Review

Cassini-Huygens’ exploration of the Saturn system: 13 years of discovery

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Science  14 Jun 2019:
Vol. 364, Issue 6445, pp. 1046-1051
DOI: 10.1126/science.aat3760

Cassini's last look at Saturn's rings

During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin.

Science, this issue p. 1046, p. eaat2965, p. eaat2349, p. eaau1017; see also p. 1028

Abstract

The Cassini-Huygens mission to Saturn provided a close-up study of the gas giant planet, as well as its rings, moons, and magnetosphere. The Cassini spacecraft arrived at Saturn in 2004, dropped the Huygens probe to study the atmosphere and surface of Saturn’s planet-sized moon Titan, and orbited Saturn for the next 13 years. In 2017, when it was running low on fuel, Cassini was intentionally vaporized in Saturn’s atmosphere to protect the ocean moons, Enceladus and Titan, where it had discovered habitats potentially suitable for life. Mission findings include Enceladus’ south polar geysers, the source of Saturn’s E ring; Titan’s methane cycle, including rain that creates hydrocarbon lakes; dynamic rings containing ice, silicates, and organics; and Saturn’s differential rotation. This Review discusses highlights of Cassini’s investigations, including the mission’s final year.

Flybys of Saturn by Pioneer 11 in 1979 (1), Voyager 1 in 1980 (2), and Voyager 2 in 1981 (3) provided fleeting glimpses of the Saturn system. Shortly after the Voyager 2 flyby, concepts for an orbiter-and-probe mission like Cassini-Huygens were proposed by the planetary science community to provide answers to important questions raised by these earlier flybys—in particular, to study the unknown surface of Saturn’s haze-enshrouded moon, Titan. In 1986, the National Research Council’s Committee on Planetary and Lunar Exploration (COMPLEX) stated that the “highest priority for outer planet exploration in the next decade is intensive study of Saturn—the planet, satellites, rings, and magnetosphere—as a system” (4). Using the COMPLEX endorsement, NASA initiated joint studies with the European Space Agency (ESA), resulting in the Cassini-Huygens mission.

The motivation for planetary science missions such as Cassini-Huygens includes seeking answers to basic questions such as how planetary systems form, how they evolve through time, and where beyond Earth can requirements for life be found (5). The high-level Cassini scientific objectives were to conduct in-depth investigations of the Saturn system, including orbital remote sensing of Saturn’s atmosphere, icy satellites, and rings; in situ orbital measurements of charged particles, dust particles, and magnetic fields; and detailed measurements with the Huygens probe during descent through Titan’s atmosphere and on the surface. The Cassini-Huygens mission met or exceeded all of these objectives. Details of the mission, orbiter, probe, history, and instruments can be found in (6, 7).

Cassini-Huygens was launched in 1997 and entered orbit around Saturn in 2004 (8), beginning its 13-year exploration of the Saturn system, which spanned almost half of a Saturn year, covering late northern winter, spring, and summer. Cassini performed a comprehensive survey of the Saturn system, including detailed mapping of Saturn’s rings, moons, and magnetosphere, as well as seasonal studies. NASA’s Cassini spacecraft carried ESA’s Huygens probe, which landed on the moon Titan in 2005 (9). This international mission was a cooperative undertaking between NASA, ESA, and the Italian Space Agency [Agenzia Spaziale Italiana (ASI)].

Cassini’s 4-year Prime Mission began its exploration of the Saturn system and raised new questions to address in the following extended missions. The 2-year Equinox Mission continued observations over the equinox crossing in August 2009, when Saturn’s rings were edge-on to the Sun. In 2010, the Cassini Solstice Mission began the final 7 years of exploration. It sent the spacecraft on equatorial orbits with many targeted flybys of the icy satellites; inclined orbits with improved views of Saturn’s rings and poles; and, finally, highly inclined ring-grazing orbits and the Grand Finale phase. In the Grand Finale, Cassini dove between the innermost D ring and the upper region of Saturn’s atmosphere before the mission’s end in 2017, a few months after northern summer solstice, with a deliberate crash into Saturn’s atmosphere.

