Titan (Saturnian Moon)


I. Introduction

II. Localization

III. Formation

IV. Composition

1. Atmosphere

2. Air

3. Soil

V. Cassini-Huygens

1. Introduction

2. Launching of the Huygens-Cassini mission

3. What is happening during the Huygens-Cassini mission ?

VI. Earth and Titan

VII. Conclusion

1. Open issues

VIII. Bibliography

Students : DHYNE Miguël, 18, Special Math-Science (Salzinnes, Namur)

                  DOUILLET Thibault, 17, Seminary of Floreffe

                  SOUGNEZ Nicolas, 17, Seminary of Floreffe

Teacher : SOUGNEZ Bernard (Master in geographic sciences)

School : Seminary of Floreffe

                 Rue du Petit Séminaire, 7

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Contact : info@flosat.be

November 2002

Titan (Saturnian Moon)

I. Introduction

Textfeld:  Titan is the largest moon of Saturn and the second largest moon in the solar system, rivaled only by Jupiter's moon Ganymede. Before the Voyager encounters, astronomers suspected that Titan might have an atmosphere. Scientists also believed they might find liquid seas or pools of methane or ethane; water would be frozen due to Titan's low surface temperature. Expecting an unusual world, Voyager 1 was programmed to take numerous close up views of Titan as it flew past in November of 1980. Unfortunately, all that was revealed was an impenetrable layer of atmosphere and clouds. Only slight color and brightness variations were observed.

Textfeld:  Although Titan is classified as a moon, it is larger than the planets Mercury and Pluto. It has a planet-like atmosphere which is more dense than those of Mercury, Earth, Mars and Pluto. The atmospheric pressure near the surface is about 1.6 bars, 60 percent greater than Earth's. Titan's air is predominantly made up of nitrogen with other hydrocarbon elements which give Titan its orange hue. These hydrocarbon rich elements are the building blocks for amino acids necessary for the formation of life. Scientists believe that Titan's environment may be similar to that of the Earth's before life began putting oxygen into the atmosphere.

In Greek mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.
Discovered by Huygens in 1655 (see picture).

II. Localization

Titan, Saturn's largest satellite, is a distant world: Car away from the Sun and its close, warmterrestial planets. A fair distance from Saturn, too. Seen from Titan, at more than a million kilometers away, Saturn looks like a big yellowish ball, permanently girdled by an icy hoop. The Sun's disk is hard to see at a distance of close to one billion five hundred kilometers, more than nine times farther away than the Earth. As a consequence, our star is a hundred times dimmer there, and the temperatures prevailing on Titan drop to 70 K (- 200 : C) in the atmosphere. This atmosphere is thick enough to make Titan an important but mysterious object in our solar system. Even from a near-by distance, as Voyager 1 found out in 1980, the dense cloud decks surrounding the small world obscure any glimpse of its surface, as on Venus.



III. Formation

Textfeld:  Titan grew by accumulating smaller objects. Several collisions have made it that big.  After a collision, the heatwave provoked by it is distributed on the satellite's surface until the inside gets colder than the surface once the accretion is over. The surface temperature can become warm enough to make the ice melt on a whole part of the satellite. The silicates that were present in the ice then settle at the bottom of this liquid layer and form a new, thick-based layer. This is how the satellite becomes differenciated for it is made up of a rocky nucleus topped by a silicate and ice layer and then surrounded by pure ice. Titan is more differenciated than other satellites such as Ganymede, which is almost as big as Titan, because the latter's ice, made of water and ammonia, melts at -100°C, while Ganymede's ice melts at 100°C. After this stage, Titan underwent several distortions that aimed at perfecting its orbit around Saturn, which was more eccentric than it now is. This stage could have led to a form of volcanism on Titan. The satellite's inner pressures are strong enough to allow several physical states to coexist. The natureof their transition affectsthe global circulation of Titan's inner convective movements, from a dynamic point of view. The heat draining loses its efficiency and the satellites thus get warmer. Indeed, the highest temperatures on titan's surface were hot enough to melt its ice made out of water and ammonia ; that is why the resulting draining of nitrogen took part in the forming process and the maintaining of Titan's atmosphere.

