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Planetary Ring Systems ASTB21H3F Planetary Ring Systems Piranavan Satkunanathan Joseph Zhang Jonguk Yoo Seyed Hossein Hosseini Nassab 1 Introduction: Planetary ring systems are rings of cosmic dust and other small particles orbiting around planets in extremely thin and flat circular disc-shapes. There are four planets that we know so far with their individual ring systems in our Solar system, and they are: Jupiter, Saturn, Uranus and Neptune. All of them belong to the Jovian planets and they are all gas giants. The discoveries of these rings were fairly recent, other than the rings of Saturn due to its spectacular appearance and size. Saturn’s rings were discovered by Galileo during early 17th century and the other three ring systems were not found for more than three and half centuries. Uranus’s rings were found in 1977, Jupiter’s rings were found in 1979 and Neptune’s ring “arcs” were found in early 1980s. There are many interesting facts and questions about the Planetary ring systems, so in this paper we are going to introduce details of these Jovian ring systems with Saturn in most focus, and explain their History/Formation, Properties/Composition/Dimensions and Historical Observations/Questions. History and Formation: As a matter of fact: Saturn, Uranus, Jupiter and Neptune are the only planets that have planetary rings in solar system, and among these planets, rings of Saturn appear most clearly. So, what are the rings? During 1610s, the existence of the rings was first discovered by Galileo, but he didn’t have precise explanation to what they were. He could only describe them as Saturn’s “ears”. In 1655, Christiaan Huygens observed and suggested that Saturn was surrounded by a ring with improved telescope. After few years, in 1675, Giovanni Domenico Cassini determined that Saturn's rings were composed of 2 multiple smaller rings with gaps between them. Pierre-Simon Laplace, in 1787, suggested that the rings were composed of a large number of solid ringlets. In 1859, however, British mathematician, James Maxwell, claimed that if the rings of Saturn were made of solid material, the rings should have broken by Saturn’s tidal force. He also said that they were made of number of small particles such as Dust or Ice but it just seemed like plate with solid materials from the Earth because of the large distance. It is like a beach that consists of large quantity of grains of sand but when viewed from a distance, it looks like a piece of solid land. So then, which theory was right? In 1850, French astronomer, Eduard Roche, had studied that what would happen if the Moon gets too close to the Earth. Roche found that the tidal force between the Earth and the Moon is inversely proportional to the cube of distance between the Earth and the Moon. In other words, if you half the distance between the Earth and the Moon, the tidal force will become 8 times larger. Also, he found that if a satellite comes within a certain distance to a primary object, the tidal force between them becomes greater than the force of gravity of the satellite itself, and the satellite will not be able to maintain its physical form. After the satellite has been disrupted by the tidal stress, its debris will end up spinning around the planet forming plate-like structure, and this is what created the ring systems. This distance limit is called Roche limit, and it is described as, 1/3 d = 2.44R (ρM / ρm) -where R is the radius, ρm is the density of the primary object, and ρm is the density of the satellite. This is the theory of Ring Formation. However, the exact causes of creation of rings, dynamic evolution, and the ingredients of them are still not fully revealed. For 3 example, some of Saturn’s rings are located outside of its Roche limit; so therefore, the creation of rings cannot be fully explained by this theory. Another explanation for existence of the rings is that the material of the proto-planetary disk that was within the Roche limit of the planet could not coalesce to form moons. So far, there isn’t enough evidence to conclude with a perfect theory with full explanation. The amount of observations by space probes such as Voyager 1, 2 and Cassini-Huygens are very limited. Gaps: If we look closely at the rings, there are gaps between them. The main cause is believed to be the orbital resonances with outer satellites. Orbital periods of the moons and the particles within the rings are explained by Kepler’s laws, and if an outer moon has a period that is an integer fraction of the particles in the ring, then the moon gives an extra pull on the particles each time they line up and this