Cassini: the Beginning of the End

A Long-running Cassini Enters Its Final Orbits

Saturn and its rings

Saturn and its rings, as seen by Cassini on April 3, 2017. Courtesy Cassini Mission.

It has been nearly 20 years since the Cassini Mission to Saturn started its grand adventure of exploration. Folks, this has been a mission for the record books. Not only has it been extended several times (and for good reasons!), it has been giving us almost a “Webcam at Saturn” view for 13+ years. That’s a pretty good return for the money and time!

Saturn is a fascinating place. The more we study it, the more we learn about its rings, atmosphere, and collection of moons. Unfortunately, the spacecraft is running low on fuel, and when that happens, the mission ends. On September 15, 2017, the spacecraft will complete its final orbit and spiral into the massive atmosphere of Saturn. Between now and then, the spacecraft has a lot to do. For one thing, it will nudge into ever-closer orbits of the planet. It will pass close by Titan, the shrouded moon that the Huygens lander revealed to us early in the mission. Eventually, the spacecraft will be doing deep dives between the rings and the planet, and send back ever-more detailed images of both.

Mission planners are calling this “Cassini’s Grand Finale”, and for good reason. NO other spacecraft has gotten this close to Saturn, and given the vagaries of space exploration funding and planning these days, it’s not likely we’ll see another do the same thing for some years.

So, Why Does Cassini Study Saturn So Closely?

You might wonder why the multinational team and NASA decided to spend so much time on one planet? Actually, planetary scientists have spent a LOT of time at nearly all the planets except Uranus, Neptune, and, of course, distant Pluto — which was the latest one to be explored. In the cases of Uranus and Neptune, they also get to watch them through orbiting space telescopes, but the Voyager 2 mission did give close-up images and data.

The reason planetary scientists do this is to observe change over time, that is — the evolution of a planet’s surface, atmosphere, and other characteristics. It’s not like you go out, take a snapshot, and that tells you everything you want to know about a world. No, if you’re smart and have the capability, you look at it for a long time, under changing conditions. We do the same with Earth’s climate and surface features. And, as anybody interested in space exploration knows, we also do it with Mars, at Venus, and Jupiter.

In the case of the Cassini mission, the rewards for watching such a long time have been amazingly great:

1) as I mentioned above, the spacecraft’s Huygens lander showed close-up details of Titan, and the analysis of Titan data shows that while it’s a frozen desert, it’s quite Earth-like in many ways. So, studying Titan gave us another way of understanding how weather, climate, and geologic systems work there, and also on Earth. That’s a win.

2) Cassini has watched the Saturnian weather systems through a good part of the 29-Earth-year-long Saturnian year, giving insight into how it changes as the planet orbits the Sun; that’s a win for learning more about weather processes in the outer solar system.

Saturn propellor seen by cassini

A “propeller” of ring material swirling around a tiny moonlet near the Encke Gap in Saturn’s rings. This one is named “Earhart” after Amelia Earhart, a famous aviator in the early 20th century. Courtesy Cassini Mission.

3) The rings! OMG, the rings. They win just for being so gorgeous and intriguing. The Voyager spacecraft visits whetted our appetites for those rings; Cassini followed up with some incredible closeups. We may not yet know exactly what objects collided to form those rings, but planetary scientists can now tell us how those ring particles react and behave. Cassini images show how the shepherding satellites corral the rings. In the case of one ring, those images how the particles spewing from the moon Enceladus make their way to orbit around Saturn to form the ring.

4) What about those fascinating moons! I just mentioned Enceladus: it tops the list as a geologically active, water-rich world. That’s the one spewing ice particles to the E ring. Titan, of course, is a huge prize, despite those heavy clouds. It continues to fascinate us with its frigid surface. At least one other moon may have come from the outer solar system, and gotten captured into Saturn orbit millions of years ago. And, the list goes on — the moons themselves could be the subject of another mission, if we had the time, money, and equipment.

5) Let’s not forget the constant study of the magnetic field and radiation environment around Saturn over the years!  And, the aurorae that sizzled at its polar regions. I remember when discovering aurorae at any planet other than Earth was a new thing. Now, it’s completely obvious that planets with magnetic fields can experience them.