Cassini’s encounters with Saturn’s moons Enceladus and Titan led to findings such as potentially habitable environments in Enceladus’ and Titan’s subsurface water oceans (1012); Enceladus’ south polar geysers (13), the source of Saturn’s E ring (14); and Titan’s methane-based hydrologic cycle (15), with methane rain that creates hydrocarbon lakes and seas (16).

Other findings include three-dimensional structures in the planet’s dynamic rings (17), a giant Saturn storm that completely encircled a northern latitude band for almost a year (18), a long-lived hexagonal jet stream encircling the north pole (19), and evidence that the captured moon Phoebe may have originated in the outer Solar System’s Kuiper Belt (20).

The final year of the mission, when Cassini moved closer to Saturn than ever before, provided an array of discoveries regarding the interior of Saturn, its upper atmosphere and rings, and the gap between the rings and the planet.

Enceladus, an ocean world

Before Cassini’s arrival, Enceladus, a small moon ~500 km in diameter, was known to reflect almost 90% of the sunlight it receives, making it the brightest moon in the Solar System. During Cassini’s early flybys, images of the moon revealed a system of extensive cracks in the surface and large regions completely devoid of craters. The south polar region contains large tectonic features, is devoid of craters, and is the youngest surface on Enceladus. It includes a system of four nearly parallel fractures, nicknamed the “tiger stripes,” centered near the south pole (21). Each tiger stripe is about 130 km long and 2 to 4 km wide and is separated from the others by ~35 km.

Activity on Enceladus was first detected by Cassini’s magnetometer as a deflection of Saturn’s magnetic field (22). During a close targeted flyby in 2005, Cassini’s thermal infrared spectrometer discovered a hot spot centered on the south pole (23). The four tiger stripe fractures were the warmest regions—more than 100 K warmer than the surrounding areas (24)—and were the source of an immense ice plume (Fig. 1A) imaged by Cassini’s camera (13). The giant plume of water vapor and ice grains is fed by both discrete jets (25) and curtains of material (26) originating inside each tiger stripe. The localized jets in the tiger stripes are the warmest regions detected on the moon (27). Enceladus’ extensive water vapor and ice particle plume was unexpected, and later Cassini mission phases were adjusted to further investigate this feature. Cassini ultimately flew through the Enceladus plume seven times, directly sampling its gas (28) and icy particles (29).

Fig. 1 Icy jets from Enceladus’ south pole create the E ring.

(A) Jets spewing ice particles and water vapor from Enceladus’ south pole form a bluish plume in this false-color image, taken by Cassini in 2005 (128). (B) False-color full-disk view of Enceladus shows softened craters and tectonic fractures. South polar region containing blue-greenish tiger stripes is crater-free and young (129).

CREDITS: (A) NASA/JPL/SPACE SCIENCE INSTITUTE; (B) NASA/JPL/SPACE SCIENCE INSTITUTE

The lack of impact craters in the tiger stripe region demonstrates that it is a geologically fresh surface that is constantly renewed (Fig. 1B). Most of the plume material eventually reimpacts Enceladus, but the smallest grains ejected in the plume are the source of Saturn’s tenuous, distended E ring (14, 30), which is densest at the orbit of Enceladus and spreads throughout the system. The E ring interacts with the inner moons, such as Mimas, Tethys, Dione, and Rhea, coating one side of them with E-ring grains (31). Cassini scientists discovered that the source of the plume is a global liquid water ocean beneath Enceladus’ ice crust (11). The ocean has a depth of ~10 km, located beneath an ice shell that is mostly about 26 to 31 km thick but is considerably thinner, as little as 5 km, in the south polar region (10).

