Leicester scientists have found a previously unknown process that fuels massive planetary aurorae on Saturn.
Saturn is unlike any other planet yet discovered in that parts of its aurorae are caused by whirling winds inside its own atmosphere, rather than the planet’s magnetosphere.
Aurorae are only created on all other planets that have been detected, including Earth, by tremendous currents that flow into the planet’s atmosphere from the surrounding magnetosphere. Interaction with charged particles from the Sun (like on Earth) or volcanic material spewed from a moon circling the planet cause these (as at Jupiter and Saturn).
This revelation alters scientists’ perceptions of planetary aurorae and addresses one of the first questions posed by NASA’s Cassini probe, which arrived at Saturn in 2004: why can’t we simply calculate the duration of a day on Saturn?
Cassini attempted to quantify Saturn’s mass rotation rate, which affects the length of its day, by tracing radio emission ‘pulses’ from the planet’s atmosphere when it first arrived. The rate seemed to have altered during the two decades since the previous spacecraft to fly by the planet, NASA’s Voyager 2, flew by in 1981, much to the amazement of scientists doing the observations.
Nahid Chowdhury, a PhD student at the University of Leicester, is a member of the Planetary Science Group at the School of Physics and Astronomy, as well as the study’s corresponding author. He said, ”
“Saturn’s internal rotation rate must be constant, but experts have known for decades that a number of periodic features associated to the planet — measures we’ve used to understand the internal rotation rate on other planets, such as radio emission — fluctuate with time. Furthermore, separate periodic characteristics may be detected in both the northern and southern hemispheres, which fluctuate over the course of a season on the globe.
“The genuine rotation rate of the planet can’t vary this rapidly, according to our knowledge of planetary inner physics, thus something unusual is going on at Saturn. Since the launch of the NASA Cassini mission, many ideas have been proposed to explain the mechanism(s) underlying the observed periodicities. This is the first observation of the basic driver, which is located in the planet’s upper atmosphere and is responsible for both the observed planetary periodicities and aurorae.
“It’s an incredible feeling to be able to deliver a solution to one of our field’s most vexing concerns. This is likely to prompt some reconsideration of how local atmospheric weather conditions on a planet influence the formation of aurorae, not only in our own Solar System but also in other parts of the galaxy.”
To answer the decades-old question, astronomers and planetary scientists at the University of Leicester collaborated with colleagues from NASA’s Jet Propulsion Laboratory (JPL), the Japan Aerospace Exploration Agency (JAXA), the Universities of Wisconsin-Madison, Boston, and Lancaster, as well as Imperial and University Colleges in London.
Using the Keck Observatory in Hawaii, they monitored infrared emissions from Saturn’s upper atmosphere and documented the shifting fluxes of Saturn’s ionosphere, which lies deep beyond the magnetosphere, over the course of a month in 2017.
When this map was compared to the known pulse of Saturn’s radio aurorae, it was discovered that a major amount of the planet’s aurorae are caused by the planet’s whirling weather pattern, which is also responsible for the planet’s reported fluctuating rate of rotation.
The system is believed to be powered by energy from Saturn’s thermosphere, with ionosphere winds ranging between 0.3 and 3.0 kilometers per second.
Associate Professor of Planetary Astronomy at the University of Leicester, Dr Tom Stallard, added:
“We’ve seen how the pulsating aurorae and the wobbling magnetic field lines reaching out into space show an apparently shifting rotation rate, and we’ve been analyzing the impacts of this new finding for a long time at the University of Leicester. Our experts, along with the rest of the scientific world, have theorized for two decades about what may be causing these unusual periodicities.
“Late-night talks at scientific gatherings have centered on whether the volcanic moon Enceladus, interactions with Titan’s dense atmosphere, or interactions with Saturn’s brilliant rings may be the reason. However, many scholars have increasingly focused on the hypothesis that the unpredictability is caused by Saturn’s upper atmosphere.
“This hunt for a new sort of aurora may be traced back to some of the first speculations concerning the aurora on Earth. We now understand that aurorae on Earth are fueled by interactions with the Sun’s charged particle stream. However, I like the fact that the term Aurora Borealis comes from the phrase “the Dawn of the Northern Wind.” Saturn possesses a genuine Aurora Borealis, the first known aurora caused by the winds in a planet’s atmosphere, according to these measurements.”
Dr. Kevin Baines, a member of the Cassini Science Team and a co-author of the paper from JPL-Caltech, added:
“By firmly pinpointing the cause of the enigmatic fluctuation in radio pulses, our research clears up much of the ambiguity around Saturn’s bulk rotation rate and day duration.”
Scientists have been unable to compute the bulk internal rotation rate using the regular pulse of radio emission because to the varied rotation speeds recorded at Saturn. Fortunately, Cassini scientists devised a novel method for measuring Saturn’s bulk rotational period, which was determined in 2019 to be 10 hours, 33 minutes, and 38 seconds, based on gravity-induced perturbations in the planet’s complex ring system. This method now appears to be the most accurate means of measuring the planet’s bulk rotational period.
The University of Leicester’s planetary study covers the whole Solar System — and beyond.
The Juno project, which is made up of a worldwide team of scientists investigating Saturn’s enormous neighbor, Jupiter, is led by Leicester researchers, who are also conducting investigations of the Solar System’s outer planets from the newly launched James Webb Space Telescope. Leicester is also a key contributor to the research and instruments for the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), which is scheduled to launch in 2022.
The NASA Exoplanet Science Institute administered a NASA Keck PI Data Award to assist this study.