On September 15, the Cassini spacecraft is going to plunge into the atmosphere of Saturn where it will be vaporized, marking the end of more than 13 years of exploration of Saturn and its environment.

Artist view of the dislocation of the Cassini spacecraft in the upper atmosphere of Saturn (Credits: NASA/JPL)

The Cassini-Huygens mission

Everything started in the 80’s, not long after Saturn flybys by the probes Voyager 1 & 2, when the idea of a space mission devoted to Saturn and to its moon Titan – the only moon in the solar system with a thick atmosphere – started to grow at the American and European space agencies, NASA and ESA. The mission was named Cassini-Huygens in honor of two astronomers of the XVIIth century, the Italian/French Giovanni Domenico Cassini and the Dutch Christiaan Huygens. Huygens discovered Titan and observed Saturn’s rings while Cassini described their structure.

The mission is composed of two parts: the Cassini orbiter and the Huygens probe developed by ESA and destined to dive into Titan’s atmosphere. Launched in October 1997, the Cassini-Huygens couple started to orbit around Saturn on July 1st, 2004, after a 7-year journey through the solar system. Initially scheduled to last for 4 years, the mission has been extended twice up to September 15, 2017. During the 13 years of the mission, Cassini will have achieved almost 300 orbits around the ringed planet, continuously collecting data of Saturn and its space environment.

The analysis of these observations enabled the scientific team of the mission, to which the Laboratory of Planetary and Atmospheric Physics (LPAP) of the STAR Research Unit of the University of Liege, to make numerous discoveries and extend our knowledge of the Saturnian system.

Moons and rings of Saturn

A first important scientific breakthrough was the plunge of the European probe Huygens through Titan’s opaque atmosphere in early 2005 and its landing on the surface of the moon, Huygens thereby becoming the first man-made object to land on a body so far away. Huygens captured unprecedented images of Titan’s surface, covered by mountains, dunes and methane lakes. Thanks to Huygens descent and to a hundred of flybys of the moon by the Cassini orbiter, scientists could determine the composition of the atmosphere and describe the atmospheric dynamics, including a methane cycle similar to the water cycle on Earth.

One of the most surprising discoveries made by the Cassini mission was the presence of geysers on the surface of Enceladus. Enceladus is a small moon of Saturn covered by a thick layer of ice. Cassini flybys have revealed jets of water vapor and icy particles at the south pole of the moon. The water of these geysers comes from a global liquid water ocean beneath the icy crust of Enceladus. This large ocean has all the characteristics required to shelter life, but only a new space mission would be able to detect signs of life.

Besides Titan and Enceladus, Cassini has observed many other moons and highlighted the strong interaction existing between the moons and the rings of the planet. Contrary to what was previously thought, images provided by Cassini cameras have shown that the rings are very dynamic and disturbed by small irregular moons orbiting within them.

Atmosphere and auroras of Saturn

The planet Saturn achieves one orbit around the Sun in approximately 30 years. The Cassini spacecraft has then accompanied Saturn during almost a half Saturnian year and seasonal phenomena could be observed in its atmosphere. Heat circulation between the summer hemisphere and the winter hemisphere has been understood, as well as the dynamics of the winds. Cassini had also the opportunity to follow the evolution of a storm, as large as the whole Earth, which persisted during 7 months in Saturn’s atmosphere. In addition, a large-scale atmospheric stream characterized by a hexagonal shape and discovered during the Voyager flybys, is located around the north pole of Saturn. Cassini has revealed the high stability of this hexagonal structure and the evolution of its color throughout the seasons because of the formation of haze in the polar regions.

Cassini image of the hexagonal atmospheric structure at the north pole of Saturn (Credits: NASA/JPL)

In the upper atmosphere, at high latitude, atmospheric gases emit light due to the precipitation of charged particles traveling along the magnetic field lines. These polar lights, usually referred to as aurorae, constitute a valuable tool to investigate the dynamics of the matter surrounding Saturn. Researchers at the LPAP of the University of Liege acquired an important expertise in the study of the auroral emissions by analyzing images in ultraviolet light provided by Cassini. They have, among others, discovered that Saturn’s inner magnetosphere, the region around Saturn dominated by the planetary magnetic field, can be strongly influenced by the charged particles coming from the Sun, an influence which was not expected before.

Superimposition of aurorae observed in 2016 in ultraviolet by the Cassini spacecraft and an image of Saturn. The human eye being insensitive to ultraviolet, the light of the aurorae has been converted to visible light.

Hence, September 15 will mark the end of the exceptional Cassini mission. Up to its final disintegration, Cassini will transmit data towards the Earth during its plunge in Saturn’s atmosphere to help scientists to know even a little more about it. Nevertheless, after the spacecraft will have disappeared in the clouds of Saturn, the large amount of data collected during 13 years by Cassini will continue being used in order to, undoubtedly, make new discoveries.

Benjamin Palmaerts  is a Post-doc researcher at the Laboratory for Planetary and Atmospheric Physics (LPAP)  – in the STAR Research Institute –  counting a lot of specialist of the study of the UV aurora emissions on Jupiter and Saturn .