April 2, 2019 : Is The Likelihood Of A Subsurface Ocean Higher On Enceladus Than On Mimas, Tethys, Dione, Rhea Or Titan ?

Planetologists believe that many worlds in the Solar System may harbor a subsurface ocean rich in liquid water. Around the Gas Giant Jupiter for instance, Europa and Ganymede may contain an ocean beneath their icy crust. The icy moons of the Ringed Planet Saturn may also contain an internal ocean. The tiny moon Enceladus where geysers or water plumes have been clearly identified during the Cassini mission appears to be the perfect candidate in the list of worlds or moons which may harbor a subsurface ocean rich in liquid water. The other icy moons of Saturn such as Tethys, Dione and Rhea may also contain a subsurface ocean even if its potential presence is harder to demonstrate than for the case of Enceladus. Titan which is the largest moon of Saturn and which contains lakes and seas of hydrocarbons on its surface is part of the list of worlds which may harbor a layer of liquid water beneath their external crust. Beyond Saturn, Triton, the largest moon of Neptune, Pluto, the largest Dwarf Planet in the Solar System and its main moon Charon may also contain a subsurface ocean dominated by liquid water.

A new study entitled  Evolution of Saturn's mid-sized moons , recently published online in the journal Nature Astronomy and proposed by Marc Neveu and Alyssa R. Rhoden, reveals numerical simulations regarding the evolution of the geology and the orbit of several moons of Saturn for 4.5 billion years. In this study, the researchers focused their attention on Mimas, Enceladus, Tethys, Dione and Rhea. The moons undergo the influence of the gravity of Saturn and the other moons. The planetologists advanced that the interior of Enceladus, Dione and Tethys must be active thanks to tidal forces or gravitational phenomena. They pointed out however that Mimas may be inactive today even if it undergoes strong tidal forces related to its particularly high proximity to Saturn. They believe that Mimas must have formed relatively recently, between 0.1 and 1 billion years ago. The icy moons studied by Marc Neveu and Alyssa R. Rhoden are located between Saturn and Titan which are the most massive bodies of the system. The simulations bring major clues regarding the surprising difference between Enceladus which is remarkably active and Mimas which seems to be a dead world.

The Cassini-Huygens spacecraft gathered a huge amount of data upon Saturn, its complex rings and its myriad of moons. More than 60 moons have been identified around Saturn, from tiny moonlets less than 1000 feet or 300 meters wide to the Opaque Moon Titan which is the second largest moon in the Solar System. Some moons are particularly close to Saturn like Mimas and Enceladus. Therefore, they must undergo, during their orbit, strong tidal forces related to the gravity of Saturn. The gravitational influence of the other moons also plays a key role in the Saturn System. From Mimas to Rhea, the diameter and the mass increase each time, Rhea being larger and heavier than Dione which is larger and heavier than Tethys which is larger and heavier than Enceladus which is larger and heavier than Mimas. The data acquired from the orbiter have shown that the structure and the density of the moon are not automatically dependent on the mass or the distance from the Gas Giant.

Previous theories advanced that those worlds orbiting the Ringed Planet took shape from rings of debris around the Gas Giant. The gravitational pull exerted by the Gas Giant is supposed to produce strong tides on those moons and to engender processes of churning and melting which are accompanied by a progressive migration of heavier rocks to the core of the moon and by the development of a layer of lighter ice above the heavier material. The worlds evolving closer to Saturn are supposed to undergo stronger tidal effects than the worlds evolving farther from the Ringed Planet. Mimas evolves closer to Saturn than Enceladus so that Enceladus undergoes 30 times weaker tides than the icy moon Mimas. Yet, Mimas seems completely inactive whereas Enceladus reveals geysers or cryovolcanoes implying the potential presence of an underground ocean or melting phenomena beneath its icy crust. Rhea which is the largest of the moons studied by the planetologists is supposed to have the highest capacity to retain the heat likely to engender processes of melting of its rock and its ice. However, researchers have shown that Rhea may be a mixed jumble of rock and ice. That may not be the case for the smaller moons Mimas, Enceladus and Dione which unveil structures of rocky cores and icy shells.

A better understanding of the origin and of the geological activity of those icy worlds is likely to help us better evaluate the potential for life beneath the icy crust of those moons where a hidden ocean dominated by liquid water may be found. Enceladus is clearly the best candidate for life on the basis of our scientific knowledge today. Marc Neveu who is the lead author of the research work and who is a planetary scientist at NASA Goddard Space Flight Center at Greenbelt, Maryland pointed out :  Fundamentally, what excites me is the search for life beyond Earth.  Planetologists try to simulate the effects of tides on the geology of the icy moon in order to gather new clues. However, researchers have to take into account the fact that the variations in the orbits engendering tides occur on daily scales and that the geology is influenced by tides on scales of millions of years. Scientists have had to study the short-term effects of tides and the evolution of the geology separately. In the new work, the planetologists have been in a position to design computer models whose goal is to reproduce the geological changes and that simplify orbital effects those icy worlds might have undergone over Saturn's roughly 4.5-billion-year life.

