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Satellites and Rings of the Giant Planets

Athena COUSTENIS, Marcello FULCHIGNONI and Françoise ROQUES

LESIA, Paris Observatory, PSL University, Paris, France

1.1. Introduction

Ground-based measurements as well as measurements from the Voyager, Galileo, and Cassini-Huygens probes have shown that the icy satellites around the giant planets (see Figure 1.1) are not the dead and lifeless objects that scientists imagined until the beginning of the 2000s. Recent observations have in fact revealed that they are unique and active worlds with, for some, surprisingly diverse characteristics on the surface and in the thin atmospheres (Schenk 2010; Lang 2011; Catling and Kasting 2017).

Figure 1.1. Main satellites of the Solar system at scale

(source: Wikimedia Commons, Bricktop, Deuar, KFP, TotoBaggins, City303, JCPagc2015). For a color version of this figure, see www.iste.co.uk/encrenaz/solar2.zip

Until the 1990s, our knowledge of the Solar System was based on the data collected by the Voyager missions, two NASA probes launched in 1977 that flew by the Solar system's giant planets in the 1980s as well as by 48 of their satellites out of the 200 or so that have been observed to date (Atreya et al. 1989).1

The Galileo mission, developed by NASA, had the goal of studying the planet Jupiter and its satellites. Launched in 1989 and in operation until 2003, Galileo allowed us to better characterize Jupiter's atmosphere, its magnetosphere, and its large satellites.2 Most notably, it provided new information about volcanism on Io, as well as substantial evidence about the presence of a liquid water ocean under the frozen surface of Europa and the presence of a magnetic field on Ganymede.

The ESA-NASA-ASI Cassini-Huygens mission, launched in 1997, included an orbiter (Cassini) for Saturn and a lander (Huygens) for Titan. It arrived in the Saturn system in 2004 (Coustenis 2015). For 13 years (2004-2017), it studied the Saturnian environment and unveiled details about the planet, its satellites, and its rings. The satellite Titan was the target of the European Huygens probe, which landed in January 2005. The mission instruments provided an impressive wealth of data that revolutionized our knowledge of the Saturn3 system.

Most satellites are made up of a mixture of water and other kinds of ice, including, often, ammonia ice and rocky material (Atreya et al. 1989). A surprising variety of unusual and spectacular geological characteristics have been observed on these satellites, including patterns that bear witness to tectonic activity, volcanism, resurfacing, and more. Manifestations of geological and atmospheric processes that occur on our planet have also been observed, helping us to improve our understanding of the formation, evolution, and conditions for life in our Solar System and beyond. The latter presuppose the presence of liquid water for around a billion years, as well as the presence of chemical elements that are essential for life (referred to as "CHNOPS," for carbon C, hydrogen H, nitrogen N, oxygen O, phosphorus P, and sulfur S) and of sources of energy in a stable environment (Lissauer and de Pater 2019). According to a categorization proposed several years ago, there are two kinds of possible habitats: those in which underground oceans of water interact directly with a core rich in silicate, as on Europa, the smallest of Jupiter's Galilean satellites, or on Enceladus, a small satellite orbiting Saturn, while another class of habitats includes satellites that have bodies of liquid water sandwiched between two layers of ice, as we expect to be the case for Ganymede, the largest satellite in the Solar system, in orbit around Jupiter or Titan, Saturn's largest satellite (Coustenis and Encrenaz 2013; Coustenis 2014; Hand et al. 2020).

