The implications of the H₂ variability in Titan's exosphere

We present in this paper an investigation of the distribution of H ₂ in Titan's exosphere, based on the measurements made with the Ion Neutral Mass Spectrometer (INMS) onboard Cassini during 32 encounters with the satellite. The observed H₂ density in Titan's exosphere shows significant variance from flyby to flyby. However, no appreciable trend with geophysical or solar conditions can be identified. A data-model comparison is made in the framework of the Chamberlain approach, taking into account two ideal cases. First, we assume that the observed variability is spatial. In this case, the damping of exobase perturbations when propagating into the exosphere is a diagnostic of the spatial scale of the perturbations. We find that for all reasonable choices of this spatial scale, the model predicts significantly more damping than implied by the INMS data. Second, we assume that at any given time, the physical conditions in Titan's upper atmosphere and exosphere are globally uniform, but these conditions evolve with time, indicating that the observed variability is temporal. In such a case, the observations can be interpreted as a result of exobase perturbations on timescales in the range of ∼10 ³-10⁶ s. The time-varying H₂ exosphere of Titan essentially reflects the varying structure and energy deposition in the upper atmosphere of the satellite, which are ultimately determined by the variations in either the solar EUV/UV radiation or the level of magnetospheric particle precipitation. However, we do not expect the considerable variability observed for Titan's H₂ exosphere to be induced by the varying solar inputs into Titan's atmosphere. Instead, we postulate that such a variability is more likely to be associated with Titan's varying plasma environment. Comparisons between different categories of Titan flybys tentatively reveal that the H ₂ exosphere tends to be more energetic and more expanded, and H ₂ molecules tend to escape more rapidly, with increasing levels of electron precipitation from the ambient plasma environment.