Saturn’s rings perplex astronomers – again


The gas giant’s brightest ring is lighter – and younger – than first thought. Belinda Smith reports.


Saturn's B ring is the most opaque of the main rings, appearing almost black in this Cassini image taken from the unlit side of the ringplane. The moon Tethys, 1,062 kilometres in diameter, is visible in the extreme upper left of the image. – NASA/JPL-Caltech/Space Science Institute

With Saturn’s rings, less really does seem to be more. The brighter opaque rings, astronomers assumed, contained more matter, and were heavier. Conversely, their transparent dull counterparts were thought to be comparatively sparse.

But in a study published in the journal Icarus, planetary scientists found the big opaque ring circling the gas giant doesn’t weigh any more than the dim rings – and it has them stumped.

“At present, it’s far from clear how regions with the same amount of material can have such different opacities,” said lead author and University of Idaho planetary scientist Matthew Hedman.

Saturn’s rings were first spotted by Galileo Galilei in 1610. Since then, telescopes and spacecraft – such as the Cassini mission, which has been exploring Saturn and its moons since 2004 – have shown the rings are made of mostly icy chunks and dust.

Looking down on, and partially through, Saturn's rings from their unlit side. The B ring is in the centre, so dark as to appear almost black. – NASA/JPL/Space Science Institute

While the rings start around 6,500 kilometres from the gas giant’s equator and extend more that 120,000 kilometres into space, they’re incredibly thin – the main rings are generally only around 10 metres thick.

So Hedman and Cassini co-investigator Phil Nicholson from Cornell University analysed Cassini data to nut out the density of the largest and most opaque ring – the B ring.

Of course, it’s impossible to “weigh” the ring, so they looked at “spiral density waves” – waves created by the complex gravitational interactions from Saturn and its moons tugging on ring particles.

From these waves, they determined the mass of the ring particles – and found the opaque rings are no denser than the transparent ones.

“Appearances can be deceiving,” Nicholson said. “A good analogy is how a foggy meadow is much more opaque that a swimming pool, even though the pool is denser and contains a lot more water.”

So why bother tease out these mysteries?

Finding a ring’s density will tell planetary scientists how old it is – a “lighter” ring evolves faster that a heavier one, for instance. So the less the B ring weighs, the younger it might be. This work dials down age estimates from a billion years to a few hundred million – meaning they formed well after Saturn did.

“By ‘weighing’ the core of the B ring for the first time, this study makes a meaningful step in our quest to piece together the age and origin of Saturn’s rings,” said Linda Spilker, a Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.

“The rings are so magnificent and awe-insipring, it’s impossible to resist the mystery of how they came to be.”

The image of the rings below has been rotated to vertical to accommodate the detail.

This natural colour mosaic was taken from 10 degrees below the illuminated side of the rings. It shows shows, from left to right, (that is, bottom to top) radially outward from Saturn, the C ring (with its Colombo and Maxwell gaps); the B ring and the Cassini division beyond, with the intervening Huygens gap; the A ring (with its Encke and Keeler gaps); and, on the far right, the narrow F ring. The total span covers approximately 65,700 kilometres.

Although it is too faint to be seen here, the D ring is located just to the left of the C ring.

NASA/JPL/Space Science Institute
  1. http://dx.doi.org/10.1016/j.icarus.2016.01.007
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