Our Solar System: Saturn
30 June 2004, 7:30 pm. The VIP room at JPL is quiet as we all watch a thin line trace horizontally across the big screen at the front of the room. It’s the radio signal from NASA’s Cassini spacecraft as it speeds toward Saturn Orbit Insertion (SOI) after seven years in transit. JPL invites some folks to planetary encounters and landings, so we often get an eclectic group of space enthusiasts like June Lockhart (the original matriarch of Lost in Space), Jeff Bezos, and will.i.am of the Black Eyed Peas. At this encounter, I’m one of the scientists available to provide commentary and answer questions. What we hope to see is a dip in the signal indicating that its frequency changed due to the Doppler effect. That would tell us the spacecraft slowed down and was captured by Saturn. It would also tell us the spacecraft survived crossing the ring plane where even a small particle could end the mission before it started. Sure enough, right on time the signal dipped and we all cheered. That was SOI; the start of 13 years of intense scrutiny of Saturn, its rings, and its many moons, which held many surprises for us.
Saturn had been visited before by Pioneer 11 in 1979, Voyager 1 in 1980, and Voyager 2 in 1981. All of those were fly-bys, but they were enough to pique our interest in Saturn, its rings, and its system of moons, particularly Titan. Like Jupiter, Saturn itself is made up almost entirely of gasses like hydrogen and helium, with minor amounts of methane, ammonia, acetylene, and other gasses that give its clouds some color. Saturn is even gassier than Jupiter, though, and its overall density is actually less than water – it would float if you had a bathtub big enough. The rings are Saturn’s big claim to fame and were even observed by Galileo 400 years ago. They’re easily seen in a small telescope, though they disappear when they’re occasionally edge-on. One of the big mysteries of the rings has been, how are they maintained? Models show that the rocks and dust that make them up should all fall into Saturn over time. But the Cassini mission found that they were far more dynamic than originally thought: Waves were seen flowing around them and many tiny moons were discovered in the gaps between rings that are now known as “shepherd” satellites as they keep the rings from disintegrating.
For a geologist like myself, the larger moons were of the most interest. Especially Titan, the largest (about the size of our moon) and only the third body in the solar system to have a significant atmosphere: one and a half times Earth’s. Annoyingly, Titan’s atmosphere contained gasses and aerosols that mimicked thick smog so the surface couldn’t be seen with optical instruments. That’s why Cassini came armed with an imaging radar that could “see” through the clouds and image the surface. We were excited about the first views of Titan’s surface as the previous fly-bys and observations from Earth had shown that the atmosphere contained a lot of methane (aka natural gas) and the temperature out there at Saturn’s orbit was about -290° F. That’s the point where methane can exist as a gas, liquid, or solid. In other words, methane could act on Titan like water does on Earth. There could be rivers and lakes of liquified natural gas on Titan! The only way to know was to pierce the veil and map the surface. Finally, on October 26, 2004, Cassini was directed close enough to Titan to capture a radar swath across the moon. My colleagues in the Cassini Radar Science Team and I sat glued to our computers as the radar processing team sent out the first swath. As we scrolled down the long strip, we discussed via telecon what we were seeing. We realized we were the first people in history to see the face of Titan. Unfortunately, we didn’t see any rivers or lakes that day, though we did discover numerous sand dunes near the equator. Later, as the Cassini Navigation Team worked their magic and the spacecraft caromed around the Saturn system getting orbital nudges from one moon so the orbit would pass by another moon, we eventually mapped much of Titan’s surface and sure enough, huge lakes appeared in the northern polar region, as big as our Great Lakes.
Titan still has its mysteries: We still don’t know what the dunes are made of, though it’s likely the “sand” is actually water-ice particles. We also don’t understand the dynamics of the large lakes. To address the lakes, a colleague with whom I’ve sailed a lot proposed a probe to Titan that would parachute down to a lake and float around, collecting data on depth and currents. That idea wasn’t accepted, but a later proposal to send a drone was. It’s called Dragonfly with a planned launch in 2027 and arrival at Saturn in 2036.
Titan wasn’t the only moon with a surprise for the Cassini team. As the spacecraft approached Enceladus for the first time, the sun illuminated wispy plumes shooting up from the south pole. Images of the area showed huge fractures in the icy crust and no impact craters: a sign that the surface was young. As the mission went on, the Navigation team got bolder and eventually commanded the spacecraft to actually pass through the plumes, passing only 30 miles above the surface of Enceladus. That allowed sensors to determine something about the composition of the plumes. They were found to be composed of water vapor and many complex carbon-containing compounds chemists call “organic,” through that’s because the compounds contain carbon atoms, not because they’re alive. The source of the heat and water is still unknown.
After 13 years orbiting Saturn, Cassini was running low on fuel and it was decided to command the spacecraft to spiral slowly into Saturn. The mission team was understandably emotional about the end of such a long-term relationship: years of planning, seven years in transit, and 13 years in orbit. JPL made an Emmy award-winning film about the final days called “Cassini’s Grand Finale,” which can be found on YouTube. Incredible images were obtained as the spacecraft closed in on the rings and Saturn’s cloud tops. The last signals were sent on September 15, 2017.