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Astrophysics

The Jewels of Saturn

Images from the Cassini probe suggests the existence of four new moons and confirm the theoretical model that explains errors in the rings

NASA/JPL/SPACE SCIENCE INSTITUTETwo ways of looking at TitanNASA/JPL/SPACE SCIENCE INSTITUTE

The God of agriculture and the Master of time, Saturn was magnificent and violent. He revolted against his oppressive father, castrated himself, and reigned over his brothers and other Gods until he was dethroned by his own son, Jupiter, the God of Gods at Olympia. He was remembered every year by the ancient Romans in celebrations that lasted a week, the Saturnalia Festival. Now with the name of a planet – the sixth out from the Sun and the only one with a vast grouping of rings -, Saturn is once again ruling in the sky as soon as the first data from the space mission formed by the spacecraft Cassini and Huygens, launched almost eight years ago in the direction of this yellow planet, became public.

The Cassini-Huygens combination is the most well equipped mission that has been sent to Saturn, even at a cost considered to be low: US$ 3.3 billion. The result of the cooperation between the American, European and Italian space agencies, it will send back information for a further four years about this planet generous in its number of moons and rings, considered to be a living model of the formation on the solar system and even on the origin of life on the earth, billions of years ago.

In January, the Huygens spacecraft separated itself and landed on Titan, the largest of Saturn’s moons and the only one in the Solar System to preserve a dense and rich atmosphere of nitrogen and carbon compounds, similar to those of Earth. The first images of the surface of Titan have suggested clouds and rivers of methane, an organic compound formed between carbon and hydrogen – perhaps the same that had favored the appearance of life on our own planet. As well, they indicate that Titan exhibited volcanic activity in the past and that currently ice could well exist on its surface.

Other good surprises had already occurred in July of 2004 when the first pictures that Cassini took of the seven main Saturn rings during its approximation to the planet arrived. Disclosed in September, the analysis to these photos revealed the possible existence of four new moons – which if confirmed would raise the total to thirty seven -, as well as a probable new ring. One of the photos especially – captured on the 21st of June, before the two spacecrafts  entered into the orbit of Saturn – was the reason for celebration by the Brazilian physicist Silvia Giuliatti Winter, from the São Paulo State University (Unesp) in Guaratinguetá, and the Irish astrophysicist Carl Murray, a member of the team that analyzed the images from the Cassini-Huygens mission.

The shadow of F
This image, in black and white, suggests that one of the new moons, of less than 10 kilometers in diameter, is situated very close to one of the outermost rings of Saturn, named F (the main rings are identified by letters, A to G). “I noticed this object almost undetectable close to the external part of the F ring”, says Murray, from Queen Mary College, of London University, in a communication from the Particle Physics and Astronomy Research Council of the United Kingdom. “It was an unbelievable privilege to be the first person to identify it.”

This is a finding of greater importance than it seems. In a paper published in 1997 in the scientific magazine Icarus, Murray, Silvia and Mitchell Gordon, from Virginia State University, in the United States, said that typical alterations of the F ring could only be justified by the existence of four small moons, at that time unidentified, close to the ring. This forecast, confirmed in part by the Cassini images, was born from a mathematical model developed by them based on the images of the F ring captured in 1980 and again in 1981, during the passage of the probes Voyager 1 and Voyager 2 close to Saturn.

Too close
The model detailed in the Icarus article described, with precision, the structure and the deformations of this ring – the fourth most external and one of the most slender of the seven rings that circulate around the planet. Back in 1997, in another study only published in 2000, the group forecast that the moon, Prometheus, would cause deformations of the ring as it would approximate to it. “In July of last year, before the congress in Paris, Murray told me that the Cassini images correspond to the forecasts of our model”, says Silvia. “I didn’t expect this spacecraft would detect the disturbances of Prometheus upon the multiple structure of the F ring nor this new moon close to it.”

A quarter of a century after its discovery by the spacecraft Pioneer 11, in 1979, the F ring is still today the one that most intrigues physicists and astronomers. It is formed by four distinct bands of particles situated on the same plane, in an elliptical orbital of distance around 140,000 kilometers from the center of Saturn, as details Silvia, Murray and Gordon. With widths that run from 48 to 55 kilometers, these four bands extend for an eighth of the extension of the ring – in the other seven eighths the structure could well vary. At certain points these bands show curious alterations and that are almost always temporary. At distances on average of 20 kilometers from each other, the strips are either intertwined like the braiding of hair or form mild undulations. Or, all of a sudden, disappear for a stretch only to reappear later, like a highway interrupted by the fall of a bridge.

Some eight years ago, Silvia, Murray and Gordon explained the surprising modifications in the bands of ring F through gravitational interaction with the two moons of the closest orbit, Prometheus and Pandora. Promethium is 100 kilometers in diameter and moves in an elliptical orbit internal to the F ring at some 139,000 kilometers from Saturn. Pandora is smaller: it is 84 kilometers in diameter, with an orbit external to the F ring, at 142,000 kilometers from the second largest planet in the solar system, whose diameter is ten times that of the Earth, smaller only than that of Jupiter. By being larger, Prometheus causes the disturbances more intensely on the structure of the ring, especially when this moon finds itself at the closest point of approximation to the F ring, a phenomenon observed every nineteen years, according to the forecast of  Silvia and Murray. It is this period of gravitational attraction of the moon on the ring’s particles that makes it more intense. “This maximum approximation is greater in some regions because the orbit both of the moons and of the rings are elliptical”, explains  Silvia.

Braids and waves
In a paper published during 2000 in Planetary and Space Science, . Silvia, Murray and Gordon used that data of the maximum approximation of Prometheus to simulate the effects upon the four bands of F – when the moon’s distance reaches a mere 2 kilometers from the most internal strip of the ring and at 280 kilometers of the most external. Almost nothing, when faced with the width of the totaled rings, which is equivalent to the distance of the Earth to the Moon.

On moving away from the ring, Prometheus drags with it a cloud of particles of the most internal portion of F, which seems interrupted, launching itself in the direction of Saturn – something that had not been detected by the Voyager missions and was now observed by Cassini. In the four bands, undulations that are clearly pronounced come about. When it is Pandora that is getting close to F, its bands do not get to the point of breaking up, but less intense undulations are formed, according to  what Silvia’s team demonstrated in their most recent simulation. With close to one tenth of the diameter of Prometheus and Pandora, smaller moons with an orbit coinciding with that of a ring have brought about the intertwining and the increases in density of the bands. In a simulation carried out last semester, based on the information provided by Cassini, Dr. Silvia confirmed the truth of this model.

Since its discovery in 1610 by the Italian astronomer Galileu Galilei, Saturn has called the attention not just for its imposing size and its moons, but for its rings. With a telescope made by himself, Galileu could not discern the rings and had believed that there were two immense satellites almost stuck to the planet. In 1656, the Dutch astronomer Christiaan Huygens – discovered Titan – and proposed that these satellites were in truth a single gigantic rigid ring going around the planet. But it was another Italian, Giovanni Cassini, who suggested in 1675 that this ring was not rigid and was not a single unit. Only the sending of the space probes revealed the details and the nuances of these rings, the most brilliant, extensive and well preserved in the solar system, formed by particles of dust and ice. Another three planets, Jupiter, Uranus and Neptune, have rings, but they are not visible.

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