Cassini

With discoveries at Saturn’s moons Enceladus and Titan, Cassini and Huygens made exploring “ocean worlds” a major focus of planetary science. Insights from the mission also help us look for potentially habitable planets -- and moons -- beyond our solar system.

Life as we know it is thought to be possible in stable environments that offer liquid water, essential chemical elements, and a source of energy (from sunlight or chemical reactions). Before Cassini launched in 1997, it wasn’t clear that any place in the icy outer solar system (that is, beyond Mars) might have this mix of ingredients. By the next year, NASA’s Galileo mission revealed that Jupiter’s moon Europa likely has a global ocean that could be habitable. Since arriving at Saturn in 2004, Cassini showed that Europa isn’t an oddball: Potentially habitable ocean worlds exist even in the Saturn system -- 10 times farther from the sun than Earth.

When the Cassini mission started, scientists presumed Enceladus was too small to generate and hold onto the heat required to maintain subsurface reservoirs of liquid water. Cassini’s discovery of intense geologic activity near the moon’s unexpectedly warm south pole—complete with towering jets of icy spray—sent shockwaves through the space science community. After over a decade of investigation, the mission eventually determined that Enceladus hosts a global liquid water ocean, with salts and simple organic molecules, and likely even hydrothermal vents on its seafloor. Thanks to Cassini, Enceladus is now one of the most promising places in our solar system to search for present-day life beyond Earth.

Saturn’s largest moon, Titan, offered tantalizing hints that it, too, could help us understand whether life could have evolved elsewhere. Cassini and ESA’s Huygens probe (which landed on Titan’s surface) found clear evidence for a global ocean of water beneath Titan’s thick, icy crust and an atmosphere teeming with prebiotic chemicals. Based on modeling studies, some researchers think Titan, too, may have hydrothermal chemistry in its ocean that could provide energy for life. On its frigid surface, which hosts vast seas of liquid hydrocarbons, scientists wonder, could Titan be home to exotic forms of life “as we don’tknow it”?

Titan is 10 times farther from the sun than Earth and much colder, but Cassini showed it to be the only other place in our solar system with stable liquid on its surface and a kind of “hydrological” cycle involving methane rather than water.

Flowing liquid hydrocarbons at Titan make for eerily Earthlike landscapes -- they carve branching channels and steep canyons into rock-hard ice; they settle into lakes and seas with gently sloping shorelines and sheltered bays; they tumble water-ice “rocks” into rounded pebble shapes like those in earthly rivers.

Titan’s landscape also shares other similarities with Earth. Large, arid swaths of dunes gird the moon’s equatorial regions. Composed of organic materials that settle out of Titan’s thick, hazy atmosphere, these dunelands are sculpted by winds in ways similar to dunes in places like Namibia and the Sahara. Scientists have also spotted volcano-like mounds that, if indeed volcanic in nature, would erupt slushy lavas made of water rather than molten rock.

From its perch in space, Cassini watched Titan’s climate cycle play out over the years, with seasonal changes bringing bright, feathery methane rain clouds that dump precipitation on the landscape. Huygens saw clear evidence of a landscape that experiences intermittent but heavy floods, not unlike places in the American desert southwest.

Titan's smoggy atmosphere resembles an extreme version of the skies above Los Angeleson a day with poor air quality. And, more importantly, Titan’s atmosphere is thought to be similar to early Earth's before life developed here. Titan provides perhaps the best stage in the solar system to watch the organic chemistry that led to the origin of life on Earth billions of years ago. Titan can also be considered a possible analog for the future Earth. Its methane cycle gives us a hint of what Earth’s water cycle might look like in the far future as the increasingly brighter-burning sun changes the stability of water in our oceans and atmosphere. The seas at Titan’s poles might be remnants of larger bodies of liquid that once covered much more of the moon’s surface.​

Cassini provided a brief glimpse into deep time in the Saturn system. The rings, for example, are a natural laboratory for processes that form planets -- a mini solar system, if you will. They show us how objects clump together and break apart. And in the ripples we can read the history of impacts into the rings. We also see “propeller” features that obey the same physical processes that form planets.

Moons in the Saturn system are also time capsules preserving histories of bombardment and other forces at play over time. At Titan, in particular, we have access to the kinds of complex carbon chemistry that might have taken place on Earth in its “prebiotic” days. From the Cassini mission’s finale, new data about the planet’s interior and the mass of the rings will provide a powerful insights about their formation and evolution.

While other missions flew past Saturn or trained telescopes periodically from afar, Cassini had a front-row seat for approximately 13 years -- nearly half a Saturn year (northern winter to the start of northern summer) -- to epic changes unfolding before its very eyes.

This long-lived robotic observing platform, bristling with science instruments, provided an unparalleled glimpse into what happens as weather and climate conditions on the planet and Titan respond to the seasons -- sometimes rather abruptly. Among the most amazing changes Cassini captured: the eruption of a once-every-30-years storm (one of the most powerful ever seen in the solar system), methane rainstorms at Titan and the appearance and disappearance of features such as the “magic island.”

