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The realLord of the Rings is Saturn, a massive outer planet boasting a set of rings about 27Earths wide. Being a gas giant likeJupiter, Saturn shares many of its attributes: a strong magnetic field generated by churning metallic hydrogen deep inside, raging storms in its gaseous upper atmosphere, and a diversity ofplanet-like moons that are worlds unto themselves. Saturn’s rings and larger moons are visible even from small backyard telescopes.
Saturn was born roughly 4.5 billion years ago in the Solar System’s early days. Both Saturn and Jupiter probably formed closer or farther from the Sun than they are today, then migrated to their current positions later. Their gravitational influence likely flung asteroids and comets all over the Solar System, some of which slammed into early Earth and may have delivered water here.
We know ofmore than 5,000 exoplanets — worlds orbiting beyond the Solar System — and the statistics show that most stars have planets. Many are Jupiter and Saturn-like worlds close to their stars. One exoplanet we’ve found appears to haverings 200 times wider than Saturn’s. What we learn about Saturn helps us understand similar exoplanets, and vice versa.
Saturn’s rings are mostly made of water ice, ranging from the size of a speck of dust, to a house, to a mountain. The largest chunks, known as moonlets, have enough gravity to clear small gaps,distort the rings’ shape, and causewave-like disturbances.
Scientists thinkSaturn’s rings formed when a large moon got too close to Saturn and was ripped apart due to intense gravitational forces — or. that the rings are leftover material from when Saturn formed. The rings are gradually being pulled into Saturn and willdisappear completely within 300 million years. The other giant planets — Jupiter, Uranus, and Neptune — each have their own ring systems, though none rival the grandeur of Saturn's.
Like our other three outer planets, Saturn is a solar system unto itself, with many compelling targets to study.
In addition to boasting an impressive ring system, Saturn hasat least 124 moons, the largest of which areactive, planet-like worlds.
Larger than Mercury,Titan has an orange, hazy atmosphere that may be similar to what Earth’s was like before life arose here around 3.5 billion years ago. Complex, organic molecules — one of the building blocks for life as we know it — form Titan’s atmosphere and rain from the skies. By studying Titan, we can learn more about potentially habitable environments for life throughout the Universe.
The surface of Titan is strikingly similar to Earth, with some key differences: Titan hasmountains and dunes made of mostly ice rather than rock, andrivers,lakes and seas filled with methane and ethane instead of liquid water.A liquid-water ocean probably exists beneath the surface, though we don’t know if the chemical mix is conducive to life, or if there’s an energy source to power life, like thehydrothermal vents in Earth’s oceans.
At just 500 kilometers (310 miles) wide,Enceladus could fitcomfortably inside the U.S. state of Arizona. But this moon has a bigsecret under its icy crust: a saltwater ocean, which leaks into spacevia geysers on the surface to form one of Saturn’s outer rings. We knowthe ocean contains complex organic materials.Could it also containlife?
Mimas boasts a massive crater that makes it eerily similar to the Death Star fromStar Wars. It too may have a subsurface ocean, though material from the ocean does not appear to leak up to the surface.
Some of Saturn’s smaller moons are among the most photogenic in our solar system.Dione andRhea are cratered snowball worlds with rocky cores.Hyperion looks like a giant sponge or coral reef.Iapetus is two-faced, with an icy half and a dark half coated with material coming from the comet-like moon Phoebe. And then there’sPan,a moon whose gravity has grabbed enough material from Saturn’s rings tomake it look like a flying saucer or dumpling, depending on who youask.
Christiaan Huygens first glimpsed Saturn’s rings and the planet’s largest moon, Titan, through a telescope in the 1650s. Shortly thereafter, Giovanni Cassini found 4 more moons and the planet’s largest ring gap, now named the Cassini Division in his honor.
