Life on other Solar System Planets
George Lebo
29 October 2012
AST 2037
1
Life on Mercury
• Can the closest planet to the Sun support life?
2
Mercury’s Orbit and Rotation
•Astronomers initially thought Mercury’s rotational and orbital
periods were the same - same side always faces sun
•Radar observations showed rotational period = 59 days (orbital
period = 88 days!)
•Mercury is not tidally locked to the sun in
the same way as the moon-earth system
•Sun’s gravity and Mercury eccentric orbit
brought it into semi-synchronous orbit
•Mercury presents the same face to the sun
every OTHER time around!
3
Mercury’s 3:2 Resonance
• Once thought to be
synchronous
• Mercury rotates 3
times for every two
orbits
• One Mercury “day”
lasts two Mercurian
years!
Mercury’s Atmosphere
• Almost non-existent - high surface temperature and
low escape velocity
• no atmosphere means drastic temperature changes
700 K (day) = 800 F
100 K (night) = -280 F
• what little atmosphere there is comes from solar wind
particles (Hydrogen and Helium)
5
Mercury’s Surface
•Similar to the moon - craters!
•Old surface
•No plate tectonics
•Craters flatter and have thinner
ejecta rims than lunar craters
due to higher gravity on
Mercury than the moon
•Craters not as dense as on the
moon - filled by volcanic
activity - but not dark like
“maria”
Mariner 10 image from mid-1970s
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Messenger Spacecraft Orbit
Summary
• The spacecraft flew by Earth once, Venus
twice, and Mercury itself three times,
allowing it to decelerate relative to Mercury
with minimal fuel. It also executed 5 Deep
Space Maneuvers (DSM’s).
Messenger Spacecraft Orbit
www.youtube.com/watch?v=GXEuQtpreXE
Messenger Spacecraft Mosaic
Messenger Spacecraft Mosaic
Mercury’s Surface
• Scarps (or cliffs) are seen
on the surface
•NOT seen on the moon
•appear to be about 4
billion years old
•not the result of plate
tectonics
•probably the result of the
surface cooling, shrinking
and splitting at this time
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Water on Mercury?
• It’s WAY HOT there – don’t be
stupid!!
• Besides, no atmosphere (water
would boil off, right?)
• We thought so …
• But Messenger’s FIPS (Fast
Imaging Plasma Spectrometer)
Experiment has now found
traces of water in its atmosphere
• Radar map made in 1991 shows large
reflections at North Pole
• Similar to radar signature from ice on
Mars polar caps
• Believe Mercury has ice at the bottom
of craters near the pole
• Permanent shade  permafreeze!
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Why Venus?
• Duh …
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OK … Really
• Physical properties of Venus:
• Diameter = 0.95 * Earth
• Mass = 0.8 *Earth
• Distance from Sun = 0.72
* Earth
• Solid surface, with
atmosphere (incl. clouds)
• Venus is commonly
known as our “twin”
planet!
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Venus’s Rotation
•Rotation direction is retrograde (opposite that of other terrestrial planets)!
•243 day rotation period
•Axis is almost exactly perpendicular to orbit
•Why? Possibly hit by large body during formation altering spin direction
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Venus’s Atmosphere
•Much more massive
atmosphere than Earth’s
(surface pressure is 90x
Earth!)
•Surface temperature is
730 K (!!!)
•Carbon dioxide (96.5%),
Nitrogen (3.5%)
•No water - the clouds are
made of sulfuric acid
droplets
Pioneer UV image taken 1979
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Venus’s Atmosphere
•Fast moving clouds 50-70km above
surface
•Haze 30-50 km
•Clear air below 30 km
•Upper atmosphere is very windy 400 km/hour
•Wind speed decreases lower in the
atmosphere
•Temperature and pressure increase
closer to the surface
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Why is Venus So Hot?
• F = L/4d2  Venus receives about twice the solar
radiation as Earth per unit area
• F_emitted =  T4  equilibrium temperature scales up
by (2)1/4 power  about 20% higher temperature
• This is 20% of 300K, or about 60K higher
• Expect Venus to be at about 170 F (but really is more
like 900 F)
•
•
•
•
Atmosphere is largely CO2 (a greenhouse gas)
This traps solar radiation more efficiently
Temperature much higher this way
What about “cool spots”? Not likely (why?)
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Runaway Greenhouse Effect
• On Earth, most CO2 from atmosphere has been locked
up in limestone (CaCO3), slowly getting
released/replenished via combo of rain and plate
tectonics
• If temps higher (i.e. move Earth closer to the Sun, by
magic), more water vapor in the air
• Water vapor increases infrared absorption from the
ground (it’s an EXCELLENT greenhouse gas!)
• Temps rise some more, more water evaporates, more
greenhouse effect, temps rise more, etc.
• Once hot enough, water vapor rises to top of
atmosphere, and solar UV light dissociates it into H +
O
• Lighter H drifts off into space (which is why Venus has
150x the deuterium of Earth!)
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Venus’s Surface
Radar (radio waves) echoes reveal the surface topology
•Elevated “continents” make up 8% of the surface
•Mostly rolling plains with some mountains (up to 14 km)
•No tectonics
•Buckled and fractured crust with numerous lava flows
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A few Soviet spacecraft have landed on Venus in 1970s
•Survived only an hour before burning up
•little evidence of erosion - young surface
•rocks are basaltic and granite
•some craters (very few) caused by meteoric impact
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Venus: Magellan Mission
• Satellite orbiting Venus in 1990s
• Precision radar mapping of entire surface with ~150yard resolution (better map of all Venus than of all
Earth!) – not so now
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Venus’s Surface: Volcanoes and Craters
•Volcanoes resurface the planet every
~300 million years
•Shield volcanoes are the most common
(like Hawaiian Islands)
• A caldera (crater) is formed at the
summit when the underlying lava
withdraws
•Largest volcanic structures are called
coronae - upwelling in the mantle which
causes the surface to bulge out - not a
full-fledged volcano.
•Usually surrounded by other volcanoes
•Venus is thought to still be volcanically
active today (Magellan lava flows)
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How did Venus get this way?
• Runaway greenhouse raises temps, gets rid of surface
water early on
• Early plate tectonics brings up internal H2O and CO2
from early limestone into atmosphere
• No surface water  no way of trapping CO2 , so it
stays in the atmosphere now (huge pressure,
greenhouse high temps)
• Internal water lost  important lubricant for plate
tectonics; plate tectonics stops
• Volcanoes continue
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Summary
• Mercury has no atmosphere, little water (frozen in
caps), and extreme hot/cold temps
• Venus is Earth’s twin superficially, but has huge
pressure, no water, sulfuric acid for rain, and temps
that can melt lead (pretty much everywhere)
• Reasons for Venus situation indicate Earth would look
like that too if it was at Venus distance from Sun
• So … in general it is unlikely that planets this close to a
star like the Sun will be able to support life (!)
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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 while looking
for comets
• 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 (Berlin Observatory) 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,
launched in 1997
•Gravity assists from Venus
(twice), Earth and Jupiter
• 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 this (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 ( and reflects short wavelength
light
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 (2.5X) than
expected from the Sun’s heating alone
Life on the Giant Planets?
• We see many “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
•
This is Ganymede
Saturn’s Titan
•
Largest moon
•
Atmosphere more
than 95% nitrogen,
The rest is methane
•
“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
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
Titan’s Surface
Lakes of liquid hydrocarbon imaged from orbit via Cassini
Huygens lander mission in 2006
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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