University of Kansas Titan Studies

Image courtesy of NASA/JPL-Caltech.

The Composition of Titan's Ionosphere

T. E. Cravens, I. P. Robertson, J. H. Waite Jr., R. V. Yelle, W. T. Kasprzak, C. N. Keller, S. A. Ledvina, H. B. Niemann, J. G. Luhmann, R. L. McNutt, W.-H. Ip, V. de la Haye, I. Muller-Wordag, J.-E. Wahlund, V. A. Anicich, and V. Vuitton

(The final version of this paper appears in Geophysical Research Letters, 32, no. 12, L12108, doi: 10.1029/2005GL023249. Copyright 2005 American Geophysical Union.)

Abstract with link to full article on the AGU website.
PDF of final submission version.

Abstract. We present Cassini Ion and Neutral Mass Spectrometer (INMS) measurements of ion densities on the nightside of Titan from April 16, 2005, and show that a substantial ionosphere exists on the nightside and that complex ion chemistry is operating there. The total ionospheric densities measured both by the INMS and the Cassini Radio and Plasma Wave (RPWS) experiments on Cassini suggest that precipitation from the magnetosphere into the atmosphere of electrons with energies ranging from 25 eV up to about 2 keV is taking place. The absence of ionospheric composition measurements has been a major obstacle to understanding the ionosphere. Seven "families" of ion species, separated in mass-to-charge ratio by 12 Daltons (i.e., the mass of carbon), were observed and establish the importance of hydrocarbon and nitrile chains in the upper atmosphere. Several of the ion species measured by the INMS were predicted by models (e.g., HCNH+ and C2H5+). But the INMS also saw high densities at mass numbers not predicted by models, including mass 18, which we suggest will be ammonia ions (NH4+) produced by reaction of other ion species with neutral ammonia.

Figures:

	Figure 1. Schematic of Titan for the Cassini encounter on April 15, 2005, with the spacecraft trajectory. The INMS ion measurements were made on the outbound part of the trajectory.
	Figure 2. Density profiles of several important ion species. Densities measured by the INMS are shown versus altitude, time from CA, and solar zenith angle (SZA) for the outbound T5 encounter of Cassini with Titan. The latitude and local solar time at CA are 73.7 deg. N and 0.7 hrs., respectively. Total INMS ion density and an electron density profile measured by the RPWS experiment are also shown.
	Figure 3. Measured mass spectra for three altitude intervals are shown - average density versus Mass (in Daltons). Red: 1027 km-1200 km, multiplied by a factor of 100. Blue: 1200 km-1400 km, multiplied by a factor of 10. Green: 1400 km-1600 km, multiplied by a factor of 1.
	Figure 4. Average ion mass versus altitude. Average masses are from INMS densities.

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Last modified May 4, 2007 Tizby Hunt-Ward tizby@ku.edu