It’s hard to believe that the Cassini mission arrived at Saturn and its moons in July 2004. It’s even harder to imagine that this remarkably successful machine explorer now has less than a year of life ahead of it.
Around September 15th, 2017 the 2,000 kilogram orbiter will plunge into Saturn’s upper atmosphere – a destructive end that will yield new data, but also ensure that our visit to this system runs little risk of contaminating pristine environments like that of icy moon Enceladus.
But before that, Cassini is performing a magnificent last waltz with Saturn. Starting with a series of 20 pole-crossing orbits that also dive through the space just outside Saturn’s main rings (the so-called F-ring orbits) Cassini will later shift to a set of 22 orbits that bring its closest approach nearer and nearer to the gas giant – actually within the rings. This animation shows the two orbital families in green and blue respectively.
At this time Cassini is in the midst of its 2nd ring-grazing orbit (green orbit). Already this trajectory has enabled the mission to capture some of the most detailed images of Saturn’s north pole that we’ve ever seen. These show the great hexagonal shape in the upper atmosphere – a possible consequence of turbulent flow between gas masses circulating at very different speeds.
Our understanding of this structure is still incomplete, but the hexagon has persisted since at least the time of the Voyager flyby in the early 1980s. Each side of the hexagon is about the size of the Earth.
These polar shots are also quite reminiscent of Juno’s recent imagery of Jupiter’s poles. The great bands of circulating atmosphere that many of us associate with gas giant planets, devolve into a peppering of small structures that look cyclonic in nature.
Different wavebands of light reveal just how strong a feature the polar hexagon is. In the images below (left to right) are Cassini data in violet (420 nanometers), red, near infrared (728 nanometers), and the infrared. These probe different depths and compounds in the atmosphere.
It’s a great start to a grand finale. Expect to see much more over the coming months.