September 24, 2020 : A New Study Proposed By Samuel W. Bell Suggests A New Chronology Based On Planetocentric Impactors For Titan And The Other Moons Of Saturn

A new research work proposed by Samuel W. Bell, published in the Journal of Geophysical Research Planets and entitled « Relative Crater Scaling Between the Major Moons of Saturn: Implications for Planetocentric Cratering and the Surface Age of Titan », suggests a new chronology for Titan and the other moons of the Gas Giant Saturn. Titan is the only moon of Saturn containing a significant atmosphere and that atmosphere engenders relatively strong erosional processes. Therefore, the age of Titan is hard to determine since researchers can't base their analyses on the concentration of impact craters on Saturn's largest moon because Titanian craters tend to disappear over time due to erosional processes. However, the study of craters on the other major moons of the Ringed Planet can allow us to infer the potential age of the Opaque Moon. The difficulty is to determine the fraction of impactors which represent planetocentric impactors and the fraction of impactors which represent heliocentric impactors. The moons of Saturn may turn out to be much younger than previously believed if most of the impact craters are related to planetocentric impactors.

Samuel W. Bell who is an Associate Research Scientist at the Planetary Science Institute and who was working at Brown University's Center for Computation and Visualization and the Planetary Science Institute at the time of his study upon the chronology of Saturn moons pointed out : « Most studies dating surfaces on the Moon or Mars rely on counting how many impact craters have formed and knowing the cratering rate, but on the moons of Saturn, we do not know the cratering rate. » Previous assessments upon the chronology or the age of the Saturn moons have been based on the assumption that most impacts or craters come from heliocentric impactors or objects orbiting our star. The author added : « If the impacts came solely from Sun-orbiting objects, the relative cratering rate would be much, much higher the closer the moons are to Saturn. However, the crater densities of the oldest surfaces of Mimas, Tethys, Dione, Rhea, and Iapetus are all relatively similar. » A giant world like Saturn is likely to act as a vacuum cleaner for any asteroid or comet entering the area of the Gas Giant due to its relatively high gravity. That's why one can assume that the nearest moons to Saturn may be more heavily cratered than the other moons of the Ringed Planet if most impacts are related to heliocentric impactors rather than planetocentric impactors.

Samuel W. Bell considers that the relatively similar crater density observed on the major icy moons of Saturn is probably closely linked to planetocentric impactors, asteroids, comets, moonlets or tiny moons. The crater density of the nearest moons to Saturn would logically appear much higher than that of the farthest moons to Saturn if we are in a configuration in which the craters are mostly related to heliocentric impactors. Most moons of Saturn unveil a roughly similar concentration of craters. If Titan is not a captured planetary body or Dwarf Planet, its age may be relatively similar to that of the other moons because the moon may have formed in one of the circumplanetary disks of the Ringed Planet. Several moons in the area of the rings have been identified from the Cassini orbiter during its long mission in the Saturn System. Therefore, numerous moonlets or small objects may evolve today around Saturn. From time to time, their orbit or trajectory can change and they can become impactors for the major moons of Saturn. A moon like Mimas whose orbit is very close to the Gas Giant Saturn is heavily cratered. A moon like Enceladus which evolves farther from Saturn appears younger because there are very active geysers in the fractures of its south polar region.

We regularly discover new objects in the Solar System, from asteroids to comets or even Dwarf Planets. Several moons present in the Solar System may represent captured objects, planetary bodies or Dwarf Planets. The inventory of the various objects orbiting Saturn or orbiting the Sun is probably far from being complete. To a certain extent, the system of Saturn appears to be a miniature Solar System because it contains many moons or moonlets which evolve, roughly, on the same plane like the eight known planets of the Solar System. Thanks to major advances in the field of technology, a more precise idea upon the structure of the Solar System takes shape and our level of understanding upon the dynamics or the evolution of the Solar System improves. Many moonlets which have not been identified around Saturn are likely to produce the craters observed on worlds like Tethys, Dione or Rhea. Titan appears to be the « center of gravity » of the moons orbiting Saturn so that the asteroids, the comets or the moonlets entering the atmosphere of the giant moon are probably much more numerous than those crashing into the other moons of Saturn.