NASA’s Pioneer 11 was the first spacecraft to visit Saturn, flying past the planet in 1979 and revealing yet another outer ring. Voyager 1 flew by a year later, swinging by Titan to get a good look at the moon’s thick orange atmosphere. Voyager 2 flew closer to Saturn itself, discovering the planet’s upper atmosphere was a chilly -200 degrees Celsius (-328 degrees Fahrenheit), and finding trace amounts of ammonia crystals that give Saturn its pale yellow hue.
In 2004, Cassini-Huygens, a joint robotic mission by NASA and the European Space Agency, became the first spacecraft to orbit Saturn. One of the most revolutionary space missions of all time, Cassini spent 13 Earth years — almost half a Saturn year — watching how the planet and its moons changed with the seasons as they orbited the Sun.
On Saturn itself, Cassini studied thehexagonal storm at the planet’s north pole first seen by Voyager 2 in 1981, anddiscovered a smaller, circular vortex on the south pole. The north pole storm is remarkably symmetric and has a central eyewall akin to Earth’s hurricanes. Cassini also observed aplanet-wide megastorm that appears roughly every Saturn year—30 Earth years. The spacecraft alsomapped the structure and shape of Saturn’s magnetic field and narrowed downthe planet’s rotation rate — less than half an Earth day, even though Saturn is 9.5 Earths wide.
Shortly after its arrival at Saturn, Cassini released the European Huygens probe, which landed on Titan’s surface in 2005 — a first for any world in the outer Solar System. As Huygens descended, it gathered data on thecomplexchemistry happening in Titan’s atmosphere. Post-landing, the probe took the first-ever images from Titan’s surface and survived for 2 hours, despite frigid temperatures of about -180 degrees Celsius (-292 degrees Fahrenheit).
As it performed long, swooping orbits around Saturn, Cassini repeatedly flew by many of the planet’s moons. The spacecraft’s cloud-penetrating radar pierced Titan’s orange haze, allowing scientists to create aglobal geologic surface map. Gravity measurements by Cassini and radio measurements by Huygens showed that Titan likelyharbors a large subsurface ocean of water. Cassini also directly imaged Enceladus spewing water from its subsurface ocean out into space. Mission operators flew the spacecraft directly through a plume, leading to thediscovery of organic materials using Cassini’s onboard mass spectrometer, which determines the chemical makeup of materials passing through it.
Cassini performed what NASA dubbed the "Grand Finale" of its mission in 2017: a number of passes between Saturn and its inner rings. In these close encounters, Cassini measured the mass of the rings based on how the spacecraft was gently tugged toward them, and found them to weigh less than even the small moon Mimas, which is just 200 kilometers (124 miles) wide. Combined with the fact that the rings are bright, undarkened by constant space weathering, this hinted that they may beless than 100 million years old, which would be very young in geological terms. Cassiniended its mission with an intentional plunge into Saturn in September 2017, becoming a permanent part of the planet it was sent to study.
Cassini left an impressive legacy for future missions. As an all-purpose flagship spacecraft, it was designed to answer general questions about Saturn and its moons, and help us figure out questions for new missions to answer.
The next — and at the moment, only — spacecraft heading to the Saturn system isDragonfly. Dragonfly is a NASA mission to Titan scheduled to launch in June 2027. The spacecraft is an eight-bladed drone-like craft called a quadcopter that will make short flights around the surface.
Dragonfly will study the chemicals that rain from Titan’s atmosphere onto the surface. Because we think Titan’s atmosphere is similar to Earth’s when life arose around 3.5 billion years ago, the mission will help us understand possible environments for life here and elsewhere.
Life as we know it needs three things: an energy source like sunlight, a liquid solvent like water, and complex, organic molecules that bond with one another. Titan has the organic molecules, but it’s very cold, and hosts liquid methane and ethane on its surface instead of water. Dragonfly will, in a sense, be studying an alternate version of Earth. It will see how Titan relates to life as we know it — and to theoretical forms of life unlike anything we’ve ever imagined.
Acknowledgements: This page was initially written by Jatan Mehta in 2020.
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