Cassini‘s greatest hits

For 13 years NASA’s Cassini orbiter has been circling Saturn, taking measurements of the solar system’s ringed jewel and its cornucopia of satellites. On 15 September that run, which spanned most of the Saturnian winter and all of spring, will come to an end. Low on fuel and still perhaps carrying a terrestrial microbe or two, the probe will crash into the gas giant to avoid contaminating one of its potentially life-friendly moons.
Along with taking one awe-inducing photo after another of the Saturnian system, Cassini has racked up dozens of scientific achievements. In January 2005, shortly after parking itself in Saturn’s orbit, the spacecraft dispatched the European Space Agency’s Huygens probe, which became the first human creation to land on a body in the outer solar system when it touched down on the planet’s thickly hazed moon Titan. Cassini proceeded to observe colossal Saturnian storms, analyze an intricate ring system that may resemble protoplanetary disks, and spot geysers of water ice emanating from Enceladus, a moon that had long played second fiddle to Titan.

If there’s one thing that Cassini—and subsequently other planetary probes such as New Horizons and Rosetta—has shown, it’s that even the most unexciting-looking celestial objects can turn out to be incredibly dynamic, once they’re studied up close. In particular, Cassini’s observations of Saturn’s active moons have forced scientists to reimagine the cosmic blueprint for life as they turn their attention to distant planetary systems. Despite headlines gushing over Earth-size exoplanets that are warm enough for water to pool on their surfaces, Cassini has demonstrated that life may very well thrive far outside the so-called habitable zone.

Here, using the stunning images from Cassini that we’ve almost come to take for granted, we round up some of the mission’s most important achievements.

The moons

Taken by Cassini in 2009, this image shows the atmosphere of Titan lit from behind. Titan is the only satellite in the solar system with a dense atmosphere —one that, like Earth’s, is rich in nitrogen (98.4%, in Titan’s case). The yellowish haze is caused by molecules called tholins, which are synthesized by UV light irradiation of methane and other simple organic molecules in the atmosphere. Credit: NASA/JPL

On 14 January 2005 the Huygens probe (named for Dutch astronomer Christiaan Huygens, who discovered Titan in 1655) became the first—and still only—human-made craft to land on a body in the outer solar system. This image was created from several shots taken by the Descent Imager/Spectral Radiometer. The shapes in the foreground, lit by a lamp on the lander’s exterior, are 10–15 cm in diameter and are believed to be primarily frozen water. Credit: ESA/NASA/JPL/University of Arizona

As the Huygens probe was descending to the surface of Titan, it took several images of the moon’s surface. The wrinkled hills are believed to be composed mainly of water ice. The surface is covered in a layer of tholins; the organic molecules are produced in the atmosphere, brought to the ground in methane rains, and then funneled into flatlands, craters, and valleys. Credit: ESA/NASA/JPL/University of Arizona

In the near-IR, sunlight can be seen glinting off the surface of Titan’s northern polar sea. The Voyager missions first hinted at the presence of liquid hydrocarbons on Titan’s surface in the early 1980s. It was not until Cassini captured specular reflections like the ones in this composite image from 2014 that the presence of large bodies of liquid was confirmed. Credit: NASA/JPL-Caltech/University of Arizona/University of Idaho

This false-color radar mapping of Titan’s northern hemisphere was released in 2013. The seas seen in the previous image as well as numerous smaller lakes and rivers are colored blue and black. The majority of Titan’s surface shows minimal variance in elevation, rarely exceeding 500 m. Most of the moon’s mountains are in the southern hemisphere, but the Misty Montes can be seen in the lower left part of the image between 50–60° N and 60–70° W. Credit: NASA/JPL-Caltech/ASI/USGS

This mosaic of Enceladus, Saturn’s sixth-largest moon, was assembled from photos taken by Cassini during two 2005 flybys. It reveals the moon’s active surface, particularly near the south pole at the bottom of the image. In those early flybys, Cassini compiled evidence that the four blue parallel cracks, known as tiger stripes, emit jets of salty ice and water vapor into space. Subsequent analyses have concluded that Enceladus hosts a global ocean , in places perhaps just a few kilometers below the surface ice. Thanks to Cassini Enceldaus is now one of the top candidates for hosting extraterrestrial life within the solar system. Credit: NASA/JPL/Space Science Institute

