Artist's rendition of Titan T-118 flyby

Titan's upper atmosphere is an active place where methane molecules are being broken apart by solar ultraviolet light and the byproducts combine to form compounds like ethane and acetylene.

Cassini's 'T-118' Titan Encounter: Fly By Once, Measure Twice

Purple Haze
Encircled in purple stratospheric haze, Titan appears as a softly glowing sphere in this colorized image taken one day after Cassini's first flyby of the moon on July 2, 2004.
Cassini’s cameras look in one direction; some of the spacecraft’s particle detectors look in another. Having both types of instrument observe Titan’s atmosphere at the same time, or the same latitude, or even on the same flyby can be difficult. This is why Cassini scientists are excited about the T-118 flyby. This encounter features a complex, joint observation by two instruments. It is the only flyby in the mission where the Ultraviolet Imaging Spectrograph (UVIS) and the Ion and Neutral Mass Spectrometer (INMS) will observe Titan's atmosphere simultaneously at the same latitude.

Earlier comparisons of atmospheric density made at different latitudes and times are difficult to reconcile. Having two instruments at work simultaneously provides less variability in the data.

The UVIS occultations are near north and south polar vortex boundaries; unusual patterns of gas abundances and temperature are seen in these regions in the south. Both hemispheres are interesting because they sample a dynamical regime that is not typical of the rest of Titan's atmosphere.