We know that the large satellites of the gas giants, which are located beyond the "ice line" (that is, the distance from the Sun corresponding to the limit for which condensation of water molecules is possible on the surface of a celestial body), contain large amounts of water. Indeed, with an average density of around 1.8 g/cm3, the largest satellites (like Titan) contain almost 45% water by mass. Among these satellites, some have a core rich in silicates and can contain underground oceans of liquid water in contact with the core (as is probably the case for Europa and Enceladus). This is important, because in such environments on Earth, we find life (for instance, in the hydrothermal vents located at the bottom of the oceans) and therefore generate the heat from the chemical sources located under their ice crust. Other satellites can have oceans or pockets of liquid water trapped between two layers of ice (as suggested by measurements on Ganymede and Titan). In the study of the emergence of the elements of life on such satellites, the time scale is essential. If this liquid environment lasted for long enough, it could see the emergence of life; but, on the other hand, this could be inhibited if the ocean was so isolated from the environment that it was impossible for the concentration of ingredients necessary for life or the appropriate chemical inventory for the pertinent biochemical reactions to occur. What is important is to realize that these satellites contain large volumes of water, from whence they get their nickname of "ocean worlds" (see Figure 1.2). Sometimes the amount of water contained in these satellites exceeds that found on Earth, sometimes by factors that can even reach an order of magnitude, as is the case for Titan and Ganymede.

Figure 1.2. Ocean worlds in the Solar system. The mass of the water in liquid or solid form is indicated, the mass of water on Earth taken as a unit. The percentage of liquid water in relation to the total mass of the planet or satellite is also given

(source: PHL@UPR Arecibo, NASA). For a color version of this figure, see www.iste.co.uk/encrenaz/solar2.zip

However, the exploration of these sub-surface oceans isn't easy, because the outer layers of ice on satellites can be very thick. We suppose that their thickness is on the order of several hundreds of kilometers on Ganymede, Callisto, and Titan. Europa may have a thinner layer of ice (possibly several dozens of km) because its proportion of water is only 10% of its mass, and it also has "chaotic" regions where the surface layer of ice may be even thinner. To understand the internal structure of the water layer, it is necessary to know the phase diagram of water under certain conditions of pressure and temperature.

On the edge of our Solar System, we find Uranus and Neptune surrounded by satellites that are unique in many ways, such as Triton and its geysers. Triton (in orbit around Neptune) and other objects beyond Neptune's orbit are thought to harbor liquid water oceans under the surface (still present today, even at the very low temperatures of their environments). The satellites of Uranus and Neptune have only been explored to date by the Voyager 24 mission.

Below, the most important discoveries about the outer Solar System's satellites from spatial and ground-based observations are described.

1.2. Jupiter's satellites

As of 2020, we are aware of 79 natural satellites around Jupiter (see Table 1.3, Volume 1, and Figure 1.1 in this volume) with diverse characteristics (Burns et al. 2004; Schenk 2010). The four largest satellites in the system, called the "Galileans" (Io, Europa, Ganymede, and Callisto) were discovered by Galileo in 1610.

1.2.1. The Galilean satellites

Among the Galilean satellites, Ganymede, Europa, and Io are caught in a 1:2:4 orbital resonance, which means that these objects periodically exercise strong gravitational influence on each other (see section 1.2.3, Volume 1, and section 4.5 in this volume). The three ice-covered Galilean satellites, Callisto, Ganymede, and Europa, present a wide diversity of characteristics on their surfaces, probably because of their different evolutions, even if they are situated in the same environment (see Figure 1.1). The unique characteristics of their geology bear witness to the parameters of their formation such as composition, density, and temperature, as well as evolving factors such as geophysical processes and the stage of differentiation (Schenk 2010). The Galilean satellites show signs of increased geological activity as their distance from Jupiter decreases (see Figure 1.3). As early as the 1980s, the observations from the Voyager probe showed large disparities between these four satellites, suggesting a strong influence of gravitational tidal forces on each of them. Thus, Io, the closest to Jupiter, is subjected to enormous tidal forces that lead to very intense volcanism. Europa may still be very active tectonically and volcanically today, because the small number of craters detected indicates a relatively young surface, while the larger Galilean satellites, Ganymede and Callisto, which are also the farthest, have old and densely cratered surfaces with a large range of sizes and internal structures (see Figure 1.3). Furthermore, the interactions with the Jovian magnetosphere and stellar winds, as well as the effects of the tides, have left traces on the surfaces that we see today in the form of æolian processes, cryovolcanism, and impact...