March 19, 2021: A New Study Suggests That Enceladus And Titan May Have Taken Shape Much Farther From Saturn Than Their Current Orbit

A new research work entitled "Formation Conditions of Titan's and Enceladus's Building Blocks in Saturn's Circumplanetary Disk", published in The Planetary Science Journal on March 12, 2021 and proposed by Sarah E. Anderson, Olivier Mousis and Thomas Ronnet reveals that Enceladus and Titan may have formed and developed in a circumplanetary disk that appeared much farther from Saturn than the orbit of Enceladus or Titan today. Enceladus is a tiny moon which evolves at a mean distance of 237,948 kilometers from the Ringed Planet Saturn whereas Titan is a giant moon which evolves much farther from Saturn at a mean distance of 1,221,870 kilometers from the Gas Giant. Mimas which is smaller than Enceladus evolves closer to Saturn than Enceladus. There are several major moons located between the orbit of Enceladus and the orbit of Titan. Those major moons are Tethys, Dione and Rhea. Rhea is bigger than Dione and Dione is bigger than Tethys. Thus, until the orbit of Titan, the moons tend to be bigger and bigger as we go away from the Ringed Planet. The orbit of Iapetus, another major moon of Saturn, is found much farther from Saturn than Titan. But Iapetus appears to be much smaller than Titan.

The new study shows that Enceladus and Titan may have formed and developed in a circumplanetary disk (CPD) that was not located at the level of their current orbit. That circumplanetary disk may have been found much farther from Saturn than the orbit of its largest moon Titan. The simulations proposed by the team of Sarah E. Anderson suggest that the disk may have had a radius representing a value found in the range between 66 Saturnian radii and 100 Saturnian radii. The mean radius of the second largest planet in the Solar System represents 58,232 km. Therefore, the lower limit of that presumed circumplanetary disk is equal to 3,843,312 km and the upper limit of that presumed circumplanetary disk is equal to 5,823,200 km. In any configuration of that CPD, the mean distance between that disk and Saturn is much higher than the mean distance between Titan and Saturn. Enceladus and Titan are thought to have formed and developed long ago in the period of the formation of the Solar System. The Solar System formed about 4.6 billion years ago. The center of gravity of the Solar System became the Sun, the only star of the Solar System. A circumstellar disk gave birth to the planets we know today. Around most of those planets, smaller worlds or moons took shape as well.

Saturn is surrounded by numerous moons, from tiny moons like Prometheus or Pan to major moons like Titan or Rhea. Researchers are particularly interested in the system of rings and in the system of moons of Saturn because the rings and the numerous moons can tell us a lot regarding the dynamics or the evolution of a system of natural satellites in the long run. The system of rings of Saturn is unique and remarkably complex. It unveils curious features and it is likely to engender new moons. Is the system of rings of Saturn the outcome of a collision between Saturn and a small icy world for instance ? Several hypotheses have been advanced to try to explain the bright rings we see around Saturn. The major moons of Saturn, from Tethys to Titan, probably took shape in the same circumplanetary disk. They are not captured planetary bodies like Triton for instance. Thanks to the Cassini-Huygens mission, we have had the opportunity to obtain a huge amount of data regarding the surface or the atmosphere of worlds like Enceladus, Tethys, Dione, Rhea or Titan. Their composition is likely to tell us a lot regarding their history or the way they formed and developed.

The researchers of the new study focused their attention on the bright moon Enceladus and on the Opaque Moon Titan. Those worlds are thought to have taken shape in a late-stage circumplanetary disk around the Gas Giant Saturn. The goal is to evaluate the evolution of the abundances of volatile compounds or elements in the disk on the basis of several factors. The factors considered by the researchers are the migration, growth and evaporation of icy grains. In fact, the scientists try to determine the origin of the material that engendered or produced Enceladus and Titan. The team mobilized a simple model in which the location of the centrifugal radius is modified in order to evaluate the time evolution of the ice lines of water ice, ammonia hydrate, methane clathrate, carbon monoxide and dinitrogen pure condensates. The composition of the moons we observe today implies that the building blocks of those worlds may have formed in an area of the circumplanetary disk located between the ice lines of carbon monoxide and dinitrogen at their outer limit from the Gas Giant and the ice line of methane clathrate at their lower or inner limit from Saturn.

