The Cassini-Huygens Space Probe and Its Mission to Explore Saturn and Its Rings
Launch and Journey to Saturn
Before Cassini-Huygens blasted into outer space, only three other probes had visited Saturn. Pioneer 10 was the first in 1979, beaming back only pictures. In the 1980s, Voyagers 1 and 2 also went by Saturn, taking limited measurements as they continued their mission to the outer planets and eventually to interstellar space (Gutrel 38). Named after Christiaan Huygens (who discovered Titan, a moon of Saturn) and Giovanni Cassini (who took many detailed observations of Saturn), the Cassini-Huygens probe was launched almost 20 years after the Voyager probes in October of 1997 (41-2). The combined probe is 22 feet in length, cost $3.3 billion, and weighs in at 12,600 pounds. It is so heavy that the probe needed gravity assists from Venus, Earth, and Jupiter just to get enough energy to arrive at Saturn, taking a total of 2.2 billion miles to make it (38). During this trip, Cassini-Huygens passed by the Moon in summer of 1999 and six months later went by Masursky, a 10-miles wide asteroid which, as discovered by the probe, differs chemically from the other asteroids in its region. In late 2000, the probe went by Jupiter and took measurements of its powerful magnetic field as well as photographing the planet (39). Finally, in June of 2004, the probe arrived at Saturn (42), and in early 2005 Huygens separated from Cassini and descended into the atmosphere of Titan.
During its mission, Cassini has implemented powerful tools to help unravel the mysteries of Saturn. These tools are powered by 3 generators containing a total of 72 pounds of plutonium that have an output of 750 watts total (38, 42). The Cosmic Dust Analyzer “measures the size, speed and direction of dust grains. Some of these bits may originate from other planetary systems.” The Composite Infrared Spectrometer “analyzes the structure of Saturn’s atmosphere and the composition of its satellites and rings” by looking at the emission/absorption spectrums, particularly in the infrared band. The Imaging Science Subsystem is what is used to capture images of Saturn; it has UV to infrared capabilities. The Radar bounces radio waves to the object, and then waits for the return bounce to measure terrain. The Ion and Neutral Mass Spectrometer looks at the atoms/subatomic particles coming from the planetary system. Finally, the Radio Science Subsystem looks at radio waves from Earth and how they change through Saturn’s atmosphere and rings (40).
These are but a small portion of what Cassini is capable of. Though originally designed for only 76 orbits, 1 GB of data per day, and 750,000 photographs (38), Cassini has seen its mission extended until 2017. Huygens has returned valuable data about Titan, which looks more like a primitive Earth every day. Cassini also has increased our knowledge of Saturn and the moons surrounding it.
Findings: Saturn's Atmosphere
In December of 2004, it was reported that a ring of radiation between Saturn's clouds and its inner rings was found. This was unexpected because radiation is absorbed by matter, so it is a mystery how it could have gotten there unscathed. Don Mitchell of John Hopkins University theorizes that positive charged particles such as protons and helium ions in the outer belt (themselves captured from cosmic sources) merged with electrons (negative particles) from the cold gas around Saturn. This creates neutral atoms that can move around in the magnetic field freely. Eventually, they lose their hold on electrons and will become positive again, potentially in that inner zone. Some could crash into Saturn, changing its temperature and potentially its chemistry. Later evidence from the end of Cassini's mission not only confirmed this but surprisingly found that the D ring had two moonlets (D73 and D68) which moved in this zone and effectively trapped protons that formed in this process due to different densities at play (Web 13, Lewis).
Anthony Delgenio, atmospheric scientist at NASA's Goddard Institute for Space Studies discovered through Cassini that Saturn has thunderstorms like those on Earth. That is, they too emit electrostatic discharges. Unlike Earth, the storms are 30 miles deep into the atmosphere (3 times deeper than on Earth). Cassini also measured the wind speeds at the equator, which clocked in at 230-450 mph, a decrease from Voyager 1's measurement of 1000 mph. Anthony is unsure as to why this change has occurred (Nething 12).
Another parallel to Earth weather was observed when Cassini spotted a storm at the south pole of Saturn. It was 5000 miles wide with wind speeds of 350 miles per hour! It was similar in appearance to hurricanes on Earth but a big difference was the lack of water. Therefore, because Earth hurricanes are governed by water mechanics, Saturn's storm must be a result of some other mechanism. Also, the storm hovers above the pole and rotates, not moving otherwise (Stone 12).
