Haze Formation on Triton

The largest moon of Neptune, Triton, possesses a cold and hazy atmosphere. Since the discovery of the near-surface haze layer during the Voyager fly in 1989, the haze formation mechanism has not been investigated in detail. Here we provide the first haze microphysical model on Triton. Our model solves the evolution of both size and porosity distributions of haze particles in a self-consistent manner. We simulated the formation of sphere and aggregate hazes with and without condensation of the C₂H₄ ice. The haze particles can grow into fractal aggregates with mass-equivalent sphere sizes of ∼0.1–1 μm and fractal dimensions of D_f = 1.8–2.2. The ice-free hazes cannot simultaneously explain both UV and visible observations of Voyager 2, while including the condensation of C₂H₄ ices provides two better solutions. For ice aggregates, the required total haze mass flux is ∼2 × 10⁻¹⁵ g cm⁻² s⁻¹. For the icy sphere scenario, the column-integrated C₂H₄ production rate is ∼8 × 10⁻¹⁵ g cm⁻² s⁻¹, and the ice-free mass flux is ∼6 × 10⁻¹⁷ g cm⁻² s⁻¹. The UV occultation observations at short wavelengths, <0.15 μm, may slightly favor the icy aggregates. Observations of the haze optical depth and the degree of forward scattering in UV and visible should be able to distinguish whether Triton's hazes are icy spheres or ice aggregates in future Triton missions.