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Cassini the Ring Grazer
As the Cassini spacecraft prepares to fly between Saturn and its rings, Will Gater looks at the latest results from the mission.
As most of us were getting up to go to work on 16 January this year, NASA's Cassini spacecraft was making a spectacular dive towards the rings of Saturn, some 1.6 billion km away. From high above the planet's pastel-yellow globe, Cassini's trajectory brought it racing down past the outer edge of the planet's main rings, in what the mission team are calling a 'ring-grazing' orbit. These thrilling close swoops, which draw to a close in April, in some ways mark the penultimate phase of Cassini's time at Saturn — a paradigm-shifting exploration that began over a decade ago and which will end in September this year when the spacecraft will be crashed into the gas giant's atmosphere. But the ring-grazing orbits are also evidence of how the Cassini team intends to squeeze every last drop of science from the veteran spacecraft, all the while capturing imagery of breathtaking detail.
One such image, of the tiny moon Daphnis, was captured by Cassini's cameras during the probe's close pass of the rings on 16 January. Daphnis is just 8km wide and looks, like many small moons throughout the Solar System do, like a pockmarked potato. Unlike Saturn's larger moons — Titan, Rhea and Dione, for example — it actually orbits within the planet's main rings, close to the outer periphery of the so-called A ring. And its presence there profoundly influences its surroundings.
Daphnis's most obvious impact on the rings is a thin parting that it has created in the icy material. The 'Keeler Gap', as it's known, spans a mere 42km and extends all the way around the A ring. But to see Daphnis's most striking creations one needs to look a little closer to the moon itself.
Either side of Daphnis, on the diagonally opposing edges of the Keeler Gap, the ring material has been swept into exquisite wave-like structures. Cassini's scientists have been scrutinising these extraordinary features from afar for years, but the spacecraft's ring-grazing orbit on 16 January afforded them their finest — and closest — sighting of the mission so far.
Many rings, one disc
To understand what's going on in the new image we first need to briefly explore the physics of the rings themselves — a system that is composed of countless individual objects. "The particles in Saturn's rings range from marble-sized to house size," says Matthew Tiscareno, a Cassini scientist based at the SETI Institute in California, US.
And while the major sections of Saturn's famous ring system certainly have their own designations — the 'A ring', the 'B ring' and so on — it's best not to think of them as a collection of rings.
"This is one of my pet peeves. It's a very common misconception," explains Tiscareno. "There are very few gaps that would separate one ring from another. Instead you should think of it more as a broad disc. But each part of the disc is orbiting Saturn at a different rate."
This motion plays an important role in the creation of the Daphnis wave structures seen in Cassini's recent image. In fact, researchers use modified equations relating to fluids to examine the physics of Saturn's rings, says Jeff Cuzzi, a Cassini scientist and ring expert at NASA's Ames Research Center in California. "[At] the top of the image the ring particles are moving towards the right the fastest. Daphnis is going a little slower. And then the material at the bottom is going the slowest," he says. "You can think of this material at the bottom as flowing past Daphnis from right to left".
It's when the ring particles drift by Daphnis that the gravity of the small moon leaves its mark. "As it goes by, it experiences a gravitational pull towards Daphnis," explains Cuzzi. "That distorts the orbits [of the ring particles] pulling them up towards Daphnis."
The end result is a series of beautiful wave-like peaks trailing the moon, two of which are seen in remarkable detail in the Cassini image shown left. Multiple waves are created — and there are even more out of shot — because, as Daphnis goes around Saturn, the slower orbiting material it has disrupted at the outer edge of the Keeler Gap lags behind the moon in its orbit; Daphnis thus has a constant stream of unperturbed edge material parading past it that it can repeat the 'rippling' process on. "So in that second wave to the left of Daphnis [are] the particles that had encountered Daphnis, just like in that first wave one orbit ago," says Cuzzi.
Daphnis's gravity also creates waves on the inner edge of the Keeler Gap. But, because the material there is orbiting faster than the moon, the waves extend in the opposite direction to those on the outer edge. In the same image you can see that, to the right of Daphnis, the ring particles on the inner edge have been subtly deflected. This is the onset of one of the inner-edge waves.
Cuzzi says there's an Earthly analogy for this remarkable interaction between Daphnis and the edge of the Keeler Gap. "Think about a river going by and there's a rock in the river. As the river goes by the rock, the water flows up and down and you get this ripple downstream of the rock. This is exactly what we're seeing here," he explains. "In the river the ripple is always fixed to the rock, that is there's always a ripple sitting right behind that rock, but the actual water molecules are moving right through that ripple."
Although it's tricky to get a sense of it in Cassini's latest picture, the wavy ripples that Daphnis creates are in fact three-dimensional features. "Daphnis actually has an orbit that's slightly inclined so it kind of slowly moves up and down relative to the rings," explains Cuzzi. "As it does this these perturbations that it causes on the edges are actually flipped up vertically." Indeed previous long-range images of Daphnis taken by Cassini — when the ring system was lit nearly side-on by the Sun — have shown the waves throwing shadows across the icy material below.
