Mohawk Valley Astronomical Society

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Cassini's Jewels

by Perry Pezzolanella

One of the most successful and scientifically rewarding planetary missions came to an end on September 15, 2017, when the Cassini orbiter, running low on stabilizer fuel, burned up in Saturn’s atmosphere. For over 13 years, since it arrived at Saturn on June 30, 2004, Cassini revealed Saturn, its rings, and moons in a way never imagined. From giant electrical storms, hurricanes, thousands of rings, lakes on Titan, geysers on Enceladus, a spongy looking Hyperion, two-toned Iapetus, and amazing discoveries among the other moons, Cassini discovered many unusual places that look and feel strangely familiar back here on Earth.

Spacecraft had already visited Saturn long before Cassini giving tantalizing clues to the unusual nature of Enceladus and Titan along with storms and rings of Saturn, but many questions were raised and a fully dedicated orbiter mission lasting at least four years was necessary. Cassini would fulfill this goal and go on for several more mission extensions lasting well into 2017. The Primary Mission ran from arrival until June 2008 when the Equinox Mission took over and lasted until September 2010. The Solstice Mission then began and lasted until the Grand Finale began in April 2017 with 22 orbits that threaded between Saturn’s cloud tops and its innermost ring. The rings define Saturn and look like a vinyl record, but closeup they are thousands of strands of ice, dust, and rock separated by gaps. Cassini discovered complex rings where particles clump together to form bigger bodies. The larger bodies grow into boulders and mini moons that affect the shape and structure of the rings often creating waves and ripples while shepherding the rings. By flying between the rings and Saturn’s cloud tops Cassini determined the mass of the rings, they formed after Saturn formed, and they will not last forever.

Saturn is a cloudy, stormy world. Cassini unveiled the churning, turbulent clouds, and areas where gases from far below erupt into long-lasting thunderstorms. Cassini was able to observe thunderstorms in fine detail and listen for lightning flashes by detecting radio static. The Great White Spot of 1990 was so big and bright that several MVAS members saw it from here on Earth. These giant storms seem to erupt every 30 years, but one luckily erupted 10 years earlier than expected, on December 5, 2010, at a time when Cassini could study it closeup and watch it evolve. It was a violent electrical storm that grew and was elongated by the powerful 1000 mile per hour jet stream winds. It eventually wrapped itself completely around the planet in a white belt 9000 miles wide. By the summer of 2011, the storm died out and the atmosphere remained relatively calm for the rest of the mission. Giant polar vortices cover both poles with clearly defined eyewalls, but only the northern polar vortex appeared like a hexagon. Saturn also has a very strong magnetic field which triggers large, bright aurora displays.

The moons of Saturn pretty much stole the show as every moon had something interesting and unique unto itself. The Voyager 1 & 2 flybys in 1980 and 1981 hinted that each moon had something to define it from the others. Phoebe was the suspected captured asteroid hardly 140 miles across and was found to be covered with bright cliffs on the rims of the largest craters along with bright crater rays revealing ice beneath a dark layer. Small impacts on Phoebe eject dust into a large, thin ring around Saturn that coats the surfaces of the inner moons, especially Iapetus. Iapetus is the two-toned world with one side as bright as snow and the other as dark as coal. It is the third largest moon of Saturn at 905 miles across and always keeps one side facing Saturn. The forward-facing hemisphere is bombarded by the dark, dusty particles of Phoebe. The dark material also absorbs heat and further keeps that hemisphere free of ice. Iapetus also appears like a giant walnut with mountains as tall as 12 miles almost completely encircling the equator. The cause of this ridge is a new mystery, but it is possible that Iapetus may have had a ring that collapsed neatly along its equator.

