European Space Agency

The Cassini Mission to Saturn and Titan

C. Kohlhase & C.E. Peterson

Cassini Project, Jet Propulsion Laboratory, Pasadena, California, USA

When Cassini/Huygens was launched from Cape Canaveral on 15 October 1997, the 5.6 t, 6.8 m-high spacecraft carried a suite of scientific sensors to support 27 investigations probing the mysteries of Saturn's system. In addition to a fascinating atmosphere and interior, the vast system contains the most spectacular of the four planetary ring systems, numerous icy satellites with a variety of unique surface features, a huge magnetosphere teeming with particles interacting with the rings and moons, and the intriguing moon Titan - slightly larger than the planet Mercury and with a hazy atmosphere denser than Earth's.

Introduction

Cassini Mission to saturn and titan

The Cassini/Huygens mission is an international venture between NASA, ESA, the Italian Space Agency (ASI) and several separate European academic and industrial partners. The mission is managed for NASA by the Jet Propulsion Laboratory (JPL) in Pasadena, California. After an interplanetary voyage of 6.7 years, the spacecraft will arrive at Saturn on 1 July 2004, where it will brake into orbit around the planet. ESA's 318 kg Huygens Probe will execute its mission in November 2004, at the end of the first of Cassini's many orbits about Saturn. Having relayed the Huygens data, the Orbiter will then continue its intensive exploration of the system through June 2008.

Cassini/Huygens was launched atop a Titan-4B/Centaur from Launch Complex 40 at the US Air Force Cape Canaveral Air Station in Florida. Though under the primary control of the USAF 45th Space Wing, launch operations also involved the efforts of many other agencies, technical centres and contractors. Once injected into space and acquired by the Deep Space Network (DSN) tracking antennas, mission control shifted to the Mission and Science Operations (MSO) teams at JPL, with Probe support from ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany (see the article by Sollazzo et al. in this issue).

On reaching Saturn in mid-2004, Cassini will swing to within 20 000 km of the cloud tops (an altitude only 1/6th the diameter of Saturn) to begin the first of 74 planned orbits. In late 2004, Cassini will release the Huygens Probe for a descent of up to 2.5 h through Titan's dense atmosphere. The instrument-laden Probe will beam its findings to the Orbiter for storage and then relay to Earth. The Huygens portion of the mission is covered in detail in the Lebreton & Matson and Hassan & Jones articles in this issue.

What we know about Titan is certainly tantalising. Its brownish-orange, hazy atmosphere of nitrogen, methane and complex array of carbon-based molecules hides a frigid surface that may contain subsurface reservoirs or perhaps even lakes of liquid ethane and methane. Much of Titan's interior and surface is probably frozen water ice, with perhaps thin patches of overlying frozen methane and ammonia. As high-energy particles and ultraviolet radiation bombard the nitrogen and methane molecules in the atmosphere, these and further reactions create a variety of organic molecules that clump together and rain slowly down. Whether this material collects on the surface or sinks into surface pores is not known. In many ways, Titan's environment may resemble the chemical factory of primordial Earth. Though the extreme cold makes the possibility of life unlikely, Titan may still provide valuable clues to the chemistry of early Earth.

The Orbiter will execute 50 close flybys of the moons, including more than 40 of Titan. In addition, there will be more than 25 distant flybys of the icy moons. Cassini's orbits will also allow it to study Saturn's polar and equatorial regions.

Throughout the mission, costs will be contained and efficiency enhanced by streamlined operations. The Cassini Project uses simplified organisational groups to make decisions. Flight controllers will take advantage of high- level building blocks of spacecraft action sequences to carry out mission activities. New technology includes powerful new computer chips, solid-state recorders, gyroscopes with no moving parts, and solid-state power switches.

Mission design

Cassini swingbys
Cassini requires several planetary swingbys to gain sufficient Sun- relative speed to reach Saturn almost 7 years after launch. (Courtesy of JPL)

Delivering Cassini and its large complement of scientific instruments to Saturn and Titan produced a spacecraft launch mass of 5548 kg, more than half of which is propellant for trajectory changes. Not even the powerful Titan-4B/Centaur can reach Saturn directly with this payload, but it can provide sufficient energy for a direct trajectory to Venus. Here, the great velocity gains from a gravity assist must be used to reach Saturn 6.7 years after launch, by flying by Venus twice and Earth and Jupiter once each - the so-called 'VVEJGA trajectory' (Venus-Venus-Earth-Jupiter Gravity Assist).

