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Credit: NASA/JPL/University of Michigan

Caption: This data is from Cassini's ion and neutral mass spectrometer,
which detects charged and neutral particles in the atmosphere


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Dec 03, 2004
Titan - The Enigma (2)

Another major enigma surrounding Titan is its atmosphere. Titan's atmosphere is believed by many scientists to be similar to Earth's early atmosphere billions of years ago. Toby Owens, principal scientist at the Jet Propulsion Laboratory, said: "What we've got is a very primitive atmosphere that has been preserved for 4.6 billion years. Titan gives us the chance for cosmic time travel . . . going back to the very earliest days of Earth when it had a similar atmosphere."

The graph above shows that the proportion of heavy nitrogen-15 in the atmosphere of Titan is much greater than that around other planets. Scientists believe that the lighter nitrogen-14 was lost over large geologic times scales for reasons that remain unknown. It could be explained if most of the atmosphere had evaporated into space, a process in which the nitrogen-14 would have escaped more easily than nitrogen-15. But it would mean that Titan once had an atmosphere 40 times as thick as Earth's - making it a dwarf version of a gas planet. 'This bizarre world may be far more complex that we have begun to imagine,' says Larry Soderblom of the US Geological Survey in Flagstaff, Arizona.

The striking disparity in nitrogen isotopes is telling us something about the way planetary atmospheres are formed rather than how they evolve. And why do we insist that a star's "children" all be born at the same time? Titan's atmosphere is primitive, but not in the sense that it is 4.6 billion years old or that it was once 40 times as thick as Earth's. Instead, there has not been time for young Titan to lose much atmosphere. Hannes Alfvén wrote in Evolution of the Solar System (NASA SP-345, 1976), "..the Laplacian concept of a homogeneous gas disc provides the general background for most current speculations. The advent of magneto-hydrodynamics about 25 years ago and experimental and theoretical progress in solar and magnetospheric physics have made this concept obsolete but this seems not yet to be fully understood."

While acknowledging Alfvén's point, it is possible to go a step further and invoke the electrical behavior of plasma, not just its magnetic behavior. The electrical model of planet birth proposes that planets are born by electrical expulsion of some of the matter of a star or gas giant in a tremendous "flare." The rings we see around the gas giant planets are evidence of former episodes of expulsion, not accretion. The rings of Saturn are the most recent. It is important to note that flaring red dwarf stars are extremely common and are an unexplained phenomenon. Red dwarf flares are like a stellar lightning flash but they may produce 10,000 times as many x-rays as a comparable flare on the Sun.

The electric discharge model would have profound effects on the new planet's atmosphere, including that of a new moons like Titan. The primary effect comes from the source and depth of the ejection from the flaring parent dwarf star or gas giant, which determines the initial bulk composition of the atmospheric components.

Chemical elements are then sorted in the plasma discharge according to their critical ionization velocity. Also isotopes will separate in the combined electric and magnetic fields of the discharge. Lastly, the plasma gun effect (seen now ejecting material from Io into space) is known from laboratory tests to be a copious source of neutrons. The neutrons may be captured to form heavy isotopes (such as nitrogen-14 to nitrogen-15) and short-lived radioactive species - we find evidence of that in some meteorites that are also formed in this birth process.

The combination of all of these effects suggest that it would be unlikely for any two bodies in the same "family" to have the same initial atmospheres. Subsequent electrical interactions between planets and moons would serve to transfer surface materials and atmospheres, transmute elements, and further complicate the picture. That fits generally with the irregular elemental and isotopic signatures found in the atmospheres of our planetary system.

There is another mechanism that could contribute to the lack of nitrogen-14 in Titan's atmosphere. Nitrogen-14 can capture an electron from the discharge to become carbon-14. Carbon-14 decays by very weak beta decay back to nitrogen-14, with a half-life of approximately 5,730 years. If the age of Titan's atmosphere can be measured in thousands of years instead of billions, then a significant amount of nitrogen-14 may still be locked up on the surface as carbon-14.

To suggest that "Titan once had an atmosphere 40 times as thick as Earth's - making it a dwarf version of a gas planet," only complicates the plainly impossible standard model of formation of the solar system. It does not explain why other large moons do not have substantial residual atmospheres. It seems far more plausible to suggest that Titan is a much newer moon than Jupiter's Ganymede or Callisto. Titan simply hasn't had time to lose its atmosphere - just as Saturn hasn't had time to lose its rings following its last discharge.

And what about Venus with its hot and heavy atmosphere?

For more information about the Cassini-Huygens mission, visit and the Cassini imaging team home page, .




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