Science during Cassini’s Grand Finale

When September 15 rolls around, Cassini will be doing science right up to the end. The plan is to measure the atmosphere as the spacecraft plunges through Saturn’s cloud tops. It will transmit back information until the pressure is more than it can bear. This is the safest way to dispose of the spacecraft, since its rocket fuel will have run out. Scientists do not want to risk inadvertent biological contamination of the moons by letting Cassini drift in the system without guidance.  Better to let it plunge into Saturn and give us the first in situ look at the clouds as it roars through. The spacecraft probably won’t last very long after it enters the atmosphere, so it will be interesting to grab a look when we can.

You can follow the mission at the Cassini Web page, and monitor the latest images and data. I can confidently predict that the pictures are going to be spectacular and well worth the wait!

(Note: I received some emails from folks about  my recent absence from the pages here. I’m working on a book, which has engaged my writing chops in overdrive; however, I’ll endeavor to post here more often. Thanks for the notes!)

Sailing Over Titan’s Seas in Summer

Cassini Captures Cloud Movement Over Ligeia Mare

An animated gif of clouds moving across Titan's northern Ligeia Mare (sea). Watch the clouds over the dark area lower center/right. Credit: NASA/JPL-Caltech/SSI

An animated gif of clouds moving across Titan’s northern Ligeia Mare (sea). Watch the clouds over the dark area lower center/right. Credit: NASA/JPL-Caltech/SSI

If you’ve ever been aboard a sailing ship, you probably know the sensation of the craft cutting through the ocean, wind at your back and a breeze in your face. It’s probably the same sensation you get when you go hang-gliding, or water (or snow) skiing.

It turns out, if you lived on Saturn’s icy but intriguing moon  Titan, you could experience the same sensations (provided you could survive the atmosphere and cold temperatures). Of course, your ship would need to be able to withstand the frigid methane sea, and the cold, largely nitrogen (with small amounts of methane and hydrogen) atmosphere.

The clouds would be made of methane, possibly some ammonia, and other hydrocarbons. Feeling the breeze on your face would require you to withstand an atmospheric pressure about 1.5 times that of Earth’s sea level, and near-surface temperatures of about 94 Kelvin (-297 F, or -179 C). Not impossible, but right now, pretty improbable. That’s why we have the Cassini-Solstice Mission — to give us a spacecraft-eye view of what it might look like from above.

So, what would a cloudy, breezy sea day on Titan be like? Cassini scientists just released an animation of clouds blowing across the surface of the northern Titan sea called Ligeia Mare. In the sequence (which you can see here), the clouds blow just over the hydrocarbon-rich sea at speeds of around 7 to 10 miles per hour (3-4.5 meters/second). These images were taken a few weeks ago (late July), and the formation of the clouds and their actions may be harbingers of summer on Titan.

Titan does indeed have seasons during its 30-Earth-year-long year. Each of those seasons lasts about 7 Earth years, giving plenty of time for seasonal change to occur. When Cassini first arrived at Saturn and began studying this moon, its northern pole was pointed away from the Sun, which put it in high winter. At that time, the north polar region was shrouded with a hazy hood. There was a lot of cloud activity in the southern hemisphere (during its summer, when things were a bit warmer (relatively)).

As equinox approached, when both northern and southern hemisphere Titan got equal amounts of light and heat from the Sun, the northern polar hood shrank. Cloud activity continued for a while, until the passing of a large storm in 2010. Then, cloud activity dropped quite a bit. In the approach to northern hemisphere summer (southern hemisphere winter), the northern hood nearly disappeared, and now that we’re starting to see northern summer and southern winter. This latest discovery of clouds above a northern hemisphere ocean could signal summer weather patterns. Their appearance also leads the science team to speculate about whether (or how) the clouds are rela ted in some way to the seas. It’s possible that clouds form over the seas as a matter of course, but it’s also possible that Cassini just happened to catch some clouds racing over the ocean surface as part of a larger-scale circulation pattern.

Cassini will continue studying atmospheric change at Titan during the upcoming northern hemisphere summer (southern hemisphere winter). Already it has given us a great deal of information about the only other world in the solar system (besides earth) that has a fully developed atmosphere (and could possibly be habitable to certain forms of life). Stay tuned!