Imagine an alien world where hydrocarbons reign (or ‘rain’) instead of water.  Here on Earth, vast quantities of water are driven in a powerful cycle of evaporation/transpiration, condensation, and precipitation.  The Earth is home to deep oceans covering most of its surface, raging rivers carving grand canyons, towering cloud systems, churning hurricanes, massive glaciers, gentle rains, and ultimately, the cradle and sustenance of life itself.  In our hypothesized hydrocarbon world, there could be oceans of ethane and methane that evaporate these compounds and generate hydrocarbon clouds and gasoline-like rains.  Perhaps there are oddly colored hydrocarbon rivers, lakes, and icebergs.  Solar radiation striking the upper atmosphere may convert some of the hydrocarbons into a smog-like haze.  Sunlight may even convert some of the hydrocarbons into an organic precipitate that falls as a snow back to the surface of this strange world.  Here on Earth, life seems to have found a niche in every conceivable nook.  Could life, even if only expressed as microbes, develop and survive on a hydrocarbon world?  Now imagine that such a world not only exists, but is one of our cosmic neighbors (astronomically speaking).  How frustrating would it be if these oddities were hidden from view by an opaque cloud cover?

More than mere speculation, such a place is believed to exist in our solar system.  This world has been named Titan as it is by far the largest of Saturn’s 31 moons.  Titan has been a long time curiosity to astronomers and now, finally, we are set to conduct some significant up-close explorations.  In 1997, NASA and the European Space Agency (ESA) launched the Cassini spacecraft, which is due to arrive at the Saturnian system within the next few months. The latest reports indicate that all is on course to begin an unprecedented 4-year survey of the famously ringed planet and its moons.  The Cassini spacecraft also carries with it a probe dubbed Huygens that it will drop to the surface of Titan.  Although Saturn is the centerpiece, Titan sits close to the heart of the Cassini mission.  And it is to that enigmatic moon that my attention is drawn.

It has been nearly 350 years since the discovery of Titan and astronomers still know little of its surface due its thick atmosphere that is opaque to visible light.  A Titanian atmosphere is intriguing in its own right given that Titan is the only moon in the entire solar system that possesses a dense atmosphere (60% denser than even the Earth’s atmosphere).  Titan is not only the largest moon of Saturn, but it is also the second largest moon in the entire solar system, second only to Jupiter’s moon Ganymede.  Titan is larger than both the planets Pluto and Mercury and its diameter is about 40% of the Earth’s.  Finally, in 1980 and 1981, NASA’s Voyager 1 and Voyager 2 spacecrafts gave astronomers their first close-up view of Titan.

The twin Voyagers were one of NASA’s most successful planetary exploration missions.  Taking advantage of a favorable positioning of the planets, NASA launched the Voyagers in hopes of exploring at least the Jupiter and Saturn systems.  Both Voyagers succeeded in this mission and Voyager 1 was essentially sacrificed from further planetary exploration in order to satisfy the desire to learn more about Titan.  A close fly-by of Titan enabled Voyager 1 to capture detailed images and other measurements of Titan’s atmosphere.  However, the trajectory pitched Voyager 1 up out of the plane of the solar system in which the planets reside.  Voyager 2 also examined Titan, but at a greater distance so that it could remain on its path within the plane of the solar system.  Voyager 2 become the first spacecraft to explore the outer planets Uranus and Neptune. 

The Voyager images showed Titan to be enshrouded in a thick, orange atmosphere with a slight blue haze hovering above.  Somewhat like the Earth, Titan’s atmosphere consists mainly of nitrogen; however, Voyager also detected low concentrations of a range of organic molecules such as methane, propane, ethane, acetylene, and other hydrocarbons [1].  The observed quantity of methane in the atmosphere remains unexplained given that solar radiation converts methane to ethane.  It is hoped that Cassini will be able to determine what processes on Titan replenish atmospheric methane levels.  Different hypotheses invoke either oceans or underground reservoirs containing large quantities of methane [2].  Solar radiation also converts methane into other molecules such as acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. Hydrogen cyanide is particularly interesting in that it is a building block of amino acids which, on Earth, are the building blocks of proteins that make up living organisms [3].

The atmosphere of Titan is believed to be similar to the atmospheric conditions on Earth during its early formational period.  Thus, Titan is being eyed for signs of Life or, at least, for some insight into the formation of life on Earth.  Given the extreme cold and scarcity of liquid water on Titan, it is not expected that life will be found (nor is the Cassini/Huygens mission designed to search directly for life); however, it may still be within the realm of possibility that a microbial kind of life could find a home on such a world with abundant organic content and potentially liquid chemistry at its surface.

Subsequent to the Voyagers, innovations in Earth-bound telescopes (e.g., adaptive optics, speckle interferometry [4]) and space telescopes (Hubble) were able to reveal some information about Titan’s surface.  The success of these observations was based on the use of radio and infrared waves as they can penetrate cloud covers such as on Titan.  A similar technique was used by the Magellan spacecraft in the early 1990s to map the surface of the planet Venus through its thick, global cloud cover.  The faint detections of radar waves bounced off Titan allowed astronomers to begin to see its surface.  It is the form of the reflected radio waves that suggest to astronomers that the surface of Titan may be covered by large bodies of liquid hydrocarbons.  Recent detections of mirror-like glints of such radar signals provided further evidence of large bodies of liquid hydrocarbon on Titan’s surface [5].  Even if these interpretations are correct, it still remains unclear whether Titan is covered by a hydrocarbon ocean, or if there are hydrocarbon seas amid continents of dry land (an Earth-like model), or if it is a mostly dry world spotted with a few hydrocarbon lakes. 