Now, with a finding like that it may come as a surprise that the awesome storms that Saturn has, which seem to cycle every 30 years, don't get much attention. But they certainly should. Cassini data seems to point to an interesting mechanism, which is as follows: First, a minor storm passes by and removes water from the upper atmosphere as precipitation. On Saturn, this takes the form of hydrogen and helium and the precipitation falls between cloud layers. This caused a transference of heat, leading to a decrease in temperature. After a few decades, enough cold air is built up to hit a lower layer and cause convection, thus a storm (Haynes "Saturnian," Nething 12, JPL "NASA-funded").
Saturn has another difference from Earth besides these thunderstorm patterns. Scientists found that the energy output from Saturn differs in each hemisphere, with the southern portion radiating about 17% more than the northern. The CIRS instrument detected this result, and scientists think several factors play into this. One is cloud cover, which has fluctuated greatly from 2005 to 2009, the window of this energy change. It does match with the changes in the seasons as well. But when compared to Voyager 1 data from 1980-81, the energy change was far greater than then, possibly hinting at a positional variance or even a solar radiance change on Saturn's cloud cover (Goddard Space Flight Center).
But I would be remiss if I didn't mention the north pole of Saturn, which has of all things a hexagonal pattern at it. Yes, that picture is real, and since its discovery by Voyager in 1981 its been a real humdinger. Cassini data only made it even cooler, for the hexagon may act like a tower by channeling energy from below the surface to the top via storms and vortexes that were spotted forming. As to how the hexagon formed in the first place or how it remains so stable over time remains a mystery (Gohd "Saturn").
Findings: Saturn's Rings
Cassini has also seen irregularities in Saturn's F ring up to 650 feet in length that are not uniformly distributed in ring, likely due to gravitational pulls from the moon Prometheus, which is just outside the Roche limit and thus plays havoc on any potential moons forming (Weinstock Oct. 2004). As a result of the gravitational interactions of this and other small moons in the ring, tons of half-mile sized objects are paving their way through it. The collisions happen at relatively slow speeds (about 4 miles-per-hour) because the objects are moving around the ring at roughly the same pace. The paths of the objects look like jets as they travel through the ring (NASA "Cassini Sees"). The collisional theory would help explain why so few of the irregularities have been spotted since Voyager, which witnessed much more in its short visit than Cassini has. As the objects collide, they break up and thus cause less and less visible collisions to be seen. But because of an orbital alignment that Prometheus has with the rings every 17 years, the gravitational interactions are strong enough to create new moonlets and a fresh cycle of collisions begins. Fortunately, this alignment happened again in 2009 so Cassini kept an eye on the F ring over the next few years to gather more data (JPL "Bright"). For the B Ring, not only were gravitational interactions with Mimas at play along the edge of the ring but also some resonant frequencies being hit. As many as three additional different wave patterns can travel through the ring at once (STSci).
Another interesting development in our understanding of Saturn's rings came in the discovery of S/2005 S1, now known as Daphnis. It resides in the A Ring, is 5 miles wide, and is the second moon to be found in the rings. Eventually Daphnis will disappear, for it slowly erodes and helps sustain the rings (Svital Aug 2005).
And how old are the rings? Scientists were not sure because models show the rings should be young but that would mean a constant source of replenishment. Otherwise they would have faded away a long time ago. Yet initial Cassini measurements show the rings to be about 4.4 billion-years-old, or just slightly younger than Saturn itself! Using Cassini's Cosmic Dust Analyzer they found that the rings usually receive little contact with dust, meaning that it would have taken a long time for the rings to accumulate the material they see. Sascha Kempf, from the University of Colorado, and coworkers found that over a seven-year span only 140 large dust particles were detected whose paths can be backtracked to show that they did not come from the local area. The majority of the ring rain comes from the Kuiper Belt with small traces of the Oort cloud and interstellar dust possible. It is unclear why dust from the inner solar system isn't a larger factor, but size and magnetic fields may be a reason. The potential for dust to come from destroyed moons is still a possibility too. But data from Cassini's death dive in the inner rings showed that the mass of the rings matches that of the moon Mimas, meaning that the earlier findings were contradicted because rings shouldn't be able to hold onto so much mass over a long span of time. The new findings point to an age of 150 to 300 million years old, considerably younger than the prior estimate (Wall "Age", Witze, Klesman "Saturn's," Haynes "Propellers").