The Huygens-Cassini probe is going to be used in order to understand how Saturn, its rings and its satellites were formed (see Cassini-Huygens).

IV. Composition

1. Atmosphere


Titan under the veil of mystery

Planet Earth

Nitrogen     Argon   Methane Hydrogen  Oxygen  Water

2.      Air

Textfeld:  Saturn's biggest satellite's atmosphere is in many respects original : On the one hand, it is a bit similar to Earth because it is mainly made of nitrogen, and its surface pressure is around 1.5 bar. But on the other hand the surface temperature is only - 170°C, due to its distance from the sun. A lot of the minor elements that make up Titan's atmosphere are methane and nitrogen compounds methane, ethane, ethylene but also HCN (hydrocyanic acid) and other nitriles. The existence of this complex organic chemistry that associates carbonaceous compounds and nitriles in a cold environment, is one of a kind in our solar system. The Cassini mission's main goal is to study Titan's environmental features by analyzing Titan's thick, cloudy layers, its composition and its surface's present state. How can methan, which is usually photodistintegrated inhigh atmosphere, be regenerated ? This phenomenon remains unexplained. Many theories have been put forward, such as methane rains accumulated in the lakes, ponds or in these sorts of mineral sponges called " clathrates "… But no actual evidence was found. Even though the pictures taken from telescopes seem to show methane clouds in Titan's troposphere, only a direct imagery of these clouds could allow the theory of these methane clouds - which is a bit similar to our water cycle - to be validated.

3.      Soil

The surface pressure on Titan is peculiarly high for such a small world : 1.5 bar, compared with only 1 bar on Earth. The temperature is extremely low because the sun rays cannot reach the satellite's surface : About -180°C. Imagining an actual landing on Titan's soil would be difficult because of the thick clouds surrounding it. If ever this temperature was to be found on earth, everything would be frozen. On Titan, there is no oxygen, but there could be methane, ethane and nitrogen where the water, or rather the ice, is buried in the soil, for these gasses can be liquid. Because of high pressure and low temperature on Titan's surface, these 3 gasses, that can hardly be kept at a liquid stateon Earth, are kept close to their condensation state, therefore they con coexist in either of these three states : solid, liquid, gaseous.

Real observations of this satellite's surface are uncommon, but do exist. On the one hand, nothing can be seen through the ultraviolet rays, neither from Earth nor from space. On the other hand, radar and spectral probes as well as images produced by infrared rays ans photometric measures have always brought new elements in debates and speculations. That is why some theoretical models can be found about physical conditions, such as Titan's mass, density, chemical composition, temperature, pressure etc. For instance, condensed ethane (which is quite similar to methane) can be found at a liquid state on Titan's surface, hence the suggestion that the satellite would be covered by oceans of ethane and methane.

Textfeld:  Nevertheless, the observations of the satellite, either from space (thanks to the Hubble telescope) or from Earth (thanks to new adaptable optical devices that allow astronomers to see through Titan's atmospheric turbulence), that have been made since 1994, managed to break through Titan's clouds. They were designed to find " windows " by sending specific frequencies similar to infrared rays. These windows are areas in which methane is less absorbed, and are a means to sound the lower part of the satellite's atmosphere - but also its surface. These pictures come as a confirmation to the 1994 spectral observations, which showed that most of Titan's soil was bright with some dark stripes. Its surface must therefore be heterogeneous, and cannot be totally covered with hydrocarbonaceous oceans. They also prove that there are rocks on Titan's soil, and also a localised lake made of hydrocarbons. There is also an uneven layer of methane clouds in the lower troposphere. The most recent image that was taken of Titan's surface is somewhat thrilling : Titan may be mainly covered with ice and organic sediments, but its morphology could be suitable for an icy relief with, perhaps, mountains or craters covered with methane snow, and whose edges, attacked by the waves, have created the numerous lakes of hydrocarbons…

The ambitious Cassini-Huygens mission was successfully launched from Cape Canaveral on October 15., 1997. In year 2004, we will finally reach that distant world. Thanks to this mission, the astronomers will find out the true nature of this small, hibernating moon and especially of its soil from which Staurn can hardly be seen.

