May 23, 2019 : How Can We Explain The Apparent Contrast Between The Relatively High Level Of Internal Activity Of Enceladus, Titan Or Dione And The Relatively Low Level Of Internal Activity Of Moons Like Mimas Or Rhea ?

The System of Saturn is rich in mysteries. Some moons like Enceladus or Titan seem particularly active in terms of geology or internal activity whereas other moons like Mimas or Rhea seem to be inactive or much less active than those well-known worlds of the Gas Giant. Among the 62 moons orbiting the second largest Gas Giant in the Solar System, one can notice that most worlds orbiting Saturn are devoid of any atmosphere and unveil a highly cratered surface. In fact, Titan is the only moon of the Ringed Planet containing a significant atmosphere. That exotic atmosphere mainly composed of nitrogen appears completely opaque from outer space in the visible spectrum due to the presence of a haze of hydrocarbons or organics. The origin of Titan's atmosphere is undoubtedly one of the major mysteries in planetology today. Thanks to the Cassini mission, we have become aware that a tiny moon can be geologically active. The south polar region of Enceladus, a bright and icy moon of Saturn, unveils geysers rich in water. The geysers appear in the famous  Tiger Stripes  which are topographic fractures.

Thanks to the Cassini-Huygens mission, we have been in a position to determine that there are lakes, seas and rivers in the high latitudes of Saturn's largest moon Titan. Titan which is bigger than the Moon or Mercury is clearly an active world with a dynamic atmosphere and a complex meteorology. Clouds of methane or ethane can take shape. Vortices can also develop above each polar region of the Opaque Moon. Planetologists believe that there is an internal ocean of liquid water beneath the external crust of Titan and Enceladus. The cratered moon Dione is also suspected to harbor an internal ocean of liquid water. By contrast, moons like Tethys, Rhea or Iapetus may be inactive or less active than Titan, Enceladus and Dione. How can we explain that dichotomy in terms of dynamics or internal activity among the numerous moons of Saturn ? Some moons of Saturn unveil strange shapes like Pan or Atlas which look like a ravioli. How can we explain those unusual shapes ? Are those moons young ? What are the interactions between those small moons and Saturn's rings ?

Researchers try to determine the history or to anticipate the evolution of the numerous moons of Saturn. Most moons of Saturn are heavily cratered implying that they may have formed long ago. The largest airless worlds orbiting Saturn look alike and are rich in craters. Therefore, they may have formed during the formation process of the Ringed Planet. One can advance that those moons may be more than 4 billion years old or as old as the major bodies of the Solar System. The moons which are relatively close to the rings or in the rings of Saturn may be much younger than moons like Titan or Iapetus. Some moons like Atlas or Mimas may be only 100 million years old or less. Why do we believe that the moons close to Saturn are younger than the moons far from Saturn ? The prevailing hypothesis is based on the fact that the gravitational interactions between Saturn and its moons lead, over long periods of time, to a progressive migration of the orbits of the moons outward, farther from the Gas Giant. As a result, the moons which are evolving at a relatively small distance from Saturn must be relatively young. That's the case for Pan, Atlas or Mimas.

The complexity of Saturn's rings and the diversity of Saturn's moons implies that some moons of the Gas Giant may be younger than the other moons or that they may have formed at a different time. The small moons which have been identified in the rings of Saturn or close to the rings of the Gas Giant may be relatively young compared to other moons like Rhea or Titan. At the present time, some tiny moons may be forming at the outer edge of the rings, on the basis of the icy material present around them. They may act as vacuum cleaners ? The small moon Mimas which evolves closer to Saturn than Enceladus and which is heavily cratered may be relatively young. That icy world may have formed on the basis of the icy compounds or particles of Saturn's rings. The rings of the Gas Giant may have been fuelled by asteroids or comets that broke up at a relatively small distance from Saturn. In parallel, a rain of comets may also have engendered numerous collisions implying the development of dust and small icy molecules around the Gas Giant.

Some planetologists believe that, during the long history of the Solar System, some Gas Giants may have migrated in their orbits and may have engendered instabilities with the development of a  snooker game  between planets and moons. Therefore, Jupiter, Saturn, Uranus or Neptune may have captured new moons during that process. The relatively dark moon Phoebe may be from another place in the Solar System because the irregular moon evolves in a retrograde orbit around Saturn and because its orbit is highly inclined to the plane of the equator of Saturn. The composition of that world which evolves far from the Gas Giant beyond Titan implies that Phoebe may have formed much farther from the Sun. Phoebe may be a captured world like Triton, the famous moon of Neptune. Are worlds like Titan, Iapetus, Phoebe or Hyperion from the Saturn System or are they from another planetary system ? The study of other planetary systems is likely to bring us new clues.

