On the amount of heavy molecular ions in Titan's ionosphere

Abstract: We present observational evidence that the ionosphere of Titan below an altitude of 1150km is a significant source of heavy (>100amu) 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 950–1300km had densities in the range 900–3000cm−3. Below these peaks the number densities of heavy positively charged ions reached 100–2000cm−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 50–150cm−3. These results imply that ~105–106 heavy positively charged ions/m3/s are continuously recombining into heavy neutrals and supply the atmosphere of Titan. The ionosphere may in this way produce 0.1–1Mt/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 (>100amu) molecular ions below 950km. [Copyright &y& Elsevier]