Enceladus’s south pole erupts in this 2010 image taken with Cassini’s narrow-angle camera. In 2005 the probe’s magnetometer and UV spectrograph detected water vapor and other chemical species above the moon’s southern hemisphere, which confirmed scientists’ previous suspicions of cryovolcanism. Cassini subsequently flew through the plumes and identified traces of molecules including ammonia, methane, and carbon dioxide. The source of the geyser’s material is a global saltwater ocean, an environment that could potentially host life. Enceladus’s eruptions also feed Saturn’s E ring. Credit: NASA/JPL-Caltech/Space Science Institute

Iapetus, discovered in 1671 by Giovanni Cassini, is Saturn’s third-largest moon. Its most notable characteristics are its equatorial ridge and two-tone coloring. The former is the result of Iapetus’s large size and low density; the moon is composed primarily of ice. The reddish-brown coloring is from material left behind as surface ice sublimated away from the warmer area of the surface. Iapetus’s large orbit, combined with the moon being tidally locked to Saturn, causes Iapetus to have a much more significant heating and cooling cycle than that of the other moons. Credit: NASA/JPL/Space Science Institute

Mimas, discovered in 1789 by William Herschel, is the smallest astronomical body discovered so far to have been rounded in shape by self-gravitation. Only 415 km in diameter at its widest, the moon is best known for the 130-km-wide crater seen in this image. The crater was discovered in 1980, three years after the first Star Wars movie, and as a result Mimas has frequently been referred to as the Death Star moon. This image was taken during Cassini’s closest approach to Mimas in 2010. Credit: NASA/JPL-Caltech/Space Science Institute

A UFO, a wrinkled bean, and a celestial ravioli are the common descriptors for three of Saturn’s moons: Atlas, Daphnis, and Pan. These images, which were taken by Cassini on different dates in 2017, were juxtaposed to compare the moons’ sizes and shapes. Atlas and Pan are of similar size and feature a distinctive central bulge. Daphnis is tiny in comparison, with a diameter of just 8 km, so small that the moon was not discovered until 2005 when Cassini imaged it. Atlas orbits near the outer edge of Saturn’s A ring; Daphnis and Pan orbit inside the A ring, within the Keeler and Encke Gaps, respectively. Credit: NASA/JPL-Caltech/Space Science Institute

The rings

Cassini captured a stunning view of Saturn’s rings when the planet was positioned between the Sun and the probe in 2006. Beyond the brightest rings are two sets of faint ones: the G ring and, farther outward, the E ring. During the occultation Cassini scientists also discovered two new rings that are coincident with the orbits of Saturnian moons—just one example of how the planet’s satellites mold its intricate ring system . The dot just inside the G ring, at about ten o’clock, is Earth. Credit: NASA/JPL/Space Science Institute

The bright blotches at the bottom right of this 2009 Cassini image are called spokes, and they are just about as mysterious now as they were when the two Voyager spacecraft first spotted them in the 1980s. The radially oriented spokes reveal that particles in Saturn’s rings, in this case the B ring, sometimes reflect sunlight differently. Scientists suspect the spoke phenomenon is due to the influence of Saturn’s magnetosphere on electrically charged ring particles, a rare case of forces other than gravity manipulating ring behavior. There also seems to be a seasonal component, as both Cassini and the Voyagers spotted the spokes near the Saturnian equinox. Credit: NASA/JPL/Space Science Institute

Saturn’s potato-shaped moon, Prometheus, makes a mess of the planet’s F ring in this 2006 Cassini image. Every 14.7 hours Prometheus’s elliptical orbit brings the moon close enough to the F ring to draw material from the ring and carve out a dark channel. Because Prometheus orbits faster than do the ring particles, the moon pillages a different portion of the ring each go-around. Credit: NASA/JPL/Space Science Institute