As the Jupiter-Saturn connection is only available for one 3-year period every 20 years (Voyager used the 1976-1978 equivalent), it turns out that departures later than Cassini's primary launch period (6 October - 4 November 1997, with contingency days available through 15 November) lose the energy gain available from Jupiter and must endure much longer flight times if they are to make it to Saturn with sufficient performance to attempt a minimum tour mission. For the VVEJGA route, the Sun-relative speed gains for each of the four swingbys are roughly 6, 7, 6 and 2 km/s, respectively. For the secondary launch period from 28 November 1997 to 11 January 1998, the speed gains from the VEEGA route are about 6 km/s for each of the three swingbys, with arrival at Saturn some 2.3 years later than the preferred arrival date of 1 July 2004, for the primary mission.

Cassini begins SOI
Cassini begins the Saturn Orbit Insertion (SOI) burn on 1 July 2004 (Courtesy of David Seal/JPL)]

The primary arrival date is favourable from three points of view. It allows a close flyby (52 000 km) of the moon Phoebe (likely a captured asteroid in a distant, retrograde orbit) some 19 days before Saturn Orbit Insertion (SOI). The tilt of Saturn's rings is more favourable for imaging and radio-science observations than it is for later arrivals. Finally, the spacecraft power available during the tour from the Radioisotope Thermal Generators (RTGs) is higher than it would be after the longer journey times of the later launches.

Daily launch windows from Cape Canaveral opened at 09:38 UT on 6 October and lasted for up to 140 min each launch day, moving earlier by about 6 min daily. During the cruise to Saturn, activities are limited primarily to engineering and science instrument maintenance and calibrations, navigation data collection, trajectory corrections and gravitational-wave searches during 40-day periods around solar oppositions beginning in December 2001.

The probability of accidental entry during the >1000 km altitude Earth swingby will be controlled to 10-6 through measures such as trajectory aim-point biasing, precision navigation, robust spacecraft design against propulsion- and micrometeoroid-induced failures, and rigorous flight-team training. Scientific observations will be made of the Saturnian system during the late cruise phase as Cassini approaches. Present funding and project planning do not allow for scientific data to be collected during the earlier planetary swingbys.

On arrival at Saturn, Cassini will make its closest approach to the planet, passing only 20 000 km above the cloud tops. It will fire one of its two redundant engines on 1 July 2004 for 96 min to slow its speed by 622 m/s for SOI; braking into a 1.33x178 Saturn radii (RS), 148-day, 16.8° orbit will consume 830 kg of the main propellant supply. A 50-min, 335 m/s burn 13 days after apoapsis of the post-SOI orbit will raise periapsis to 8.2 RS to target Cassini for a Titan encounter and Huygens' entry on 27 November 2004. If any problem with the spacecraft or ground system prevents execution of the Probe mission on the first Titan pass, a decision can be made as late as a few days before Probe separation to delay until the second Titan encounter on 14 January 2005.

On 6 November 2004, 22 days before the first Titan flyby, the entire spacecraft will be manoeuvred into an impact trajectory with Titan. Two days later, the Orbiter will turn to orient the Probe to its entry attitude, spin it up to just over 7 rpm, and release it with a separation velocity of about 0.3 m/s. Two days after separation, the Orbiter Deflection Manoeuvre (ODM) of 45 m/s ensures that it will not follow the Probe into Titan's atmosphere (by aiming 1200 km off Titan's limb) and establishes the proper geometry (by slowing down) for the Probe Relay Link. Huygens is targeted for an entry angle of -64° and a dayside landing 18.4°N of Titan's equator and some 200°E of the sub-Saturn point.

After Huygens enters Titan's atmosphere at 6 km/s, decelerates to 400 m/s in less than 3 min, and deploys its series of parachutes, it will transmit its findings to the Orbiter for up to 2.5 h during descent, and possibly for another 30 min from the surface. The Orbiter will receive these data over its High-Gain Antenna (HGA) for redundant storage aboard its two Solid- State Recorders (SSRs), then turn later to play back these precious data to the waiting radio telescopes on Earth.

Cassini's tour phase begins after completion of Huygens' mission and ends four years after SOI. The baseline tour consists of 74 orbits of Saturn with various orientations, orbital periods ranging from 7 to 155 days, and Saturn-centred periapses ranging over about 2.6 - 15.8 RS. Orbital inclinations with respect to Saturn's equator range from 0° to 75°, providing opportunities for ring imaging, magnetospheric coverage and assorted Earth, Sun and stellar occultations by Saturn, Titan and the ring system. Most of the 43 Titan encounters have flyby altitudes of 950 - 2500 km. As a result of Titan's considerable mass, the Saturn-relative total gravity swingby gains amount to about 33 km/s (more than that gained during the interplanetary journey), easily enough to move the Saturn-relative orbits through a wide range of desired observational geometries. The baseline tour also contains seven close flybys within 1000 km of icy satellites, and 27 additional distant flybys of icy satellites within 100 000 km.

