Rings of Ice and Dust
Galileo Galilei was the first to see them. In 1610 he observed Saturn’s rings through a simple telescope. However, he did not recognise them as such because he thought that they were moons of the planet. It was Christiaan Huygens who identified a ring structure around Saturn in the middle of the 17th century. Nowadays the spacecraft Cassini delivers impressive pictures from a very close distance that reveal the fine structures of Saturn’s rings. Professor Frank Spahn from Potsdam is among the scientists who analyse the data collected by the Cassini Mission.
On the morning of 15 October 1997 a spacecraft was launched from the Cape Canaveral Air Force Station in Florida. Its destination is more than one billion kilometres far from Earth. On board there are two space probes – Cassini und Huygens. Seven years later, on 30 June 2004, the international community of space scientists cheered. Cassini und Huygens successfully entered the orbit of Saturn. Then Huygens was separated from Cassini with three little explosive charges and started its journey to Titan, the moon of Saturn, to explore the atmosphere and surface of this satellite. While the Huygens Mission was finished two hours and 10 minutes after the landing of the space probe on Titan, Cassini still orbits Saturn and sends data and photos to the Earth.
Frank Spahn, Professor of Non-Linear Dynamics at the Institute for Physics and Astronomy of the University of Potsdam, has been dealing with Saturn for almost 30 years. The Cassini Mission is a lucky chance for him. “That was like hitting the jackpot,” he says remembering how it all began. The knowledge that Cassini reveals about Saturn, its moons and rings is revolutionary, says the physicist. He is especially taken with the disc-shaped rings surrounding Saturn. They consist of ice and dust particles, which are something between just a few micrometres and several metres in size. They move in circular orbits around the planet. The extension of the rings from the outer edges is 300,000 kilometres, and they have a height of only three to five metres. There are several tonnes of ice per square metre. Due to the high density of this structure, these extremely thin rings are even visible from Earth through a simple telescope.
For planetary scientists, the rings of Saturn are more than just fascinating artwork of nature. “Imagine these rings as dynamic laboratories for the evolution and formation of planets,” Spahn explains. Similar to preplanetary discs, which are the stellar nurseries of planets, there is a lot of material in these rings in a confined space. The particles constantly bump against each other and move around a centre of mass. Astronomers assume that bigger bodies, so-called planetesimals, can form under such conditions. They are the basic components of planets. The areas where such formation of stars and planets takes place are far beyond our solar system – out of reach for humans today. But Spahn is convinced, “One day we will have telescopes with such a high resolution that we will be able to watch the planets grow.” But before this happens, scientists need to make do with models. They can check the validity of these models with the help of the planetary rings. “To these places we can fly,” the scientist says.
Nevertheless, there is a big difference between the processes in preplanetary discs and Saturn’s rings. The latter are subject to the tidal power of Saturn. Since the rings are very close to the planet, enormous, disruptive forces act on the particles. They prevent the formation of objects that are bigger than 30 metres. If they exceed this size, they are torn apart by tidal power. Nevertheless, planetary scientists can learn a lot from this due to fortunate circumstances. Saturn is surrounded by numerous moons that orbit the planet beyond the rings. For a long time scientists have assumed that small natural satellites are hidden within the ring system. The so-called moonlets can have diameters of several kilometres. If they consist of solid ice, the tidal powers cannot do them any harm. Already in the 1980s Frank Spahn wondered what effect moonlets might have on the ring structures of Saturn. He ran computer simulations with bodies in a viscous disc and calculated the structures that such moons of Saturn might leave in the ring due to the effect of gravitational forces. In theory there was the hope that these traces would tell where the moons were hidden.
The calculations showed: Moonlets with a diameter of more than one kilometre should produce gaps with bow waves. Later the scientists found out that smaller moonlets can be identified by propeller-shaped features in the rings. They themselves are too small to be directly pictured. In fact, with the help of these hypotheses, they were able to identify moonlets in the rings of Saturn, first with pictures taken by the space probe Voyager 2 and later with pictures from Cassini. “So far we have discovered more than 300 bigger objects in Saturn’s rings,” Spahn explains. “The most beautiful are the huge-scale propellers that almost create a gap,” he says with a smile. “They have all the qualities we predicted.” New estimates by astronomers say that the number of moonlets and bigger chunks of ice in the rings is enormous. There are perhaps more than a million. “Pan,” with a diameter of 30 kilometres, is the biggest moon that has been discovered so far.
For the scientists, the moons in Saturn’s rings not only bear strong implications for the pattern that forming planets leave in gas and dust clouds. They also provide information about the formation and history of the rings around the gas planet. So far there have been two theories. One theory says that the rings are remnants from the time when Saturn was formed five billion years ago. However, in that case there should not be any objects that have a diameter of more than 30 metres. In the meantime, it was possible to provide evidence of many such moonlets with diameters of between 100 metres and several kilometres. This indicates that the ring around Saturn is younger than the planet itself. It probably formed at the time of the Late Heavy Bombardment about four billion years ago when numerous asteroids crashed into the planets of our solar system and their satellites. One or more icy moons of Saturn were destroyed by impacts, perhaps. The remnants evenly distributed around the planet and finally formed the impressive rings.
The Cassini Mission will come to an end in 2017. Then the spacecraft will fly through the rings to Saturn and eventually hit the planet. “I am really looking forward to this moment,” says Professor Spahn. “Then we will see the particles of the rings directly for the first time.” Heike Kampe
The second biggest planet in our solar system has a diameter of 120,500 kilometres. Being a gas planet, it consists of 96 per cent hydrogen and is approximately 1.4 billion kilometres from the Sun. The temperatures in the Saturn system are between -150 and -200 degrees Celsius. Several icy moons orbit the planet. Saturn has a prominent ring system consisting of water ice. It takes Saturn about 30 years to make one revolution around the Sun.
Professor Frank Spahn is a theoretical physicist. Since 2006 he has been an associate professor at the Institute of Physics and Astronomy at the University of Potsdam. He is a member of the Cosmic Dust Analyzer Team of the Cassini Mission.
Institut für Physik und Astronomie
Text: Heike Kampe, Web Content Editing: Julia Schwaibold, Translation: Susanne Voigt