Over a longer span of years, the color of Saturn’s northern hemisphere shifted as the ring shadows retreated southward -- changing from the surprisingly bluish tones seen upon arrival to the hazy, golden hues most observers are familiar with. On Titan, Cassini witnessed a vortex filled with complex organic chemicals forming over its south pole, and saw sunlight glinting off of the lakes in its northern hemisphere as the sun rose over them.

The spacecraft’s patient eyes also were rewarded with new views of Saturn’s north pole as winter ended there and the sun rose once more. Cassini’s infrared sensors measured temperatures across the rings as the sun set on one side and rose on the other, revealing new details about the structure of ring particles. It used the onset of wintry darkness at the south pole of Enceladus to obtain an unambiguous reading of the amount of heat coming out of the moon’s interior. And it saw the mysterious ring features called spokes (wedge-shaped features in the rings that rotate along with the rings like the spokes in a wheel) appear and disappear -- apparently a seasonal phenomenon.

Planet-size Titan and diminutive Enceladus stood out in Cassini’s in-depth survey of Saturn’s moons. But the mission showed that every moon in the Saturn system is a unique character with its own mysteries, and many of Saturn’s satellites are related in surprising ways.

For example, Cassini data enabled scientists to confirm earlier suspicions that Phoebe is likely an object from the outer solar system beyond Neptune, captured by Saturn’s gravity long ago. Phoebe also turns out to be key to the two-toned appearance of the moon Iapetus: As Phoebe sheds its dark dust, it coats the leading side of Iapetus and causes ice to heat up and migrate to the moon’s opposite side.

Cassini also gave scientists a better understanding of why Hyperion looks like a giant sponge or wasp’s nest tumbling through space. Researchers determined that the moon’s density is so low that impacts tend to compress its surface rather than blasting it out, and the material that is launched into space tends to escape for good, thanks to Hyperion’s low gravity.

Cassini found that Enceladus is not only active, but that its geologic activity is creating Saturn’s E ring and spray-painting the surfaces of several of the other moons with its highly reflective ice particles.

The mission also followed up on a mystery from the early 1980s when NASA’s Voyager spacecraft flew by the Saturn system and saw bright wispy terrains on Dione. Cassini found that the features were in fact a vast network of canyons. Cassini also detected hints of a faint atmosphere that might have been outgassed from the moon’s interior.

And Cassini watched closely over many years how Prometheus interacts with Saturn’s F ring to create features like “streamers,” “plumes” and “drapes.”

Although Cassini scientists are still working on determining the exact origin of Saturn’s main system of rings -- and hope to answer this question with data collected during the spacecraft's final orbits -- they have learned along the way that there are in fact, many ways to form rings around a planet.

There is a diffuse ring that is created out of the bits of water ice jetted out by the moon Enceladus (the E ring). There are rings that were created because of the material thrown off when meteorites hit moons (such as the G ring and the two rings discovered by Cassini in images from 2006 -- the Janus-Epimetheus ring and the Pallene ring). There are rings controlled by interactions with moons, like the F ring, which is regularly perturbed by Prometheus, and the narrow ringlets that share the Encke Gap with Pan.

In addition to the rings’ origins, Cassini’s close-up examination has also revealed propeller-shaped features that mark the locations of hidden moonlets. The processes involved in the formation of such objects are thought to be similar to how planets form in disks around young stars.

Cassini also helped explain Saturn’s “spokes,” first spotted during the Voyager flybys of the early 1980s. Cassini scientists figured out that they are made of tiny ice particles that are lifted above the surface of the rings by an electrostatic charge, the way a statically-charged balloon held over a person’s head will lift hairs. Their charge appears to be related to the angle of sunlight striking the rings -- a seasonal effect.

The changing angle of the sun also showed scientists an array of vertical structures in the rings, including fluffy peaks of material as high as the Rocky Mountains at the outer edges of the A and B rings. The vertical structures and the shadows they cast also revealed wavy patterns in the parts of the rings that resemble a miniature Milky Way, giving scientists insight into the way galaxies form.

You can only get to know a planet so well with remote and sporadic observations. To truly understand the dynamics of a place as complicated and interesting as Saturn, you have to go there and stay to explore.

Towering jets of ice and water vapor pouring out of a moon as tiny as Enceladus were a huge surprise (explaining why Voyager flybys in the early 1980s saw that the moon had a young surface), as was the later finding that the moon has an ocean under its icy crust. Scientists also had not expected to find Saturn’s magnetosphere -- the region around the planet strongly influenced by Saturn’s magnetic field -- to be filled with an electrically excited gas, or plasma, of oxygen. It turned out this was another surprise from Enceladus, as the water vapor from its plume is broken apart by sunlight and the liberated oxygen spreads out through Saturn’s magnetic bubble. Cassini detected this oxygen on approach to Saturn, but its origin was perplexing at first.