Inward of Iapetus is Hyperion, a potato-shaped moon about 203x161x133 miles that looks like a giant sponge. The low density and low gravity combine to preserve crater impacts making it look spongy. Hyperion also has erratic rotation caused by its close orbit to Saturn’s largest moon, Titan, along with its irregular shape. It is literally tumbling as it orbits Saturn so the spin rotation axis and rotation period can change unpredictably in as little as a few days. Rhea is Saturn’s second largest moon at 949 miles across and Cassini found unusual bluish spots along the equator, possible evidence of a very thin ring of material hardly six miles wide that surrounded the moon not too long ago. Dione, only 698 miles across, has giant fractures and walled depressions in the smooth terrain, which might be volcanic vents. Half of Dione is covered in what might be volcanic ice deposits, which hints at a possible subsurface sea. Tethys, 668 miles across, has a huge rift zone named Ithaca Chasma that runs nearly three-quarters of the moon’s circumference and a set of mysterious red arcs across the surface that may have formed very recently. It is possible that Tethys may also be intermittently active. Mimas is the moon, at 243 miles across, that perfectly resembles the Death Star from Star Wars. The huge crater, Herschel, at 81 miles across, nearly destroyed the moon. Cassini imaged the battered icy surface in fine detail.

One of the highlights during Cassini’s mission had to be Enceladus. This small 311-mile moon was thought to be something special when Voyager 2 flew past it in 1981. It was found to be quite smooth and reflected nearly all the sunlight that strikes it. It had all the indications of being geologically active orbiting within the faint E-ring, which hinted at geysers, but proof would have to wait. Cassini finally solved the mystery in 2005 when it discovered geysers erupting from trenches near the south pole. These plumes contain water vapor and the source turned out to be a vast underground ocean. Tidal heating caused by the gravitational tugs of Saturn and Dione heat the interior of Enceladus to deform it, especially at the poles, causing the surface to fracture and water to be forcibly ejected as geysers hundreds of miles high. Why only the south pole has geysers is a mystery. Cassini flew through the plumes several times without harm and found that nearly 98% of the gas is water vapor. The rest is a mixture of compounds necessary for life. Enceladus has all the necessary ingredients for life: a warm subsurface sea, and plentiful hydrogen that organisms could potentially harness as a chemical energy source. The geysers make Enceladus the best target for future exploration in the search for life as the water ice can be easily collected and sampled from the erupting geysers.

Saturn’s largest moon is Titan at 3270 miles across and far different than any other moon in the Solar System as Cassini discovered liquid methane and ethane on the surface, the only world besides Earth to have liquid on its surface. Cassini delivered the Huygens probe to Titan where it made a dramatic landing on January 14, 2005. As Huygens descended it sent back images of branching river channels flowing into deltas and into what appeared to be a sea or vast dark mudflat complete with patchy fog or low clouds near the shoreline. Huygens landed on the moist mudflat where liquid methane may have recently flowed. The scenery was dim and bleak, bathed in an orange light from the thick, smoggy atmosphere 1.6 times as thick as Earth’s. Cassini pierced the smoggy veil, mapped the seas that are mainly clustered in the north polar region, and found them to be quite calm with steep, rugged shorelines, several larger than Lake Superior and up to 600 feet deep. Cassini also imaged lake effect clouds forming over the larger seas traveling downwind, and possible icebergs or sea foam that appeared and disappeared. Methane rainstorms were discovered that flooded lowlands and would later dry up. Vast sand dunes were also discovered straddling parts of the equator composed of organic compounds similar in texture to coffee grounds, hinting at strong winds, along with towering mountains, with the tallest 10,948 feet tall and possible cryovolcanoes that may be active. A large underground ocean of liquid water was also discovered making Titan a future mission candidate for the search for life.

Cassini revealed the splendors of Saturn and its moons with every day bringing new discoveries, which raised more questions that still need to be answered. Mysteries were solved. New mysteries were revealed. Theories and predictions were confirmed, or completely tossed away. Cassini’s discoveries during its 13 years orbiting Saturn were like jewels for all to admire and appreciate. It will be missed, but its legacy will usher in a new wave in the exploration of Saturn, its rings, and moons.