HR: 08:30h
AN: P21D-03    [Abstracts]
TI: Photochemical Origin of Nitrogen on Titan and Enceladus
AU: * Atreya, S K
EM: atreya@umich.edu
AF: Univ. of Michigan, Space Research Bldg., Ann Arbor, MI 48109, United States
AU: Matson, D L
EM: dennis.l.matson@jpl.nasa.gov
AF: Jet Propulsion Lab., 4800 Oak Grove Dr., Pasadena, CA 91109, United States
AU: Castillo-Rogez, J
EM: jccastil@jpl.nasa.gov
AF: Jet Propulsion Lab., 4800 Oak Grove Dr., Pasadena, CA 91109, United States
AU: Johnson, T V
EM: torrence.v.johnson@jpl.nasa.gov
AF: Jet Propulsion Lab., 4800 Oak Grove Dr., Pasadena, CA 91109, United States
AU: Adams, E Y
EM: eya@umich.edu
AF: Univ. of Michigan, Space Research Bldg., Ann Arbor, MI 48109, United States
AU: Lunine, J I
EM: jlunine@lpl.arizona.edu
AF: Univ. Arizona, Lunar and Planetary Lab., Tucson, AZ 85721, United States
AB: Saturn's largest moon, Titan, and one of its smallest, Enceladus, contain nitrogen (N₂) in their atmospheres. N₂ comprises 95% of the volume of Titan's air, whereas it is about ~4% in the Enceladus' plume environment. At Titan, gravitational escape of N₂ is relatively slow. However, the gas is subject to rapid escape from the smaller moon. This implies that any nitrogen present in Enceladus' environment must be constantly replenished. The Huygens GCMS data at Titan show that the abundance of primordial argon (³⁶Ar) is several factors of ten below that expected if nitrogen accreted as N₂. This implies that N₂ was converted from nitrogen-bearing compounds —primarily ammonia (NH₃), in the dense and relatively warm subnebula of Saturn. NH₃ could then be dissociated back into N₂ in Titan's past by (a) photochemistry (Atreya et al., 1978), (b) shock induced chemistry (Jones and Lewis, 1987; McKay, et al., 1988), and (c) thermal dissociation. Mechanism (b) does not seem plausible in view of water-ammonia chemistry—which prevents N₂ formation—and untenable amounts of H₂ resulting from the dissociation of NH₃ and CH₄. Photochemistry is capable of producing 5-8 bars of nitrogen—an amount needed originally to explain the current 1.5 bars after accounting for escape—in 17-27 Myr (Adams 2006, Wilson 2002, Atreya 1986, Atreya et al., 1978). In this talk we discuss details of this likely process. Photochemical production of N₂ is not viable at Enceladus because of the long time constants of the process and the exospheric type atmospheric densities. On the other hand, mechanism (c), first invoked by Matson et al. (2007a) could work. It was suggested that dissociation of ammonia in the interior of Enceladus at temperatures in excess of 650K could produce the N₂ detected in the moon's environment. In a companion paper (Matson et al., 2007b) we examine the feasibility of such a mechanism at Titan also to assess the contribution of thermal dissociation of NH₃ to Titan's primordial nitrogen. References: Adams EY, thesis, U. Michigan, 2006. Atreya SK et al., Evolution of a Nitrogen Atmosphere on Titan, Science 201, 611-613, 1978. Atreya SK, Atmospheres and Ionospheres of the Outer Planets and their Satellites, Springer-Verlag, New York- Berlin,188-190,1986. Jones and Lewis, Icarus 72, 381-393, 1987. Matson DL et al., Icarus 187, 569-573, 2007a. Matson et al., Endogenic Origin of Titan's N₂, Fall AGU Meeting 2007b. Wilson EH, thesis, U. Michigan, 2002. McKay et al., Nature 332, 520-522, 1988.
UR: http://www.umich.edu/~atreya
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 0343 Planetary atmospheres (5210, 5405, 5704)
DE: 5455 Origin and evolution
DE: 6281 Titan
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting