The Symmetry of Titan’s Seasonal Haze Variation Observed by HST-STIS

Titan’s haze displays a seasonal variation with a peak brightness in late summer in both hemispheres. In strong methane bands, the hemispheric contrast is reversed. The explanation is a peak haze opacity in late winter that absorbs more in continuum wavelengths but reflects more in strong methane bands. This seasonal variation causes a north-south asymmetry of Titan’s haze that has been known since the Voyager fly-bys. The evolution of the seasonal change is less well known since Titan’s annual cycle lasts almost 30 years and since few observations exist that have tracked Titan with the same instrument in a consistent way over 30 years. Cassini’s time in the Saturn system did not even last 15 years. The Space Telescope Imaging Spectrograph (STIS) observed Titan in the years 1997, 1999, 2000, 2002, 2004, 2017, 2018, and 2019. This 22-year period includes observation pairs spaced apart by 15 years. It allows us to check how well the seasonal cycle reverses after two seasons. Most of these observations cover the whole disk of Titan at 0.05 arc-sec spatial sampling and 1000 spectral samples between 520 and 1020 nm wavelength. Such image cubes allow an accurate evaluation of the haze opacity as function of altitude and latitude. Our analysis of these eight data cubes revealed that the seasonal variation is remarkable symmetric. The duration between two reversals was within three months of half of Titan’s year, accurate to two percent. In addition, the latitudinal variation of Titan’s haze opacity was an almost perfect mirror image of the condition two seasons earlier. The last three sets of STIS observations were planned to catch a reversal of the north-south asymmetry that was predicted based on the earlier STIS observations. The prediction was right on even though a baseline of seven years was extrapolated 15 years into the future. Our analysis also confirmed that the seasonal haze variation has two separate components, a major component at altitudes below 80 km that causes most of the observable variation, and a minor component at altitudes above 150 km. This was suspected based on the limited STIS data set of 1997 to 2004. The major component has a phase lag of exactly one season and lags 2.3 ± 0.3 years behind the minor component. Such long phase lags are indicative of a system of slow response with respect to the solar illumination, a system with a long time constant. On top of the seasonal variation of sinusoidal shape, we detected other variations with 10 times smaller amplitudes and much shorter time scales. This research is published in a current issue of Icarus. It was supported by STScI General Observer Programs 14612, 15122, and 15490.