The warmer area of the system that led to the formation and the development of the planet and its various moons was found inside the Proto-Saturn. The coldest parts of the system were found farther from the Proto-Saturn. Once we determine the ice line of key molecules or elements, we can deduce the potential area in the circumplanetary disk where the various moons took shape and developed. According to the team of Sarah E. Anderson, the ideal zone for the formation of the building blocks of Enceladus and Titan is found farther than the ice line of water and farther than the ice line of ammonia around Saturn from the Gas Giant. The planetologists determined that a source of dust in the area of the centrifugal radius does not allow the replenishment of the disk in the volatiles considered to be primordial in the moons analyzed by the specialists. They concluded that the simulations based on a centrifugal radius representing from 66 to 100 Saturnian radii lead to the formation and development of solids whose composition is in line with the composition determined for the moons Enceladus and Titan. Most worlds around Saturn are dominated by water ice but they also contain other molecules or elements.

Titan is relatively rich in molecular nitrogen and methane for instance. Titan is sufficiently massive to contain a stable atmosphere. Titan has the right combination of gravity and environmental temperature to contain a deep and thick atmosphere. Titan is in fact the only moon of Saturn to contain a significant atmosphere. The other moons are too light to retain a significant atmosphere. A tiny moon like Enceladus is devoid of any significant atmosphere but that world has a remarkably young surface because there are geysers in the fractures of its south polar region. The tidal forces or the gravitational influence of Saturn and the other moons of the Gas Giant play a major role in the internal activity or in the dynamics of a world like Enceladus. Titan evolves much farther from Saturn than Enceladus but its interior may also be active with the potential presence of a subsurface ocean dominated by liquid water. Enceladus and Titan are part of the big list of the worlds of the Solar System that may contain a subsurface ocean or subsurface pockets dominated by liquid water. Europa, a major moon of Jupiter, Tethys, Dione and Rhea, some major moons of Saturn, Triton, the largest moon of Neptune, and Pluto are also thought to contain a subsurface ocean rich in liquid water.

Are Enceladus and Titan differentiated worlds or relatively homogeneous worlds ? Planetologists try to gather clues regarding the internal structure of the various icy bodies orbiting the Ringed Planet Saturn. Titan is believed to contain several layers like the Earth which contains a crust, a mantle and a core. Enceladus is much smaller than Titan. Therefore, its internal structure may be quite different from the internal structure of Titan. The elements or molecules found in the plumes of the geysers are likely to tell us a lot regarding the internal structure of Enceladus. Enceladus and Titan may have emerged and developed in the same circumplanetary disk so that their composition may not be fundamentally different. Heavier elements will tend to be found at a higher depth whereas lighter elements will tend to be found at a lower depth. In the systems of the Gas Giants, molecules like water, ammonia, methane, carbon monoxide or molecular nitrogen are regularly encountered. The atmosphere of Titan resembles the atmosphere of the Blue Planet to a certain extent. Both atmospheres are dominated by molecular nitrogen. However, oxygen is absent or almost absent in the atmosphere of Titan.

The atmosphere of Titan is in fact completely opaque due to the presence of a haze rich in hydrocarbons or organics. Titan's atmosphere may resemble the atmosphere of the Early Earth in terms of composition. The troposphere of Titan contains relatively high concentrations of methane. Methane plays a key role in the meteorological cycle of Titan. Methane can condense to form lakes, seas or rivers on Titan. Methane can form clouds as well on that intriguing moon. The clouds can engender rainfall events like on Earth. Thus, the meteorological system of Titan is likely to tell us a lot regarding our own meteorological cycle. The level of stability of Titan's atmosphere is clearly a major topic of research. The elements or compounds of the disk that engendered the formation and the development of Enceladus and Titan evolved toward solid forms for relatively long periods of time, via equilibrium states, allowing the development of bigger and bigger moons regularly fuelled by the disk of gas and dust until that circumplanetary disk completely disappeared. The System of Saturn is often regarded as a miniature Solar System. Thus, the study of the dynamics of the Saturn System can help us better understand the dynamics of the Solar System.

The image above reveals a portion of Saturn, a portion of its rings as well as the Orange Moon Titan in front of the Gas Giant. The view was generated on the basis of data obtained with the Narrow-Angle Camera of the Cassini spacecraft on May 21, 2011, at a distance of about 1.4 million miles or 2.3 million kilometers from the Opaque Moon. The images mobilized to produce the color view were captured using red, green and blue spectral filters. One can notice the remarkable contrast between the color of Titan's atmosphere and the color of Saturn's atmosphere in particular. Image credit: NASA/JPL-Caltech/Space Science Institute.

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