The study upon the concentration of craters on the major icy moons of the Gas Giant Saturn appears more complex than the study upon the concentration of craters on the Moon. The crust of Saturn moons appears to be dominated by water ice whereas the crust of the Moon is dominated by denser materials like silica. Moreover, the surface of the Moon is rich in regolith, a layer of dust related to erosion and impact events. The dust is likely to cover some small craters over time. Is there a layer of snow on the surface of the icy moons of Saturn likely to cover the small craters as well ? How long do the craters last on the icy moons of Saturn ? Many questions regarding the soil or the crust of Saturn moons must be answered. The new research work brings new clues for the study of the history of the Saturn System. Samuel W. Bell argued : « There are many important implications of this new chronology. » In the configuration in which most impactors are heliocentric objects, asteroids or comets, one can't envisage that the moons of the Gas Giant are younger than 4 billion years old. In the configuration in which most impactors are planetocentric objects evolving around Saturn, the major moons of the second largest planet in the Solar System can be considered much younger as has been proposed from astrometric observations of tidal orbital evolution.

The age of Titan is a major topic of research because the giant moon unveils remarkable erosional processes on its surface that are reminiscent of erosional processes encountered on Earth. The relatively heavy atmosphere of the Orange Moon implies regular winds which can be relatively strong at the level of the surface. Thus, dunes can take shape like on Earth. Due to the various erosional processes on Titan and due to the fact that the Titanian atmosphere can represent a shield for any potential impactor entering the atmosphere of the giant moon, the impact craters tend to disappear in the long run. That's why a very limited concentration of craters on Titan has been observed from the eyes of the Cassini orbiter during its mission in the Saturn System from 2004 to 2017. If most craters are related to heliocentric impactors, Saturn's largest moon is very old. Samuel W. Bell advanced : « The assumption of impactors orbiting the Sun results in the conclusion that the surface of Titan is probably at least 4 billion years old, even though Titan shows clear evidence of active weathering. » The new research work suggests that Titan may be much younger than previously thought.

The Cassini spacecraft as well as the Huygens probe have shown that the surface of Titan appears relatively young. Linear and parallel dunes extending over long distances at a low or mid-latitude have been clearly observed with the Radar Mapper of the Cassini orbiter. The high latitudes of the northern hemisphere are relatively rich in lakes, seas or rivers. Moreover, there may be a subsurface ocean dominated by liquid water beneath the external crust of the giant moon. Fractures or canyons can take shape and mountains can develop as well implying a relatively dynamic world. The nature of the soil and of the external crust of Titan is a major topic of research. Are there pockets or subsurface lakes, seas or oceans of methane beneath the external crust of the Opaque Moon ? Are there geysers or cryovolcanoes spewing water ice or methane for instance ? Geysers spewing water ice as well as hydrocarbons or organics have been clearly identified in the fractures of the south polar region of the tiny moon Enceladus in particular. Those observations demonstrate that similar processes are likely to take shape on Titan.

A heavily cratered surface can hide an active interior. Several moons of Saturn may contain a subsurface ocean of liquid water due to tidal heating in particular. Enceladus demonstrates that tiny worlds can remain warm in their interior over time thanks to gravitational phenomena related to the planetary bodies or moons evolving in their region. Therefore, the age of the moon or planetary body can potentially be underestimated. Samuel W. Bell argued : « With the new chronology, we can much more accurately quantify what we do and don't know about the ages of the moons and the features on them. » He concluded : « The grand scale history of the Saturn system still hides many mysteries, but it is beginning to come into focus. » The system of Saturn appears particularly dynamic due to the presence of many rings which may be relatively young. Those rings unveil moonlets in particular. The major rings of the Gas Giant are likely to engender new moons. But are those moons stable over time ? Will those rings be swallowed by a new moon emerging from them in the long run ? That hypothesis is not unlikely due to the potential aggregation processes or accretion processes that those rings are likely to engender.

The image above reveals the cratered moon Rhea in front of the Opaque Moon Titan. The natural-color view was generated on the basis of data acquired using red, green and blue spectral filters. The images were captured with the Narrow-Angle Camera of the Cassini spacecraft during the Cassini-Huygens mission in the Saturn System on June 16, 2011 at a distance of about 1.1 million miles or 1.8 million kilometers from Rhea and at a distance of about 1.5 million miles or 2.5 million kilometers from Saturn's largest moon. One can infer the age of Titan by studying the concentration of craters on Rhea and on the other icy moons of the Ringed Planet. Image credit: NASA/JPL-Caltech/Space Science Institute.

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