Saturn is considered the beauty king among the planets. Even a small telescope reveals a distinctive ring system in which the sphere of the gas giant seems to float weightlessly. The rings have a diameter of almost a million kilometers and are only a hundred meters thick. They consist of countless tiny crumbs of ice cream, some of the debris is the size of a single family home. They all orbit the second largest planet in the solar system like mini moons.
Astronomers have long puzzled over when and how Saturn’s rings might have formed. Were they there from the beginning, i.e. did they form four and a half billion years ago from the same primordial cloud as the planet itself? Or do they owe their existence to a cosmic accident that happened much later?
According to one theory, there once was an icy moon called Chrysalis. It was thrown out of its orbit about a hundred million years ago under the influence of other satellites. According to the models, Chrysalis then came too close to Saturn and was crushed by its strong tidal forces. The rings are said to have formed from the fragments. Scientists around Jack Wisdom from the Massachusetts Institute of Technology in Cambridge described this scenario a few months ago in the journal Science.
For an older age, the rings are not dusty enough
Now this idea has gained support based on the age of the rings. A group led by Sascha Kempf from the US University of Colorado has discovered that the rings have existed for no more than four hundred million years. That writes the team in the magazine Science Advances. For Harald Krüger from the Max Planck Institute for Solar System Research, who was not involved in the study, this result is convincing: “The question was open for a long time, now it actually seems to have been answered,” he says.
The basis for the work are measurements of the space probe Cassini. Before it burned up in Saturn’s dense atmosphere at the end of its twenty-year mission in September 2017, the onboard instruments had transmitted a great deal of data to Earth. This estate has not yet been fully viewed. One of the scientific devices, called the Cosmic Dust Analyzer, studied Saturn dust as well as tiny interplanetary meteoroids – solid bodies that fly through the planetary system. This was the key to determining the age of the rings.
Kempf and his colleagues took advantage of the fact that the ring particles do not consist exclusively of ice, but are somewhat contaminated by dust. This is ensured by the constant bombardment of micrometeorids. Over time, more and more dirt should settle, similar to the surface of a piece of furniture: the longer it is not wiped, the thicker the layer of dust.
The researchers now estimate the dust content of the rings to be only 0.1 to a maximum of two percent. From this very low level of pollution, the scientists conclude that there is no way the crumbs in the rings could have been under bombardment for a period of billions of years. In addition, all ring particles combined have a surface area 10,000 to 100,000 times larger than that of a single moon of the same mass. Therefore, according to the publication, the originally bright ice rings are extremely susceptible to contamination and are becoming darker and darker. During the measurement period of 13 years, the Cosmic Dust Analyzer registered an influx of 163 dust particles from the planetary system in the rings.
Extrapolation, the observed color and the different reflectivities of the rings depending on the distance from Saturn allowed the researchers to estimate the age of the rings. It is therefore between one hundred and four hundred million years. The authors categorically rule out that the rings could have formed at the same time as the planet. Saturn did without its rings for most of its existence.