Seasons on Saturn Sightseeing What is en-route toSaturn Cassini Huygens was launched Cassini in 1997 on a Titan IV/Centaur Huygens? rocket. In order to build up the energy needed to reach distant Saturn, the spacecraft followed a Cassini Huygens is an international space mission spiral route past the Earth, Venus to explore Saturn and its largest moon, Titan. The and Jupiter. As it swung by each mission is made up of NASA’s Cassini orbiter and planet, it picked up speed using a the European Space Agency’s Huygens probe. technique called “gravity assist”. Cassini Huygens launch Taking this roundabout route meant that The mission is named after two 17th Cassini could carry out science investigations Century astronomers: Jean Dominique during its journey. The spacecraft’s ISS cameras Cassini, the discoverer of four of took spectacular images of the Moon during an Saturn’s orbit Saturn’s moons, and Christiaan Huygens, Earth flyby in 1999 and picked out Asteroid Jupiter who discovered Titan and realised that 2685 Masursky from a distance of 1.6 million Earth’s orbit the “ears” Galileo had observed either kilometres – good practice for its task of Jupiter’s orbit Between November 2000 and February 2001, Cassini teamed up side of Saturn were actually rings. finding new moons at Saturn. Venus’ orbit with NASA’s Galileo mission to study Jupiter. Besides taking some beautiful pictures of the gas giant, Cassini found that Jupiter’s Jean Dominique Cassini Christian Huygens Cassini’s trajectory magnetic field is lopsided, leaks high-energy particles and changes size as space weather events pass by. Cassini trajectory

After the Jupiter flyby, Cassini participated in an experiment to search for gravitational waves. It looked for frequency shifts in radio signals transmitted between the spacecraft and ground- stations in NASA’s Deep Space Network. Results from this experiment have given further proof to Einstein’s theory of Cassini spacecraft general relativity.

Background image: Cassini The Moon Asteroid 2685 Masursky

• By the time Cassini-Huygens reaches Saturn, it will have travelled almost 3.2 billion kilometres. It would take around 30 million years to drive that distance Cassini in a car. Further Information Particle Physics and • Sending a radio signal to or from the spacecraft, once in orbit, will take If you would like to know more about Astronomy Research Council between 68 and 84 minutes. The exact time depends on the relative positions the Cassini Huygens mission, try the The Particle Physics and Astronomy Research after Iapetus of the Earth and Saturn. following resources: Council (PPARC) is the UK Government strategic Orbiter Did you World Wide Web sites science investment agency for research in: • Cassini’s closest approach to Saturn is during the Saturn Orbital Insertion http://saturn.jpl.nasa.gov/index.cfm Particle Physics Cassini arrived at Saturn on 1st July 2004. manoeuvre, when it is 18,000 kilometres from the top of Saturn’s atmosphere http://sci.esa.int/science- Astronomy It will spend at least four years in orbit to • Christiaan Huygens was the inventor of the pendulum clock e/www/area/index.cfm?fareaid=12 Space Science Cassini give us our first long-term, close-up look http://www.sp.ph.ic.ac.uk/cassini/ This support is provided by funding UK • Saturn has not been visited by a spacecraft since Voyager 2 flew by in 1981. http://pssri.open.ac.uk/missions/ researchers and by ensuring that they have access at the Ringed Planet. Pioneer 11 and Voyager 1 also swung by Saturn in 1979 and 1980 respectively. to world-class facilities in the UK and overseas. Huygens mis-cas.htm The Council pays the UK's subscription to the http://www.esa.int science programmes of the European Laboratory know... http://www.ex.ac.uk/Mirrors/nineplanet for Particle Physics (CERN) and the European Exploring Saturn s/saturn.html Space Agency (ESA). http://www.uk2planets.org.uk PPARC recognises the importance of its science in assisting wealth creation; in post-graduate Books training; and in motivating young people towards Ralph Lorenz & Jaqueline Mitton and interest and careers in science generally. Lifting Titan’s Veil Cambridge University Press, 2002 The UK contribution to the Cassini Huygens Huygens mission is part of its wider involvement in David M Harland planetary exploration. For more details, visit Mission to Saturn: Cassini and the www.uk2planets.org.uk Huygens Probe Springer Praxis, 2002 Probe Fredric W. Taylor The Cambridge Photographic Guide In December 2004 Cassini releases the Huygens to the Planets www.uk2planets.org.uk probe and, on 14th January 2005, Huygens will Cambridge University Press, 2001 descend through Titan’s atmosphere to discover PPARC publishes information on a With over 30 known moons, one of the most what lies beneath the opaque, orange haze. variety of topics related to physics and astronomy, to help increase public complex magnetospheres in the Solar System Scientists believe that the mixture of complex Particle Physics and awareness of the UK's achievements in Astronomy Research Council and its famous ring system, Saturn is a organic molecules in Titan’s atmosphere science and technology. Find out more Polaris House fascinating target. Like Jupiter, it is a gas giant by visiting the PPARC website: resembles the atmosphere of the primordial North Star Avenue http://www.pparc.ac.uk with a colourful, cloudy atmosphere. Comparing Swindon Earth. Although missions to date have not been Wiltshire SN2 1SZ and contrasting these two giant planets will give able to see through the thick haze, it’s possible Tel: +44 (0) 1793 442000 us insights into how they behave as a family and Fax: +44 (0) 1793 442002 that the moon’s surface may be covered with Email: [email protected] how the Solar System formed. oceans of methane and ethane. Data collected by Huygens may help us work out how chemical Acknowledgements: Photos supplied by: Cassini @ NASA/PL/Space Science Institute, soup turned into life on Earth. NASA/JPL/Cassini Imaging Team/Arizona,University of Arizona, NASA/Kennedy Space Centre, Cassini @ JPL, NASA/JPL-Caltech, Bands in Saturn’s atmosphere Smithsonian Institutiion Libraries, Washington, DC, Spce Telescope Cover image: Saturn Approach Science Institute, JPL, Gregor Kervina, Ljubijana, Slovenia, J.T. Particle Physics and Astronomy Rings Trauger (JPL) and NASA, US Geological Society, NASA Janus. by Cassini Design and production by Holly Benson Communications, April 2004. Research Council Saturn’s moons Titan’s haze

