In its final year, Cassini plunged where no spacecraft had plunged before, down in the space between Saturn and its rings. Again and again it dove, for a total of 22 orbits. In the data collecting during those breakneck dives, astronomers have just found new information about the way tiny moons sculpt and carve those rings.
It is, they say, not only new evidence that Saturn’s rings are much younger than the planet, but also a window into the way planets form in the giant rings of dust and debris that circle newborn stars.
The new data, collected using four of Cassini’s instruments, show the rings in more detail than ever before.
“It’s like turning the power up one more notch on what we could see in the rings. Everyone just got a clearer view of what’s going on,” said Cassini Project Scientist Linda Spilker of JPL-NASA. “Getting that extra resolution answered many questions, but so many tantalising ones remain.”
They reveal the delicate straw-like textures and clumps within the rings, and patterns produced by the movement of the shepherd moons, such as Daphnis. Scientists have also compiled new maps of the colours, temperatures, and chemistry of the rings.
In turn, this information answers some intriguing questions. For instance, a series of streaks generated by impacts in the F ring – that’s the outermost of the main rings – are all the same length and orientation.
That implies a flock of impactors that is orbiting Saturn, not a swarm of rogue cometary debris in orbit around the Sun.
The data also ponied up some new information about Daphnis. The shepherd moon’s shenanigans in its clear lane we call the Keeler gap are already pretty well documented, but new images have revealed thin strands of ring material separating from the crests in the moon’s wake.
But not everything is enlightening. Cassini scientists spotted something they don’t yet understand: three distinct textures – smooth, clumpy, and streaky. These features occur in the rings in distinct belts, with sharp, well-defined edges.
The problem is, so far the textures cannot be linked to any feature of the rings identified to date.
“This tells us the way the rings look is not just a function of how much material there is,” said astronomer Matt Tiscareno of the SETI Institute.
“There has to be something different about the characteristics of the particles, perhaps affecting what happens when two ring particles collide and bounce off each other.
“And we don’t yet know what it is.”
There were more mysteries in the rings’ chemistry, revealed by Cassini’s Visible and Infrared Mapping Spectrometer.
In the outermost part of the A ring, the spectral map revealed an abundance of weak water ice. This was a surprise, because the region is highly reflective, which usually indicates water ice of high purity, or strong water ice. Exactly how it is so reflective is a puzzle.
And the spectral analysis also detected no methane or ammonia ice in the rings. This is also a head-scratcher, since last year scientists had found, among other organics, ammonia and methane raining down on Saturn from its innermost ring.
But that’s OK. Because, even though the probe’s mission ended nearly two years ago now, there’s a lot more Cassini data yet to be unravelled.
“We see so much more, and closer up, and we’re getting new and more interesting puzzles,” said astronomer Jeff Cuzzi of NASA.
“We are just settling into the next phase, which is building new, detailed models of ring evolution – including the new revelation from Cassini data that the rings are much younger than Saturn.”
Better hurry though. In 100 million years, those glorious rings might be entirely gone.
The research has been published in Science.