The Cassini spacecraft has been in orbit around Saturn for seven years. The objectives of the mission have been to study the atmosphere of that planet, its satellites, its magnetic field and its ring system. NASA has extended the life of this successful mission until 2017.
Three moments of great emotion have marked the life of the Cassini-Huygens mission. The first occurred when the spacecraft finally entered orbit around Saturn in July 2004, after almost seven years of travel. The second momentous moment occurred when mission controllers managed to get the small Huygens probe to undock from the main spacecraft and penetrate Titan’s dense atmosphere in January 2005; As the probe descended and landed on the surface, its cameras captured the landscape of this enigmatic moon, Saturn’s largest. The third moment occurred in October 2008 when, on a low flight over the icy moon Enceladus, the lead spacecraft passed through the spectacular geysers of steam and ice dust erupting at its south pole.
The Cassini-Huygens mission is a joint project of NASA and the European Space Agency (ESA), to which 18 European nations contribute, but scientists from many other countries participate. Its name honors two 17th century astronomers: the Italian-French Giovanni Domenico Cassini (1625-1712) and the Dutchman Christiaan Huygens (1629-1695). Both made notable contributions to the study of Saturn, its moons and its rings.
Mysterious rings
Despite what we have learned about Saturn’s rings since Galileo Galilei first observed them with his rudimentary telescope in 1610, we still don’t know enough to explain with certainty how they formed, what they are made of, and how they are organized. We assume that the main rings (those seen with a simple telescope) are composed of a multitude of ice rocks a few meters in diameter and that these rocks are distributed in several layers no more than a few tens of meters thick, But not even the Cassini probe has been able to get close enough to see individual rocks and thus determine their size and composition. Even the best photos show us the rings as a uniform sheet without thickness. However, the main rings cannot be a continuous, rigid disk. Here’s why: The force of gravity that a planet exerts on an object orbiting it is stronger on the parts of the object that are closer to the planet and less intense on those that are farther away. The difference is called tidal force and tends to break the object. If the object orbits very far from the planet, the tidal forces are very small, but they multiply if the object approaches less than one or two times the radius of the planet, a region known as the Roche limit, or if the object is very big. Within the Roche limit, tidal forces exceed the cohesive forces of bodies with relatively large sizes, in such a way that they stretch them until they break and fragment into pieces small enough so that the tidal and cohesive forces are balanced. .
Throughout the mission, Cassini’s cameras and especially two of its instruments, the CIRS and VIMS spectrometers, have devoted a good part of their attention to the main rings. Spectrometers are instruments that decompose and measure the properties of the light (or electromagnetic radiation) they receive from bodies in order to identify their composition. Each spectrometer operates in specific ranges of the electromagnetic spectrum; The CIRS analyzes the infrared radiation and the VIMS the visible part and also the infrared. In particular, the objective of CIRS has been to create temperature maps of both the rings and Saturn and its moons and, with these maps, we seek to better understand these bodies and the processes associated with them.
The images from the Cassini spacecraft show details impossible to achieve even with the most powerful telescopes in Earth orbit. These images were taken near the equinox, when the Sun illuminates the rings almost edge-on. Like sunsets, the shadows on the rings are very elongated and this magnifies irregularities in the structure of the rings.
It was long thought that the main rings formed when a satellite about 300 kilometers in diameter in an unstable orbit entered the Roche limit and was torn apart by tidal forces. The idea seems reasonable: Saturn has many moons of those sizes. However, we know even before the Cassini spacecraft that the rings are made of more than 90% ice, so the unfortunate moon would have to have been a ball of pure ice. Perhaps that is why some scientists preferred the hypothesis that, instead of an unstable moon, the object that gave rise to the rings was a comet captured by Saturn. In 1994, comet Shoemaker-Levi 9 (SL9), which had a nucleus about five kilometers in diameter, was projected into Jupiter’s atmosphere. Upon crossing the Roche limit, the comet broke into several pieces that one by one were engulfed by the planet. In the fragmentation process, tidal forces stripped the comet of its outer layers of gas and dust, and these formed a faint halo around Jupiter. From that event and with the recent observations of the Cassini spacecraft; An intermediate theory was proposed. The new theory assumes the destruction of a satellite as large as Titan, which is more than three times larger than the Moon. This satellite would have been composed of a rocky core and a thick mantle of water ice. In the early stages of Saturn’s formation this hypothetical moon was gradually stripped of its outer layers of ice as it approached closer and closer to the planet, as suggested by the case of comet Shoemaker-Levy 9. Ultimately, the core and part of the mantle The icy planets sank into Saturn’s atmosphere, but the ice in the outer layers remained in orbit. Over time this material formed the flattened disk we see today. The problem is still open, but for the moment this is the best theory we have.
