectacular ring system
Uranus
• Discovered by William
Herschel in 1781
• Named after father of Saturn
• Barely visible to naked eye
• Featureless atmosphere
• Deviations in the expected
orbit of Uranus pointed to the
possibility of another planet
influencing its motion
Neptune
• There had to be another planet influencing Uranus
• 1845 - John Adams determined the planet’s mass and orbit
• 10 months later - Urbain Leverrier, independently came up with the
same result
• 1846 - Johann Galle found the new planet Neptune
• Cannot be seen with naked eye
• “Bluish” Jupiter atmosphere
Space Craft Exploration of Jovian Planets
•Voyager 1and 2 left Earth in 1977
• reached Jupiter in March and July of 1979
• Used Jupiter’s strong gravity to send them on to Saturn - gravity assist
• Voyager 2 used Saturn’s gravity to propel it to Uranus and then on to Neptune
• Studied planetary magnetic fields and analyzed multi-wavelength radiation
• Both are now headed out into interstellar space!
Space Craft Exploration of Jovian Planets
• Galileo - launched in 1989 and reached Jupiter in December 1995
• Gravity assists from Venus and Earth
• Two components: atmospheric probe and orbiter
• Probe descended into Jupiter’s atmosphere
• Orbiter went through moon system
• Cassini mission to Saturn
• Studying Saturn’s moon
Titan in much the same way
as Galileo studied Jupiter
Jovian Planet Properties
•Most of their mass is Hydrogen and Helium – light elements = low densities
•High surface gravity allows their atmospheres to retain these light elements
•Dense compact core at the center
•But, NO SOLID SURFACE – gaseous atmosphere becomes denser (eventually
liquid) at core
•Differential Rotation – outer regions rotate slower than inner regions
Jovian planets
- axis tilt and magnetic fields
•Uranus has the most inclined rotational axis - extreme
seasons!
•All appear to have strong magnetic fields - rapid rotation and
liquid conductive cores or mantles
Jupiter’s Atmosphere
Two main features: colored bands and
Great Red Spot
• molecular hydrogen – 86%
• helium – 14%
• small amounts of methane, ammonia, and water vapor
•Darker colored belts lie atop downward moving
convective cells
•Lighter zones are above upward moving cells
•Belts are low-pressure, Zones are high pressure
•As on Earth, wind moves from high to low
•But rotation causes wind patterns to move
East/West along equator
•Temperature difference between bands is main
reason for color difference
Jupiter’s Atmosphere
•Haze lies at the upper edge of the
troposphere
•Thin layer of white ammonia clouds –
125 – 150 K
•Colored clouds below that
• Warmer - 200 K
• clouds are mostly droplets or
crystals of ammonium hydrosulfide
• At deeper levels, clouds of water
ice or water vapor
The Galileo probe survived for
about an hour before being crushed
at this altitude.
Weather on Jupiter
Main weather feature – Great Red Spot!
• swirling hurricane winds
• has lasted over 300 years!
• diameter twice that of Earth
• rotates with planet’s interior
• the spot appears to be confined and
powered by the zonal flow
Smaller storms look like white ovals (this one is over 40 years old)
Why do the storms last so long?
On Earth, hurricanes lose power when then come upon land
No continents on Jupiter – nothing to stop them once they start
Saturn’s Atmosphere
• molecular hydrogen 92.4%
• helium 7.4%
• traces of methane and ammonia
•Layer of haze
•Troposphere contains 3 cloud layers
•ammonia ice
•ammonium hydrosulfide ice
•water ice
•Overall temperature is cooler than
Jupiter
•Atmosphere thickness is 3 times that
of Jupiter (caused by lower surface
gravity on Saturn)
•Thicker clouds result in less varied
visible colors
Weather on Saturn
•Computer enhanced image shows bands, oval storm systems, and
turbulent flow patterns like those seen on Jupiter
Atmospheres of Uranus and Neptune
•molecular hydrogen 84%
•helium 14%
•methane 2% (Uranus) 3% (Neptune)
Abundance of methane gives these planets their blue color
Methane absorbs longer wavelength light (red) and reflects short
wavelength light (blue)
Weather on Uranus and Neptune
Uranus
•Few clouds in the cold upper atmosphere – featureless
•Upper layer of haze blocks out the lower, warmer clouds
Neptune
•Upper atmosphere is slightly warmer
than Uranus (despite its further distance
from Sun)
•More visible features (thinner haze, less
dense clouds lie higher)
•Storms – Great Dark Spot
•Seen in 1989 – gone in 1994
Internal Structures – models that fit the data
Metallic hydrogen is like liquid metal
Jupiter
Saturn
Uranus/Neptune
•Increasing temperature and pressure deeper in core
•Jupiter bulges at radius (7% larger)
•Saturn less assymetric – larger core – same basic overall structure
on a smaller scale
•Uranus/Neptune have a high density “slush” below cloud level compressed water clouds
Internal Heating
•Primordial Heat
-Jupiter’s heat source results from
strong heating during formation by the
collapse of material onto the core
•Generation of Heat
-Saturn generates some heat due to the
gravitational contraction of helium gas
•Effect of internal heating - raises the temperature of the
interior and atmosphere to higher values than expected
from the Sun’s heating alone
Life on the Giant Planets?