Why is Jupiter richer in relatively big moons than Saturn ? Jupiter is surrounded by the four Galilean Moons Io, Europa, Ganymede and Callisto. Saturn has only one moon which can rival those moons of Jupiter in terms of size. Saturn's largest moon Titan turns out to be the second largest moon in the Solar System. The moons of the largest planet in the Solar System seem to have been more stable or less unstable than the major moons of Saturn over time, probably because they have undergone fewer collisions and because they orbit farther away from the Gas Giant. In parallel, the System of Saturn contains the most massive and complex system of rings in the Solar System. Saturn's rings can generate new moons and engender instabilities in the orbits of the other moons. The major moons of Jupiter can't undergo the destructive influence of Jupiter's gravity because they evolve sufficiently far from the Gas Giant. That may not be the case for moons like Mimas or Enceladus which evolve at a relatively small distance from Saturn.

Some moons of the Ringed Planet may harbor a subsurface ocean of liquid water. Enceladus, Titan and Dione to a lesser extent are the best candidates for an internal ocean rich in water. Enceladus evolves at a relatively small distance from Saturn so that it undergoes relatively strong tidal forces from the Gas Giant. How to explain that the icy moon Mimas which is a little bit smaller than Enceladus and which evolves closer to Saturn is probably devoid of any internal layer of liquid water today ? The researcher Marc Neveu performed a study, published in Nature Astronomy on April 1st 2019, in which he explains that the paradox may be related to the relatively young age of Mimas. In fact, Mimas may be less than 1 billion years old and may have formed from compounds or particles from Saturn's rings. The debris mobilized in the formation process of Mimas may have progressively lost their potential radioactive heat prior to that development of the icy moon. In that configuration, the gravitational influence of Saturn may not be strong enough to engender the level of heat required to allow the development of a subsurface ocean beneath the icy crust of Mimas.

Yet, the possibility of a subsurface ocean beneath the irregular crust of Mimas is not completely ruled out since we had identified small wobbles as the small moon rotates on its axis on the basis of data obtained from the Cassini spacecraft. The wobbles are likely to be linked to a subsurface ocean. The presumed subsurface ocean of Enceladus is particularly interesting since it is salty like the oceans of the Blue Planet. The salinity of the substance in the geysers of Enceladus implies that the water molecules of the subsurface ocean may have chemical interactions with rocks or the presumed rocky core of the tiny moon. Can an exotic lifeform develop in that environment since the potential chemical interactions are likely to bring energy or potential nutrients for any organism ? An exotic ecosystem is also likely to take shape inside the presumed subsurface ocean of Titan if there are interactions between water molecules and the presumed layer of rock beneath the subsurface ocean but we don't have clues yet regarding potential interactions between rock and liquid water for that giant moon.

The presumed ocean of Titan may be old. That liquid layer may have formed more than four billion years ago, fuelled by the heat generated by the radioactive decay of the material inside the Opaque Moon. Impacts of comets or meteorites may also have played a key role in the development or evolution of the presumed ocean in the past. The gravitational influence of the Gas Giant Saturn may not be strong enough to account for the presence of an internal ocean inside Titan. The presence of the presumed subsurface ocean of Enceladus may be explained by a strong impact event in its history. However, the gravitational forces exerted by Saturn can account for the presence of an internal ocean beneath the icy crust of Enceladus. The possibility of life in the presumed ocean of liquid water inside Enceladus is seriously considered because there are organics, carbon, hydrogen as well as energy inside it. During a close flyby of Enceladus in 2015, the Cassini probe had identified molecular hydrogen in the plume of water. Hydrogen plays a key role in hydrothermal vents beneath the oceans of the Earth. That's why we believe that extremophiles can develop in that type of environment.

Titan may also host an exotic lifeform since it contains an atmosphere with a haze rich in organics as well as lakes, seas or rivers of methane or ethane. Titan whose atmosphere resembles our own atmosphere in terms of composition represents a soup of organics or hydrocarbons that can tell us a lot regarding the origin of life. Titan is relatively stable in its orbit so that its complex chemistry involving organics, hydrocarbons or nitrogen can continue to develop over a long period of time. On the other hand, some small moons which evolve closer to Saturn are likely to vanish or to be destroyed by potential collisions in the area of the rings. Over time, the moons of Saturn tend to move away from the Gas Giant due to gravitational factors. Therefore, the small moons which are close to the rings or inside the rings are likely young. Their orbit is progressively increasing like the orbit of the Moon around the Earth. Since the birth of the Solar System or over the past 4.6 billion years, the System of Saturn may not have been as quiet as we could imagine. Some planetologists believe that the rings of Saturn and their moonlets may have disappeared in only about 100 million years because the Gas Giant is attracting them and there is a rain of dust and ice from the rings toward the Gas Giant.

The image above reveals the crescent of Enceladus and Titan, two active worlds orbiting the Gas Giant Saturn. The image represents a natural color view produced on the basis of data obtained from the Narrow-Angle Camera of the Cassini orbiter on February 5, 2006. Views acquired using red, green and blue spectral filters were mobilized to generate the final image. The tiny moon Enceladus appears 4.1 million kilometers or 2.5 million miles from the camera whereas the Opaque Moon Titan appears 5.3 million kilometers or 3.3 million miles from the camera. Image credit: NASA/JPL/Space Science Institute.

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