Saturn is the second-largest planet in the Solar System and the most visually distinctive body humans have ever observed. Its largest moon, Titan, is the only place besides Earth where you can stand on a solid surface in a thick nitrogen atmosphere and watch it rain. The rain is methane. That turns out to matter enormously.
What Cassini Did
Almost everything known about Saturn at the detailed level comes from the Cassini–Huygens mission, which reached the planet in 2004 and operated there until 2017. Cassini was a NASA-led flagship with a major ESA contribution: the Huygens probe that descended through Titan's atmosphere and landed on its surface in January 2005 — the first, and so far only, landing on a body in the outer Solar System.
Over 13 years in orbit, Cassini mapped Saturn's ring structure at unprecedented resolution, confirmed and characterised the active plumes of Enceladus (see the Enceladus article), flew close to dozens of Saturnian moons, and ended its mission with a planned "Grand Finale" of orbits threading between Saturn and its innermost ring, followed by a controlled destruction in the atmosphere. The deorbit was chosen specifically to avoid any risk of the spacecraft ever contaminating Titan or Enceladus.
Cassini is a hard act to follow. No mission to the Saturn system is currently in flight. But the data it returned is still being worked through, and one major successor mission is in development for Titan.
The Rings
Saturn's rings are made almost entirely of water ice, with trace amounts of rocky material. They extend outward from roughly 7,000 km above Saturn's cloud tops to over 80,000 km, but the main rings are only about 10–30 metres thick vertically. If you shrank Saturn and its rings to the size of a dinner table, the rings would be thinner than a sheet of paper.
The rings are probably young, by Solar System standards — somewhere between 10 million and a few hundred million years old, based on their mass and the rate at which micrometeorite impacts dirty them up. They will also not last forever. Ring material is slowly drained into Saturn's atmosphere by gravity, and the rings are probably being lost on a timescale of a few hundred million years. The rings are, in effect, a temporary feature we happen to be around to see.
Titan in Detail
Titan is the second-largest moon in the Solar System and the only one with a dense atmosphere. That atmosphere is 94 percent nitrogen — a composition closer to Earth's atmosphere than any other body in the Solar System — and about one and a half times Earth's surface pressure. But Titan is cold: −179 °C at the surface, cold enough that methane and ethane exist as liquids on the ground.
The result is a methane hydrological cycle. Methane evaporates from liquid methane-ethane lakes, forms clouds, rains back down, erodes channels in water-ice "bedrock," and collects again in lakes. The lakes are real: Cassini's radar mapped dozens of them, mostly near the north pole. The largest, Kraken Mare, is the size of the Caspian Sea. Methane rainstorms have been observed from orbit.
Under the ice surface, Titan almost certainly has a global subsurface water ocean, similar in principle to Europa's. Titan is therefore two potentially habitable environments stacked: a water ocean deep down, and a weird but chemically rich methane surface above it. No other body in the Solar System offers that combination.
Dragonfly
Dragonfly is NASA's next flagship mission to Titan. It is a nuclear-powered rotorcraft — a drone, in plain terms — the size of a small car. Titan's thick atmosphere and low gravity make powered flight much easier there than on Mars; the same rotorcraft would be grounded on Earth but flies comfortably on Titan with modest power draw.
Dragonfly is planned for launch in 2028 and arrival at Titan in 2034. Once it lands, it will spend years doing short flights from site to site across the dunes near Titan's equator, sampling surface chemistry at each stop and looking specifically for complex organic molecules that might have pre-biotic relevance. It is the first powered, repositionable spacecraft ever to operate on another world, and it is deliberately designed around Titan's unique flyability.
The mission design also takes advantage of something Titan has that Mars lacks: a forgiving atmosphere. Titan's air is thick enough to parachute through easily, cold enough that thermal management is a heat-retention problem rather than a cooling problem, and radio-quiet enough that communication to Earth is straightforward over the long cruise distance.
Why This Matters
Saturn's system is the best nearby showcase of how much diversity a single planetary system can have. Its rings are a billion times older than any civilisation and may vanish before the next one arises. Enceladus is a small moon spraying its ocean into space. Titan is Earth-sized with a hydrological cycle that does not use water. Between the four of them — Saturn, the rings, Enceladus, Titan — there is more planetary science per light-hour than anywhere else in the Solar System outside the Jupiter system.