The distribution of Titan's high-altitude (out to ∼50,000 km) exosphere from energetic neutral atom (ENA) measurements by Cassini/INCA

Abstract

We report observations of Titan's high-altitude exosphere detected out to about 50,000km altitude. The observations were made by the Ion Neutral Camera (INCA) on board the Cassini spacecraft. INCA detects energetic neutral atoms (ENA) that are formed when the ambient magnetospheric ions charge exchange with Titan's neutral atmosphere and exosphere. We find that Titan's exospheric H2 distribution follows closely a full Chamberlain distribution including ballistic, escaping and satellite distributions. As expected, neutral densities are dominated by a satellite distribution above about 10,000km. The maximum detectable extent of the exosphere (∼50,000km) coincides with the radius of the Hill sphere of gravitational influence from Saturn. While we find no direct indications of a neutral Titan torus with densities greater than about 1000cm−3, we observe interesting asymmetries in the distribution that warrants further investigation. Based on these findings we compute the average precipitating ENA flux to be about 5×106keV/(cm2s), or 8×10−3erg/(cm2s), which is directly comparable to that of precipitating energetic ions (Sittler, et al., 2009) and slightly higher than that of solar EUV (Tobiska, 2004). Thus, the energy deposited by precipitating ENAs must also be taken into consideration when studying the energy balance of Titan's thermosphere.