Cassini was put to rest while it still had enough propellant to control itself, preventing any accidental collision and contamination of the moons Enceladus and Titan. Both have conditions that could possibly support life.
As we look back on Cassini’s extraordinary mission to Saturn, what are some of its major discoveries? What are scientists learning as they continue to comb through 13 years of mission data?
Enceladus and Titan revealed
For many people who worked on the mission, Cassini’s dramatic dive into the planet’s atmosphere is still fresh in their minds.
“The memories of that final plunge into Saturn and of those final weeks are still very, very vivid,” said Linda Spilker, who was the project scientist for the Cassini mission.
Also on her list is the discovery of Enceladus’ subsurface ocean. Enceladus sits in the middle of Saturn’s hazy E ring. When Cassini arrived at the planet, scientists hoped to learn what the relationship was between Enceladus and the E ring.
At only 500 kilometers (310 miles) wide, Enceladus seemed far too small to be an active world. The notion that the moon was shooting material into space and creating the E ring seemed far-fetched.
“People just didn't think it was likely,” said Spilker. “They expected Enceladus to be frozen solid.”
In 2005, Cassini discovered water plumes shooting from the moon’s south pole. Later in the mission, the spacecraft directly sampled the plumes and discovered organic molecules — the building blocks of life as we know it.
Another highlight was Titan, which also surprised scientists.
NASA’s two Voyager spacecraft imaged Titan during their tours of the outer solar system and found hints that there could be liquid methane and ethane on Titan’s surface. However, the probes were not equipped with instruments that could gaze through the moon’s hazy atmosphere.
Cassini was. The spacecraft discovered hydrocarbon lakes and helped scientists learn that the moon had a hydrologic cycle that works like Earth’s. The European Space Agency’s Huygens probe, which deployed from Cassini shortly after arrival at Saturn, landed on Titan to show us what the surface looked like close-up. Images from Huygens revealed rocks that were probably shaped by liquid, similar to river rocks on Earth.
An interconnected system
In the years following the end of the Cassini mission, scientists have come to appreciate that it’s impossible to consider any part of the Saturn system on its own.
“It's a real interconnected system where everything interacts with everything else,” said Matthew Hedman, an associate professor of physics at the University of Idaho who was a Cassini participating scientist.
Hedman published data that helped solve a longstanding mystery about Saturn: How long is a day on the ringed planet?
On a terrestrial planet like Earth, this question can be answered by watching landforms make a lap around the planet. Since gas giants like Jupiter and Saturn have no landforms, an alternative way to determine the length of a day is by tracking how long it takes one of the planet’s magnetic field poles to travel around the planet.
Unfortunately, that doesn’t work on Saturn because the magnetic field pole is aligned almost perfectly with the planet’s rotation axis, meaning the magnetic pole hardly moves at all.
So scientists tried something different: They learned that Saturn’s interior vibrates like a bell, creating variations in its gravitational field. These variations show up as waves in the planet’s rings. By tracking these waves, scientists were able to determine that a Saturn day lasts 10 hours, 33 minutes, and 38 seconds.
Studying the frequency of the ring waves may also yield clues about Saturn’s interior, including what its core is like.
“If the planet's more centrally condensed, it rings at one frequency. If its center is puffier, it rings at other frequencies,” Hedman said.
Rings that live fast and die young
When NASA made the call to end the Cassini mission, it sent the spacecraft on a daring series of final orbits between the rings and the planet itself. That phase of the mission was called the grand finale.
The way that Cassini’s path was perturbed by the mass of Saturn on one side versus the rings on the other side allowed scientists to measure how heavy the rings were.
Spilker said the team took a poll on how heavy the rings would be. Most scientists predicted their mass would be the same or larger than that of Saturn’s moon Mimas. Although an image of Mimas next to the rings gives the impression that the rings should overwhelm the small moon, they are in fact only 40% as massive.
This means the rings are relatively young — possibly between 10 to 100 million years old — and could have formed during the reign of the dinosaurs on Earth. Lightweight rings also support the theory that they formed from one or more comets, asteroids, or moons that broke apart.
Cassini also discovered that the rings are raining back into Saturn at a much higher rate than previously predicted. They may have as little as 100 million years to live.
A magnetic field ties it all together
Perhaps no aspect of the Saturn system connects everything together more than the planet’s magnetic field.
Saturn’s magnetic field works similar to Earth’s: Deep within the planet, fast-flowing material generates an electric charge. That charge creates a magnetic field that stretches out from the planet. When charged particles from the Sun hit this field, they are shunted into Saturn’s atmosphere at the poles and generate beautiful aurorae.
The magnetic field also traps material shed from Saturn’s rings and moons — particularly Enceladus, said William Kurth, a research scientist and engineer at the University of Iowa who served as the principal investigator for Cassini’s Radio and Plasma Wave Science instrument.
“We thought that the dominant material in the magnetosphere would be nitrogen from Titan's atmosphere. We were quite surprised when we discovered it was really water-group ions coming from Enceladus,” he said.
Saturn’s magnetosphere is a bustling place. Studying its structure and strength can reveal unseen processes happening throughout the system, including Saturn’s interior. Recent papers have found that plasma waves known as Z-mode waves and whistler-mode waves can energize electrons, contributing to Saturn’s version of the Van Allen radiation belts at Earth. Other studies are revealing a more complex radio spectrum at Saturn than originally found by the Voyager spacecraft.
With so much Cassini data available from each of the spacecraft’s science instruments, there are plenty of discoveries yet to be made.
“There are new students that are coming online who are finding that Cassini has left a treasure trove of data,” said Kurth. “We original investigators on Cassini kind of took the cream of the crop of the data. These newer generations of researchers and students are digging into the details of those.”
What Cassini left behind
All Cassini data is publicly available through NASA’s Planetary Data System. As part of the mission’s closeout process, the team behind each science instrument wrote a guide to help future users comb through their data.
NASA still funds a small amount of Cassini research and analysis work through the Cassini Data Analysis Program. Through CDAP, researchers ask for two or three years of funding to study specific Cassini topics.
“I think part of the lasting legacy of Cassini really is that 13 years of data,” said Spilker. “That's a tremendous legacy and really a gold mine that certainly hasn’t been mined completely yet.”
Just as the Voyager flybys of Saturn shaped the Cassini mission’s science questions, Cassini is paving the way for new missions that will follow up on its findings. Dragonfly, an eight-bladed flying spacecraft scheduled to launch in 2027, will explore Titan and investigate whether the hazy orange moon could support life.
When Cassini discovered Enceladus’ plumes, it revealed one potential way for a future spacecraft to sample the moon’s subsurface ocean without having to drill deep beneath its icy crust. A proposed mission called Orbilander could carry out that task.
An Enceladus mission was recommended as a priority to NASA after the agency completes its Mars Sample Return, Europa Clipper, and Uranus orbiter missions. This means a return to Enceladus may not happen for a decade or even two. But it could be worth the wait to answer a very important question: Are we alone in the universe?