Titan’s Twilight and Sunset Solar Illumination

We calculate the illumination conditions at Titan’s surface using the Monte Carlo radiative transfer model SRTC++, motivated by the proposed Dragonfly Titan lander. We find significant surface illumination during twilight after sunset, with the twilight flux maximized near 1.0 μ {{m}} wavelength. Out to 30° past Titan’s terminator, the twilight illumination exceeds that of Earth’s Moon at full phase in visible red wavelengths (0.65 μ {{m}}). Imaging at night should be quite effective for stationary surface landers if they use long integration times, though it would be less effective for platforms floating on Titan’s seas. Titan sunsets should be underwhelming events at visible wavelengths, with the Sun fading out while still well above the horizon and overall illumination diminishing slowly as the Sun falls below the horizon. Shadows below the lander should receive illumination from diffusely scattered light low in the sky near Titan’s horizon. The total near-horizon illumination maximizes when the Sun is highest in the sky owing to the intensity of multiple scattering.