Astrobiology of Titan and
early results from Cassini-Huygens
Lecture at Penn State Astrobiology Workshop for Educators
July 28th 2005
Dr Conor A Nixon
University of Maryland/NASA GSFC/Cassini CIRS Team
Overview
• Saturn system summary.
• Titan - the enigmatic moon.
• Why go back?
• The Cassini-Huygens spacecraft.
• Latest and greatest Cassini pictures and
results.
• Huygens landing.
• Where do we go from here?
Facts about Saturn (i)
• Saturn is the 6th planet from the Sun, 9.5
times as far as the Earth.
• Saturn is the 2nd largest by mass and radius.
• Saturn is mostly composed of the same
materials as the Sun: H2 and He.
• Other common elements such as C, N, O, P
are hydrogenated: CH4, H2O, PH3, NH3 etc.
• Condensates such as NH3 ice form the visible
cloud deck: there is no ‘surface’ like the Earth.
Facts about Saturn (ii)
• Saturn has the most extensive ring system of
any planet in the solar system.
• Saturn rotates with a period of 10 hours 40
minutes (interior) with an axial tilt of 26°.
• Saturn has 2nd most moons (47, for now!),
after Jupiter (63).
• Saturn’s largest moon, Titan, is the 2nd largest
in the solar system.
The Giant Moon
• Titan
was the sixth moon ever to be
discovered, in 1655 by Dutch
astronomer Christian Huygens.
• Named due to its massive size,
Titan was originally thought to be
the solar system’s largest moon.
• Through a large telescope, Titan
appears as a fuzzy pale orange ball.
Even Voyager 1 saw little detail.
Obscured by clouds
• Titan’s
size was originally
over-estimated: we are looking
at dense, thick layers of opaque
haze, not the surface.
• Voyager 1 snapshots of the
planet’s edge (1980) showed
that the haze was multi-layered.
• Voyager 1 finally solved the question of size, by using
radio waves to penetrate the haze.
• Titan’s diameter of 5150 km is larger than Mercury, but
smaller than Ganymede.
Atmospheric Composition
• We now know that the atmosphere is largely composed
of nitrogen. Which other planet is like this?
• In addition, the atmosphere contains several percent of
methane, and many compounds of H,C,N: but no free O2.
Earth vs Titan Atmospheres
Titan’s surface
• This
artist’s impression shows lakes of liquid hydrocarbons (‘natural gas’), such as ethane on the surface of
Titan - is this likely?
Got to have chemistry…
• Titan’s atmosphere is like one giant chemistry lab: UV
light from the distant Sun splits up some of the native
methane and nitrogen molecules.
• The pieces of these molecules are then free to bond
together in new ways, forming heavier chemicals which
are expected to condense and rain out.
• Over billions of years, huge lakes of liquid
hydrocarbons should have accumulated on the surface,
at least, that’s what we thought…
The mystery deepens…
• This image shows one
of the best-ever pictures
of Titan taken from the
Earth (HST).
• A huge bright, icy
continent was spied on
the leading side of Titan.
• BUT - no dark lakes of
liquid hydrocarbons.
• What could be wrong
with our theory?
Organics to pre-biotics?
• The presence of simple organic (‘carbon containing’)
molecules on Titan led scientists to speculate whether
any more complex, ‘pre-biotic’ molecules may have
formed too.
• Khare, Sagan and
colleagues experimented
by passing a DC current
though a reaction vessel
containing 90% N2 and
10% CH4 to simulate
Titan’s atmosphere.
Tholins
• This experiment produced a tarry, reddish-brown
goo which formed on the walls of the reaction
vessel.
• This was named ‘tholin’, after the Greek word for
mud!
• Tholins have been intensively studied in the lab as
possible analogs for Titan and Triton hazes.
• Khare et al also found that by the simple addition
of HCl, 16 amino acids were produced.
• What are amino acids and why are they so
important?
Amino Acids From Tholin
• Chemically, an amino acid is a carbon molecule which has three
types of bonding other than simple C-H bonds: C=O, C-OH, and CNH2.
