preview.tinyurl.com/3b5q37 -> www.newscientist.com/article/dn13421-saturn-satellite-reveals-first-moon-rings.html?feedId=online-news_rss20
Geraint Jones of the Mullard Space Science Laboratory at University College London, UK. The rings revealed their presence by the way they block the flow of energetic electrons that zip around Saturn, trapped by the planet's magnetic field. The effect was observed by the Cassini spacecraft as it passed about 500 kilometres away from Rhea in November 2005. As Cassini approached the moon, the flow of electrons had a mostly gradual decline that included three sharp downward spikes before the moon itself completely blocked electron flow to the spacecraft. When the spacecraft emerged on the other side of the moon, the flow had an overall gradual rise, which included another three sharp downward spikes. In contrast, when Cassini passed near another icy moon, Tethys, electron intensity remained steady except when the moon itself blocked the electron flow. Stable disc The ring idea appears to be the only one that fits the observations. Although charged atoms and molecules, called ions, could scatter electrons away from Rhea, Cassini did not find them in sufficient abundance to explain the dips, and calculations show that observed levels of gas and dust could not explain them either, the team says. They believe a disc of larger particles, probably from millimetre to metre scale, soaked up the electrons. Orbital evolution studies show such particles would form a stable disc around Rhea's equator. The electron flow began dropping when it crossed into the so-called Hill sphere, where Rhea's gravitational force is stronger than Saturn's, which means the disc must extend to that point. The disc's density increases closer to the moon, causing the observed decline in electrons. Jones says the three sharp spikes reveal narrow rings of higher particle density within the disc extending around Rhea. Unknown origins The origin of the disc and rings remains unclear. They might be leftovers from Rhea's formation, or from an impact later in its history. Cassini found no trace of rings around Saturn's similar icy moons Dione and Tethys, which Jones speculates cannot hold onto rings because they are much closer to Saturn than Rhea and therefore subject to more interference from the planet's gravity. Nothing has been seen around the more distant Titan, either, perhaps because rings would have a hard time persisting there due to its thick atmosphere, which could drag particles down from orbit. Although Cassini's cameras have failed to see the rings, the electron measurements are solid, says Larry Esposito of the University of Colorado in Boulder, US, who was not involved in the study. Missing moon That means the rings must be clumpy, he says, and made of large particles that are harder to see than the fine dust that makes other rings more apparent. In February, scientists announced finding similar drops in electron flow around Saturn, pointing to some extra rings around the planet that also remain unseen. That study was led by Elias Roussos of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, who also participated in the Rhea study. Normally, narrow rings require a small moon to shepherd the particles and keep them confined, but no satellites of Rhea have been spotted so far. "Whatever is going on there is a real puzzle, so ultimately there may be another explanation, but from what we have at the moment a debris disc and embedded rings are the best explanation," Jones told New Scientist.
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