In addition to water vapor (32), Cassini’s Ion and Neutral Mass Spectrometer (INMS) showed that the plume contains gaseous carbon dioxide and simple hydrocarbons such as methane, propane, and acetylene (33). During Cassini’s closest dive through the plume in 2015, INMS also detected molecular hydrogen (H2) (34). Cassini’s Cosmic Dust Analyzer (CDA) found salt-rich ice grains containing sodium and potassium (35) that are probably frozen droplets from the underground salty ocean (36). CDA also detected tiny grains of silica, <10 nm in size, originating from Enceladus’ ocean (37). These tiny silica grains most likely condensed from hot water spewing from hydrothermal vents on Enceladus’ seafloor (37). The excess hydrogen discovered by INMS could also be produced by the hydrothermal vents. CDA and INMS also detected evidence for large organic fragments, indicative of complex organic molecules (38). The source of energy for the hydrothermal activity on Enceladus is tidal interactions among Enceladus, Dione, and Saturn (39). With liquid water, a hydrothermal energy source, and carbon-bearing organic molecules, the subsurface ocean of Enceladus is potentially habitable.

Titan, an Earth-like analog

Titan is the largest moon in the Saturn system, slightly larger than the planet Mercury. It is the only moon in the Solar System with a dense atmosphere, first characterized in detail by Voyager 1 (40, 41). Titan and Earth are the only bodies that have surface liquids: water on Earth, liquid hydrocarbons on Titan. Titan’s atmosphere is mostly nitrogen with some methane and a haze layer of organics that give this moon its orange color. Cassini mission results on Titan have been reviewed in two books (42, 43).

The Huygens probe was built and operated by ESA and carried on the Cassini spacecraft. Huygens separated from Cassini in December 2004 and landed on Titan 3 weeks later, on 14 January 2005 (44). The probe’s 2 hour 27 min parachute descent provided an in situ atmospheric profile of temperature, pressure, density, wind, and composition, as well as detailed images of the surface. The surface pressure was 1.47 times that on Earth (45); super-rotating prograde, zonal winds peaked at 430 km hour−1, much greater than Titan’s equatorial rotation velocity (46); and atmospheric composition included the noble gases argon, krypton, and xenon (47), whereas nitrogen and methane were confirmed as the primary constituents (48). After landing, the Gas Chromatograph Mass Spectrometer (GCMS) measured an increase in abundance of methane gas as the relatively warm GCMS inlet heated Titan’s surface (47).

Cassini flew overhead to receive and relay the Huygens data, including data from ~72 min on Titan’s surface, before Huygens’ link to Cassini was lost as Cassini set over the horizon. Huygens landed in a dry lakebed filled with rounded pebbles (Fig. 2), near the Xanadu region, an equatorial area about the size of Australia. Images taken at an altitude of ~10 km captured erosional patterns on a hillside with very steep slopes. Rounded, smoothed pebbles at the landing site are evidence of fluid flow (49). For more results from the Huygens probe, see (44).

Fig. 2 Huygens probe image taken just after landing on Titan’s surface in 2005.

This image of Titan’s surface was generated with spectral reflection data added for color. The largest icy pebbles are 5 to 10 cm across. Flowing methane created their rounded shapes (130).

CREDIT: NASA/JPL/ESA/UNIVERSITY OF ARIZONA

Cassini-Huygens revealed an array of complex hydrocarbons in Titan’s atmosphere, created as methane gas is broken apart by sunlight in the upper atmosphere, generating active chemistry (50) and a link to haze formation (51). The mechanism by which methane is replenished remains a mystery. Over the course of the mission, methane rainfall darkened parts of Titan’s surface (52, 53), and methane clouds formed and dissipated (54). As the seasons changed, the evolution and breakup of Titan’s northern winter polar vortex and the early formation of Titan’s southern winter polar vortex were observed (55, 56). The polar vortices appear to be tilted by a few degrees relative to the rotational pole of Titan, and the entire stratosphere is also tilted by several degrees (57).

The surface of Titan was observed from orbit using radar and imaging at infrared wavelengths, which showed broad regions of light and dark terrain (58) and geologic processes reminiscent of those on Earth. However, with surface temperatures around 93 K, methane plays the role of water on Earth, and solid water ice is an ingredient of Titan’s bedrock. Titan’s rain is composed of methane, its lakes and seas are filled with liquid methane and ethane (59), and its long, linear equatorial dunes are particles of organic solids or ice covered by organics (60, 61). Some seas are about the size of North America’s Great Lakes (62) and are ~160 m deep (59). Titan’s hydrocarbon lakes and seas are found near the poles, with most of them located in the north (16). As northern summer approached, increasing surface winds (63) produced possible surface waves (64). Strong evidence exists for a subsurface liquid water ocean (12).

