Photoelectric Charging of Submicron Aerosols and Macromolecules in the Titan Haze

We quantify the charge states of submicrometer aerosols and aromatic macromolecules in Titanʹs organic haze. The aerosol charge is balanced between the recombination of positive ions with the aerosol plus the ejection of electrons from the aerosol via the UV-driven photoelectric effect and the recombination of electrons with the aerosol. During the day, the dominant charge state for submicro-meter aerosols is positive. Macromolecules composed of fewer than 32 carbon atoms with low electron affinities (<1.0 eV) are neutral, while the rest are mainly neutral and negatively charged with a small fraction (∼10%) becoming positively charged at higher (≥300 km) altitudes. At night, Titanʹs aerosol population becomes uniformly neutral and negatively charged. The time taken for a nighttime aerosol to change from being negatively charged to its most probable daytime positive charge is on the order of a few seconds for the largest submicrometer aerosols, while macromolecules tend to persist in an anionic charge state for one to several Earth days. Charging strongly influences aerosol agglomeration via Coulomb attraction and may account for the seasonal variations in the albedo of the Titan haze at midrange (∼200–250 km) altitudes. Enhanced agglomeration may also efficiently produce a source of condensation nuclei for the daily rainout of methane. In addition, the difference in aerosol charge between Titanʹs day and night (or summer and winter) phases will produce dramatically different chemistries which must be accounted for in future photochemical models. Finally, if there are PAH-like macromolecules in the Titan haze, Cassini Huygens should be able to observe these charge differences, with neutral macromolecules emitting strongly at 3.3 and 11.2 μm, cationic macromolecules emitting between 6.2 and 8.6 μm, and anionic macromolecules emitting in both infrared spectral regions.