On the amount of heavy molecular ions in Titan's ionosphere
Gurnett, D. A.; Coates, A. J.; Garnier, P.; Kurth, W. S.; Wahlund, J. -E.; Young, D. T.; André, M.; Ågren, K.; Galand, M.; Eriksson, A. I.; Waite, J. H.; Yelle, R. V.; Cravens, T. E.; Magee, B.; Müller-Wodarg, I.; Vuitton, V.; Mandt, K.; Crary, F. J.; Cui, J.
Sweden, United Kingdom, United States, France
Abstract
We present observational evidence that the ionosphere of Titan below an altitude of 1150 km is a significant source of heavy (>100 amu) molecular organic species. This study is based on measurements by five instruments (RPWS/LP, RPWS/E, INMS, CAPS/ELS, CAPS/IBS) onboard the Cassini spacecraft during three flybys (T17, T18, T32) of Titan. The ionospheric peaks encountered at altitudes of 950-1300 km had densities in the range 900-3000 cm -3. Below these peaks the number densities of heavy positively charged ions reached 100-2000 cm -3 and approached 50-70% of the total ionospheric density with an increasing trend toward lowest measured altitudes. Simultaneously measured negatively charged ion densities were in the range 50-150 cm -3. These results imply that ~10 5-10 6 heavy positively charged ions/m 3/s are continuously recombining into heavy neutrals and supply the atmosphere of Titan. The ionosphere may in this way produce 0.1-1 Mt/yr of heavy organic compounds and is therefore a sizable source for aerosol formation. We also predict that Titan's ionosphere is dominated by heavy (>100 amu) molecular ions below 950 km.