Based on the analysis of data from the NASA Cassini probe, which orbited Saturn between 2004 and 2017, an international group of researchers, led by the French National Center for Scientific Research, has announced the existence of an ocean of liquid water beneath the surface of Mimas. Although this possibility had been speculated in recent years, until now the models developed by scientists had difficulty fitting with Cassini observations and explaining the lack of evidence on the surface of the satellite that could suggest a fluid interior.
In this way, Mimas would join the select group of moons of the gaseous planets of the solar system in which the presence of large concentrations of underground water has been certified, as is the case of Europa (Jupiter) or Titan and Enceladus (Saturn). ), especially interesting places for the search for life.
The importance of the discovery lies in the fact that the Mimas ocean could be a recent phenomenon, which would allow us to study how the processes that transform the interiors of icy satellites develop to create liquid masses. Furthermore, if confirmed, this finding would suggest that underground seas may be abundant in our solar system and be found even on moons that do not show visible signs that reveal their presence.
Mimas became famous from the images captured by the Cassini spacecraft in some of its approaches, and which showed a world marked especially by an enormous impact: a crater 142 kilometers in diameter called Herschel and which represents, nothing more and nothing less than a third of the diameter of the satellite. With this suggestive shape, Mimas earned the informal nickname of the Death Star, due to its similarity to one of the ships from the Star Wars saga.
This satellite, one of the so-called interiors of Saturn (moons that orbit within the planet’s extensive and tenuous E ring), presents enigmatic characteristics that have fueled intense research activity. To begin with, it is subject to the intense gravitational interaction of the colossus Saturn due to the proximity of its orbit, a phenomenon that should cause notable internal heating and, consequently, visible geological activity. However, the surface of Mimas does not show signs that reveal said activity.
In clear contrast, the moon Enceladus, which is 1.3 times farther away from Saturn than Mimas and therefore receives less gravitational stretch, is a geologically active world, with hydrothermal phenomena that heat underground liquid water, the They rise through cracks and launch it into space through geysers observed in some regions of its surface. Another example is Jupiter’s satellite Europa, which shows a frozen but fragmented terrain, with evident signs of movements between large blocks of ice caused by the existence, below, of a global ocean of liquid water.
Second, the data captured by the Cassini mission indicated, from the beginning, the presence of a wobble motion in Mimas, a swaying motion as it orbits Saturn (in astronomy, this type of swaying is called libration). Although these oscillations are observed in other satellites of the solar system (such as our Moon), the significant amplitude of Mimas’s libration led us to suspect that there must be important irregularities inside this world. In addition to the libration sway, small variations were also observed in the satellite’s orbit (specifically, in the position of the periastron, the orbital point closest to the planet).
In order to explain the characteristics of Mimas’ libration and orbit, scientists have created, in recent years, several possible models of the structure of the satellite’s interior. Since each of the hypotheses generates different observable patterns, based on the Cassini data, the first studies that were carried out were able to rule out the existence of an interior composed of rock layers of different density, or the concentration of mass below the large crater. of Mimas (which could correspond to a fragment of the object that formed it). Obviously, the simple model of a solid and homogeneous interior was also invalidated.
Finally, the list was reduced to two potential schemes: Mimas could have a highly deformed rock core, or significant amounts of underground liquid water. But determining the correct model requires in-depth analysis, with complex computer simulations that take into account many factors, such as disturbances created by other nearby moons, including some of Saturn’s smaller ones.
These simulations are precisely what the authors of the new research have been able to carry out. In them, the hypothesis based on a distorted rocky interior is only demonstrated to be consistent with Cassini’s orbital and libration observations in the case of an extreme deformation, consisting of a silicate structure very stretched in the direction of Saturn (a shape that researchers describe as a flattened cake). However, a deformation of this caliber should be visible in the external morphology of Mimas, which is not the case.
Having discarded this model, the scientists’ work has focused on simulating various prototypes of internal structures, consisting of the combination of an icy surface, an underground sea and lower layers formed by mantle rocks. This analysis has allowed us to eliminate many of these variations, incompatible with the Cassini data, and has led to the determination that Mimas must have an interior ocean under an ice layer between 20 and 30 kilometers thick.
However, to be correct, the proposed hypothesis must be able to resolve the issue of geological inactivity shown on the surface of Mimas. This aspect is surprising if one takes into account that the presence of an interior ocean would be a consequence of the heat generated by the gravitational interaction with Saturn, an energy that should fuel other phenomena on the outside of the satellite. In fact, the accepted values ??for the energy flow that Mimas emits into space should not only cause visible activity on the surface, but are also sufficient to progressively modify its trajectory (specifically, to reduce its orbital eccentricity, a parameter that measures the deviation that an orbit presents with respect to a perfectly circular figure).
The study carried out provides a good fit with these observations in the case that the formation of the ocean is a relatively recent phenomenon, with an estimated age that, according to these simulations, could range between 2 and 25 million years. According to this model, ocean growth would have been rapid in geological terms, still active, and not yet causing substantial surface effects. On the other hand, the hypothesis foresees values ??for the eccentricity of Mimas’s orbit, in the most recent past, that are plausible and that resemble those observed in Enceladus, another of Saturn’s main interior satellites.
The study of Mimas could be fundamental to understanding the processes that developed in the past on other icy moons in the solar system and that gave rise to the existence of masses of liquid water below the surface.
But in addition, the alteration mechanisms in the mineral composition caused by the interaction between water and rocks tend to be geologically rapid and develop in periods ranging from millions to a few tens of millions of years. Therefore, as the authors of the new research emphasize, “Mimas offers a unique opportunity” to study these processes while they are taking place.