The moons Mimas, Iapetus, and Phoebe

The Saturn system contains 62 known moons, many of them small, captured objects that are distant from Saturn and irregular in shape. Closer to Saturn are 24 regular moons that probably formed from the same gas-and-dust cloud as Saturn. Some of these moons have strong gravitational interactions with the ring system, opening gaps and sculpting the rings. Saturn’s icy moons have been reviewed previously (65).

Mimas is the innermost and smallest of the intermediate-sized moons, with a diameter of 394 km. During a close flyby in February 2010, Cassini’s infrared spectrometer discovered that the leading side of Mimas—the hemisphere that faces forward in the moon’s orbit around Saturn—is ~15 K colder than the trailing side (66). This thermal anomaly, which has a shape reminiscent of Pac-Man, is the result of differing thermal inertia between the leading and trailing sides of Mimas. The leading face is altered by bombardment of highly energetic electrons, which increases the contact between regolith grains, decreases their porosity, and increases thermal inertia (67). Models of Saturn’s E ring suggest that the side of Mimas that faces away from the direction of motion should be preferentially coated by particles from the E ring (31). Mimas’ high reflectivity (it is one of the most reflective moons in the Solar System, after Enceladus) supports this hypothesis. Water ice is the primary compound detected on its surface (68). Its heavily cratered appearance demonstrates that there is little current geological activity on the moon (69).

Farther out in the Saturn system, one hemisphere of Iapetus is as dark as soot, whereas the other half is nearly as bright as fresh snow. The reason for this light–dark dichotomy was not known before Cassini’s arrival at Saturn. Spitzer Space Telescope observations have shown that a very tenuous ring exists at the orbit of Phoebe (70), probably due to dust ejected from the moon by tiny meteor impacts. This dust is swept up by Iapetus as it orbits Saturn. The darkest side of the moon is centered in the direction of motion of the satellite. A handful of small impact craters on the dark side punch through to bright material below, suggesting that the dark surface material is only a few meters thick (71).

Cassini imaging showed a 20-km-high ridge that circles most of Iapetus’ equator (72). The ridge breaks up into mountains in some of the dark regions. The ridge must have formed early in the history of Iapetus because it is heavily cratered and eroded. The surface of Iapetus is mainly water ice with small amounts of carbon dioxide, carbon, and complex organic molecules present (73).

Early in the mission, Cassini-Huygens flew close to Phoebe, the largest outer irregular moon with a diameter of 220 km. Phoebe moves in an inclined, elliptical, retrograde orbit 13 million kilometers from Saturn. It is covered with impact craters that were probably created by collisions with smaller outer moons. Some of the craters contain icy patches and layered structures; others have unusual conical shapes (72). Phoebe reflects only a few percent of the sunlight that falls on it, about the same as that reflected by the dark regions of Earth’s Moon. In addition to water ice, Phoebe’s surface is composed of carbon and carbon dioxide (74). The presence of carbon and its higher density than the other medium-sized moons of Saturn suggest that Phoebe was formed near the edge of the Solar System and migrated inward before being captured by Saturn (20). Phoebe might have originated in the Kuiper Belt, the reservoir of ice-and-rock bodies beyond the orbit of Neptune.

Discovery of seven small moons

Before the Cassini mission, Saturn was known to host a family of small satellites, including the co-orbital moons Janus and Epimetheus, which switch orbits every 4 years and appear to have been one body until their violent separation (75). Other previously known moons include the F-ring shepherds, Prometheus and Pandora, as well as Atlas, orbiting just outside the A-ring edge, and Pan, which clears the Encke gap in the rings (76).