And with all that dust, objects can sometimes form in the rings. In June 2004, data indicated that the A ring had moonlets. Images from Cassini taken on April 15, 2013 show an object at the edge of the same ring. Nicknamed Peggy, it is either a moon forming or an object falling apart. After this discovery, scientists looked back at over 100 past images and saw interactions in the area of Peggy. Other objects near Peggy were spotted and could be a result of gravitational forces pulling ring material together. Janus and Epimetheus also happen to orbit near the A ring and could contribute to the bright clumps on the edge of the A ring. Unfortunately, Cassini will not be in a viewing position to follow-up until late 2016 (JPL "Cassini Images", Timmer, Douthitt 50).
Though it was long thought to be true, scientists did not have observational evidence for Enceladus feeding Saturn's E ring until recent observations showed the material leaving the moon and entering the ring. Such a system is unlikely to last forever though as Enceladus loses mass each time it ejects the plumes (Cassini Imaging Central Lab "Icy tendrils").
Sometimes the rings of Saturn fall into shadow during eclipses and offer a chance to be studied in detail. Cassini did this in August of 2009 with its Infrared Spectrometer and found that as expected the rings cooled off. What scientists did not expect was how little the A ring did cool off. In fact the middle of the A ring stayed the warmest during the eclipse. Based on the readings, new models were built to try and explain this. The most likely reason is ina reevaluation of the size of the particles, with the likely diameter of the average A ring particle being 3 feet in diameter and with a small coating of regolith. Most models predicted a heavy layering of this around the icy particles but these would not be as warm as needed for the observations seen. It is not clear what is causing these particles to grow to this size (JPL "At Saturn).
Interestingly, the rings were key to getting a precise fixing on the length of Saturn's day. Normally, one could use a fixed feature on a planet to find the rate, but Saturn doesn't have that feature. If one understands the interior below, then one could use the magnetic field to help piece it together. This is where the rings come into the picture, for changes to Saturn's interior caused gravity shifts that manifested themselves in the rings. By modeling how those changes could have arisen using Cassini data, scientists were able to understand the distribution of the interior and find a length of 10 hours, 33 minutes, and 38 seconds (Duffy, Gohd "What").
The Grand Finale
On April 21, 2017, Cassini initiated the end of its life as made its final close approach to Titan, getting to within 608 miles to gather radar data and used a gravitational slingshot to push the probe into its Grand Finale flybys around Saturn, with 22 orbits.During the first dive, scientists were surprised to find that the area between the rings and Saturn is...empty. A void, with very little to no dust in the 1,200 mile area the probe passed through. The RPWS instrument only found a few pieces less than 1 micron in length. Maybe gravitational forces are at play here, clearing the area out (Kiefert "Cassini Encounters," Kiefert "Cassini Concludes").
Also detected by RPWS was a drop in plamsa between the A and B rings, otherwise known as the Cassini Division, indicating that the ionosphere of Saturn gets impeded as UV light is blocked from hitting the surface of Saturn, generating the plasma in the first place. But another mechanism may be making the ionosphere, for plasma changes were still seen despite the blockage. Scientists theorize that the D-ring may be creating ionized ice particles that are moving around, generating plasma. Differences in particle count seen as the orbit went on indicated that this particle flow (consisting of methane, CO2, CO+N, H2O, and other various organics) can cause differences in this plasma (Parks, Klesman "Saturns ring").
As the final orbits continued, more data was gathered. Closer and closer did Cassini get to Saturn, and on August 13, 2017 it completed its closest approach at the time at 1,000 miles above the atmosphere. This helped position Cassini for a final flyby of Titan on September 11 and for the death dive into Saturn on September 15 (Klesman "Cassini").
Cassini fell into Saturn's gravity well and transmitted data in real-time for as long as possible until the last signal arrived at 6:55 am central time on September 15, 2017. The total time of travel in Saturn's atmosphere was about 1 minute, during which time all the instruments were busy recording and sending data. After the ability to transmit was compromised, the craft likely took another minute to break up and become part of the place it called home (Wenz "Cassini Meets."
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© 2012 Leonard Kelley