The tour designers have developed an elaborate sequence of Titan swingbys to achieve the many scientific remote-sensing and in-situ data-collection conditions requested by the scientists. It is crucial that the navigation accuracy for each swingby be very precise, because the total delta-V available for flying the entire 4-year tour is only 500 m/s, i.e. less than the average delta-V assist (770 m/s) from each Titan swingby. By using radiometric tracking data and optical navigation images of Titan and other satellites against a star background taken on each orbit, the Navigation Team predicts control errors at the 10 km level for the Titan swingbys, sufficient for the available propellant.

The mission designers are developing an integrated plan to allow the flight and ground systems to collect and return the desired science data, while Cassini remains 'on the tour'. Sequences of operational routines are used rather like building blocks to execute the necessary engineering support and scientific activities. These various 'operational modes', 'data modes' and 'templates' can be strung together to ensure that spacecraft subsystem and instrument capabilities are used to best advantage. As the three RTGs do not provide sufficient power to turn all the instruments on simultaneously, the various instruments must be operated in logically related subsets. Hence, such operational modes as 'Optical Remote Sensing', 'Radar/INKS' (Ion and Neutral Mass Spectrometer) and 'Downlink Fields/ Particles/ Waves' convey their intent.

The majority of the scientific instruments are body-mounted, making it is necessary to turn the entire spacecraft, point in different directions to perform the desired measurements, record these data on the two SSRs (which can hold 1.8 Gbit each), and finally turn to Earth to radio these data to the ground. During each orbit about Saturn, there are 4 to 7 days of 'high activity', with the remainder spent in 'low activity'. The former generally occurs near Saturn and the satellite flybys, with intensive data- collection periods lasting about 16 h daily, followed by 8 h of playback to either a 70 m antenna or a 34/70 m array, capturing up to 4 Gbit daily (by interleaving real-time and SSR data during each playback). During low activity, the Orbiter may simply roll to collect fields and particles data to broadcast to a smaller 34 m antenna each day, though off-Earth turns are still allowed as long as downlink data return levels do not exceed about 1 Gbit daily.

after delivering the Huygens Probe
After delivering the Huygens Probe, the Orbiter will make 43 close gravity-assisted swingbys of Titan to achieve a variety of Saturn geometries for its 12 instruments (Courtesy of JPL)

The Cassini Orbiter

Pincipal features of Cassini/Huygens spacecraft
Principal features of the Cassini/Huygens spacecraft (Courtesy of JPL)

The Cassini Orbiter is one of the largest and most complex robotic spacecraft ever built. Together with the Huygens Probe, it is twice the size of Galileo: 6.8 m tall and 4 m across. Carrying over half its mass in propellant (3132 kg), the total spacecraft with its instruments and Huygens weighs 5548 kg.

The Orbiter's main body is formed by a stack consisting of the lower equipment module, the propulsion module, the upper equipment module, and the High-Gain Antenna (HGA). Attached to this stack are the Remote-Sensing Pallet, the Fields and Particles Pallet, and the Huygens Probe. Some instruments, such as the Titan Radar and the Radio and Plasma Wave Subsystem (RPWS), are attached to the upper equipment module. The two equipment modules are also used for externally mounting the magnetometer boom and the three power- providing RTGs. The spacecraft electronics bus is part of the upper equipment module, supporting data handling (including the command and data subsystem and the radio-frequency subsystem), instruments and other spacecraft functions. During the inner Solar System cruise and science tour, the 4 m-diameter HGA communicates with the Deep Space Network at a maximum of 166 kbit/s, using its X-band transmitter and only 20 W power (19 W at end of mission). Two Low-Gain Antennas (LGAs) transmit data and receive commands when the HGA cannot be pointed at Earth.

Cassini HGA
Cassini's High-Gain Antenna (HGA) is able to operate at S-, X-, Ka- and Ku-band. In addition to communications (X-band with Earth and S-band with Huygens), radio-science measurements will probe Saturn and satellite gravity fields, rings, atmospheres and surfaces. This artist's concept illustrates the radar mapping of Titan's shrouded surface of Titan at Ku- band. Radar images will be taken at a typical resolution of 500 m. Altimetry and passive radiometry measurements will also made. Approximately 1% of Titan's surface can be mapped during a flyby. Full coverage will be accomplished by combining the high-resolution radar mapping with lower-resolution passive radiometry (Courtesy of JPL)

Once on its way to Saturn, the Orbiter uses two 445 N main engines and 16 smaller 0.5 N thrusters clustered in groups of four in redundant pairs for propulsion and manoeuvres. The primary and backup main engines have separate feed systems. A gimbal mechanism directs thrust through Cassini's centre of gravity and can swivel ±12.5° in two orthogonal axes. The main engines use the helium-pressurised hypergolic combination of monomethyl hydrazine (N2H3CH3) fuel and nitrogen tetroxide (N2O4) oxidiser. A separate 132 kg tank of hydrazine (N2H4) is used for the thrusters. In general, the main engines are used for all manoeuvres requiring a delta-V greater than 0.8 m/s. The thrusters can provide as little as 0.015 N/s for attitude control.

