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Cassini spacecraft Flies Past Mysterious Titan Again.

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Cassini Spacecraft.
NASA's Cassini spacecraft swept past Titan.

Cassini spacecraft swept past Titan for the second time yesterday, this time on a more direct course, and passed by 1,200 kilometres above the surface. Once again the spacecraft's cameras took hundreds of images as Cassini drew closer to Titan, revealing the same surface features now associated with the landing site of the Huygens probe that is set to decend to Titan's surface in mid January.

With a diameter of 5,150 kilometres, Titan is the largest of Saturn’s family of moons; it's even bigger than the planets Mercury or Pluto. It has an atmosphere of orange-yellow smog composed mostly of nitrogen with an abundance of hydrocarbon organic compounds including methane; although, it seems to have very few clouds. On October 26, Cassini passed close to Titan revealing a first glimpse of the moon's strange surface. It discovered a rugged yet level landscape with few craters, meaning that the planet must be geologically active. Mysterious oily flows of cryogenic ice ooze across the surface. Planetary scientists have been thrilled by the results so far.

Titan is cold. Its surface temperature is -180 C - way too cold for liquid water, yet it's close to the triple point of methane, where this hydrocarbon gas can exist in all three physical states at its surface: solid ice, liquid or gaseous.

Cassini turned its Ultraviolet Imaging Spectrograph (UVIS) towards the star Spica (Alpha Virginis), then Lambda Scorpi, and for the next 8 hours observed the stars as they were obscured by Titan's atmosphere. This sensitive instrument is different from other types of spectrometers because it can take both spectral and spatial readings. It's particularly adept at determining the composition of gases. Spatial observations take a wide-by-narrow view, only one pixel tall and 60 pixels across. The spectral dimension is 1,024 pixels per spatial pixel. Additionally, it's capable of taking so many images that it can create movies to show the ways in which this material is moved around by other forces. This provided a vertical profile of the main constituents of the atmospheric layers that have a similar temperature profile to the Earth.

Close approach occurred before Cassini passed up through Saturn's ring plane, and returned some of the best close up images of the ring system to date. Then Cassini began using its radar to map part of Titan's surface terrain at a small solar phase angle. The experiment was looking for signs of hot spots on the moon's surface that would indicate the presence of active cryo-volcanoes, and even lighting in Titan's atmosphere.

The 2.6-meter Huygens lander probe will separate from its mother ship on Christmas Eve, travelling towards Titan and entering the moon's atmosphere on 14 January. Much of Huygens's science will take place during its atmospheric decent, which will be relayed to Cassini, and then transmitted back to Earth's waiting scientists and the media. If Huygens actually lands successfully on Titan, it'll be a major bonus for the mission.

Huygens will be attempting to determine the origin of Titan's molecular nitrogen atmosphere. Planetary scientists want to answer the question: "Is Titan's atmosphere primordial (accumulated as Titan formed) or was it originally accreted as ammonia, which subsequently broke down to form nitrogen and hydrogen?"

If nitrogen from the Solar nebula (out of which our solar system formed) was the source of nitrogen on Titan, then the ratio of argon to nitrogen in the Solar nebula should be preserved. Such a finding would mean that we have truly found a sample of the "original" planetary atmospheres of our Solar System

Huygens will also try to detect lightning on Titan. The extensive atmosphere of Titan may host Earth-like electrical storms and lightning. Although no evidence of lightning on Titan has been observed so far, the Cassini Huygens mission provides the opportunity to determine whether such lightning exists. In addition to the visual search for lightning, the study of plasma waves in the vicinity of Titan may offer another method. Lightning discharges a broad band of electromagnetic emission, part of which can propagate along magnetic field lines as whistler-mode emission.

By Science Correspondent Richard Pearson

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