• Amino acids are the building blocks of proteins, and essential to
cellular life as we know it. They are sometimes called prebiological
molecules.
Water on Titan
• Titan is much too cold for surface liquid water,
with a temperature of 95 K (-178°C).
• Life as we know it on Earth cannot survive
without liquid water.
• However, it has been proposed that water could
exist temporarily in melt pools produced by
impacts.
• Large melt pools hundreds of meters deep might
take centuries or even millenia (if mixed with
ammonia) to freeze completely.
Liquid assets
• This is long enough for interesting organic
chemistry to take place!
• Chemical reactions in surface melts could add
oxygen from water to the N, C, H in the tholin to
make carboxylic acids, purines and pyramidines.
• As well as the surface, Titan may well have a subsurface ocean like Europa.
• This is needed in many models to allow methane
to escape from the interior, and replace the methane
lost in the atmosphere by chemistry.
Astrobiology on Titan?
A New Mission to Saturn
• The Voyager 1 and 2 missions which had flown by Saturn and
Titan in 1980-1981 had raised almost as many questions as they
had answered.
– What is the surface like? Are there lakes or seas?
– Why is Titan the only moon with an atmosphere?
– Where does the methane go to/come from?
– Is Titan similar to a prehistoric Earth?
• A new mission was needed, one which wouldn’t just fly past
while snapping a few shots, but go into orbit for years,
examining the system in detail.
Cassini-Huygens
• Cassini was designed to answer these questions. The
orbiter is designed for a 4-year prime mission.
• The European Space Agency added a Titan descent
probe, called Huygens.
• Cassini-Huygens is
a huge international
science cooperation…
Cassini Instruments
• Cassini carries 12 separate scientific
instruments:
– 4 are cameras or spectrometers for UV, IR,
and visible light.
– 6 are fields and particles instruments: dust
analyzers, magnetometers, charged particle
detectors…
– 2 are radar and radio science experiments.
Both of these can ‘see’ through the haze to the
surface, if close enough to Titan.
Huygens Instruments
• Huygens
carries six scientific instruments onboard:
– ACP - the Aerosol Collector Pyrolyzer.
• ACP collects aerosol samples for onboard chemical analysis.
– DISR - the Descent Imager/Spectral Radiometer.
• Takes Pictures!
– DWE - the Doppler Wind Experiment.
• Uses the radio signal to measure probe swinging and drifting.
– GCMS - the Gas Chromatograph and Mass Spectrometer.
• Analyzes the composition of the atmosphere.
– HASI - the Huygens Atmospheric Structure Instrument.
• Measures the physical and electrical properties of the atmosphere.
– SSP - the Surface Science Package.
• Determines the physical properties of the surface.
Huygens is prepared…
• Huygens is
2.7 meters
across and
built like a
clam-shell.
• The probe is
here attached
to the front
heat shield (r).
Ready to go…
• Cassini on display before
launch, fall 1997
• Cassini at launch pad, on
rocket, awaiting fairing.
Blast-off!
• Cassini-Huygens was launched at 4:43 a.m. EDT on October 15th 1997
from Cape Canaveral Air Force Station
Cassini takes the scenic
route to Saturn…
Going Into Orbit!
A Grand Design
• Having
finally reached
Saturn orbit,
Cassini is now
on the 13th of
76 planned
orbits in the
prime 4-year
mission,
including 45
close Titan
encounters.
What has Cassini discovered so far?
• Trying
to tell all of Cassini’s accomplishments in the
first 12 months is already impossible in a 1-hour talk!
• They include:
– new information about the fast-moving clouds on Saturn,
and its atmospheric composition.
– fascinating insights into the nature of the moons: Phoebe,
Iapetus, Enceladus, Rhea, Hyperion, Dione, Tethys.
– Fabulous pictures and scientific information about the rings
and magnetosphere.
• Let’s concentrate however on Cassini’s new insights
about Titan.
From a
distance…
• Cassini first Titan
fly-by on July 2nd
2004 was quite
distant, but already
showed amazing
new details of the
‘veiled world’.
• This UV filter
image has been
falsely colored to
show detail.