Cassini orbiter and Hyugens probe
The Cassini Orbiter and Huygens Probe in the solar thermal vacuum test chamber (Courtesy of JPL)

instrument on the remote sensing pallet
The locations of the imaging science instruments on the Remote Sensing Pallet (Courtesy of JPL)

particle experiment on fields and particle pallet
The locations of some of the fields and particles experiments on the Fields and Particles Pallet (Courtesy of JPL)

Mounted below the main engines is a retractable cover that protects them from micrometeoroids during cruise. The thin disilicide refractory ceramic coating on the inside of the engines is especially vulnerable to micrometeoroid damage: it could lead to burn-through and engine loss. The main engine cover can be extended and retracted many times and has a pyrotechnic ejection mechanism should there be a mechanical problem that interferes with main-engine operation. During cruise, the cover remains closed when the main engines are not in use.

Power is supplied to the spacecraft by three RTGs, providing about 700 W. Solar electric power generation is impractical so far from the Sun, as the enormous size of an effective solar array would be too massive and bulky to fit on any launch vehicle.

The Command and Data Subsystem (CDS) receives ground commands via the Radio Frequency Subsystem (RFS). The CDS then distributes the commands designated for other subsystems or instruments, executes those commands that are decoded as CDS commands, and stores sequence commands for later execution. There are two CDSs so that the mission can continue should one fail.

Cassini carries two identical 1.8 Gbit SSRs, each capable of transferring data at more than 470 kbit/s. Each CDS is linked to the SSRs such that each can communicate (read/write) with one SSR, but not both simultaneously. The CDS receives data destined for the ground on the data bus from other subsystems, processes it, formats it for telemetry and delivers it to RFS for transmission to Earth.

CDS software contains algorithms that provide protection for the spacecraft and the mission in the event of a fault. In the case of a serious fault, the spacecraft will be placed in a safe, stable, commandable state (without ground intervention) for at least two weeks to give the operations team time to solve the problem and send the spacecraft a new command sequence. It also automatically responds to a pre-defined set of faults (problems) needing immediate action.

The X-band RFS provides the telecommunications facilities for the spacecraft and is used as part of the radio-science instrument. The Ultra Stable Oscillator (USO), the Deep Space Transponder (DST), the X-band Travelling Wave Tube Amplifier (TWTA), and the X-band Diplexer are also used as part of the radio-science instrument.

The Attitude and Articulation Control Subsystem (AACS) provides dynamic control of Cassini's orientation. It keeps the spacecraft orientation fixed for HGA and remote-sensing pointing and performs target-relative pointing as well as repetitive motion required during imaging such as scans and mosaics. Spacecraft rotation during the Saturn tour that requires high pointing stability is normally controlled by the three main Reaction Wheel Assemblies (RWAs), although modes requiring faster rates or accelerations may use the thrusters. The AACS is capable of supporting a pointing accuracy of 1 mrad with a stability of 8 mrad/s, and rotation rates of 0.02 - 1°/s. The AACS also controls the main-engine gimbals.

The AACS uses Inertial Reference Units (IRUs) for angular-motion measurements about three orthogonal axes. Two of the three are operational at any one time, with one providing backup in case of equipment failure. Together with the Stellar Reference Unit (SRU) star tracker, the IRUs form the basis of Cassini's attitude-determination system.

The heart of each IRU is a set of four solid-state hemispherical resonator gyroscopes (HRGs) developed by the Delco Division of Hughes Aircraft Co. The inertially sensitive element in each HRG is a fused-silica shell, the hemispherical resonator. If a standing wave is established on the shell (much like making a wineglass 'sing' by sliding your finger around the rim) and the shell is rotated about its axis, the oscillating mass elements experience forces that cause the standing wave to precess with respect to the shell. The precession angle is a constant fraction of the angle through which the shell has rotated, allow precise measurement of angular motion in the axis of the HRG.

Each IRU weighs less than 8 kg. The units are designed to meet all performance requirements over 2500 h of testing and 30 000 h of in-flight operation. They must also meet requirements over 200 on/off cycles in testing and 500 on/off cycles in flight.

The SRU is a 15 deg-square field of view star tracker that provides three-axis attitude measurements. The redundant SRU can provide the AACS flight computer (AFC) with up to 50 000 pixels of information per second. AFC software algorithms can establish and maintain stellar reference by comparing incoming pixel frames to an onboard catalogue of some 5000 stars. Three to five stars are commonly tracked at any one time.

