37th DPS Meeting, 4-9 September 2005
Session 6 Cassini I
Invited, Monday, September 5, 2005, 2:00-3:50pm, Music Concert Hall

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[6.07] INMS Titan Observations

J.H. Waite, Jr. (UM), H. Niemann (NASA GSFC), R.V. Yelle (UA), W.T. Kasprzak (NASA GSFC), T.E. Cravens (KU), J.G. Luhmann (UC Berkeley), R.L. McNutt (APL), W.-H. Ip (National Central Univ. Taiwan), V. De La Haye (UM), S. Ledvina (UC Berkeley), I. Mueller-Wordarg (Imperial College, London), N. Borggren (UA)

The Ion Neutral Mass Spectrometer (INMS) aboard the Cassini Orbiter has obtained the first in situ composition measurements of the neutral densities of molecular nitrogen, methane, hydrogen, argon, and a host of stable carbon-nitrile ion and neutral compounds in the first and sixth flybys of Titan. The bulk composition and thermal structure of the moon's upper atmosphere appear to vary with latitude and local time. The new data set provides strong evidence for atmospheric waves in the upper atmosphere and for the existence of a warm, chemically complex corona. Furthermore, the data set provides direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atmosphere.

The atmosphere likely formed from outgassing as planetesimals composed of silicates, water ice, clathrates of methane, and ammonia hydrates coalesced. Subsequent photochemistry and/or shock-induced chemistry likely converted the atmospheric nitrogen into molecular nitrogen, which is inferred by the absence (<0.6 ppm) of 36Ar in the INMS data. (Ice clathrate delivery of N2 would have presumably also delivered 36Ar to the proto Titan.) The decrease of the 14N to 15N isotopic ratio with respect to the terrestrial value allows us to suggest an early atmosphere >1.5 to 100 times more substantial that was lost via escape over the intervening 4.5 billion years. Carbon in the form of methane has continued to outgas over time from the interior (as inferred from the elevated 12C to 13C ratio as compared to terrestrial values) with much of its subsequent photolysis products being deposited in the form of complex hydrocarbons on the surface (~5 x 1027 s-1 as estimated from the H2 escape rate of 6.1 ± 0.2 x 109 cm-2 s-1 measured by INMS).

This talk will highlight the composition, vertical structure, wave processes, and escape of Titan's atmosphere.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.