AST 309
part 2:
Extraterrestrial Life
Life in the outer Solar System
Overview:
Prospects for life on:
1. Europa (Jupiter moon)
2. Titan (Saturn’s moon)
3. Enceladus (Saturn’s moon)
Life on Europa?
NASA’s JUNO mission to Jupiter:
Juno arrives at Jupiter
in 2016
Science goals:
1.
2.
3.
4.
abundances (e.g. ratio of O/H)
determine core mass
map the gravitational and magnetic field
map the variation in atmospheric composition, temperature structure,
cloud opacity and dynamics
Life on Europa?
• Europa is the sixth of Jupiter's known satellites and the
fourth largest; it is the second of the Galilean moons.
Europa is slightly smaller than the Earth's Moon.
Galileo Galilei
Europa
1610!
Life on Europa?
• Spacecraft exploring the Galilean satellites:
Voyager 1 & 2
Galileo
Life on Europa?
Basic Properties of Europa:
Semimajor axis = 671079 km
Orbital period = 3.551810 days
Heliocentric Distance = 5.203 AU
Rotational Period Synchronous
Orbital inclination = 0.464 degrees
Eccentricity = 0.0101
Radius = 1565 km
Mass = 4.797E22 kg
Mean density = 2.99 g/cm3
Surface Gravity = 0.135 of Earth's
Escape Velocity = 2.02 km/s
Geometric Albedo = 0.6
Surface Temperature = 128 K (-145 C)
Surface Composition = Water Ice
Tenuous O2 Atmosphere = Surface Pressure about 10-11 Earth's
Europa:
Europa’s surface is smooth and young (no craters), and is covered with
cracks:
Europa is tidally heated like Io (just less)
and has maybe the youngest surface in
the outer solar system!
Icebergs on the surface moved
by liquid water that later froze or
by slushy warmer ice beneath?
Europa:
Europa is heated by tidal forces from nearby massive Jupiter and has
forced orbital eccentricity of 0.0094 from the gravitational interactions
with the other Galilean moons:
That is exactly the same process
that drives Io’s intensive volcanism:
Europa is tidally heated like Io (just less)
and has maybe the youngest surface in
the outer solar system!
Europa:
2 models of Europa’s
interior:
Artist’s conception of the icy
surface of Europa
Europa:
Cycloidal features (“flexi”) near Europa’s south pole. These cycloidal cracks form in
Europa's solid-ice surface with the daily rise and fall of tides in the subsurface ocean
(Gregory V. Hoppa, Randall Tufts, Richard Greenberg and Paul Geissler of the Luna
and Planetary Laboratory, University of Arizona). This image shows what appears to be
the most convincing evidence yet for a global ocean under Europa's icy crust.
Europa:
Reasons why Europa is so interesting:
• the likely presence of a sub-surface ocean of liquid water
(perhaps as much as 150 km deep) which could provide a
medium and solvent for life.
• intense radiation from Jupiter's magnetosphere striking ice
on Europa's surface and releasing oxygen, which if it finds
its way into ocean could provide a fuel for life;
• the possible presence of undersea volcanic vents, which
could furnish energy and nutrients for organisms.
Europa:
Reasons why Europa is so interesting:
Chaotic features seen in many images of Europa's icy surface are probably created
by Europa's tides, and are believed to be evidence of melt-through needed for
exposing the oceans. The mixing of substances needed to support primative life
may be driven by the tides on Europa, with maximum heights of 500 meters
(much larger than Earth tides). Circulation of liquid water through cracks
produced by tidal forces could bring salts and organic compounds dissolved in
the water up to Europa's surface. This circulation also brings biologically useful
chemicals, such as formaldehyde (as well as organic compounds dumped on
Europa's surface by cometary impacts) down to the subsurface ocean. Other
chemicals, formed by radiation near the surface, such as sulfur, hydrogen
peroxide, and free oxygen, would also provide primative life with sources of
energy and nutrients. Hydrothermal vents would produce organic compounds
(seen as dark material coloring cracks?) and provide a heat source. Undersea
volcanism could also lead to large melt-throughs, and tidal heat, created by
internal friction could also melt the ice. The melted-through ice provides light
and surface chemicals to the oceans. Any creatures inhabiting these oceans
could use photosynthesis for energy.
The question
really is: is
the ice thin or
thick?
Tides and
undersea
volcanoes
could play a
role too!
Europa:
Let’s go there and find out!
Before we drill, we would
send a radar mapping
probe to measure the
thickness of the ice crust!
Indirect evidence from large craters suggest a
thickness of 19 to 25 km. On Europa, larger
craters become smoothed out with gentle
concentric rings. They are shallower than their
counterparts on other moons. This is because
of the influence of a very thick cushion of ice
(Schenk, 2002, Nature)
Titan:
• The biggest moon of Saturn
• Cold! 75 K (-180 C)
• Thick atmosphere! ~ 1.5 x pressure of Earth
mostly nitrogen (95%) and methane (CH4)
Origin?