BACK FRONT Background image: Cassini

Saturninternal structure Saturn is a gas giant, composed mainly of hydrogen and helium. It does not have a surface on which one could land. At the bottom of the atmosphere, the hydrogen fog becomes thicker and pressure increases until droplets condense into liquid hydrogen. As pressure increases this turns into liquid metallic hydrogen, a substance that we can’t produce on Earth as the temperatures and pressures required are too enormous. At the centre of the planet is a “rocky” core that has a chemical composition similar to the nebula from which the Solar System formed. Saturn Although we can’t study Saturn’s internal structure directly we Saturn’s rings are made up of particles of

Saturn’s atmosphere can measure the planet’s magnetic field, which is generated by the ice, dust and rock that range in size from rotation of the molten metal. The magnetometer instrument, MAG, millimetres to several kilometres. Although is building up a three-dimensional picture of Saturn’s magnetic field, allowing scientists to draw conclusions about the planet’s the main rings would stretch over halfway internal structure. The MAG team is led by scientists from Imperial from the Earth to the Moon, they are less Rings College London. than one kilometre thick. The rings are SaturnAtmosphere separated by gaps maintained by “shepherd” Saturnmagnetic field moons that orbit within them.

Saturn’s yellowish colour is due to the amount of sulphur in its Saturn’s magnetic field is one of the most complex in the Solar In places ring particles are clumped together and some of the atmosphere. The cloud patterns look less spectacular that those System. It is around 8,000 times as strong as the Earth’s magnetic rings appear to have become twisted or braided. Images from seen at Jupiter because Saturn has a thicker layer of haze above field, but less than half the strength of Jupiter’s field due to Saturn’s Voyager show structural features that appear then disappear. the cloud tops. However, images from Voyager and the Hubble smaller metallic core. Cassini’s Imaging Science Subsystem (ISS) cameras are ten times Space Telescope show that Saturn also has massive storm systems. more powerful than those carried by Voyager, so they will pick The planet’s striped appearance is due to bands of winds blowing The main sources of plasma (electrically charged particles) in out far more detail in the rings. The ISS cameras will have in opposite directions. Unlike on Earth, where weather is driven Saturn’s magnetosphere are the Solar Wind and gases escaping opportunities to image the rings from different angles as Cassini by radiation from the Sun, winds on Saturn are powered by energy from Titan’s atmosphere and the surfaces of the icy moons. The follows its complicated orbital tour. The Saturn Orbit Insertion emitted from the planet’s core. Cassini Plasma Spectrometer (CAPS) instrument monitors the manoeuvre is the closest that Cassini will get to Saturn and gives interaction of plasma with Saturn’s magnetic field and determines the ISS team the best view of the rings. The Composite Infrared Spectrometer (CIRS) instrument measures the sources and sinks of plasma by measuring the composition and infrared radiation emitted by Saturn’s atmosphere, rings and moons. direction of motion of particles. Scientists from Queen Mary, University of London are part of It is mapping Saturn’s atmosphere in three dimensions, showing how the ISS cameras team. chemical composition and pressure vary with altitude. Scientists from Scientists from Mullard Space Science Laboratory and CCLRC Oxford University built a major part of the CIRS instrument. Rutherford Appleton Laboratory are involved in the CAPS instrument. Saturn’s rings

