2017 was a great year for observations of Saturn. Cassini took its closest view of the planet before plunging into it and Hubble has now used the planet's approach to the northern summer solstice to produce an incredible view of the polar aurora around the ringed planet.
The released image, as reported by the European Space Agency, was taken over a period of seven months before and after the solstice, the best time to observe the auroral region. This is the third observation campaign of Saturn’s aurorae. In 2004, Hubble looked at the southern aurorae just after the southern solstice. In 2009, it snapped both poles during the rare time that its ring appears edge-on from Earth.
Saturn’s aurorae are different from Earth’s one. On Earth, electrically charged particles from the Sun are trapped within our magnetic field, interacting with the oxygen and nitrogen in the atmosphere and creating the spectacle we can witness at high latitudes.
On Saturn and other gas giants, the aurorae are instead formed by interactions with hydrogen, which is the main component of those planets’ atmospheres. The light emitted by such an interaction is a lot more energetic than what we get on Earth and can only be seen in ultraviolet.
The saturnine aurorae show a lot of variation. They are obviously influenced by the solar wind and by the rapid rotation of the planet that spins on its axis in roughly 11 hours. The aurorae are highly variable localized features with two distinct peaks.
The image is a composite of several auroral observations in combination with a splendid view of Saturn taken by Hubble in the early months of 2018. The observations are more than just a pretty picture. Hubble’s work was coordinated with Cassini’s Grand Finale flybys. As the space telescope was studying the changes in the aurorae, the probe was deep within the magnetosphere of Saturn collecting precious measurements.
Researchers are interested in the magnetic field of Saturn because it is the second largest of any planet in the Solar System, after Jupiter. At its widest extension, the magnetosphere is 1.28 million kilometers (800,000 miles) in diameter.