Cassini uses a digital Sun Sensor Assembly (SSA) to detect the Sun when it is in the sensor field of view. Following detection, the measured Sun location determines the spacecraft attitude to sufficient accuracy to facilitate star identification by the SRU. The SSA also provides Sun reference for spacecraft thermal 'safing' (i.e. shutdown in case of thermal overload). The SSA has 2-for-1 redundancy, and at least one SSA will be powered on at all times during the mission.

Thermal control is accomplished by several means, the most visible being the black-and-gold Multi-Layer Insulation (MLI). In addition to the automatically positioned reflective louvres covering the 12-bay electronics bus, strategically-placed heaters and radiators also help to provide thermal control for systems and instruments as Cassini travels between 0.61 and 10.1 AU from the Sun. The thermal-control elements must dissipate the waste heat from the RTGs, as well as the 700 W consumed by the various electronics subsystems. The majority of the electronics must be maintained within 5-50°C. In the case of VIMS and CIRS, where substantial thermal isolation from the platform and spacecraft is required, temperature control is provided as an integral part of the instruments themselves. The thruster clusters are temperature- controlled with Variable Radioisotope Heater Units (VRHUs) and catalyst bed electrical heaters. Electrical heaters are also used on the main engines. A heat shield protects the rest of the engine from radiant heating during and after main- engine firings.

Cassini's instruments are capable of observing from the infrared to the ultraviolet, as well as detecting charged particles, dust and magnetic fields. Its radar will pierce the clouds surrounding Titan to provide detailed images and measurements of its surface. During the four-year orbital tour of Saturn, hundreds of thousands of images in many frequencies will be sent back. The science instruments and their purposes are listed in Table 1.

Cassini Orbiter instruments
Table 1. Cassini Orbiter instruments

Cassini and Huygens the Scientists

Jean Dominique Cassini was born in Perinaldo, Italy on 8 June 1625, and given the name Gian Domenico Cassini; he changed his name in 1673 on becoming a French citizen. Christiaan Huygens was born to a prominent Dutch family in The Hague, The Netherlands on 14 April 1628. His family was deeply involved in the sciences, literature and music.

Jean Dominique Cassini
Jean Dominique Cassini with the Paris Observatory in the background (Painting by Duragel, courtesy of the Observatoire de Paris)

Cassini became the head of the Paris Observatory in 1668, and spent much of his time observing Saturn, its moons and rings. He was an excellent and assiduous observer, discovering the moons Iapetus, Rhea, Tethys and Dione between 1671 and 1684, as well as the large gap (1675) between the A and B rings now known as the Cassini Division. He also measured the rotation rate of Mars, determined the orbits of Jupiter's satellites, and created a complete and accurate map of the Moon.

Cassini had great skills as an organiser and in making science exciting to the public; he was also a first-class courtier in a patronage economy that valued novelty. In Bologna, he transformed a cathedral into an observatory, and in Paris he moved the Marly water tower to the Paris Observatory grounds for supporting very long telescopes. He personally supervised and participated in measuring the latitude and longitude of most French towns and villages. Though resistant to some new scientific ideas of his time, he and his sons and grandsons were a major presence at the Paris Observatory for almost 120 years.

Huygens, in addition to his cultural pursuits, also studied law and mathematics, and conducted experiments in mechanics and optics. Though his health was delicate, he was an accomplished dancer. Huygens discovered Saturn's large moon Titan in 1655, and was also the first to deduce (in 1656, but not reported until 1659) that Saturn was surrounded by a ring. He invented the pendulum clock, the first accurate time-keeping device, and was chosen as 'primus inter pares' ('first among equals') to organise the Academie Royale des Sciences in Paris when it was founded in 1666. Young scientists were often attracted by his brilliance, but Huygens preferred solitary contemplation to team efforts. His contributions to mathematics, astronomy, time measurement and the theory of light are considered to be of fundamental importance.

Christian Huygens
Christiaan Huygens (1629 - 1695) by Vaillant, (courtesy of Huygens museum Hofwyck, Voorburg, The Netherlands).

Saturn

view of Saturn
This view of Saturn's northern hemisphere, taken by Voyager-1 on 5 November 1980 from a range of 9 million km, shows a variety of cloud features. Small- scale convective clouds are visible in the brown belt; an isolated convective cloud with a dark ring is seen in the light brown zone; and a longitudinal wave is visible in the light-blue region. The smallest visible features are 175 km across. Such time-lapse sequences show how these storms evolve and allow the measurement of wind speeds. Winds blow mainly along lines of constant latitude on the gas giants. Near Saturn's equator, they blow eastward (with Saturn's rotation) at 500 m/s (Courtesy of JPL)

Although Saturn has been known since pre-historic times, its ring system was not discovered until the 17th Century, and much of what is now known came out of the Voyager flybys of 1980-81. Although its equatorial diameter is about 80% that of Jupiter, it has less than one third the mass, making it the only planet less dense than water (70%). Saturn's interior is suspected to be similar to Jupiter's, with a small rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer.

