Negative ions at Titan and Enceladus: recent results

Andrew J. Coates; Anne Wellbrock; Gethyn R. Lewis; Geraint H. Jones; David T. Young; Frank J. Crary; J. Hunter Waite; Robert E. Johnson; Thomas W. Hill; Edward C. Sittler Jr.

doi:10.1039/C004700G

Negative ions at Titan and Enceladus: recent results

Andrew J. Coates,*^ab Anne Wellbrock,^ab Gethyn R. Lewis,^ab Geraint H. Jones,^ab David T. Young,^c Frank J. Crary,^c J. Hunter Waite,^c Robert E. Johnson,^d Thomas W. Hill^e and Edward C. Sittler Jr.^f

Author affiliations

* Corresponding authors

^a Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, UK

^b Centre for Planetary Science at UCL/Birkbeck, Gower Street, London, UK

^c Southwest Research Institute, Division of Space Science and Engineering, 6220 Culebra Road, San Antonio, USA

^d University of Virginia, Engineering Physics, Thornton Hall, Charlottesville, USA

^e Physics and Astronomy Department, Rice University, MS 108, Houston, USA

^f NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, USA

Abstract

The detection of heavy negative ions (up to 13 800 amu) in Titan’s ionosphere is one of the tantalizing new results from the Cassini mission. These heavy ions indicate for the first time the existence of heavy hydrocarbon and nitrile molecules in this primitive Earth-like atmosphere. These ions were suggested to be precursors of aerosols in Titan’s atmosphere and may precipitate to the surface as tholins. We present the evidence for and the analysis of these heavy negative ions at Titan. In addition we examine the variation of the maximum mass of the Titan negative ions with altitude and latitude for the relevant encounters so far, and we discuss the implications for the negative ion formation process. We present data from a recent set of encounters where the latitude was varied between encounters, with other parameters fixed. Models are beginning to explain the low mass negative ions, but the formation process for the higher mass ions is still not understood. It is possible that the structures may be chains, rings or even fullerenes. Negative ions, mainly water clusters in this case, were seen during Cassini’s recent close flybys of Enceladus. We present mass spectra from the Enceladus plume, showing water clusters and additional species. As at Titan, the negative ions indicate chemical complexities which were unknown before the Cassini encounters, and are indicative of a complex balance between neutrals and positively and negatively charged ions.

This article is part of the themed collection: Chemistry of the Planets

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Negative ions at Titan and Enceladus: recent results

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