Scientists Analyze Final Images from Cassini

Toward the completion of its 13-year mission, which ended in September 2017, the Cassini spacecraft performed a set of orbits that brought it closer to Saturn than ever before. By passing over the planet’s polar regions at a low altitude, Cassini’s ultraviolet camera could observe Saturn’s auroras at an unprecedented resolution. Scientists at Lancaster University have analyzed the high-resolution imagery showing the auroras’ small-scale structure.

The images from the last leg of Cassini’s journey show for the first time the detailed structure of the main auroral arc, which varies between a smooth and a rippled shape, likely depending on how quiet or disturbed the plasma near Saturn is. The images show multiple parallel arcs near dusk, whose origins are not readily explained with the current understanding of how Saturn’s auroras are driven. An outer emission, although variable in brightness, is always present, and scientists think it could be driven by hot electrons from the ring current.

Composite of a true color image of Saturn, observed by Cassini in 2016, overlaid with a false color representation of the ultraviolet aurora in the northern hemisphere as observed on Aug. 20, 2017. Courtesy of NASA/JPL-Caltech/Space Science Institute/A. Bader (Lancaster University).

Lancaster University researcher Alexander Bader said, “This last set of close-up images gives us unique, highly detailed views of the small-scale structures which couldn’t be discerned in previous observations by Cassini or the Hubble Space Telescope. We have some ideas about what their origin could be, but there is still a lot of analysis to be done.”

Saturn’s auroras, located in the planet’s polar regions, are known to be highly dynamic, often pulsating and flashing as different dynamic processes occur in the planet’s plasma environment.

Even though Cassini’s mission is over, the data it provided will continue to help researchers understand the workings of giant planet auroras, especially in combination with Juno observations of Jupiter’s magnetosphere. This final image set will be the only high-resolution data for the foreseeable future, the researchers said, and so will form an important basis for future auroral research on Saturn.

The research was published in Geophysical Research Letters (www.doi.org/10.1029/2019GL085800) and in JGR Space Physics (www.doi.org/10.1029/2019JA027403).