Saturn's hazy yellow hue is marked by broad atmospheric banding similar to, but less well defined than, that found on Jupiter. The atmosphere is primarily composed of hydrogen with a small amount of helium and traces of other gases (e.g. methane and ammonia). Near the equator, upper-atmosphere winds can reach 500 m/s, blowing mostly eastwards, but they appear to slow at higher latitudes. At latitudes beyond ±35*#176;, these winds can alternate east and west with increasing latitude.

Despite receiving only 1% or so of the sunlight that reaches the Earth, Saturn maintains a relatively high temperature. In fact, it radiates more heat than it receives. Some can be explained by Saturn's immense gravity compressing its interior (the Kelvin-Helmholtz mechanism), and by the condensation and 'raining out' of helium, which generates heat as the drops of liquid helium loose accumulated kinetic energy through friction with lower layers.

Saturn's northern hemisphere
Saturn's northern hemisphere defined by bright features from 43 million km by Voyager-2 on 12 July 1981 (Courtesy of JPL)



   Saturn Facts

   Mass (kg)                                           5.69x1026
   Equatorial Diameter (km)                            120 660
   Mean density (kg/m3)                                690
   Escape velocity (m/s)                               35 600
   Average distance from Sun (AU)                      9.539
   Rotation period (length of day in Earth hours)      10.6
   Revolution period (length of year in Earth years)   29.46
   Obliquity (tilt of axis in degrees)                 26.7
   Orbit inclination (degrees)                         2.49
   Orbit eccentricity (deviation from circular)        0.056
   Mean temperature (K)                                88 (1 bar level)
   Core temperature (K)                                12 000
   Visual geometric albedo (reflectivity)              0.46
   Atmospheric components                              97% hydrogen
                                                       3% helium
                                                       0.05%methane


Saturn's Rings

Saturn's system of rings and moons
Saturn's system of rings and moons is vast, with rings labelled in order of discovery. The faint but far-reaching E-ring is many thousands of kilometres thick, but comprised mostly of micron-sized particles that are not a threat to Cassini (David Seal/JPL)

Though various discoveries have been made over the past 330 years, it was the remarkable images returned by Voyager-1 and 2 in 1980 and 1981 that really made a quantum leap forward in our understanding of the rings. Cassini will help to answer the many questions raised. Saturn's rings are a frigid cast of billions of particles and icebergs, ranging in size from that of fine dust to that of houses. The ring fragments are primarily loosely packed snowballs of water ice, but slight colourations suggest there to be small amounts of rocky material, possibly even traces of rust (iron oxide).

Although the distance from the inner edge of the C-ring to the outer edge of the A-ring is about 13 times the distance across the United States, the ring disc thickness is no more than 100 m (perhaps as small as 10 m!), with waves or 'corrugations' in this sheet rising and falling by a couple of kilometres. If a model of the ring sheet were to be made from material about the thickness of a coin, its diameter would need to be at least 15 km.

Numerous simple and complex patterns form within this rotating sea of icy fragments. They are variously described as circular rings, eccentric rings, clumpy rings, resonance gaps, spokes, spiral density waves, bending waves and shepherding moons. There are, no doubt, also tiny moonlets too small for the Voyager cameras to have detected. The elaborate choreography of this complex ring system of patterns is produced and orchestrated by the combined gravitational tugs from Saturn and its moons that lie beyond the ring sheet, as well as by the tiny tugs from and gentle collisions with neighbouring particles.

How did the rings form in the first place? If one could collect all of the ring particles and icebergs into a single sphere, its diameter would not exceed about 300 km - roughly midway between the sizes of the moons Mimas and Phoebe. Are the rings simply leftover material that never formed into larger bodies when Saturn and its moons condensed aeons ago? Or, as is believed from Voyager data, are they the relatively young (within the last 100 - 200 million years) shattered debris from one or more broken worlds? The subtle compositional variations suggest that more than one parent body was broken apart.

One explanation for the breakup argues that a body (either from within or outside of the Saturn system) passed close enough to the planet to be broken apart by tidal forces, but there would then need to be an energy-loss mechanism to allow the resulting fragments to be captured by Saturn. A more likely explanation attributes the breakup to impacts from meteoroids. If that theory is valid, small ring moons may still be awaiting disruption.

