Saturn’s rings are not “flat” as we see them through telescopes, but extend above and below the plane of the main disk, like a doughnut.
In 2017, NASA’s Cassini mission ended with a spectacular descent into Saturn’s atmosphere. Before the final dive, however, the probe made a series of close passes, the so-called Grand Finale Orbits (GFO), crossing the region between the planet and its rings several times. Precisely during these extreme maneuvers Cassini collected data that continues to rewrite our knowledge of the Saturn system.
The research. A new study published in The Planetary Science Journal reveals that the planet’s rings are not thin structures rigidly confined in their plane, as they appear through the telescope, but generate a vast halo of dust that extends much higher and lower than previously thought. Thanks to its Cosmic Dust Analyzer (CDA), Cassini sampled particles above and below the plane of the rings, reaching distances up to about three Saturn radii (RS).
The surprise. Over the course of 20 orbits, the probe collected 1,690 dust spectra, which were then subjected to detailed analysis. Of these, 155 were positively identified as mineral particles, specifically silicates. Their distribution is symmetrical: the dust is present in similar quantities both above and below the plane of the rings, forming a sort of diffuse envelope around the main system.
Chemical composition. Perhaps the most unexpected aspect emerges from the analysis of the chemical composition. The researchers found that silicates detected at high latitude have a composition that is virtually indistinguishable from that of particles closer to the rings.
In both cases magnesium and calcium dominate, in proportions consistent with cosmic ones, while iron is significantly depleted. A resemblance that the authors themselves define as “surprising”. «Within the precision limits of the method», explains Simon Linti, head of the research, «these grains of mineral dust show the same composition, suggesting that the silicates observed at latitudes higher than three radii of Saturn also originate in the main rings of Saturn».
Simulations. But how can such small particles move so far from the plane of the rings? To answer this question, the team performed a series of dynamic simulations, concluding that the particles must be extremely small — less than 20 nanometers — and projected at high speeds, exceeding 25 kilometers per second. An energy compatible with the impacts of micrometeoroids, tiny cosmic fragments that constantly bombard the rings.
According to the researchers, the increase in particle density as one approaches the plane of the rings strengthens this hypothesis.
Most of the expelled dust would quickly end up falling back on the rings or falling on Saturn, while only a small fraction would be able to reach the most distant regions, contributing to the formation of the observed halo. The most plausible mechanism, the authors explain, would be the condensation of vapor plumes produced by the impact of micrometeoroids. These hot and fast jets, upon cooling, would give rise to the nanosilicates detected by the probe, while explaining the iron deficiency observed in the samples.
Another hypothesis. An alternative hypothesis has also been considered: that the particles come from outside the Saturn system, attracted by its gravity. However, this possibility appears less convincing, because the composition of the dust does not coincide with that of the original grains coming from space already observed by Cassini in other regions of the Saturnian system. Since micrometeoroid impacts are common phenomena on a planetary scale, the study opens up broader questions: Could the rings of other planets also be surrounded by invisible halos of dust? And how much does this dynamic influence their evolution over time?
Questions that show how, years after the end of the mission, Cassini continues to reveal new and unexpected secrets of the “Lord of the Rings” of the Solar System.
