So far in this module we’ve looked at the general differences between Terrestrial and Jovian planets, at how and when the ring systems of the outer planets were discovered, and at why the moons of Jupiter and Saturn are so much more diverse and geologically interesting than the rocky moons of the inner Solar System. Now it’s time to look at the planets themselves in a bit more detail, and to connect what we know to the major spacecraft that have actually been there.
Almost everything we know about Saturn’s rings, atmosphere, and inner moons was pulled together by a single, long-lived mission: Cassini–Huygens. Cassini launched in 1997, did a series of gravity-assist flybys (Venus–Venus–Earth–Jupiter), and settled into Saturn orbit in 2004. For the next 13 years it became the Saturn mission — mapping the rings, flying past Enceladus two dozen times, tracking Titan’s weather and lakes, and imaging the whole system from every possible geometry.
By 2016 it was running low on fuel, and because Saturn has at least two places we now worry might be habitable (Enceladus’ ocean and Titan’s subsurface water–ammonia layer), the team ended the mission in a controlled way. In 2016–2017 Cassini was steered through a set of narrow orbits between Saturn and its innermost rings, measuring ring mass, sampling the atmosphere, and taking spectacular images. Then, on 15 September 2017, Cassini dived into Saturn’s atmosphere and was destroyed, so we would not accidentally contaminate any of the moons.
Learning Activity — Cassini highlights
Given Cassini’s role in transforming our knowledge of Saturn, the best source is still the Cassini team. The video below is a 2015 lecture by Cassini Imaging Team Lead Dr. Carolyn Porco. Even though Cassini’s Grand Finale happened in 2017, this talk lays out the logic of the whole mission and shows the key discoveries as they were being made.
As you watch, look for answers to these questions (you can pause and take notes as needed):
- Which planets and moons were used for Cassini’s gravity-assist trip out to Saturn?
- What did Cassini discover soon after arrival that wrapped all the way around Saturn and then drifted?
- What produces Saturn’s north polar hexagon — and what common explanations does Porco say do not work?
- How much mass is actually in Saturn’s rings, and why is that number important for working out whether the rings are young or ancient?
- What creates gaps, waves, and “propeller” structures in the rings? How do embedded moonlets behave?
- Why was the edge-on view of the rings so valuable, and what did it tell us about tiny moons such as Daphnis?
- What’s the difference between Saturn’s inner moons and its outer / irregular moons, and why are the inner moons such a high science priority?
- How do ring features and the crater record on Mimas help us think about how the rings formed?
- What were the science results of the Huygens landing on Titan (January 14, 2005)? When were Titan’s liquid methane–ethane lakes first confirmed?
- What did Cassini find by flying through the Enceladus plume, and why did the detection of molecular hydrogen matter?
- And finally: which famous astronomer’s idea inspired the photo The Day the Earth Smiled?
Note: this talk is long, but it’s the single best narrative of the Cassini mission. Students who watch it carefully tend to do very well on Saturn / rings / Enceladus / Titan questions.
Saturn is the best-observed of the two giants because of Cassini; Jupiter, in contrast, has been visited many times — Pioneer 10/11, Voyager 1/2, the long-lived Galileo orbiter in the 1990s, several New Horizons flyby observations on its way to Pluto — but the current, deep, pole-to-pole explorer of Jupiter is Juno.
NASA’s Juno spacecraft arrived at Jupiter in July 2016. It was designed to fly over the poles in a long, looping orbit and to measure Jupiter’s gravity field, magnetic field, auroras, deep atmospheric flows, and the structure of the Great Red Spot. In 2021 NASA extended Juno’s mission to 2025 so it could make close passes not only of Jupiter but also of several of its moons, including spectacular late-mission flybys of Io that caught major eruptions in late 2024 and early 2025. (That is, Juno is now doing some of the work we once hoped a dedicated Io mission would do.)
Before Juno arrived, the team released a time-lapse made from its approach images — it’s still an excellent way to show students the scale of the Jupiter system and to remind them that the Galilean moons are planets in their own right:
Juno’s instruments are measuring:
- Jupiter’s gravity and interior structure (is there a fuzzy/diffuse core?)
- its auroras and magnetosphere — thousands of times more energetic than Earth’s
- its ring system (yes, Jupiter has one!)
- its polar cyclones and the vertical structure of its belts and zones
- and, in the extended mission, the surfaces and environments of Europa, Ganymede, and especially Io
Because Jupiter formed first and contains more mass than all the other planets combined, every new constraint on its interior helps us tell the story of how the Solar System formed.
Learning Activity — Juno, plus what’s coming next
The Juno mission website is still being updated during the extended mission. There you can browse JunoCam images, see which targets the public has voted on, and watch short mission videos.
To get a concentrated dose, watch the stitched YouTube compilation of the Juno scientist/engineer interviews (the interviews start around 11:18):
Then watch the Why With Nye! playlist for student-level explanations of the mission goals:
As you watch, take notes on:
- How Juno’s polar orbit is different from Galileo’s equatorial orbit in the 1990s.
- What Juno is telling us about Jupiter’s magnetic field and how it compares to Earth’s dynamo.
- How Juno’s close flybys of Io are helping to connect tidal heating, volcanism, and Jupiter’s auroras.
- Why we still need future missions — especially ESA’s JUICE (on its way to Ganymede) and NASA’s Europa Clipper (launched 14 October 2024, arriving early 2030s) — to finish the job on the icy moons.
Finally, exploration of the outer planets is far from over. After Cassini and Juno, a new generation of spacecraft is on the way. NASA’s Dragonfly rotorcraft will launch later this decade to fly across the surface of Titan, studying its organic chemistry and methane lakes. At Jupiter, ESA’s JUICE mission (JUpiter ICy moons Explorer) is already en route to study Ganymede, Callisto, and Europa in detail. And NASA’s Europa Clipper, launched in October 2024, will arrive in the early 2030s to make dozens of close flybys of Europa, searching for evidence of a subsurface ocean and the conditions for life. These missions—together with the data still coming from Juno—mean that our understanding of the Jovian and Saturnian systems will continue to grow for many years to come.
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