The planetologists were in a position to determine that tidal forces are strong enough to engender the formation of past or present oceans beneath the external crust of Enceladus, Tethys and Dione. They believe that a subsurface ocean is still present beneath the icy crust of the tiny moon Enceladus thanks to the gravitational interactions with the other worlds orbiting Saturn such as Tethys and Dione which keep the orbit of Enceladus relatively elliptical. The shape, the flexing, the heating or the activity of Enceladus are constantly modified by the variations in the gravitational pull of the Gas Giant. Marc Neveu argued :  This is the first explanation consistent with data returned from NASA's Cassini spacecraft for how a tiny moon such as Enceladus, which is only about as big as Washington state or the British Isles, has a subsurface ocean when other sibling moons that are bigger or closer to Saturn, and therefore more likely to have such oceans, do not. 

The planetologists were in a position to advance that Mimas probably took shape only 100 million to 1 billion years ago because if the cratered moon were older, interactions between the rings of the Ringed Planet and Mimas would have engendered a configuration in which the moon is in a higher orbit than the current orbit. The young age of Mimas implies that the small world would have emerged after all the radioactive material present at the start of the formation process of the Solar System, approximately 4.6 billion years ago, had already decayed. In fact, at the start of the evolution process of Mimas, the interior of the moon may have been too cold or too hard for the development of significant heating phenomena inside the icy moon. Marc Neveu explained :  When ice is warmer and squishier, it becomes more responsive to tidal forces that deform it.  He added :  Mimas lacked the minimum amount of heat to start with to get tidal forces to trigger runaway heating. 

Regarding the fact that the interior of the relatively large moon Rhea might not be composed of several layers,  the measurements from the Cassini spacecraft of the interior of Rhea have a degree of uncertainty in them, so it's possible the deeper layers of its interior are rockier and the near-surface layers are icier,  Marc Neveu pointed out. The research work mobilizing numerical simulations implies that the hypothetical subsurface ocean of Enceladus is approximately 1 billion years old. Marc Neveu argued :  One billion years is enough for life to emerge 1 billion years after Earth was born, there was life.  On the basis of what we know about the history of our biosphere, he added :  1 billion years means there may still be enough chemical activity between Enceladus' rocky core and its ocean to provide energy for any potential microbial life, similar to the chemical energy in Earth's seafloor that helps sustain ecosystems there. Enceladus' ocean is not too young and not too old. It may be just the right age for life. 

Obviously, we are here in the field of speculation or intuition regarding the potential for life beneath the external curst of Enceladus. Marc Neveu pointed out that the study was performed on the basis of simplifications that are likely to lower the accuracy of the outcome. He advanced :  We didn't compute the high tides and low tides that each of the five moons experienced at the same time every few hours for 4.5 billion years to do that right, it would take about 20 years on a desktop computer.  The planetologists plan to simulate more complex configurations mobilizing more features about the Ringed Planet and its moons. For instance, the researchers intend to take into account the dynamics in the orbit or in the migration of moons or worlds in the environment of Saturn over time. Has there been a crash between Enceladus and another moon of Saturn in the recent past for instance ? Is the remarkable internal activity of Enceladus related to a recent collision ? The contrast between the dynamics of Enceladus and the dynamics of Mimas is clearly remarkable and fuels some theories regarding the emergence and the development of any subsurface ocean.

The image above reveals the largest moons of Saturn at scale. From the left part of the view to the right part of the view, one can observe Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus. Mimas is the closest moon to the Gas Giant whereas Iapetus is the farthest moon to the Ringed Planet. The moons are represented in an order of distance to Saturn from the left part of the view to the right part of the view. The dichotomy between Mimas and Enceladus in terms of geological activity draws the attention of researchers. Enceladus unveils geysers and fractures in its south polar region. The tiny moon may contain a subsurface ocean. By contrast, Mimas seems completely inactive. Credit for the original image of Mimas, Enceladus, Tethys, Dione and Rhea: NASA/JPL-Caltech/Space Science Institute; Credit for the original infrared view of Titan: NASA/JPL-Caltech/University of Nantes/University of Arizona; Credit for the original view of Iapetus: NASA/JPL/Space Science Institute; Credit for the montage: Marc Lafferre, 2019.

- To get further information on that news, go to: https://www.space.com/saturn-moons-different-traits-chances-life.html and https://www.nature.com/articles/s41550-019-0726-y.



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