UVIS Titan T0, TA Analysis D. Shemansky J. Hallett 07/25/05

Titan TA

Figure 9 Titan T0 encounter UVIS FUV spectrum: UVIS FUV spectrum against model calculations of nitrogen, hydrogen and solar reflection. The observation bin is 0A as indicated in Figures 5 and 6. The altitude range of the bin is 1030 – 1416 km. The model calculation for nitrogen is for electron impact at a temperature of Te=300000 K. The atomic carbon model includes electron and solar fluorescence. Extinction by CH4 is included with abundance as indicated on the plot. Intensities of selected emissions are indicated on the plot compared to Voyager results. The solar reflection albedo at 1826 A is 0.0037. The solar reflection model includes foreground hydrocarbon absorption.

Consistency of TB and T0 analyses Extinction of NI emission at T0 at 1030 km sub-solar limb indicates CH4 abundance of 3X1016 cm-2 compared to TB value of 3.75X1016 cm-2 Solar reflection spectrum at h= 0 km shows extinction by C2H2 and C2H4 abundances of 4.X1016 cm-2 and 2.2X1016 cm-2 . From TB this corresponds to the abundance of C2H2 at 630 km (peak of 2.5 X1017 cm-2 at 546 km), and to the value of peak abundance of C2H4 at 600 km.

Figure 13 Titan T0 encounter: Cassini UVIS planet disk mean spectrum against model of excited N2, NI, CI, and solar reflection. Transitions in the modeled spectra are indicated. No detectable NII lines appear in this spectrum. The NII 1085 A line is strong in the UVIS EUV spectrum.

Physical chemistry model calculations

Conclusions Electron excitation and the ionosphere peak at least 1325 km above the surface. In the ionosphere atomic nitrogen can be as much as 10% of the N2 population. The subsolar ionosphere peak shows 70% of the NI is in the 2D state indicating strong cold electron pumping through direct e + NI excitation.

Conclusions Based on the TA observations, the region of strong NI 2D pumping can be confined within a 1000 km region of the equator. The solar reflection albedo shows a similarly confined peak in TA, and the reflection is detectable to 1300 km in the T0 subsolar spectra.The relationship between these phenomena if any is obscure.

Conclusions The non uniform distribution of aerosols in these observations infer that the Titan atmosphere is subject to significant dynamic processes.