New ring quirks revealed during Saturn equinox
DR EMILY BALDWIN
Posted: September 22, 2009
A slate of new images taken during Saturn's equinox season reveal the finer detail of ruffles, kinks, dust clouds and even impact events occurring within the giant planet's rings.
On 11 August, sunlight hit Saturn's rings exactly edge on, creating a unique illumination geometry that has the effect of making the rings seemingly disappear, disclosing out-of-plane structures by making these features anonymously bright and causing them to cast long shadows. This phenomenon occurs twice a year when the Sun is directly overhead at the planet's equator, but since Saturn's year is equivalent to nearly 30 Earth years, this is the first time scientists have been afforded the opportunity to get up close and personal with the ring's architecture. And the ideally located Cassini spacecraft was ready to soak up the glorious sights.Seen from our planet, the view of Saturn's rings during equinox is extremely foreshortened and limited, but in orbit around Saturn, Cassini had no such problems. From 20 degrees above the ring plane, Cassini's wide angle camera shot 75 exposures in succession for this mosaic showing Saturn, its rings, and a few of its moons a day and a half after exact Saturn equinox. Image: NASA/JPL/Space Science Institute.
“The biggest surprise was to see so many places of vertical relief above and below the otherwise paper-thin rings,” says Linda Spilker, deputy project scientist at JPL. “To understand what we are seeing will take more time, but the images and data will help develop a more complete understanding of how old the rings might be and how they are evolving.”
Saturn's rings are made out of chunks of ice and debris, and were previously thought to be about ten metres thick, but the heights of some newly discovered bumps are comparable to the elevations of the Rocky Mountains. “We thought the plane of the rings was no taller than two stories of a modern-day building and instead we’ve come across walls more than 2 miles [3 kilometers] high,” says Carolyn Porco, Cassini imaging team leader at the Space Science Institute. “Isn’t that the most outrageous thing you could imagine? It truly is like something out of science fiction.”The bright streaks visible in these Cassini images taken during Saturn's August 2009 equinox are exciting evidence of a constant rain of interplanetary projectiles onto the planet's rings. The image on the left shows an impact in the A ring. The streak stretches from the right of the image to the middle, and it does not quite follow the arc of the rings. The image on the right shows an impact into the C ring. The brightest part of this streak is 200 kilometres long and 10 kilometres wide. Image: NASA/JPL/Space Science Institute.
Scientists also observed bright streaks in two different rings – A and C – that appear to be clouds of dust kicked up by collisions between ring particles or debris. These particles are sheared out by orbital motion into streaks some 5,000 kilometres long, and were likely caused by impactors of around one metre in diameter within one or two days of the images being captured. The data lends confidence to images taken of similar streaks seen in the C ring in 2005, and to the belief that debris continuously rains down on the rings contributing to their erosion and evolution.
Equinox also provided new insight into gentle spiraling undulations discovered earlier in the Cassini mission. These features, which exhibit wavelengths of around 30 kilometres, were first seen in 2006 spanning across the D-ring but the latest images show that this phenomenon stretches fully across the C-ring and up to the B-ring, too, a distance of some 17,000 kilometres. Previous speculation suggested that this 'corrugation' seen in the rings was caused by an impact in the early 1980s. Matthew Hedman, an imaging team associate at Cornell University in Ithaca thinks that “something happened in the early 1980’s to get this pattern going, but we are still trying to figure out what could have disturbed such a large part of the rings.”An unusually large propeller feature is detected just beyond the Encke Gap in this Cassini image of Saturn's outer A ring. From the 350-kilometre length of the shadow cast by this 130-kilometre-long propeller, the heights of these features above the ring plane have been measured to reach about 200 metres, indicating the moonlet responsible for the propeller in this image is likely to be 400 metres. Credit: NASA/JPL/Space Science Institute.
The equinox season also brought with it a fuller appreciation of the behaviour of ring particles as they fall under the gravitational influence of nearby moons. In one extreme case, at the edge of the Keeler Gap, the eight-kilometre wide moon Daphnis was found to force structures nearly four kilometres high, compared with the 1.5 kilometre high waves observed earlier in the mission. The result suggests that Daphnis approaches the inner edge of the ring more closely than previously thought, therefore inflicting a greater disturbance on the ring particles.
Small moonlets within the rings were also found to create unusual features, such as 'propeller' structures. These are typically a few kilometres long and centred around small moons that are only 100 metres across. Although these features were known before equinox, the unique viewing conditions of last month revealed a surprisingly large propeller spanning 130 kilometres and casting a shadow 350 kilometres long. These properties suggest that these features reach approximately 200 metres above the ring plane and that the moonlet responsible for this propeller is likely 400 metres across. A previously released image had revealed a moonlet in the outer B ring about matching these dimensions and the finding confirms that objects forming Saturn’s rings likely span the full spectrum of sizes, from the smallest dust-sized ring particles to the ring-moons like Daphnis, providing a significant advance in learning about the origin of the rings.
“This has been a moving spectacle to behold, and one that has left us with far greater insight into the workings of Saturn’s rings than any of us could have imagined” says Porco. “We always knew it would be good. Instead, it’s been extraordinary.”
The imaging team also created a mosaic of the giant planet and its rings in natural colour, taken a little over a day after equinox. The image shows one half of the rings only dimly lit by sunlight reflected off the planet, the other in near total darkness. “To know that in this one image we are witnessing, for the first time, up close, one of the Solar System’s greatest wonders is to feel truly blessed,” says Porco.
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