With data from the CIRS infrared spectrometer it is possible to construct temperature maps of Saturn’s main rings. This sequence of images combines observations from the CIRS instrument with images from the Hubble Space Telescope to study temperature variations from the solstice (up) to the equinox (down). We use a color scale where violet represents the lowest temperatures and red represents the hottest areas.
Chiaroscuro
On August 12, 2009, the most recent equinox occurred on Saturn (the equinox occurs when a planet reaches one of the two points in its orbit where the Sun’s rays fall perpendicular to the axis of rotation). With the equinox the first part of the Cassini-Huygens mission officially concluded. The objectives of this first stage were to put the ship in stable orbit around Saturn and release the Huygens probe on Titan. During the second stage, called the Solstice Mission, the spacecraft will simply wait for the next Saturnian solstice, which will occur in 2017. Interest in Saturn’s equinoxes and solstices has to do with the orientation of the rings relative to the Sun.
The axis tilt of planets like Earth and Saturn is essentially constant, but their relative tilt with respect to the Sun changes depending on position along their orbit, making their axes appear to oscillate vertically. We call the equinox the moment when the planet’s axis is not inclined and the solstice when it has its maximum inclination. In a complete cycle around the Sun (what we call the year of the planet and which on Saturn is equivalent to 29 Earth years) there are two alternating equinoxes and two solstices. When Saturn is at its equinoxes, its rings are almost invisible because we see them edge-on and when it is at its solstices, the rings have their maximum opening in one case showing their south side and in another their north side. Due to this variable orientation of the rings with respect to the Sun, they receive a highly variable amount of light, which also changes their temperature. The spectrometers on the Cassini spacecraft allow us to infer the composition, surface texture and structure of the rings from these temperature variations. The idea is that the different materials the rings are made of should react differently to changes in temperature. For example, all objects emit some radiation that depends on their temperature; At hundreds of degrees, any object will emit visible light (we then say that they shine). At extremely high temperatures, thousands of degrees or more, they could emit ultraviolet radiation and even X-rays and gamma rays, like stars. Colder bodies, tens of degrees above zero, for example, emit infrared radiation. The way light is reflected, on the other hand, depends on the structure of the object and other physical properties.
Our planet receives a flow of solar energy of about 1,400 watts per square meter of surface, but Saturn and its rings, which are almost 10 times farther from the Sun, only receive about 16 watts per square meter (about one hundredth of the energy that the Earth receives). In the case of the rings, Saturn also bathes them in radiation, but this remains more or less constant, at one watt per square meter.
The rings absorb on average 50% of the energy they receive. The other half reflects it. The absorbed energy warms them slightly until they reach an equilibrium temperature of between 40 and 120 kelvin (i.e. between -230 and -150° C). For this reason the rings emit infrared radiation, making the CIRS spectrometer ideal for studying them.
Outside of the equinox the rings always exhibit a hotter illuminated side and a cooler dark side. In particular, at the solstice the illuminated side reaches the highest temperature and at the equinox, when none of its sides is illuminated by the direct energy of the Sun, they cool down to their minimum temperature.
The temperature variations measured by Cassini confirm that the rings are composed almost entirely of ice, but contain a minimal amount of other compounds that are added by the continuous bombardment of meteorites that Saturn attracts from the outside. These same data also tell us that the rings have a structure that is not homogeneous, as they show tenuous regions that allow the passage of energy from the Sun or Saturn itself from the illuminated side to the dark side, but they also have colder, denser regions. and opaque, where solar radiation penetrates with difficulty.
ring family
Saturn is not the only planet with rings: Jupiter, Uranus and Neptune have their own ring systems, discovered in the last 40 years. It has recently been suggested that Mars could have two rings of dust due…