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Titan (moon)



Main articles: Cassini–Huygens and Huygens probe

Even with the data provided by the Voyagers, Titan remained a body of mystery—a planet-like satellite shrouded in an atmosphere that makes detailed observation difficult. The intrigue that had surrounded Titan since the 17th-century observations of Christiaan Huygens and Giovanni Cassini was finally gratified by the spacecraft named in their honor.

The Cassini–Huygens spacecraft reached Saturn on July 1, 2004 and has begun the process of mapping Titan's surface by radar. A joint project of the European Space Agency (ESA) and NASA, Cassini–Huygens, has proved a very successful mission. The Cassini probe flew by Titan on October 26, 2004 and took the highest-resolution images ever of the moon's surface, at only 1,200 km, discerning patches of light and dark that would be invisible to the human eye from the Earth. Huygens landed on Titan on January 14, 2005, discovering that many of the moon's surface features seem to have been formed by flowing fluids at some point in the past.[79] On July 22, 2006, Cassini made the first of a series of 21 planned, targeted, close fly-bys, each at only 950 km from Titan; the last is scheduled for May 12, 2008.[80] Present liquid on the surface may have been found near the north pole, in the form of many lakes that were recently discovered by Cassini.[42] Titan is the most distant body from Earth that has seen a space probe landing.[81] Titan is also the second moon in the solar system to have a man-made object land on its surface.

Huygens landing site

Huygens image from Titan's surface
Huygens image from Titan's surface

The Huygens probe landed just off the easternmost tip of a bright region now called Adiri, where it photographed pale hills with dark "rivers" running down to a dark plain. Current understanding is that the hills (also referred to as highlands) are composed mainly of water ice. Dark organic compounds, created in the upper atmosphere by the ultraviolet radiation of the Sun, may rain from Titan's atmosphere. They are washed down the hills with the methane rain and are deposited on the plains over geological time scales.[52]

After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.[52] The two rocks just below the middle of the image on the left are smaller than they may appear: the left-hand one is 15 centimeters across, and the one in the center is 4 centimeters across, at a distance of about 85 centimeters from Huygens. There is evidence of erosion at the base of the rocks, indicating possible fluvial activity. The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. It is believed that the "soil" visible in the images is precipitation from the hydrocarbon haze above.

In March 2007, NASA, ESA, and COSPAR decided to name the Huygens landing site the Hubert Curien Memorial Station in memory of the former president of the ESA.[82]

Prebiotic conditions and possible life

See also: Planetary habitability

Scientists believe that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan. Many hypotheses have developed that attempt to bridge the step from chemical to biological evolution. The Miller-Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane, forming hydrocyan and ethyne. Further reactions have been studied extensively.[83]

All of these experiments have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. While the analogy assumes the presence of liquid water for longer periods than is currently observable, several theories suggest that liquid water from an impact could be preserved under a frozen isolation layer.[84] It has also been observed that liquid ammonia oceans could exist deep below the surface;[9][85] one model suggests an ammonia–water solution as much as 200 km deep beneath a water ice crust, conditions that, "while extreme by terrestrial standards, are such that life could indeed survive".[10] Heat transfer between the interior and upper layers would be critical in sustaining any sub-surface oceanic life.[9]

Detection of microbial life on Titan would depend on its biogenic effects. That the atmospheric methane and nitrogen are of biological origin has been examined, for example.[10] Hydrogen has been cited as one molecule suitable to test for life on Titan: if methanogenic life is consuming atmospheric hydrogen in sufficient volume, it will have a measurable effect on the mixing ratio in the troposphere.[86]

Despite these biological possibilities, there are formidable obstacles to life on Titan, and any analogy to Earth is inexact. At a vast distance from the Sun, Titan is frigid (a fact exacerbated by the anti-greenhouse effect of its cloud cover), and its atmosphere lacks CO2. Given these difficulties, the topic of life on Titan may be best described as an experiment for examining theories on conditions necessary prior to flourishing life on Earth.[87] While life itself may not exist, the prebiotic conditions of the Titanian environment, and the possible presence of organic chemistry, remain of great interest in understanding the early history of the terrestrial biosphere.[88] Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiment, and chemical and photochemical modelling on Earth.[83]

An alternate explanation for life's hypothetical existence on Titan has been proposed: if life were to be found on Titan, it would be statistically more likely to have originated from Earth than to have appeared independently, a process known as panspermia. It is theorized that large asteroid and cometary impacts on Earth's surface have caused hundreds of millions of fragments of microbe-laden rock to escape Earth's gravity. Calculations indicate that a number of these would encounter many of the bodies in the solar system, including Titan.[89][90]

Conditions on Titan could become far more habitable in future. Six billion years from now, as the Sun becomes a red giant, surface temperatures could rise to ~200K, high enough for stable oceans of water/ammonia mixture to exist on the surface. As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will deplete, lessening the anti-greenhouse effect on the surface and enabling the greenhouse created by atmospheric methane to play a far greater role. These conditions together could create an environment agreeable to exotic forms of life, and will subsist for several hundred million years, long enough for at least primitive life to form.[91]

While the Cassini–Huygens mission was not equipped to provide evidence for biology or complex organics, it did support the theory of an environment on Titan that is similar, in some ways, to that of the primordial Earth.[88]

There are a wide range of options for future missions to Titan that might address these and other questions,[92] including orbiters, landers, balloons etc.

See also

Solar System Portal

References

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Further reading

  • Lorenz, Ralph; Jacqueline Mitton (May 2002). Lifting Titan's Veil: Exploring the Giant Moon of Saturn. Cambridge University Press. ISBN 0-521-79348-3. 

External links

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v • d • e
Moons of Saturn
Generally listed in increasing distance from Saturn. Temporary names in italics.
Ring shepherds
Co-orbitals
Inner large (and Trojan)
Mimas · Methone · Anthe · Pallene · Enceladus · Tethys (Telesto • Calypso) · Dione (Helene • Polydeuces)
Outer large
Rhea (rings) · Titan · Hyperion · Iapetus
Inuit group
Norse group
Gallic group
Rings of Saturn · Cassini-Huygens · Themis · Chiron
v • d • e
Natural satellites of the Solar System
Planetary satellites Rhea, Saturn's second-largest moon
Other satellite systems
Largest satellites
Ganymede · Titan · Callisto · Io · Moon · Europa · Triton
Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys  · Enceladus · Miranda · Proteus · Mimas
Inner satellites · Trojans · Irregulars · List · List by diameter · Timeline of discovery · Naming




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