Cassini data led to the discovery of seven additional small moons in the Saturn system: Pallene, Aegaeon, Anthe, Methone, Daphnis, S/2009 S 1, and Polydeuces (77). Four of these new moons are the sources of particles for co-orbiting rings or ring arcs. Pallene and Aegaeon create diffuse, dusty rings, whereas Anthe and Methone are associated with ring arcs (78, 79). The tiny moon Daphnis was discovered orbiting in the Keeler gap in the A ring, may help to clear the gap, and produces horizontal and vertical waves along both gap edges (80). An even smaller moon (S/2009 S 1), only 300 m in diameter, was discovered at Saturn equinox by the shadow it cast inside the B ring (81). Aegaeon was discovered inside an arc in Saturn’s G ring (78), whereas Polydeuces co-orbits with Dione in a gravitationally stable zone known as a Lagrangian point (76).

Rings

Saturn’s rings are complex, and the processes observed there provide a laboratory for planet formation. Some of Cassini’s highest-resolution data on the rings were obtained during Saturn orbit insertion (SOI) in 2004 as well as during the final year of the mission. Tiny propeller shapes and strawlike clumping in the strongest density wave peaks were detected in SOI images taken on the unilluminated side of the rings (82). Detailed reviews of the rings are available (83, 84).

Cassini’s 13 years in orbit provided an opportunity to observe temporal changes in Saturn’s dynamic ring system on both the sunlit and unilluminated sides of the rings. Some regions in Saturn’s rings changed on time scales of weeks or months. Dozens of objects (0.1 to 1 km) orbiting in the A ring shifted their locations as they interacted with neighboring material (85). Although these objects were too small to be seen directly, they opened propeller-shaped gaps as large as thousands of kilometers in length that were captured by the Cassini cameras and in stellar occultations of the rings (85). These migrating propeller-shaped structures (85), an analogy for protoplanets forming in a planetary nebula, were followed during the mission. Cassini also witnessed signs of the possible formation of a new moon at the outer edge of Saturn’s A ring (86).

Throughout the mission, Cassini scientists monitored one of the most active, chaotic rings in our Solar System, Saturn’s F ring (87). Clumps and dusty jets appeared and disappeared in the F-ring core, generated by embedded moonlets and disturbed by objects with orbits eccentric enough to dive through the F ring (88). Cassini observed channels opening and closing in the F ring in response to periodic close approaches by the tiny moon Prometheus (89).

Several new ringlets, composed mostly of fine dust grains, appeared during Cassini’s 13 years in orbit (79). One ringlet in the outer Cassini Division was barely visible when Cassini arrived but was among the dustiest features in the rings by the end of the mission. Meteoroids impacting Saturn’s rings generated ejecta clouds of debris that may be a source of these dusty ringlets (90).

Once every half Saturn year the ring plane aligns with the center of the Sun, and for a brief time the northern and southern sides of the rings receive essentially no sunlight. During equinox in August 2009, when the Sun was edge-on to the rings, vertically extended objects cast shadows across the rings (17). During this period, Cassini observed shadows up to 2.5 km long (Fig. 3) created by objects larger than the 5-m vertical thickness of the rings, allowing the determination of the heights of structures within the rings, including kilometer-sized objects near the outer edge of the B ring, and the vertical extent of edge waves in the Keeler gap created by the tiny moon Daphnis (84). The rings also cooled to their lowest temperatures, as for a few days they were heated only by sunlight reflected from Saturn (91).

Fig. 3 Vertical structures rise from Saturn’s B-ring edge.

Among the tallest structures seen in Saturn’s main rings, large objects rise abruptly from the edge of the B ring to cast long shadows across the ring. This image was taken by Cassini 2 weeks before the August 2009 equinox; sunlight is from above (131).

CREDIT: NASA/JPL/SPACE SCIENCE INSTITUTE

The particles forming Saturn’s A and B rings are almost pure water ice but show a strong ultraviolet absorption that varies in strength across the rings, indicating variation in minor constituents (92). A reddish color of varying intensity in the rings is deeper where the ice signature is strongest (93). The rings probably darken with time as they are polluted by meteoroid bombardment. The less massive C ring and Cassini Division are redder than the more massive A and B rings (94).

