The Changing Face of Titan’s Haze: Is it all Dynamics?

Titan’ atmosphere shows some similarities with that of the Earth, in terms of composition and surface pressure. Also, its seasonal cycle is similar, as Titan’ obliquity is about 27^°(23^°,5 for the Earth), although it is about 30 times as long. Titan’ haze exhibits an albedo contrast (NSA for North-South Asymmetry) that is changing seasonally. From the analysis of Voyager and Hubble Space Telescope data, we learned that at short visible wavelengths, the albedo of the winter hemisphere is lower by 10-20% than that of the summer hemisphere. This asymmetry peaks at 450 nm and reaches maximum amplitude around Titan’ equinoxes. It reverses in about five years, faster than a season which spans seven years. At longer wavelengths, longward of 700 nm, the asymmetry is inverted. The NSA reversal process in the red and in the UV seems to lead the reversal in the blue by 1 or 2 years. No valid explanation exists for this lag, at least in the red. The results from a recent model which couples atmospheric dynamics, haze microphysics and transport, as well as photochemistry, show that the NSA and its seasonal changes can be explained by an accumulation of haze particles at the winter pole. This is due to the pole-to-pole Hadley circulation pattern that is present during most of Titan’ year and rapidly disrupts at the time of the equinoxes. This model can also explain the observed cooler stratospheric temperatures and higher abundances of heavy hydrocarbons and nitriles in the winter polar region. In addition, it provides a mechanism for the formation of a detached haze layer around 300–400 km altitude, as well as the existence of a polar hood. Thus, it appears that the latitudinal contrasts we observe on Titan are conveniently tracing for us the dynamical behavior of its atmosphere.