While the Cassini orbiter, which was developed by the NASA, is achieving its scrupulous, four year long inspection of Titan through up-to-date digital software, the Huygens probe, which was built by the ESA (European Space Agency), will break through Titan's atmosphere and will try to land on its surface. These experiments should issue exciting results about what could become the great space mission for the upcoming years. In the meantime, there is no harm in dreaming of sci-fi landscapes, that would depict this cold, dark world that could soon become ours.

V. Cassini-Huygens

1. Introduction

On Sunday, November 12. 1980, Voyager I flew over Saturn, its rings and its 18 satellites… Eighteen hours and five minutes before the planet arrived, the robot came near an unidentified object. The Earth-controlled probed turned to it and Titan, Saturn's satellite, appeared in front of the measuring instruments…

The fog surrounding Titan puzzled all the mission's investigators. But after a single day of calculations, the researchers' perplexity gave way to amazement, because Titan's atmosphere is made up of 90 % of nitrogen, but also of hydrocarbons and nitrogen compounds such as hydrocyanic acid, cyanogen and cyanacetylene. Such a number of different elements could be Textfeld:     Cassini-Huygens
a could criterium for living conditions.

The pictures taken during the Voyagers I and II missions only show a thck, impenetrable, orange-coloured mist covering Titan's soil (Voyager II had been launched 9 months after Voyager I but had been 100 times as far). In 1988, the ESA decided to build the Huygens probe in order to penetrate the mystery of a possible " pre-biotic formation " on Titan's soil.



2.      Launching of the Huygens-Cassini mission

Textfeld:  The NASA then decided to finance the Cassini project, launching a large, unoccupied probe towards Saturn in order to study this planet, its rings and satellites from the probe's orbit. The Huygens probe will then be released when reaching the vicinity of Titan. Both probes weigh 5.8 tons in total and are as big as a small lorry.

They were launched on October 13., 1997 from Cape Kennedy Center in Florida, by the American rocket Titan IV - Centaur (whose name is actually mere coincidence).

The total cost of the project, including the cost of the flight and of the ground staff until 2008, has been estimated to 120 billion Francs (3 billion Euros). The European share it this cost is of about 15 billion Francs (0.38 billion Euros).

3.      What is happening during the Huygens-Cassini mission ?

Cassini will take a roundabout way : Titan IV is not powerful enough to throw a huge machine as Cassini directly into Saturn's sphere of influence, that is why il will follow a complex route by using the own gravity of several planets.

On October 23. 1997, space navigation and trans mission systems were successfully checked by the space centre for ESA's operations. Huygens' descent upon Titan should take place on November 27. 2004.

On April 26. 1998, the probe flew over Venus for the first time, at an height of 284 km from the planet's soil. The probe began then to fly at a speed of 39 km/sec.

On December 3. 1998, the first correction on the probe's flight path was made .

On January 21. 1999, all measuring instruments were inspected. They still worked.

On June 22. 1999, the probe flew over Venus for the second time, but at a height of 600 km this time. A few scientific observations were made on this occasion.

In July 1999, according to astronomers from the Lawrence Livermore National Laboratory in California, oceans of hydrocarbons and rocky-icy continents were seen on Titan's surface.

On August 18. 1999, the probe flew over Planet Earth and measured its magnetical field after taking pictures of the Moon. American anti-nuclear associations protested against it, finding it dangerous that the probe should fly over the Earth, because of the possibility of a crash (but they didn't say anything about the 32 kg plutonium contained in the probe).

On April 15. 2000, Cassisni was heading for Saturn, after safely breaking through the asteroid belt between Mars and Jupiter. The probe passed at 1.5 million km from the 2685 Masursky Asteroid, a very tiny object, and managed to measure its size and albedo.

