10 things we've learned from Cassini-Huygens

This is an artist’s concept of Cassini during the Saturn Orbit Insertion (SOI) manoeuvre
(Credit: NASA/JPL)

October 1997 saw the launch of NASA’s Cassini spacecraft, complete with ESA’s Huygens probe. After a long journey through the solar system, Cassini became the first spacecraft to enter Saturn’s orbit in 2004. Initially given a four-year mission, Cassini’s work has been extended twice – the Equinox mission finished in 2010. Cassini’s Solstice mission is ongoing, scheduled to continue beyond the Saturnian summer solstice in May 2017. On the 10th anniversary of Cassini’s arrival at Saturn, we’re delighted to look back at some of the things we’ve learned from the data Cassini and the Huygens probe have returned to Earth.

  1. Saturn has lots of moons. When Cassini launched from Cape Canaveral, we knew that Saturn had 18 moons. During Cassini’s seven-year voyage, observers on Earth discovered 13 more. But before 2004 had come to a close, Cassini had added three more to the tally – Methone, Pallene and Polydeuces. The total is now 62, of which 53 have been officially named. In April 2013 Cassini witnessed the formation of a small, icy object within Saturn’s rings. Affectionately known as ‘Peggy’, this may well be a new moon. This exciting and unexpected event will help us to understand the formation of Saturn’s other moons, and even give us some insights into the formation of the planets in our solar system and beyond. Peggy may be the last moon to emerge from Saturn’s rings, which are now believed to be depleted, and was first spotted by Professor Carl Murray, of Queen Mary University of London, during research funded by STFC.
  2. Saturn’s moon Enceladus is one of the places in the solar system that is most likely to be home to microbial life. Cassini found evidence of liquid water beneath the surface of Enceladus, as well as complex organic chemicals. In 2005, Cassini spotted water vapour and ice spewing from the moon’s south pole. In 2008 the spacecraft flew through one of the plumes and sampled carbon-based molecules, and precise measurements during three later fly-bys strongly suggest that there’s an underground ocean there. Enceladus has proven to be so exciting that it may become the focus of a future space mission.
  3. Titan, Saturn’s largest moon, has an internal, liquid water-ammonia ocean. Cassini’s imaging radar allowed us to see past Titan’s thick, smog-filled atmosphere and revealed vast methane lakes and wind-sculpted hydrocarbon sand dunes. With its open lakes and active weather system, including rainfall, Titan is the most Earth-like body in our solar system. Its atmosphere has been found to host the most complex chemical processes known in our solar system, and Cassini’s data is contributing to our understanding of exoplanets that are similarly hazy.
  4. Saturn’s F ring is a “a bustling zoo of objects from a half mile in size to moons like Prometheus a hundred miles in size”, says Professor Murray. Scientists from Queen Mary, University of London combed through 20,000 images taken with Cassini's Imaging Science Subsystem (ISS) to discover 500 ‘mini-jets’, strange objects that are over half a mile long and leave glittering trails through the ring.
  5. First colour view of Titan's surface
    (Credit: ESA/NASA/JPL/University of Arizona)

    Titan’s surface is covered in ‘pebbles’, which may be made of water ice. The Huygens probe that sent back pictures of the surface was not only the first spacecraft to land on Titan, but the first to land in the outer solar system. RAL Space worked with the Open University on the build of Huygen’s Surface Science Package, as well as contributing hardware to Cassini’s Plasma Spectrometer (CAPS) and Cosmic Dust Analyser (CDA) instruments.
  6. Saturn’s poles are home to giant, swirling storm systems that resemble hurricanes here on Earth. Saturn’s north polar storm was first seen glowing in infrared light in 2007 by Dr Leigh Fletcher and colleagues from Oxford University, using Cassini’s Composite Infrared Spectrometer (CIRS), while the north pole was still in darkness. (A similar hurricane-like structure was also discovered at the south pole during summer sunshine.) The team also re-discovered a mysterious, six-sided weather pattern called the hexagon, first seen by the Voyager spacecraft in 1981 and imaged for the first time in infrared light. In 2013, when the hurricane and hexagon had emerged from the darkness of winter, Cassini’s cameras could capture images of the swirling maelstrom for the first time.
  7. There’s plastic in Titan’s atmosphere. Cassini’s composite infrared spectrometer (CIRS) detected small amounts of propylene in Titan’s lower atmosphere. Here on Earth, that’s the same kind of plastic that’s used to make plastic storage containers, where propylene molecules are formed into long chains to make polypropylene. Hydrocarbons such as propylene are found in Titan’s atmosphere when sunlight breaks down methane into fragments that can combine into hydrocarbon molecules.
  8. Icy moon Dione has a whiff of an atmosphere (or, more correctly, an exosphere). Cassini’s Ion and Neutral Mass Spectrometer (INMS) “sniffed” molecular oxygen ions during a very close flyby in 2010. With one oxygen ion for every 11 cubic centimetres of space, Dione’s atmosphere is only as dense as that of Earth at an altitude of 480 kilometres. Dione’s oxygen probably comes from solar photons or energetic particles from space hitting the moon’s surface and liberating oxygen molecules from the ice. Cassini detected an exosphere around Rhea (another of Saturn’s moons) in 2010.
  9. Enormous planet-engulfing thunderstorm systems sometimes erupt from Saturn’s atmosphere, affecting the climate of the gas giant for many years. One such eruption occurred in 2010, spawning billowing white ammonia and water ice clouds that were blown around by Saturn’s powerful winds, eventually engulfing the whole northern hemisphere. (On Earth, such planet-wide weather systems are prevented by boundaries such as continents, valleys and mountain ranges.) Saturn’s superstorms appear to happen once a Saturnian year (once every 30 Earth years), and Cassini’s presence in the Saturn system allowed scientists to capture unique glimpses of the weather systems - lightning bolts, convective clouds and vortices. Oxford researchers used infrared observations by Cassini’s CIRS to discover that the storm had spawned an enormous circulating vortex in the planet’s stratosphere that lasted from 2011 to 2014. Known as the ‘beacon’, it was some 80 degrees Kelvin warmer than the surrounding, air and three times larger than Jupiter’s Great Red Spot.
  10. Just like the Earth, Saturn experiences seasons (due to its 27° tilt) that affect the climate and cloud systems. When Cassini arrived, Saturn’s wintery northern hemisphere appeared blue due to a lack of clouds and hazes. Today, as the aerosols grow during the spring, it’s taking on the familiar yellow-ochre appearance, whereas Saturn’s south is descending into darkness. UK scientists have been monitoring these slow seasonal changes over the past decade to learn more about the forces shaping the climate and environment of a giant planet.

Although Cassini has spent 10 Earth years in orbit around Saturn, that’s only about a third of a Saturn year. Cassini arrived just after the northern winter solstice of 2002, when the north was shrouded in darkness. During the spring equinox of 2009, the rings were edge on. The summer solstice of 2017 will mark the end of the mission, but there’s plenty more science left in this spacecraft in the meantime. As Professor Murray says, "Cassini-Huygens has to be one of the most successful planetary missions ever launched. In the last decade we have been able to study the Saturn system in unprecedented detail and see it change before our eyes. The end of the mission is still over three years away and I am convinced that there are many more exciting discoveries to be made."

Cassini: Coming Attractions at Saturn
(Credit: NASA Jet Propulsion Laboratory)

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