A spiral of powder-sized particles winds around Saturn鈥檚 outermost ring, according to new observations with the Cassini spacecraft. The new feature may be caused by small moons ploughing through the ring, but scientists are at a loss to explain the process.
快猫短视频s led by Sebastien Charnoz of the University of Paris in France have used Cassini鈥檚 Imaging Science Subsystem to identify the spiral around the F ring, which lies beyond the planet鈥檚 main ring system. It curls around and into the ring, and stretches about 400 kilometres to either side of the ring鈥檚 outer edge.
鈥淲e鈥檝e never seen anything like this before,鈥� says Carolyn Porco, an imaging team member at the Space Science Institute in Boulder, Colorado, US. The spiral structure appears to be associated with clumps of matter seen occasionally in the ring and in particular with one small moon, called S/2004 S6, that has collided with the ring twice over the last year or so on its orbit around Saturn.
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鈥淚t is very possible that the spiral is a consequence of moons crossing the F ring and spreading particles around,鈥� says Charnoz. 鈥淚t may be telling us that the F ring might be a very unstable, or even an ephemeral, structure.鈥�
Prometheus in passing
The observation is surprising because a moon about 1000 times more massive than S6 is required to fling the particles out to the distance of the spiral using its gravity alone. But Charnoz says S6 may still be able to scatter the particles outward by simply bumping into them.
鈥淲e have a suspect, but we don鈥檛 have the weapon,鈥� Charnoz told 快猫短视频. Once free of the ring, however, Saturn鈥檚 gravity is thought to sculpt the particles into the spiral shape as the motes nearer the planet move more quickly than those farther away.
Astronomers are also puzzled over how the little moon S6 survives these brush-ups. That is because the F ring straddles the planet鈥檚 so-called Roche limit. This is the gravitational threshold outside of which a celestial body 鈥� such as a moon 鈥� will coalesce, but inside of which potential constituents of a body will collapse and contribute to a planet鈥檚 rings.
Small objects that cross the Roche limit tend to have a difficult time holding themselves together. The F ring is in a region with a split personality, says Porco. 鈥淪omething colliding with the ring should be ripped apart, but S6 isn鈥檛.鈥�
She described the research into the spiral on Monday at a meeting of the American Astronomical Society鈥檚 Division of Planetary Sciences in Cambridge, UK.
Warm and fluffy
Another team of researchers has found a mystery in Saturn鈥檚 largest rings 鈥� the A, B, and C rings. 快猫短视频s had expected the particles to collide, spinning at different rates and showing a relatively even temperature. But Cassini鈥檚 composite infrared spectrometer (CIRS) has revealed the particles can vary by as much as 15掳C.
They are warmer when Cassini viewed the rings鈥� day-lit side but cooler when Cassini viewed them from behind, where the ring particles were shaded from the Sun. The observations, by NASA鈥檚 CIRS team at the Goddard Space Flight Center in Maryland, US, suggest the particles rotate slowly enough to cool off when they are not directly heated 鈥� a rate that may mirror Saturn鈥檚 own rotation of about once every 10 hours.
This hints that the larger particles may always keep one side facing Saturn 鈥� a phenomenon seen in much larger objects, such as the Moon, which always shows the same face to the Earth. It is not clear what causes the slow rotation, but the observation does suggest the particles are fluffy rather than solid like ice cubes. That is because they seem to be able to heat up and cool off quickly, even with a range of spin rates.
Cassini: Mission to Saturn 鈥� Learn more in our continually updated .