In September 2000, the probe's data transmission stopped functioning.

In October 2000, a test was made to check Cassini's camera system. It was successful.

In December 2000, Cassini-Huygens swang by Jupiter at a distance of 9.72 million km. It sent magnificent high-resolution images of Jupiter to Earth.

In July 2001, Cassini shifted its course after a major problem in Huygens' data recovery system was discovered.

In April 2002, the main engine's ignition corrected the probe's route during 9.8 sec while Cassini Huygens was 3 million km away from Earth. This operation, which is the thirteenth to be achieved since the rocket was launched, allowed the navigators to practise a manoeuver that is going to become common as soon as the probe is in orbit around Saturn, since they will have to switch on the engine every five days.

Cassini should theoretically work until 2008, maybe even longer. During the last four years spent in the vicinity of Saturn, it will achieve almost 60 revolutions around the planet, amongst which 33 will allow it to fly over Titan.

On November 27. 2004, the Huygens probe will be dropped by parachute  into Titan's thick atmosphere. During its descent, it will take pictures whick hopefully will show something else than a foggy cloud. It will also measure the temperature, the atmospheric pressure and the speed of the wind. The probe will also search for possible lightnings and will carry out a chemical analysis of the atmosphere's composition. All these results will be sent by radio to the Cassini ship that will send them to Earth afterwards. Nobody can tell what Huygens will come across at the end of this long descent : the soil could be solid, or liquid. The probe could land on a rocky floor but could also dive into a methane or ethane ocean, or in a thick layer of " chemical humus ".

Textfeld:  Huygens' designers tried to take each possibility into account. The device is robust enough to survive to a landing on a rocky soil, but also light enough to float if it falls into one of these oceans. It was designed to resist the wind's - and even the lightnings' - attacks. If there is no light on Titan, a powerful lamp was provided. Huygens is equipped with an instrument that can measure the soil's hardness, and another one can measure the waves' impacts in the sea. In the case that there would be some noise on Titan, a microphone has also been provided. The Huygens team hopes that the probe will " survive " during at least half an hour after its landing, so it will be possible to get the data from it.

Cassini and Huygens will not only bring scientific instruments. Thousands of signatures and messages from all over the world are also travelling through the universe inside the probe. The NASA and the ESA have given the possibility to send their signature into space for free to anybody who would like to. The messages are stocked as electronic data on two compact discs.

On the European CD carried by the Huygens probe, the following message can for instance be found :

" I"m a French Earthman (1m83), looking for a tall, attractive and preferably romantic Martian Girl. "

VI.             Earth and Titan ?

Textfeld:  With a size half of that of the Earth's, Titan as challenged from its position as the largestsatellite in our solar system by Ganymede, Jupiter's biggest satellite, round by Voyager to possess a few kilometers more in diameter than Saturn's champion. The latter is, however, still the ninth largest abject around our star (by Car bigger than the Moon and Pluto and barely bigger than Mercury), tenth by its mass (much lighter than Mercury: 140 billions of billions tons for the satellite against 330 billions of billions tons for the small planet). The satellite's mass, made of a mixture of water ice and rock (with the ice apparently containing even greater amounts of frozen, trapped and adsorbed gases than the other icy satellites, as the distance from the Sun increases), gives Titan a mean density of about 1.90.



Orbital Parameters

Semi major axis

149.6 million Km

1.221 million Km

Perihelion & periastron

147.1 million Km

1.186 million Km

Aphelion & apastron

152.1 million Km

1.257 million Km

Sidereal period (Revolution)

365.242 earth days (a rotation around the sun)

15.945 earth days (a rotation around Saturn)

29.46 earth years (a rotation around the sun)

Mean orbital speed

29.79 Km/sec

5.6 Km/sec

Orbit inclination

0.00 degrees

0.33 degrees

Sidereal period (Rotation)

