Cassini UVIS Observations of Saturn's Auroras and Polar Haze

In 2016 and 2017, the Cassini Saturn Orbiter executed a final series of high inclination, low- periapsis orbits ideal for studying Saturn's polar regions. The Cassini Ultraviolet Imaging Spectrograph (UVIS) obtained an extensive set of auroral images of both poles, some at the highest spatial resolution obtained during Cassini's long orbital mission (2004-2017). In some cases, two or three spacecraft slews at right angles to the long slit of the spectrograph were required to cover the entire auroral region to form images of auroral H2 and H emission.
The long wavelength part of the northern UVIS polar images contains a signal from reflected sunlight with absorption signatures of acetylene
and other Saturn hydrocarbons. Saturn's UV-dark polar hexagon is now seen in the new UVIS long- wavelength data, surrounded by a circular collar that is less dark. There is a definite spatial relationship between the UV-bright auroras and the dark material, with the dark material concentrated under or just inside of the main auroral oval. The outer dark collar roughly corresponds with the previously reported weaker outer auroral oval (Grodent et al., 2011; Lamy et al., 2013). Time variations in the dark material are seen. The spectroscopy of the different regions will be discussed. As has been previously discussed using Voyager data (Lane et al., 1982, West et al., 1983, Pryor and Hord, 1991), Hubble data (Ben Jaffel et al., 1995; Gerard et al., 1995) and Cassini data (Sayanagi et al., 2018), Saturn's auroras appear to be generating, through both neutral and ion chemistry, UV-dark material that is probably composed of complex hydrocarbons.