A small moon sits in the center of the propeller-shaped feature in this April 2017 photo of Saturn’s A ring. Named Blériot after the early 20th-century French aviator, this feature is the largest of the more than 150 propeller structures spotted by Cassini. First predicted in the early 2000s, the features are the result of the gravitational influence of ring-embedded moonlets between 100 m and 1 km in diameter. The Blériot moonlet is obscured by the disturbed ring material around it. Credit: NASA/JPL-Caltech/Space Science Institute

The atmosphere

A planet-encircling storm churns through the atmosphere in Saturn’s northern hemisphere in February 2011. Known as a Great White Spot, such a storm occurs about every 30 years. Six of them have been observed since they were discovered in 1876; Cassini was able to track the most recent one, which first appeared in 2010. The giant, infrequent storms may be caused by water vapor in Saturn’s atmosphere. The heavy water molecules could suppress convection between the planet’s hot and cold atmospheric gases for decades until a tipping point is reached, which triggers a massive thunderstorm. Credit: NASA/JPL-Caltech/Space Science Institute

This false-color image, called The Rose, was created in 2013. It depicts a giant storm spinning above Saturn’s north pole. The colorization reveals information about the storm’s altitude: Low clouds are depicted in red and high clouds in green. Although 50 times as wide as a typical hurricane on Earth, the Saturnian hurricane has many similar features, including a central eye and eyewall. Unlike those on Earth, however, the hurricane is locked in place and has probably been swirling for years. Credit: NASA/JPL-Caltech/SSI

The hexagonal wave pattern surrounding a hurricane at Saturn’s north pole continues to intrigue scientists. The colors blue, green, and red represent wavelengths of light from UV to visible to IR. The hexagon may be a jet stream that acts as a barrier separating small haze particles inside the hexagon from large particles on the outside. Numerous small vortices appear as reddish ovals; some spin clockwise, while the hexagon and hurricane spin counterclockwise. Credit: NASA/JPL-Caltech/SSI/Hampton University

This view of the roiling clouds above Saturn’s north pole, which was captured by Cassini on 26 April 2017, has been called Top of the World. It was taken during the first of 22 planned dives through the 2400 km gap between Saturn and its rings, dubbed the Grand Finale. As Cassini approached Saturn, it achieved speeds in the range of 121 000–126 000 km per hour, coming within about 3000 km of the tops of Saturn’s clouds and within about 300 km of the innermost visible edge of its rings. To protect itself from hitting particles in the planet’s rings, Cassini used its large, dish-shaped antenna as a shield. Credit: NASA/JPL-Caltech/Space Science Institute

Other favorites

Engineers at NASA’s Jet Propulsion Laboratory take measurements of one of Cassini’s three radioisotope thermoelectric generators, or RTGs, prior to launch in 1997. At launch the probe carried about 50 pounds of plutonium-238 ; heat from the isotope’s decay has been used to generate about 900 W of electricity to power the craft’s instruments throughout the mission. Cassini and the Huygens lander launched on 15 October 1997 aboard a Titan IVB/Centaur rocket. Credit: NASA/JPL

In 2000 Cassini passed within 10 million km of Jupiter during its voyage, a vantage point close enough to capture this true-color portrait. Mission scientists spent six months studying the giant planet, in part to test instruments and operations procedures for the main event at Saturn. The craft’s IR spectrometer mapped Jupiter’s temperature and atmospheric composition and tracked the movement of clouds, haze, and storms. Cassini also joined NASA’s Galileo spacecraft, which was already patrolling the Jovian system, to take the first simultaneous multiprobe measurement of Jupiter’s magnetosphere . The Cassini observations set the stage for a return to Jupiter with NASA’s Juno spacecraft . Credit: NASA/JPL/Space Science Institute

The Earth–Moon system is visible below Saturn’s rings in this photo captured by Cassini’s wide-angle camera on 19 July 2013. Imaging science lead Carolyn Porco , who helped set up Voyager 1’s famous Pale Blue Dot photos, led the effort to take a similar shot with Cassini. In the wide shot the Moon protrudes off the right side of Earth (see small box); both objects can be clearly seen in the inset, which shows a magnified view. Credit: NASA/JPL-Caltech/Space Science Institute