• We see lots of “life chemicals”! (Water, methane, etc.)
• Lightning observed too
• Similar to early Earth atmosphere (?)
• Expect complex hydrocarbons (Urey-Miller) (?)
• At some depth, have warm temps (~300K)
• But … these temps at high pressure
• Methane is not a sign of life here – just a sign of LOTS
of free hydrogen, some carbon, relatively little oxygen
• No solid surface – no oceans, no tidal pools, no clay
matrix – in short, no (Earth-like) places for life formation
Life on the Giant Planets?
• What about gas matrix life? (i.e. no solid surface, but life in
the air)
• Wind speed turbulence problem – try to put together a house
of cards outdoors in a hurricane
• That’s easier than forming life in Jupiter’s atmosphere!
• Convection
• Causes any chemicals from warm lower layers to rise
• This gives exposure to solar UV radiation, which breaks
it down
Does this mean NO life here?
• Nope
• Floater possibility
• But, this is pure speculation
Moons
•
Almost all moons in
the Solar System orbit
the Jovian planets
•
Jupiter’s 4 major
moons are the
Galilean satellites: Io,
Europa, Callisto, and
Ganymede
•
Jupiter has 100’s of
smaller moons
Io
•
Innermost Galilean
satellite
•
Reddish color
•
Smooth “young”
surface
•
Extreme volcanic
activity
•
Interior tidally heated
by Jupiter (& Europa)
Io – Tidal Heating
Europa
•
Second Galilean
satellite
•
Whitish, highlyreflective color
•
Smooth surface with
patterns similar to ice
caps on Earth
•
Young surface
•
May be a liquid
“waterworld” with ice
crust
Life on Europa?
• Possible view of Europan
ocean
• Note thermal segregation
Life on Europa?
• Europa “seabed” may resemble “black smoker” environment on
Earth (!)
• Probably little O2 in the water (where would it come from?)
• But anaerobic bacteria are the basis for life in Earth vents (even if
larger life O2-dependent)
• NASA developing preliminary plans for a drilling/submarine
exploration mission
Callisto & Ganymede
•
Rock/ice moons
•
Evidence for some
“glacial” flows
Saturn’s Titan
•
Largest moon
•
“Smooth” surface due
to thick atmosphere (!)
•
Infrared images show
evidence for
“continents” beneath
Titan’s Surface
Lakes of liquid hydrocarbon imaged from orbit via Cassini; T
~85K (probably ethane/methane)
Huygens lander mission in 2006
Titan’s Surface
Lakes of liquid hydrocarbon imaged from orbit via Cassini
Huygens lander mission in 2006
Life on Titan?
• Solid/liquid phase environment similar to Earth’s surface
• Totally different chemicals and temp ranges; will not be H2Obased like on Earth
• But … still a possibility
Neptune’s Triton
•
Largest Neptunian
moon
•
Retrograde orbit
•
Orbital tilt 20-degrees
•
Could indicate KBO
origin ??
Other moons
Enceladus
•
Saturn moon
•
Water here too?
Enceladus
•
Saturn moon
•
Water here too?
Summary
• The Giant Planets lack solid surfaces and have
extreme wind speed, turbulence and convection
in their atmospheres
• Still .. we cannot rule out life there (floaters?)
• Moons such as Europa and Enceladus may have
liquid water oceans with Earth-like temperatures
and geothermal vents; these may be promising
places to search for life
• The moon Titan has a thick atmosphere and
(apparently) lakes/rivers of liquid hydrocarbons;
temps are COLD, but we can imagine some lowtemperature, slow-reaction life developing here
too (?)
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