Voyager-2 image of Saturn's rings
Numerous 'spoke' features appear in this Voyager-2 image of Saturn's rings. They are believed to arise from electromagnetic forces acting on charged dust grains that have been dislodged from ring bergs struck by meteoroids (Courtesy of JPL)

Titan

Titan's surface with saturn dimly
This view shows Titan's surface with Saturn dimly in the background through Titan's thick atmosphere of methane, ethane and (mostly) nitrogen. Cassini flies over with its HGA pointed at the Huygens Probe. Thin methane clouds dot the horizon and a narrow methane spring or 'methane fall' flows from the cliff at left and drifts mostly into vapour. Smooth ice features rise out of the methane/ethane lake and crater walls can be seen far in the distance (David Seal/JPL)

Titan is possibly the most unusual moon in the Solar System. Larger than Mercury, more massive than Pluto and only slightly less massive than Jupiter's largest moon Ganymede, it has an atmosphere for some reason yet unknown with a surface pressure 1.5 times that of Earth's at sea level. Although scientists had speculated that Titan had some sort of atmosphere, few were prepared for the layers of hazes and clouds that prevented Voyager from making detailed surface observations. The first hints of surface detail have come from the Hubble Space Telescope, which noted a relatively IR-bright region 4100 km across in the southern hemisphere.

Titan is denser than Saturn's other satellites, possibly due to gravitational compression. Its composition is not precisely known, although its density suggests mostly water ice. Whether it is differentiated into layers or whether there is a molten core is not yet known. No magnetosphere was discovered by Voyager, so Titan might be geologically inactive.

Titan's atmosphere is composed primarily of molecular nitrogen (as is Earth's) with no more than 1% argon and a few percent methane. There are also trace amounts of several other organic compounds (ethane, hydrogen cyanide, carbon dioxide, propane, acetylene, etc.). Other, more complex, chemicals in small quantities must be responsible for the orange colour as seen from space. It is suspected that the organics are formed as methane in the upper atmosphere is destroyed by sunlight. As high-energy particles and UV bombard the nitrogen and methane molecules in the upper atmosphere, these and further reactions could create a variety of organic molecules similar to the smog found over Earth's large cities, but much thicker. These molecules could then clump and rain slowly to the surface, where they may collect in pools, lakes or subsurface reservoirs.

In atmospheric terms, Titan is thought to represent conditions on the early Earth before life appeared. At the surface, Titan's temperature is a frigid 94 K. Water ice does not sublimate at this temperature and so any water at the surface should not be part of the atmospheric chemistry. Nevertheless, there appears to be some kind of complex chemistry going on. Though life in any form familiar to us is unlikely to exist, due to the extreme cold, Titan may still provide us with information that could apply to the chemistry of early Earth.

It has been speculated that methane clouds produce a rain of liquid methane, resulting in large bodies of a liquid ethane/methane mixture up to 1 km deep. However, recent ground-based radar and Hubble Space Telescope observations make it clear that such global oceans are unlikely.

Voyager-2 photograph of titan
This Voyager-2 photograph of Titan, taken on 23 August 1981 from 2.3 million km, shows some detail in the cloud systems. The southern hemisphere appears lighter in contrast, a well-defined band is seen near the equator, and a dark collar is evident at the north pole. All of these bands are associated with cloud circulation in Titan's atmosphere. The extended haze, composed of submicron-size particles, is seen clearly around the satellite's limb (Courtesy of JPL)

Icy Moons

surface of icy enceladus
The bright surface of icy Enceladus. In the foreground, an ice geyser projects a vapour jet into space. Enceladus may be the source of the E-ring (which can be very faintly seen along Saturn's equatorial plane); icy geysers may sustain the ring's supply of micron-sized particles (David Seal/JPL)

All of Saturn's moons are likely primarily water ice with some rocky material, with the sizes and surface characteristics differing greatly, indicating widely ranging conditions during their formation and early existence. Some of the smaller irregular moons, such as Hyperion, might be the remnants of a larger satellite. Others inhabit the rings themselves, and might be leftovers from the cataclysms that created the rings.

Most of the moons for which the rotation rates are known orbit synchronously, keeping one face towards Saturn. This frequently leads to a dramatic difference between the leading and trailing hemispheres. Iapetus has an extremely dark leading hemisphere, and a brightly reflective trailing hemisphere. This dichotomy was first noted by Cassini, who observed that the satellite was visible only on one side of its orbit. Dione is remarkably free of large impact craters on its trailing hemisphere, probably due to a combination of being sheltered from impact gardening and the escape of icy fluids onto the surface through cracks in the crust, leaving the giant crisscrossing, wispy, bright marks observed by Voyager. Rhea shares many of Dione's characteristics.

Enceladus also shows the results of some kind of icy volcanism, with relatively smooth regions interrupting the otherwise cratered terrain. In addition, linear sets of grooves over 100 km long traverse the surface, probably due to faulting caused by crustal deformation, implying that Enceladus may have undergone relatively recent internal melting. Its relatively new surface makes it the brightest of Saturn's moons.