Partly corroborating the water cycle analogy to Earth, astronomers recently used Earth-bound telescopes to find that Titan possesses methane clouds [6].  Upon entry into Titan’s atmosphere, the Huygens probe will try to detect any thunderstorms from such cloud systems.  Given the frigid temperatures on Titan, the organic molecules in the atmosphere are expected to condensate out as a hydrocarbon rain.  It is also possible that photochemistry in the upper atmosphere converts organic molecules into a solid form that would fall to the surface, accumulate, and persist as a kind of snow.  Carl Sagan and his colleagues, who were able to recreate these conditions in their laboratory, dubbed this material “tholin” from the Greek word for ‘mud’ [7]. 

Peering into the infrared part of the spectrum, the Hubble Space Telescope recently may have detected a large "continent" amid the hydrocarbon seas, based on distinct variations in the surface brightness.  Although still not confirmed to be such a continent, the landing site for the Huygens probe is targeting a nearby “offshore” area [8].

By this time next year, after 350 years of collective waiting, astronomers should uncover a wealth of information about what lies below the cloud tops of Titan.  Even upon first arrival by Cassini in July 2004, the images of Titan will be 50 times better than those from the Hubble Space Telescope and we will be able to learn more than we did from the previous visits by the Voyager spacecrafts. Over the course of the mission, Cassini’s cloud-penetrating radar will map a portion of Titan’s surface at a resolution far greater than currently available. [1]

In December 2004, Cassini will release ESA’s 319-kilogram (703-pound) Huygens probe, which is named after the astronomer who first discovered Titan.  The Huygens probe will coast toward Titan, enter its atmosphere on January 14, 2005, and spend the next couple hours parachuting down to the surface below.  Huygens will collect real-time weather data and atmospheric composition measurements as it descends.  The information will be used to determine the mechanisms and processes (chemical, photochemical, magnetic, etc.) at work in Titan’s atmosphere [2, 9].  The Cassini orbiter will relay the data back to Earth.

Huygens is designed to land safely on either solid ground or in a splashdown in one of the hydrocarbon lakes or oceans, as hypothesized from telescopic evidence [10].  It is hoped that Huygens will still have a few minutes or hours of life left in it so that it can relay additional data about the surface upon which it landed.  Perhaps because I live near a coastline on Earth, I am hoping for a splashdown of Huygens near the coastline on Titan.  There, we would not only confirm the presence of hydrocarbons lakes or oceans, but also to obtain photos and other measurements such as density, depth, and wave action [1].  A solid landing would also be interesting with various measurements of the physical state, topography, and composition of the local surface [2, 9].  But the solar system is replete with dry worlds of rock and ice that we continue to explore and a world with liquid chemistry/dynamics presents so many new possibilities.

After Huygens’ work is done, Cassini will go on to study Saturn, its rings, and its multiple moons for what is planned to be another four years.  But Titan will remain an important piece of the overall mission, not only as a primary feature in the study, but also as a navigation aid to slingshot the Cassini spacecraft into various regions of the Saturnian system.  Cassini will be making some 30 or 40 flybys of Titan during its design mission.  Cassini will use those flybys to further study Titan’s atmosphere and map a large portion of its surface using radar to a far greater resolution than ever achieved previously.

Through my backyard telescope, Saturn has one significant companion – Titan.  Saturn’s other moons are more or less invisible to me, their dimly reflected light is washed out in the glare of city lights that drown my night sky.  Fortunately, the view is nevertheless spectacular thanks to the cumulative reflection of light from the countless bits of micron-to-meters sized bits of ice and dust that comprise Saturn’s famous rings.  These rings never fail to impress a newcomer to the eyepiece.  To be sure, I am looking forward to seeing all of the results of Cassini’s inspections of Saturn and its other moons.  But it is Titan that captivates my attention thanks to the mystery of exploring an unseen world with so many odd parallels to Earth, including the potential for furthering our understanding about the most fascinating aspect of the universe – Life.

References

[1] McEwen, Alfred S.  Journey to Saturn.  Astronomy magazine.  January 2004.

[2] Huygens Homepage – Fact Sheet.  http://sci.esa.int/science-e/www/area/index.cfm?fareaid=12

[3] http://voyager.jpl.nasa.gov/science/saturn_titan.html

[4] http://antwrp.gsfc.nasa.gov/apod/ap000820.html, http://antwrp.gsfc.nasa.gov/apod/ap990804.html

[5]  Campbell, Donald.  As reported by Blaine P. Friedlander Jr. in “CU-led team detects evidence of hydrocarbon lakes on Saturn moon” in the Cornell Chronicle on 10/9/2003. http://www.news.cornell.edu/Chronicle/03/10.9.03/Titan_hydrocarbon.html

[6] Bouchez, Antonin.  As reported by CNN on 12/19/2002. http://www.cnn.com/2002/TECH/space/12/19/titan.clouds/index.html

[7] Sagan, Carl.  Pale Blue Dot. 

[8] Arnett, Bill.  The Nine Planets.  18 Dec 2002. http://www.seds.org/billa/tnp/titan.html

[9] Cassini homepage.  http://saturn.jpl.nasa.gov/index.cfm