– Probably outgassing after formation by accretion of icy
planetesimals/comets and subsequent evolution
– Methane from cryovolcamism
Titan:
• Exploring Titan:
Voyager 1 & 2
Cassini / Huygens
Titan:
• Titan as seen from space:
visible
938nm methane window
composite
Titan:
• Titan is big! 2nd largest moon in our solar system:
Size comparison: our Moon,
Earth and Titan
Simple model of internal structure
Titan:
• Titan’s thick atmosphere:
Layered structure in Titan’s atmosphere
Titan:
Chemical composition determined by spectroscopy:
Titan:
Titan:
Titan:
Source of Titan's atmosphere?
Internal - original planetesimals contained dissolved/trapped volatiles -> interior
and are now outgassing to provide persistent atmosphere
External - delivery of comets (same as Earth, Venus, Mars) - evidence in high
altitude H2O and CO
Losses of Titan's atmosphere?
Thermal escape - particularly H, also other atoms produced by dissociation of
molecules at high altitudes
Ionization, stripping by surrounding magnetosphere and solar wind
Condensing -> ocean
Recycling?
Methane is recycled in a similar way to water in the Earth's atmosphere through
the hydrological cycle......
Titan:
Why is this important for astrobiology?
Titan’s atmosphere is remarkably similar to the Urey-Miller experiment and early
Earth (except temperature)
Is Pre-biotic chemistry possible on Titan?
But life?
Remember that Titan is cold and all chemical reactions slow down, but still….
Ok, so let’s land on Titan!
Titan:
The Huygens lander:
Titan:
The Huygens lander’s descent:
Titan:
The Huygens lander, impact data:
Surface is like wet clay,
packed snow or sand
Titan:
The Huygens lander:
Evidence for
liquid methane
on the surface
Heating of the
surface by the
probe caused
methane outgassing
Titan:
The Huygens lander:
Surface reflectivity
measured on the landing
site with a lamp turned on
(red line). The visible
portion of the spectrum is
consistent with laboratoryproduced tholins, thought
to be analogs of Titan's
photochemical aerosols
(black curves). Water ice is
likely responsible for the
absorption seen at 15001600 nm. The decrease of
the reflectivity with
wavelength beyond 830
nm is due to an
unidentified material.
Enceladus:
Enceladus:
Enceladus has the highest albedo (>0.9) of any body in the
solar system. Its surface is dominated by fresh, clean ice.
At least five different types of terrain have been identified on
Enceladus. In addition to craters there are smooth plains and
extensive linear cracks and ridges. At least some of the surface
is relatively young, probably less than 100 million years.
This means that Enceladus must have been active until very recently (and
perhaps is still active today).
Enceladus is much too small (500km) to be heated solely by the decay of
radioactive material in its interior at present. But briefly after its formation 4.5
billion years ago short-lived radioisotopes may have provided enough heat to
melt and differentiate the interior. That combined with modest present day
heating from long-lived isotopes and tidal heating may account for the
present day activity on Enceladus.
Enceladus:
Enceladus orbits in the densest
region of Saturn’s E-ring
Is it the source?
Enceladus is currently in a 2:1 mean motion orbital resonance with
Dione, completing two orbits of Saturn for every one orbit completed by
Dione. This resonance helps maintain Enceladus's orbital eccentricity
(0.0047) and provides a heating source for Enceladus's geologic activity
Enceladus:
Cassini discovers jet-like plumes rising from the south polar regions.
During “fly-throughs” the spacecraft detected mostly water vapor, as well
as minor components like molecular nitrogen, methane and CO2.
Enceladus:
Cassini measures mean density:
1.6 g/cm3 => some % of silicates & iron
Now two heat sources:
1. Radioactive decay
2. Tidal forces
Possible molten core or magma pockets that
drive volcanism!
Enceladus:
Enceladus:
The “Tiger Stripes” in the south
polar region.
Thermal map of cracks near south
pole of Enceladus. High temperatures
around 180 K (-135 F), normal = 72 K
Enceladus:
Pockets of water of salty ocean?
Postberg et al. (2011, Nature):
"Alkaline salt water, together with the observed organic compounds
and the thermal energy obviously present in the south polar region,
could provide an environment well suited for the formation of life
precursors.“
Cassini's Cosmic Dust Analyser instrument found that about 6% of the grains
forming the E-ring were rich in sodium salts.
“Saturn's tiny moon Enceladus may be the best place to look for life
elsewhere in the Solar System.” Bob Brown (Cassini mission)
Summary:
•
Europa likely has a liquid sub-surface ocean and
is heated by tidal forces
•
Titan has complex organic chemistry and a thick
nitrogen atmosphere (early Earth)
•
Enceladus is geological active, has lots of water,
plus some organic material and a heated interior