Hundreds of stellar and radio occultations of the rings were obtained throughout the mission at a large variety of ring geometries. Detailed horizontal and vertical structure (95, 96) and particle sizes (97) were revealed at multiple wavelengths, providing a detailed map of Saturn’s rings. Three-dimensional measurements were taken of tendril-like, ephemeral structures in the rings called self-gravity wakes (98). These transient clumps form only briefly before being torn apart by Saturn’s tides. Similar behavior in a protoplanetary disk might play a role in planet formation. A different kind of microstructure, which behaves like self-gravity but is due to viscous forces in the rings, was seen throughout the densest parts of the rings (99).

Modeling the damping behavior of dozens of spiral density and bending waves, generated by resonant interactions between ring particles and Saturn’s nearby moons, constrained the mass of most of Saturn’s main rings (100). However, occultations were not able to probe directly the densest parts of the B ring, so its mass remained uncertain. Gravity measurements during the Grand Finale orbits showed that the total mass of the rings was less than all previous estimates, hinting at a young age for the rings (101).

Saturn

Cassini’s 13-year mission provided opportunities to observe the formation and dissipation of a giant storm, the characteristics of Saturn’s polar vortices, northern hexagonal jet stream, lightning and aurora, and seasonal change. Cassini arrived 2 years after northern winter solstice and the mission ended just after northern summer solstice, providing almost two complete Saturn seasons of observations. Detailed reviews of Saturn are in (102). The shadows cast by the rings, originally covering the northern hemisphere, shifted through the equator at equinox and covered the southern hemisphere by the end of the mission. The northern winter hemisphere changed color from blue to golden as sunlight once again fell on that part of Saturn’s atmosphere (103). Aurorae and lightning were also observed on Saturn. Curtains of auroral emission rose more than 1200 km above the planet (104). Lightning was generally associated with Saturn storms (105).

Late in 2010, a giant storm erupted quickly in Saturn’s typically bland atmosphere. This type of storm occurs roughly every 30 years, but this one arrived 10 years early. Within months, the storm completely encircled the planet with a swirling band of clouds and vortices (18). Large temperature increases were measured in the stratosphere, and previously undetected molecules were observed in Saturn’s upper atmosphere (106, 107). The storm began to fade shortly after wrapping itself completely around one latitude band of the planet, about 9 months after it began (102).

Saturn’s alternating eastward and westward jet streams define the bands of cloud that circle the planet. The bands follow lines of constant latitude up to within 1° of each pole. One of the jet streams, near 75° north latitude, forms a hexagonal pattern that is two Earth diameters across (19). Voyager first discovered the hexagon, and it is still there after 35 years. Small clouds move eastward around the corners of the pattern. This hexagonal-shaped jet stream (Fig. 4) is remarkable for its stability and longevity; its source remains a mystery. Using Cassini CIRS (Composite Infrared Spectrometer) thermal data of Saturn’s north pole, a thermal hexagonal structure, precisely matching the well-known hexagon, was discovered towering hundreds of kilometers above the cloud tops (108). The presence of this thermal hexagon in Saturn’s northern summer stratosphere, which is connected to the familiar jet stream hexagon in some way, suggests that there is more to be learned about the dynamics of Saturn’s atmosphere.

Fig. 4 The hexagon-shaped jet stream encircling Saturn’s north pole.

This polar hexagon is about two Earth diameters across, more than 35 years old, and very stable. A hurricane-like storm circles at the center of the hexagon in this pair of Cassini images taken in two different seasons: 2013 (left) and 2017 (right) (132).

CREDIT: NASA/JPL/SPACE SCIENCE INSTITUTE

Cassini scientists observed hurricane-like vortices, 50 times larger than a typical Earth hurricane, centered on both of Saturn’s poles (109); these phenomena were stable throughout the mission. In the south, Cassini showed a hurricane-like vortex, with eyewall clouds rising 70 km above the clouds in the center (110). A warm vortex with well-developed eye walls circles Saturn’s north pole as well (111).

