18 March 2010 Moonlets and maelstroms: mysteries of Saturn revealed
During its six-year scrutiny of Saturn, the Cassini spacecraft has resolved many questions about the ringed world - but it has captured unexplained phenomena too.
NASA's Cassini probe took almost seven years to reach Saturn, but the lengthy trip has turned out to be well worth the wait. Since 2004, the probe has scrutinised the ringed planet in unprecedented detail, revealing new rings, flying-saucer-shaped moons and record lightning storms. The information Cassini has collected, summarised this week in two papers in the journal Science, will help resolve a number of questions - but also raise new mysteries.(Image: CICLOPS/JPL/NASA) Small moons carve gaps in Saturn's rings. But Cassini has found the rings are also scattered with "moonlets" that are too lightweight to clear a path. Some of these moonlets are too small for Cassini to see directly, but they can be identified by propeller-shaped disturbances (pictured) that they create in the ice and dust that surrounds them. It is not known whether these moonlets are the remnants of broken-up bits of larger bodies or whether they grew from scratch from material that accumulated around solid "seeds". Studying the propellers could reveal more about how the seeds of planets grow in dusty discs around stars. (Image: Space Science Institute/JPL/NASA) The north pole of Saturn is covered by a bizarre hexagon-shaped storm (shown in red in this infrared image of the planet, with its aurora in blue). Parts of this feature were detected by the Voyager mission in 1980, but Cassini snapped the first full views of the weather system, which spans nearly 25,000 kilometres - the width of two Earths. The hexagonal shape is thought to be due to a jet stream in the atmosphere that creates a wave which wraps around the planet six times at this particular latitude. But it is unclear why this jet creates such a wave and why it has been stable for decades. (Image: University of Arizona/JPL/NASA) Saturn's rings may seem immutable, but images taken by Cassini suggest something might have shaken them up 25 years ago. Alternating light and dark bands that correspond to spiralling grooves can be found the planet's innermost two rings. The origin of these ridges and troughs is unknown, but they point to a significant event that tugged on the rings: a body much larger than a typical asteroid or a dramatic change in weather that shifted Saturn's mass may have been responsible. (Image: University of Arizona/JPL/NASA) Saturn boasts marathon maelstroms that can generate lightning bolts about 10,000 times stronger than those on Earth. The lightning is not bright enough to be seen through Saturn's thick clouds, but it creates radio waves that Cassini's radio antenna can measure. Several storms have cropped up at a latitude of about 35 degrees south of the equator, in a region dubbed "Storm Alley" (pictured). But the reason the storms concentrate there is unclear. (Image: Space Science Institute/JPL/NASA) The 28-kilometre-wide moonlet Pan (shown here) and 8-kilometre-wide Daphnis are small embedded moons that clear out gaps in Saturn's rings which span the full circumference of the moons' orbit. Studies of this behaviour have helped support the idea that Jupiter-sized exoplanets can carve out similar cavities in discs around their host stars. But despite extensive searching, Cassini has not yet found similar moonlets inside more than a dozen other prominent gaps in Saturn's rings. The gravitational tugs from the distorted edge of Saturn's B ring may be responsible for five of these gaps, but the origin of some others remains a mystery. (Image: NASA/JPL/Space Science Institute) In 2005, Cassini found the first evidence of plumes of water vapour and ice emanating from the south pole of Saturn's moon Enceladus. Recent measurements of salt in the plumes suggest they could originate from a liquid ocean beneath the moon's icy surface. Heat is needed to warm the interior, but the source of this energy - some 15 gigawatts of heat must be involved - is still unclear. (Image: Space Science Institute/JPL/NASA) Advertisement