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: H 2 and He. Other common elements such as C, N, O, P are hydrogenated: CH 4, H 2 O, PH 3, NH 3 etc. Condensates such as NH 3 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 In addition, the atmosphere contains several percent of methane, and many compounds of H,C,N: but no free O 2. We now know that the atmosphere is largely composed of nitrogen. Which other planet is like this?

Earth vs Titan Atmospheres

Titan’s surface This artist’s impression shows lakes of liquid hydro- carbons (‘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% N 2 and 10% CH 4 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 Amino acids are the building blocks of proteins, and essential to cellular life as we know it. They are sometimes called prebiological molecules. 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 C- NH 2.

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 sub- surface 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 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 co- operation…

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 sunlight.

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

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 ‘cat- scratches’, 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 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 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!