Length of Saturn’s day and probing its interior

Saturn emits long-wavelength radio waves known as Saturn kilometric radiation (SKR). SKR was first observed by Voyager in the early 1980s and interpreted as an indicator of Saturn’s internal rotation period (length of day). Cassini results showed that the SKR signals are not coming from the interior of Saturn as originally assumed. The SKR period changed from year to year, incompatible with an interior origin. When Cassini first arrived at Saturn and measured the SKR period, data from the Radio and Plasma Wave Science instrument also showed that the radio waves’ frequencies were different in the northern and southern hemispheres (112). Planetary period oscillations observed in magnetic field data had a similar variability: The period of these oscillations (although close to the expected planetary period of ~10.7 hours) changed over time, particularly with season, and was different in the northern and southern hemispheres (113, 114).

An alternative method for estimating the length of Saturn’s day came from the detection of small, Saturn-driven waves in the rings (115). Saturn’s ring system acts like a seismograph, providing a measure of Saturn’s internal oscillations (or normal modes) that directly probe the interior of the planet (116) and provide a means for measuring its deep rotation rate. These vibrations, determined by Saturn’s nonuniform internal structure, are probably driven by convection inside the planet, which cause oscillations in Saturn’s gravity field that manifest themselves as waves in the rings. Preliminary modeling of the propagation behavior of this collection of waves provides an interior rotation rate for Saturn of ~10.6 hours (117).

The final year

As Cassini ran low on fuel, it embarked on a series of orbits that took it closer to Saturn. The spacecraft transmitted its final data on 15 September 2017, as it plunged into Saturn’s atmosphere, vaporizing to avoid accidental contamination of the potentially habitable moons (118). Cassini’s final phase covered roughly 10 months.

In late 2016, Cassini transitioned to a series of 20 ring-grazing orbits with closest approach (periapsis) just outside Saturn’s F ring, facilitating close flybys of tiny ring moons and high-resolution views of Saturn’s A and F rings. A final close Titan flyby in late April 2017 propelled the periapsis across Saturn’s main rings to initiate the Grand Finale orbits. During these 22 orbits, Cassini repeatedly dove between Saturn’s innermost D ring and the upper atmosphere. The last orbit turned the spacecraft into a Saturn atmospheric entry probe.

Close to Saturn

In the ring-grazing orbits, Cassini performed close flybys of the ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus. The surface characteristics of these moons are regulated by accretion of both a reddish material from Saturn’s main rings and icy grains originating in the Enceladus plume (119). The color and brightness of the moons inside or closest to the main rings (Pan, Daphnis, and Atlas) strongly resemble that of the rings. Figure 5 shows a central core surrounded by an equatorial ridge of ring particles for each of these three moons.

Fig. 5 Montage of Cassini color images of Atlas, Daphnis, and Pan.

These moons were individually photographed for this montage during separate flybys (133). Their colors are similar to those of the ring particles that form their equatorial ridges and coat their surfaces.

CREDIT: NASA/JPL/SPACE SCIENCE INSTITUTE

The Cassini spacecraft passed close to Saturn’s main rings during its final year, during which it obtained high–spatial resolution images, spectral scans, and temperature scans of the rings (120). High-resolution images revealed streaky C-ring plateaus and previously unseen bands of particle clumping throughout the rings. Weaker ice bands were identified in the outer A ring, outside the Keeler gap.

The top of the SKR emission region was sampled directly and repeatedly to determine its source (121) and elucidate how these planetary radio emissions are generated. SKR was strongly time-variable from orbit to orbit, with a dependence on local time around Saturn. Only three SKR source regions were identified, all on the dawn side of Saturn and controlled by the electron densities in the vicinity (121). These regions were embedded in upward currents associated with Saturn’s auroral oval.

Saturn’s interior

The Cassini Grand Finale at Saturn and the Juno mission at Jupiter provide a detailed view of the magnetic and gravity fields of these giant planets and offer windows into their interiors. Comparing these datasets will aid understanding of planet formation in the early Solar System.