All of the moons show some level of impact cratering, with Mimas being perhaps the most dramatic example. The large impact crater Herschel on Mimas (130 km diameter) was the result of a collision that nearly shattered the moon. Tethys also boasts an immense (400 km) crater. Tethys must have been at least partly liquid to absorb the impact without breaking up. It is speculated that many of the moons may have been shattered and gravitationally reassembled many times in their early geological history. Tethys contains Ithaca Chasma, a huge trench, 100 km wide, stretching across three quarters of its circumference. This feature may have been formed when Tethys solidified and expanded, cracking the crust.

There are complex gravitational tidal resonances between some of Saturn's moons, each other and the ring system. The 'shepherding satellites' - Atlas, Prometheus and Pandora - appear to help keep the rings in place. Mimas may be responsible for the lack of material in the Cassini Division. Pan is in the Encke Gap. Tethys has Telesto and Calypso caught in the region of its Lagrange points. Helene orbits in Dione's leading Lagrange point. Janus and Epimetheus also nearly share an orbit, apparently switching places every four years or so.

Three pairs of moons - Mimas-Tethys, Enceladus-Dione and Titan-Hyperion - maintain stable relationships between their orbits, due to their gravitational interaction. The ratio of Mimas' orbital period to Tethys' is 2:1, as is Enceladus:Dione. Titan's and Hyperion's orbits are in a 3:4 resonance. These resonances can result in tidal heating of the moons, although it is not believed that this process alone could account for the icy volcanism that may exist on Enceladus.

While the majority of Saturn's moons orbit nearly in the plane of its equator, Iapetus' orbit is inclined almost 15°. Phoebe's orbit is upside down, with an inclination of almost 175°. It is possible that Phoebe may be a captured asteroid or a comet remnant.

In addition to the 18 named satellites, at least a dozen more have been reported and given provisional designations, although none has yet been confirmed.

icy satellites voyager-2 images
Some of the interesting variety among Saturn's many known icy satellites is revealed in these Voyager-2 images. Enceladus' bright, relatively uncratered terrain is coated with water ice. The smooth areas suggest that internal heating has melted portions of the surface, possibly even leading to eruptions feeding Saturn's tenuous E-ring. Iapetus, on the other hand, has a leading face as dark as asphalt, while its trailing face is six times brighter. The dark side is presumably some type of carbon-based material, but was it swept up as the moon orbited Saturn or did it rise from the moon's interior? (Courtesy of JPL)

Saturn's Magnetosphere

saturn's magnetosphere
Saturn's magnetosphere and its major features (Courtesy of Univ of Michigan)

Saturn's magnetic field is probably generated by the planet's rotating layer of liquid metallic hydrogen. Equatorial ring currents as high as 107 A flow inside the resulting magnetosphere. The magnetic field is about 0.21 gauss at the cloud tops. Unlike most planets with magnetic fields, however, Saturn's dipole lies within 1° of its spin axis. This has important implications because dynamo theory requires some offset to permit regeneration of the magnetic field. Other Cassini objectives are to improve understanding of the source of the planet's intermittent radio bursts, as well as the many interactions among Saturn's magnetic field and the rings, moons and solar wind.

Public Outreach

design for Cassin's DVD
In this cover design for Cassini's DVD carrying 616400 signatures from 81 different countries, elements include flags, Earth, Saturn, Titan, spacecraft, probe, and Golden Eagle feathers (symbolic of the beauty and power of flight, as well as the quill pen used in writing for almost 14 centuries). Design by Charley Kohlhase (JPL)

The Cassini project maintains websites at http://www.jpl.nasa.gov/cassini/ and http://www.estec.esa.nl/spdww/huygens/html/. The sites also carry details of other material: videos, three 20-slide sets, a newsletter and an historical brochure and poster.

Educational products include a Teacher Guide (US grade levels 5-8), two interactive CD-ROMs (one on Saturn and the other on sensor 'ways of seeing'), a NASA special publication on what we know and hope to learn about the Saturnian system, and a 23-min computer animation depicting the journey of a photon from the Sun's core to Mimas, from there into Cassini's optics and through the spacecraft circuitry, and finally down to Earth where it is ultimately received by the brain of a young observer seeing this icy moon for the first time.

For community relations, the most dramatic product is the Digital Versatile Disk (DVD) containing over 616 400 signatures from 81 different countries. People of all ages mailed their signatures to JPL to be delivered to Saturn. ESA separately collected 100 000 signatures and messages for a CD-ROM attached to Huygens' Descent Module.


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Right Left Up Home ESA Bulletin Nr. 92.
Published November 1997.