Rings of Saturn. Source: NASA
Scientists have studied the flow of cosmic dust that hits Saturn from the outside, and estimated the age of the rings surrounding the planet from it. As follows from the publication on Science Advances, they turned out to be no older than 400 million years.
Previously, scientists believed that the rings were about the same age as Saturn and the rest of the planets in the solar system, that is, about 4.5 billion years. The rings were discovered by Galileo Galilei in 1610, and in the 19th century, astronomers realized that these were not monolithic formations, but a combination of many small objects. In modern times, it has been found that the rings are composed almost entirely of pure water ice.
In this case, from 0.1% to 2% of the material of the rings is cosmic dust. The particles of Saturn's rings should look like a badly cleaned pavement by spring: snow and ice inside, mud outside. Scientists estimated the amount of dust by the deterioration in the reflectivity of ice: so the amount of dust on the mirror can be estimated by the dimness of the reflection. Cosmic dust is captured from micrometeorites that Saturn is bombarded with from outside its sphere of gravitational influence. The intensity of the bombardment was measured from 2004 to 2017 by the Cassini space probe in orbit around Saturn. His instrument, a cosmic dust analyzer, determined how much small space "debris" hit Saturn.
The authors of the publication on Science Advances analyzed Cassini data and calculated the time of occurrence of the rings. The cosmic dust analyzer caught cosmic microparticles and determined from their trajectories whether they were exogenous, that is, whether they arrived from outside the sphere of Saturn's predominant gravitational influence. In total, scientists caught 163 dust particles. Of these, only 73 were definitely exogenous. The scientists also determined the radius of the particles and their speed. This was necessary in order to calculate how many kilograms of dust arrives per square meter of rings per second. At the same time, it was also taken into account that Saturn effectively "sucks in" dust due to its gravity from a large area – more than the area of the rings. It was also taken into account that not all dust actually reduces the reflectivity of water ice, if only because the micrometeorites themselves may partially consist of ice.
Knowing how much dust this cosmic “vacuum cleaner” accumulated in itself and how much it “sucked in” every second, scientists estimated the age of the rings in the range from 100 to 400 million years. On an astronomical scale, this time is very short compared to the duration of the existence of the solar system. The main conclusion of the article: the rings could not have formed along with the planet. They could not have formed during the “Late Heavy Bombardment” (an intense bombardment of the planets of the solar system by comets that happened about 4 billion years ago).