Six Cassini orbits were optimized for gravity measurements to determine the mass distribution in Saturn’s interior and the mass of the main rings. The measured values of Saturn’s gravitational harmonics require deep differential rotation to a depth of ~9000 km inside the planet (down to ~0.7 Saturn radii), in line with Juno results for Jupiter (122). This depth may correspond to the levels of magnetic dissipation. The ring mass is about 1.5 × 1018 kg (0.41 Mimas masses), pointing to a ring age of perhaps 107 to 108 years (101).

Magnetic field measurements of Saturn’s internal and external fields showed precise alignment (within 0.008°) between Saturn’s spin-axis and its magnetic axis (114). This is considerably more axisymmetric than any other measured planetary magnetic field, which is inconsistent with current dynamo field theories. Magnetometer data provide insights into Saturn’s conducting interior where zonal flows imply differential rotation, which is consistent with the gravity measurements (101). Higher-order magnetic moments suggest secondary dynamo action in the conducting interior of Saturn (114).

Magnetometer data also showed a strong, low-latitude, field-aligned current system situated between the inner edge of the D ring and the top of Saturn’s atmosphere (114). This current is similar in strength to the currents observed in the auroral zone and, as such, may be part of a global current system.

Traversing the gap

While traversing the gap between Saturn and the rings, Cassini’s in situ instruments directly measured the composition and mass flux of the infalling ring material at various latitudes. This material is composed of nanograin particles (1 to 20 nm), similar in size to small smoke particles, with the highest concentration within 2° of Saturn’s equator (123, 124). Near the equator, collisions with hydrogen atoms provide enough gas drag to decelerate the grains until they plunge into Saturn’s atmosphere. The Magnetospheric Imaging Instrument (MIMI) measured at least 5 kg s−1 of 1- to 3-nm grains entering Saturn’s equatorial atmosphere directly from the inner D ring, possibly from the bright D68 ringlet (123). INMS measured the volatile species derived from the equatorial ring grains—which included water, methane, ammonia, carbon monoxide and/or molecular nitrogen, and carbon dioxide—also entering Saturn’s atmosphere along the ring plane (125). The estimated mass influx rate was 5000 to 40,000 kg s−1, considerably higher than the MIMI estimates, which covered only a portion of the particle size range. This influx of organic-rich nanoparticles from the rings modifies Saturn’s equatorial ionosphere and atmosphere (125).

Grains at higher latitudes are charged and transported along Saturn’s magnetic field lines on each side of the rings (124), consistent with previously detected “ring rain” (126). These high-latitude grains are primarily impact ejecta from the B and C rings. CDA directly measured the mass and composition of the ~20-nm nanograins to characterize this material falling into Saturn. Two separate nanograin compositions were identified: water ice grains and silicate grains (124). Silicate grains accounted for about one-third of the identified nanograins, much greater than the estimated bulk silicate composition of the rings, which is a few percent (94). The absence of larger particles, observed in the rings farther from Saturn, indicates that some unidentified process is grinding up particles, gradually transforming the rings to become part of the planet.

During the final orbits, an inner radiation belt was detected by MIMI in the gap between the D ring and the top of Saturn’s atmosphere (127). Saturn’s main rings inhibit the inward passage of trapped charged particles that form radiation belts. Hence, the radiation belts outside the main rings cannot interact with the inner radiation belt, providing an opportunity to study the inner belt and its interactions with the D ring. Ringlets in the D ring, including D68 and D73, control the structure and outer boundary, respectively, of this radiation belt.

Cassini-Huygens’ 13-year exploration of the Saturn system has set the stage for future missions to Saturn, as well as to the ice giants, Uranus and Neptune. This mission leaves a legacy of discoveries that have changed our views of the Saturn system, how solar systems form, and the potential for life beyond Earth.

References and Notes

Acknowledgments: I thank S. Edgington and T. Spilker for helpful conversations and feedback on this paper. I also gratefully acknowledge all of the Cassini team members who designed, developed, and operated the Cassini-Huygens mission, which is a joint endeavor of NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) and is managed by JPL/Caltech under a contract with NASA. This Review is based on (134), with permission of the Astronomical Society of the Pacific Conference Series. Funding: The research described in this paper was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. United States government sponsorship is acknowledged. Competing interests: There are no competing interests.
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