|Volume: 04, Issue: 05||11/02/2005|
Cassini Uncovers More Saturn MysteriesThe Cassini spacecraft is continuing to make dazzling discoveries about Saturn and its moons. Recent observations have uncovered unparalleled phenomena in Saturn’s F ring and peculiar clouds on Titan.
Saturn's narrow, contorted F ring has recently shown phenomena not previously detected in any planetary ring. The F ring is notorious for exhibiting unusual structures such as knots, kinks, and clumps, which continue to puzzle astronomers. However, Cassini images have shown that the gravitational effect of the inner shepherding satellite, Prometheus, appears to produce regular patterns on the ring. These patterns include a series of channels or gores as well as 'streamers' of particles that temporarily link the ring to the satellite. As an example of a satellite that enters a ring on a regular basis, the phenomena posed unique challenges to the understanding of ring-satellite interactions.
The findings of the Imaging Science Subsystem team show that channels and streamers can be understood in terms of a simple gravitational interaction as Prometheus approaches and recedes from the F ring every 14.7 hours. Using Cassini data, the team developed a model that shows the mechanism by which Prometheus, as it recedes from its closest approach to the F ring, gravitationally extracts material from the ring. The affected particles do not escape the F ring: the changes to their orbits produced by Prometheus cause them to oscillate back and forth across the ring. One orbital period after the encounter, the effect is visible as a channel - in excellent agreement with the Cassini images. In this way, Prometheus leaves its mark on the F ring long after it has moved on.
"As the closer and more massive of the F ring's two shepherding satellites, Prometheus was always the likely culprit for causing changes to this narrow ring,” said Professor Carl Murray from Queen Mary, University of London. “Our model provides a plausible mechanism for the origin of intricate structures detected in the F ring and suggests that streamers, channels and a variety of other phenomena can all be understood in terms of the simple gravitational effect of a satellite on ring particles."
Over time, Prometheus is expected to drive deeper into the F ring - with more extreme perturbations - culminating in December 2009 when the two orbits approach their minimum separation.
"We see the model we have developed very much as a first step in understanding the processes at work. There are many features of the F ring that we have yet to explain but at least we have uncovered one of its secrets. Ultimately this type of research will help us to understand how planets form and evolve," Murray added.
Meanwhile, scientists at the University of Arizona have made another interesting discovery. They have determined peculiar clouds at middle latitudes in Titan's southern hemisphere may form in the same way as distinct bands of clouds form at Earth's equator.
Titan's weather forecast has remained the same for years, and that baffles scientists. They don't understand why clouds a thousand miles long stretch over the temperate latitude. Griffith compared the situation on Titan to how odd it would be if on Earth, clouds existed only at the poles and the latitude that crosses New Zealand, Argentina, and Chile.
Scientists believe the highly localized nature of the clouds suggests that they have something to do with Titan's surface, according to Griffith. Ice volcanoes must be venting methane -- the gas that condenses as clouds -- into Titan's hazy, mostly nitrogen atmosphere. Otherwise, the moon's atmospheric methane would have vanished billions of years ago because methane is destroyed by ultraviolet sunlight.
"The structure of the clouds turns out to be complicated," Griffith said. "We detected not one region, but many regions of cloud formation. Each long cloud consists of a number of vigorous storms where clouds rise to 40 kilometers altitude (25 miles) in a couple of hours and dissipate in the next half hour. The rate of cloud ascent and dissipation suggests that we are witnessing the formation of convective clouds, likely similar to thunderstorms, that disappear through rainfall.
"Over the next several hours we see the clouds form long tails, indicating that strong westerly winds stretch out the clouds and carry the particles downwind a thousand kilometers (more than 600 miles). This detailed look into the structure of these clouds reveals that the clouds evolve from a number of small active cloud formation centers lined up like an uneven string of beads long 40 degrees south latitude. These localized storms cause a healthy rain, and very long clouds, once the wind has stretched them out."
Griffith argues that it's improbable that many ice volcanoes, all aligned at 40 degrees south latitude, are forming these clouds. In addition, the scientists estimate that the cloud activity at zero degrees longitude, if volcanic, does not appear to spew out enough methane to create the mid-latitude cloud band. Smaller clouds actually lie upwind of the main cloud at zero degrees longitude, they note. The team also concluded the clouds aren't obviously caused by Saturn's tidal pull on Titan's atmosphere. In addition, no evidence existed that mountains and lakes might cause mountain clouds or marine clouds.
The researchers suggest that global circulation may cause the air to rise at this latitude on Titan, much as clouds form in a band around the Earth's equator and rain on the Caribbean islands.
"Such rising air would cut off air from the south polar region from mixing with the rest of the moon's atmosphere, causing smog to build up and form a cap over the pole," Griffith said.
The next mystery is why Titan's southern mid-latitude clouds are bunched at zero degrees longitude. There's no evidence yet that volcanoes, mountain ranges or Saturn's tides are involved.
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