Purple
haze
• This false
color UV
night-side
image from
TB
(12/16/04)
shows the
many haze
layers as
revealed by
scattered
Full Moon
• This is a
composite image
(TA flyby).
• Red and green
show methane:
north hemisphere is
redder.
• Blue shows UV:
high atmosphere
and detached hazes.
Beyond the visible
• This T0 sequence shows VIMS images of the day/night terminator, taken in
several near-IR filters (?, 3.3, 4.7 m)
CIRS Infrared Spectrum
• June
10 2004
The missing atmosphere…
• From TA INMS atmospheric sampling.
The upper atmosphere possesses much heavier
chemical species than we would have guessed.
Through the haze..
• On 10/26/04, Cassini made its first close pass of Titan.
• This series (l-r) shows how multiple frames and image
processing techniques are used to sharpen the initial picture.
• Here we see bright and dark terrain, and S polar clouds.
Land O
lakes?
• This image of the
south pole shows
white clouds and an
intriguing dark
feature with a sharp
boundary.
• This could be a lake
of hydrocarbons, or
just a depression
filled with solid
tholin.
Radar
Mapping
• TA was the first
chance to use RADAR
up close!
• Here is the same
image twice, but the
top one has been
falsely colored to aid
interpretation: pink
shows ‘rougher’
terrain, green is
‘smoother’.
• Size: 300x150 km.
‘Cat-scratches’
• The second RADAR pass in February showed
dramatically different terrain.
• These E-W linear markings have been dubbed ‘catscratches’, but their origin (aeolian, tectonic?) is actively
debated at the present time.
‘Circus Maximus’
• RADAR also confirmed that an annular feature seen
by ISS was a huge, 440 km impact basin.
Xanadu Smile
• The VIMS instrument saw this bright spot coinciding with the
Xanadu ‘smile’. This could be an impact crater or volcano.
Titan Cryovolcano?
• The VIMS team recently revealed this picture (TA) of
a possible volcano, with two outflowing ‘arms’.
Destination Titan!
• Huygens
entered
Titan’s
atmosphere at
5:13 am ET on
January 14th
2004.
• The entry angle
was 65 degrees,
with Huygens
moving at 6 km/s.
Mission
Timeline
• 180
km - 2.5m pilot chute
opens: removes back-cover.
2.5 seconds later the main
8.3m chute opens.
• 160 km - front shield
released: instruments deploy.
• 125 km - 3m drogue chute
opens for quicker descent.
Targeting Titan
• The Huygens landing site was targeted for
190°W, 10°S.
• This image
shows the landing
site of the probe as
seen from Cassini.
• The red circle
shows the Huygens
camera FOV at 20
km altitude.
Jigsaw Puzzle
• This mosaic was reconstructed from 30 images, and
shows an area 30 km with 20 m resolution.
• The probe was
swaying more and
rotating in the
opposite direction
to expectations: so
piecing the
snapshots together
was not easy!
River to the shore?
• This composite of
three images shows
what looks like a
branching river
draining to a
shoreline.
• Rainfall on Titan
would presumably
be liquid methane.
Splash
or
Bump?
Titan ‘boulders’
• Huygens bumped
rather than splashed on
Titan’s surface.
• This image shows the
ground near the
spacecraft.
• The ‘boulders’ are
probably water ice.
• The below-center
‘boulder’ is actually just
a pebble, 6 in across.
What have we learned?
• Titan has not given up its secrets easily!
• But we have learned:
– Titan appears to be mostly dry; no widespread
methane or ethane oceans or seas.
– Methane condensation produces clouds around
the south pole with possible rainfall.
– The surface is young, indicating resurfacing by
outflows, tectonics, impacts in last 100-300 Myr.
– Photochemistry is active high in the ionosphere.
What is still unanswered?
• Again, we have raised many new
questions:
– are we seeing lakes near the south pole, or
something else?
– are we seeing cryovolcanos?
– if not, where does the methane come from and
go to?
– what are the cat scratches?
– when, what and how were the ‘shorelines’
near the landing site made?
THE END…
… FOR NOW!