Tiny Moon, Huge Influence
What's abundantly clear from Cassini's new image is that even a diminutive moon like Daphnis can have a dramatic effect on the rings. Yet there are even smaller inhabitants of the rings that Cassini's recent orbits have been revealing in exceptional detail. And though these objects may be tiny, and their interactions with the ring system less obvious, they still could have an important story to tell us.
As Cassini was making another one of its ring-grazing orbits on 18 December last year, it turned its wide-angle camera towards a section of the A ring. The image it captured revealed a blizzard of artefacts from radiation and cosmic rays striking the camera's sensor. But it was the subtle features that the picture also revealed embedded within the immense, striated, swathe of icy material that were of interest to Cassini's scientists. Across much of the frame were numerous small, bright streaks within the rings — features known as 'propellers'. Cassini has been scrutinising propeller features in the rings ever since it first spotted them during the early phases of its time at Saturn, says Tiscareno. They come in two types, essentially large ones and small ones. "These are the smaller ones," he says. "We call this part of the ring the 'propeller belts'. They're just swarming here."
The bright streak of the propeller itself is caused by the gravity of a tiny, icy, moonlet disturbing the material around it. "You should probably think of the moonlet as a snowball about the size of a football pitch [roughly 100m]," says Tiscareno. There's even something of a connection between the propellers and their fellow ring-inhabitant Daphnis. "The propellers here and the gap that Daphnis is orbiting in are fundamentally the same thing," explains Tiscareno. "The only thing is that with these propellers [the moonlet] tries to start excavating a gap in the ring, but the ring is so massive that it fills the gap back in before it is able to extend all the way around."
The December 18th image represents Cassini's finest view yet of the smaller propellers. But Tiscareno and his colleagues have also been using the close ring-grazing orbits to capture spectacular pictures of some of the larger propellers — those that are thought to be created by slightly more substantial icy moonlets. On February 21st the spacecraft imaged one such example informally dubbed 'Santos-Dumont' by the mission team. The image is shown above; although it does not reveal the moonlet itself, it shows fine detail in the 'blades' of the propeller structure that the moonlet has made within the rings.
"This propeller is one of about a half-dozen whose orbits we know well enough that we had the ability to target them with flyby imaging, and it is one that turned out to be passing relatively close by during this particular flyby of the Cassini spacecraft," says Tiscareno. "The central moonlet is the size of a city block [around 500-1,000m], and the disturbance it creates in the rings can stretch for a few thousand kilometers, though it's only a few kilometers wide."
What is it, then, that studying the detailed nature of these features can tell researchers? Why might the Cassini team be using this time in the mission's final months to capture images like those of Santos-Dumont and the propeller belts? Part of the answer is that the propellers could illuminate our understanding of an enigmatic process that we have much to learn about. "It opens a window onto how planetary systems form because, when you have a baby planet forming itself out of the disc around [a] nascent star, it's a very similar situation to this moonlet that's embedded in the disc of Saturn's rings," explains Tiscareno.
It's not just with the propellers that Cassini's ring-grazing orbits are offering broader insights either. One recent high-resolution image reveals what Cassini scientists call 'straw' — conglomerations of icy material that have gathered to form huge clumpy structures within the rings. Examining this 'straw' in detail could shed light on how icy rubble 'sticks' together, which in turn could tell us something about planet formation says Cuzzi. "There are lots of things, big-picture problems, that we are understanding better by looking at the rings." he adds.
Edging towards the end
On April 22nd, once Cassini has completed its ring-grazing orbits, it will switch to a final set of trajectories that the mission team have dubbed the 'Grand Finale' orbits. These will take the spacecraft between the inner edge of the ring system and Saturn itself, with the last orbit hurtling the spacecraft into the planet's atmosphere. As the probe loops around the planet, Cassini will still be gathering data of immense interest to researchers back on Earth. "We're going to be directly measuring the mass of the rings," says Tiscareno. "That will help us distinguish between different models that we have for the origin and operation of the rings and might give us more clarity on how old the whole ring system is."
Cassini will also acquire unprecedented radar observations of the ring material. And its dust instrument will analyse the particles' chemical composition says Cuzzi. "So, finally, we'll be able to answer the big question that we've always had: why are the rings red," he says. "They're actually not white, like pure ice should be, they're actually a little red and we really don't know why that is."
Cassini's grand finale promises to be a period of intense excitement tinged with inevitable sadness then. But perhaps it's Cuzzi who best sums up the spirit for the weeks and months ahead: "We're definitely not done yet," he says.
Cassini's Orbital Timeline
October 1997 — Cassini launches from Cape Canaveral in Florida, US.
July 2004 — The spacecraft enters into orbit around Saturn.
June 2008 — The probe finishes its primary mission. It moves into a new set of orbits for Saturn's equinox in summer 2009.
September 2010 — The Equinox Mission complete, Cassini starts its Solstice Mission orbits, many of which take it far from Saturn.
November 2016 — Cassini starts its series of close 'ring-grazing' orbits. 22 April 2017 - The 'Grand Finale' trajectories will begin; Cassini will dive between the inner edge of the rings and Saturn itself.
15 September 2017 — The mission will come to an end as Cassini enters Saturn's atmosphere.
ABOUT THE WRITER
Will Gater is an astronomy journalist and presenter. Follow him on Twitter at @willgater.
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