No one knew for sure what kind of environment ESA’s Huygens probe would find when it came to rest on Titan’s surface, so Huygens was built either to land on hard ground or float, if need be. Cassini later showed scientists that most of the moon’s lakes and seas were near the north pole, and most of the moon’s landscape was more like the Arizona desert. Cassini also observed a surprisingly rich variety of complex, organic chemicals forming in Titan’s atmosphere.

Another unexpected finding -- which endures as a mystery -- is the irregularity of Saturn’s day (how long the planet takes to make one rotation on its axis). At Jupiter, a beacon-like burst of radio waves known as “kilometric radiation” beams out with clock-like regularity once a day. But Saturn’s kilometric radiation isn’t consistent. It’s somewhere between 10.6 and 10.8 hours. That might not seem like a big discrepancy, but for such a fundamental property as the planet’s rotation period, it’s frustratingly imprecise for scientists. They hope to settle the matter after thoroughly analyzing instrument data from the spacecraft's final orbits, when Cassini flew close enough to the planet to tease out the true answer from the magnetic field.

The Cassini-Huygens mission is an international collaboration involving three space agencies, with 19 countries contributing hardware to the flight system. The Cassini spacecraft carried 12 instruments, Huygens carried six more, and scientists from 26 nations are participating in the investigations. Among the many pioneering technologies of the mission were new solid-state data recorders with no moving parts that have since replaced tape recorders, solid-state power switches (space-based versions of circuit breakers), and advanced solid-state electronics. The spacecraft had over 9 miles (14 kilometers) of cabling and 22,000 connections.

Cassini was able explore the entire Saturn system in a way inconceivable with conventional propulsion. Building on the techniques used by the Galileo mission to Jupiter, Cassini mission planners designed flybys of the moon Titan to utilize the moon’s gravity to navigate around the Saturn system and maximize the science return of the mission. Titan became, in a way, Cassini’s virtual “gas station” since the spacecraft couldn’t possibly have brought enough fuel for a tour this long and complex. Each of Cassini’s 127 targeted Titan flybys changed the spacecraft’s velocity (on average) by as much as the entire Saturn orbit insertion burn. The exquisite optimization techniques developed during Cassini will enable planning for future exploration that can use similar approachs. Chief among these opportunities is NASA’s planned mission to explore Jupiter’s moon Europa using multiple flybys, known as the Europa Clipper.

Cassini required an extremely complex schedule for determining which instrument’s observations could be made at any given moment. Cassini’s intricate observation sequences, often timed to fractions of a second, were frequently planned many months or years before they were executed by the spacecraft. The collaboration between multiple teams with often differing objectives has become an exemplary model for future missions.

Over the course of almost 20 years in space, Cassini also showed that you can teach an old dog new tricks, as the mission team found new ways to use its instruments and engineering systems that their designers had not foreseen. These include using the radar instrument to plumb the depths of Titan’s seas; tasting the plume of Enceladus with instruments meant to sample Titan’s atmosphere; scanning the rings with a radar originally designed to bounce signals off of Titan’s surface; and having the Deep Space Network’s highly accurate frequency reference fill in for the radio science instrument’s lost ultra-stable onboard frequency reference. In a unique collaboration, the attitude control and navigation teams joined with the instrument teams to develop a consolidated model of Titan’s atmosphere. Cassini finished its mission by repurposing the instruments that sniffed Titan’s atmosphere and Enceladus’ plume once more, this time sampling the Saturn atmosphere itself.

The mission also had some rather surprising earthly benefits. A Cassini resource exchange, created prior to launch to help team members trade and effectively share power, mass, data rates and budget, has become a model for how to manage other types of international collaboration, including carbon trading.

When Cassini plunged into Saturn’s atmosphere, it had spent nearly every last drop of fuel it was carrying, a fitting end to a spacecraft that pushed itself to the limit...and in many ways, beyond.

Earthlings have cast their gaze upward at Saturn since ancient times, but it was Cassini’s decade-plus odyssey in orbit there that revealed the true splendor of what is arguably the most photogenic planet in our solar system.

The mission returned stunning views of complex, swirling features in Saturn’s atmosphere, draped by the graceful ring shadows that slowly shift with the seasons.

The spacecraft also revealed the bewildering variety of Saturn’s moons and helped us see each one as a unique world in its own right. One has a noticeable ridge around its equator and a two-toned color pattern (Iapetus); one looks like the “Death Star” from Star Wars (Mimas); one looks like a sponge (Hyperion); another looks like a flying saucer (Atlas); another looks like a potato (Prometheus); another looks like a ravioli (Pan).

Cassini showed us icy ringscapes that are at once magnificent in their sheer physical extent and exquisitely delicate in their expression of the subtle harmonies of gravity. These ringscapes mesmerize with the myriad designs embossed in them -- the changing pattern of thick and thin, ruffles that stand as high as the Rocky Mountains, icy waves generated by small moons interacting with the rings, and “streamers" and “mini-jets” created in the ribbon-thin F ring by interactions with Prometheus.

Perhaps the most awe-inspiring views were panoramic scenes that encompassed the entire Saturn system, including those with the planet and rings backlit, and the tiny glow of our far-off, blue home planet visible far across the gulf of outer space.