throws the particles into a new orbit. Orbital resonances would, after billions of years, eventually sweep all the particles out of a ring, but the effects of the inner moons counteract the pull from the outer moon. This is how gaps formed and its relation to the existing rings. For a long time, astronomers had the question “Is Saturn the only planet with rings?” This question was eventually answered by a discovery, which found Uranus also had the rings in 1977. However, Uranus’ rings were too thin and dim to observe from Earth since it couldn’t reflect enough sun light. The discovery accidently happened, when a nearby star was passing by. Astronomers saw observed the light from Uranus darkened briefly, and it was found to be caused by the ring system of Uranus. In 1986, space probe, Voyager 2, on observational mission to the giant planets, made clear observation as evidence of Uranus’s ring system with total number of eleven rings discovered including very faint ones. With the improved technology, the Hubble Space Telescope detected two 4 more rings, but they were located at twice the distance from the previously found rings. So, they are also called, ‘outer’ ring system of Uranus. 2 years after the discovery of Uranus’ rings in 1979, Jupiter’s rings were discovered by the Voyager 1. Unlike the rings of Saturn, the rings of Jupiter are composed of small bits of dust, therefore having less reflectivity. In 1989, Neptune’s rings were also found by Voyager 2. However, Neptune's ring system is very different from the ring system found at Saturn, they were much darker than that of Saturn's. This was caused by the differences of their composition; Saturn's rings are mostly composed of ice where as Neptune's appear to be made of small rocks and dust much more similar to the rings of Uranus’s. Observations by NASA: Many questions have risen since the rings of Saturn were discovered with an Earth based 20-power telescope by Galileo, who was puzzled by its odd appearance. From what he saw, the rings looked like bumps on the planet and he wasn’t able to imagine these bumps as rings we know today. Later in 1655, Christian Huygens observed Saturn with an improved telescope, where he was able to describe the bumps observed by Galileo to be a disk surrounding Saturn. However, only from ground base observations it was not possible to fully understand the true nature of these planetary rings. As technology improved, the image and characteristics of the rings became more and more clear and understandable. From the missions by NASA such as Voyager 1/2, Cassini Mission to Saturn, and Pioneer 11, astronomers were able to collect data to study the nature of the rings. However these new samples of data raised even more questions. 5 The first spacecraft to visit the Saturnian system was Pioneer 11. Pioneer 11 reached its closest approach to the planet on September 1st 1979. Unfortunately Pioneer 11 was designed primarily as a “fields and particles” spacecraft. The visible light sensor that was equipped was not suited for high resolution imaging. Even though some of the images revealed Saturn’s rings, the machine wasn’t able to provide a more detailed spectrum of data for astronomers in order to expand their study on planetary rings. However, it was able to discover a faint, narrow F and G ring, but its complexity became evident only when Voyager 1 and 2 took closer looks. Moreover, Pioneer 11 played an essential role in the discovery of the Jupiter’s ring system. “In 1974, the processed data taken from the Pioneer 11 mission revealed an unexpectedly low energetic charged particle around Jupiter’s little satellite, Am althea. Scientists suggested that an unseen ring or undiscovered satellite might be responsible for absorbing some of the charged particle flux inward of Am althea’s orbit, but little attention was given to the suggestion until after the voyager 1 Jupiter encounter”(Ellis D. Miner, Randii R, 2007). During the voyager flyby of Jupiter it discovered a faint ring of matter surrounding Jupiter in the plane of the planet’s equator. In the summer of 1977 from Cape Canaveral, Florida NASA launched Voyager 1 and Voyager 2 on August 20 and September 5. Voyager 1’s primary target was to conduct close up studies of Jupiter and Saturn including Saturn’s rings, and the larger moons of the two planets, while Voyager 2 was launched to study the gas giants. Voyager 1 and 2 flew by Saturn on November 1980 and August 1981 respectively, and photographs collected from them allowed scientists to confirm that the main rings (A, B and C) were composed of tens of thousands of narrow ringlets.
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