Artist’s concept of Cassini orbiter crossing Saturn’s ring plane.
NASA Jet Propulsion Laboratory
It was arguably the landmark space mission of the last decade, and it’s still delighting astronomers even after its death. After 13 years at Saturn, the ringed planet, in its dying days NASA’s Cassini spacecraft conducted a spectacular “Grand Finale” in which it got close to Saturn’s poles. Scientists are busy analysing some of its final data from the flybys—including images of Saturn’s ultraviolet auroras in unprecedented resolution.
Aurora are known as the Northern Lights and Southern Lights on Earth. On Earth they’re caused by charged particles from the Sun—the solar wind—interacting with Earth’s magnetosphere.
Aurora Borealis, the Northern Lights, over the Vestrahorn mountain in the east of Iceland. (Photo by … [+]
PA Images via Getty Images
At Saturn, it’s all very different. Its atmosphere is dominated by hydrogen, so auroras can only be seen in ultraviolet light. The same applies to auroras around Jupiter, Uranus and Neptune. Another main difference is that the aurorae rotate together with the planet.
“Man questions about Saturn’s auroras remain unanswered, even after the outstanding success of the Cassini mission,” says Alexander Bader, Lancaster University PhD student and lead author of two new studies published in Geophysical Research Letters and JGR:Space Physics. “This last set of close-up images gives us unique highly detailed views of the small-scale structures that couldn’t be discerned in previous observations by Cassini or the Hubble Space Telescope.”
Composite of a true colour image of Saturn, observed by Cassini in 2016, overlaid with a false … [+]
NASA/JPL-Caltech/Space Science Institute/A. Bader (Lancaster University)
Have aurorae been photographed before at Saturn?
Back in 2018, the Hubble Space Telescope took a series of photos of fluttering auroras at Saturn. The observations were taken in ultraviolet light and gave astronomers the most comprehensive picture so far of Saturn’s northern aurorae.
This image is a composite of observations made of Saturn in early 2018 in the optical and of the … [+]
ESA/Hubble, NASA, A. Simon (GSFC) and the OPAL Team, J. DePasquale (STScI), L. Lamy (Observatoire de Paris)
What do the new images show?
Now that’s been surpassed, with Bader’s research using images from Cassini in 2017 revealing more about the mechanism that causes them. “The images are different from previous data in that they show an incredible amount of detail we have never seen before,” says Bader, who was able to observe aurorae on the dusk side of Saturn for the first time. “What is puzzling to us is the incredible variability we observe in the data,” he continues. “In comparison to Earth, Saturn has a much larger magnetosphere and the auroral dynamics are much less controlled by the solar wind.”
A magnetosphere is the region around a planet dominated by the planet’s magnetic field.
Selection of (nearly) full views of the (a-d) northern and (e-f) southern auroral oval obtained … [+]
Bader et al
How do Saturn’s aurorae work?
Saturn’s aurorae are known to be highly dynamic, often pulsating and flashing. It’s thought that the massive size of Saturn and its rapid rotation (which takes a mere 11 hours) are the main causes of its aurora, which should mean they remain fairly static. “In this set, there are rarely even any similarities or repetitive features at the dusk side,” says Bader, “which makes it very hard to correlate what we see with other observations and find out what generates these small patches, blobs and fine arcs.”
It’s all very different to how aurorae are caused on Earth. “The solar wind doesn’t have a direct impact on the aurora,” says Bader about Saturn. “After a coronal mass ejection it is the increased number of solar wind particles and their increased energy that causes the Earth’s aurora to brighten, whereas at Saturn a brightening does not necessarily happen.”
Overview of Cassini particle and wave measurements from 2017-009. (a) Cassini INCA proton … [+]
Bader et al
During its two low passes over Saturn’s aurora zones, Cassini was sometimes embedded in the particle stream that connects aurorae to the planet’s magnetosphere. Analysis of the spacecraft’s particle measurements recorded during these times showed that Saturn’s auroras, like Jupiter’s, are generated by much more energetic particles than Earth’s.
olar projection of two Cassini UVIS images of Saturn’s aurorae from (a) 2017-009 (southern … [+]
Bader et al
However, although Saturn appears to have strong electric fields that accelerate electrons down into the atmosphere, causing bright auroras, the process appears to more closely resemble Earth’s auroras than previously thought.
Why is Jupiter important for Saturn’s aurorae?
How do giant planet auroras work? It’s still a mystery, but the researchers now intend to compare Cassini’s data with NASA’s Juno observations of Jupiter’s magnetosphere for more clues. “ Earth’s aurora is largely driven by the solar wind, whereas Jupiter’s aurora is powered from within the magnetosphere,” says Bader. “Saturn is often claimed to sit somewhere in the middle, between the Earth and the Jupiter view, showing signs of clear internal driving—for example periodic changes in brightness related to the rotation of Saturn—and solar wind driving, phases where the aurora near completely disappears.”
Particle energies are significantly higher at the giant planets. “There are still many differences in how the particles are energized enough to create aurora that we’re trying to make sense of,” says Bader.
Auroras are also known on Mars, Uranus and Neptune. “Our knowledge of them is at best highly limited,” says Bader. “A future mission visiting either or both of those systems is highly important to advance our knowledge of auroral physics.”
Wishing you clear skies and wide eyes.