   0.9972 earth days

  23.9345 earth hours

15.945 earth days

Physical Properties


5.9736 x10    Kg

1.34 x10 ²³ Kg

Mean radius

6371 Km

2575 Km

Mean density

5520 Kg/ m³

1880 Kg/ m³

Surface gravity

+/- 9.7 m/sec²

1.35 m/sec²

Escape velocity

11.2 Km/sec

2.6 Km/sec

Visual geometric albedo



Visual magnitude

- 3.86



VII.         Conclusion

Titan is one of the more interesting places in the solar system. What kind of landscape lies below the layers of clouds? What mysteries are held beneath these orange curtains? These questions will have to wait until future spacecraft are launched to visit this unusual moon. In October 1997, the Cassini spacecraft was launched for a rendezvous with Saturn in June 2004. Later in 2004, it will release the European-built Huygens probe for a descent through Titan's atmosphere. Cassini will have more than 30 encounters with Titan, mapping the moon's surface with a synthetic aperature radar similar to the one Magellan used to map Venus.

1.      Open Issues

- Are there liquids on the surface?

- Is the interior still hot?

- Why does Titan have a dense atmosphere while the other large moons do not?

- There are a lot of organic compounds and a potentially liquid environment on Titan. It is extremely cold for life, but could it be possible? Titan is one of the best possibilities. In any case, it's interesting as a comparison with the early environment on Earth.

- The Cassini radar mapper and Huygens probe will provide the hard data so frustratingly lacking on this enigmatic moon.

VIII.      Bibliography

- "Atlas du Système Solaire" 1.0, cd-rom, Asyd Multimedia, décembre 1999

- "Cassini/Huygens : A Mission to the Saturnian Systems", The International Society of Photo-Optical Instrumentation Engineers, 5-6 august 1996, Denver, Colorado.

- "Cassini-Huygens : Cap sur Titan et Saturne", http://www.astrobale.com/

- "Chimie prébiotique : expérience de simulation en laboratoire et 'vérité terrain' "L'Environnement de la Terre Primitive, Chapter 15, François Raulin, Presse Universitaire de Bordeau.

- "Contraintes sur la Formation de Titan à partir d'un Modèle Turbulent de la Subnébuleuse de Saturne", Ouliver Mousis and Daniel Gautier, http://www_lpg.obs.ujf-grenoble.fr/base_documentaire/seminaire220501.pdf

- Encyclopedia of Astronomy and Astrophysics, Paul Murdin, Particle Physics and Astronomy Research Council, British National Space Center, Vol. 4 p.3319-3326, 2001

- ESA, http://www.esa.int

- La Recherche #103, october 1997

- NASA, http://www.nasa.gov

- "Satellite formation around gas giant planets", R. M. Camps and Wm. R. Ward, Southwest Research Institute, http:// www.aas.org/publications/baas/v34n3/dda2002/38.htm

-"Saturne et Titan", Space Connection #31, p. 15-17, April 2000

- Science et Vie hors-série #196, september 1996

- "Sous le Voile de Tian", Daniel Gautier and François Raulin, La Recherche #302, p.42-47 october 1997

- "Titan", http://www.lisa.univ-paris12.fr/GPCOS/Pc/Titan.htm

- "Titan, the Earth-like Moon", Athéna Coutensis and Frederic Taylor, Oxford University.

- "Titan's Organic Chemistry and Exobiology, François Raulin, ESA SP 1177, 1997

- "The Aerosol Collector Pyrolyser (ACP) Experiment for Huygens", ESA SP1177, 1997

- "The Cassini Mission to Saturn", Office of Space and Defense Power systems, match 1996, http://nuclear.gov/space/ spacemission.html

- "The Descent Imager/Spectral Radiometer (DISR) Aboard Huygens", ESA SP1177, 1997

- "The Gas Chromatograph Mass Spectometer Aboard Huygens", ESA SP1177, 1997

- "The Huygens Probe : Science, Payload and Mission Overview", J.-P. Lebreton and D. L. Matson, ESA SP 1177, 1997

- "The Huygens Probe System Design", J. C. Jones and F. Giovagnoli, ESA SP1177. 1997