First of Many Returns

Abstract

### Contents #### Viewpoint Intensive Titan Exploration Begins P. R. Mahaffy #### Reports Cassini Radar Views the Surface of Titan C. Elachi et al. Titan's Atmospheric Temperatures, Winds, and Composition F. M. Flasar et al. The Cassini UVIS Stellar Probe of the Titan Atmosphere D. E. Shemansky et al. [Ion Neutral Mass Spectrometer Results from the First Flyby of Titan][1] J. H. Waite Jr. et al. Cassini Measurements of Cold Plasma in the Ionosphere of Titan J.-E. Wahlund et al. Energetic Neutral Atom Emissions from Titan Interaction with Saturn's Magnetosphere D. G. Mitchell et al. [Titan's Magnetic Field Signature During the First Cassini Encounter][2] H. Backes et al. T itan, Saturn's largest moon, and Earth are the only two bodies in our solar system with nitrogen-rich atmospheres. Although Earth's atmosphere has evolved partly under the influence of life (and includes carbon dioxide as the main carbon species), Titan's remains more primitive, reducing, and rich in methane, and may resemble that hypothesized for the early Earth. Our first close view of Titan came from Pioneer and the two Voyager spacecraft, which flew by in the late 1970s and early 1980s. These spacecraft provided a taste of Titan's atmosphere and its structure, determined Titan's size and density (a mixture of rock and organic ices), and showed that the moon lacked a significant magnetic field, although it is affected greatly by its orbit in and out of Saturn's magnetic field. Detailed study of Titan, however, has been limited because its atmosphere is so thick that, like those of Venus, Jupiter, and Saturn, it is difficult to see the surface beneath the haze. Cassini is equipped with a variety of instruments to both sample the haze and reveal the surface beneath it, and observations of Titan were a primary goal of the mission. Cassini also carried and released the Huygens probe, which successfully sampled Titan's atmosphere and landed on its solid surface late last year. The papers in this issue, including a summary by Mahaffy (p. 969), describe results from the first two passes of Cassini by Titan, including through its uppermost atmosphere. To see through the haze, Cassini carries a radar mapper that with each pass will build up a map of Titan's surface. The initial flyby has already revealed much (see the cover); as discussed by Elachi et al. (p. 970), Titan apparently has a relatively young flat surface with only a few large and degraded impact craters, but with a striking variation in the deposits. Together these features seem to indicate that there has been active resurfacing, likely through some form of ice volcanism. ![Figure][3] CREDIT: NASA/JET PROPULSION LABORATORY/U.S. GEOLOGICAL SURVEY A variety of other spectrometers on Cassini sampled and viewed Titan's atmosphere, providing new information on its composition and structure and showing some parallels with Earth's atmosphere [see Waite et al. (p. 982), Shemansky et al. (p. 978), and Flasar et al. (p. [975][4])]. Methane is abundant close to the surface, and Cassini has revealed several complex hydrocarbon species higher in the atmosphere. A tantalizing result is that although radiogenic argon is present (probably produced by outgassing of the moon's interior), the other argon isotopes that should have been present in an initial atmosphere are absent. Cassini also measured the effects of Titan's passage through Saturn's magnetic field [see Wahlund et al. (p. [986][5]), Mitchell et al. (p. 989), and Backes et al. (p. 992)]. It found and passed through a magnetic ionopause in Titan's atmosphere, below which the field strength dropped to zero, confirming Voyager's observation that Titan lacks a magnetic field. Cassini is planning to revisit Titan more than 40 times during its mission, repeating many of these analyses and mapping different parts of the moon's surface. Together with the Huygens data, these data should help us understand the origin and evolution of Saturn's largest moon and, by analogy, Earth's early history. [1]: /lookup/doi/10.1126/science.1110652 [2]: /lookup/doi/10.1126/science.1109763 [3]: pending:yes [4]: /lookup/doi/10.1126/science.1111150 [5]: /lookup/doi/10.1126/science.1109807