Titan is Saturn’s largest moon. It is larger than the planet Mercury and Saturn is covered with a thick, opaque haze Aurora Saturn that has prevented previous missions As on Earth, electrically charged Icy Moons from seeing Titan’s surface. particles cause spectacular aurorae when they interact Before Cassini’s arrival at Saturn, we knew of 31 moons orbiting with Saturn’s upper atmosphere. the planet. Images from the ISS cameras will be used to look for The University of Sheffield has undetected moons. Flicking quickly between images of the same provided software for the Radio area of space makes a moving object stand out against the and Plasma Wave Science (RWPS) background of stars. Using time-lapsed series of images, a moon’s experiment, which will study orbit can be plotted. these ionospheric interactions. The CAPS instrument also Saturn’s Aurora Cassini’s orbital tour includes fly-bys of a number of Saturn’s measures particles that produce intriguing moons: aurorae and may electrically charge the rings. Mimas looks rather like the Iapetus appears to be made of Death Star in Star Wars. An contrasting materials that give enormous crater, called it one very bright hemisphere Herschel, is evidence of an and one very dark hemisphere. asteroid impact that must have nearly shattered this moon.

Saturn Mimas Iapetus The atmosphere is mainly nitrogen but contains large quantities of Huygens carries six instruments that will send back information Hyperion is a potato shaped Epimetheus and Janus swap Titanmethane and traces of other hydrocarbon gases. Scientists believe it about the descent and whether Huygens lands with a crash or a Dust moon that tumbles chaotically orbits once every four years. may be similar in composition to the atmosphere of the primordial splash. The Huygens Atmosphere Structure Instrument (HASI) will along its orbital path. It is the In an exchange of momentum, Earth before life formed. monitor changes in atmospheric properties during the descent. The bubble of space around Saturn is very dusty, particularly in largest known irregular one moves out and the other shaped object – if it were any moves in. By observing this The Surface Science Package (SSP) will measure the physical, the plane of the rings. The Cosmic Dust Analyser is analysing the bigger, its gravity would pull overtaking manoeuvre, we Cassini will study Titan’s atmosphere and use Radar to map its chemical and electrical properties of the surface, whether solid or composition and trajectory of dust particles detected. Scientists can it into a spherical shape. can find out about the moons’ surface. The Huygens probe will descend through Titan’s smog liquid. If the probe lands in an ocean, it will determine the size then work out whether particles come from interstellar space or internal structure. Hyperion and measure how the physical and chemical properties of the and period of the waves. whether they come from a nearby moon. The Open University Scientists can then work out whether particles come from atmosphere change with altitude. Three British companies were and CCLRC Rutherford Appleton Laboratory were involved in the interstellar space or whether they come from a nearby moon. The responsible for developing the probe’s descent system. At an The Open University (OU) has led the development of the SSP and development of The CDA. Open University and CCLRC Rutherford Appleton Laboratory were altitude of around 50km, Huygens should get its first glimpse OU scientists are involved in the HASI instrument. involved in the development of The CDA. of Titan’s surface through the clouds. It could see icy plains,

rocky outcrops or even oceans of methane or ethane. Epimetheus Janus