Tag Archives: Cassini

Pan, the Space Ravioli

Seriously, that’s what Saturn’s moon Pan looks 

Cassini has just made the closest ever pass to Pan, one of Saturn’s shepherd moons.  It orbits within (and basically creates) the Encke Gap in Saturn’s rings.  The wide flat band of material around its equator is material accreted from the rings themselves.  The best part, though, is that the Cassini imaging team at JPL put together this neat animated GIF collecting all of the raw images from this particular imaging sequence, and its glorious:

 

It’s bittersweet, knowing that these close orbits are dooming Cassini to eventually fall into the giant planet, but we’d never get these incredible images otherwise, images of the greatest natural lab we’ve ever found for studying the way a planetary system can form.

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Daphnis, the ring-sculptor

Strictly speaking, Daphnis is one of many ring-sculptors — the shepherd moons that maintain some of the notable visual features of the rings of Saturn.  In the case of Daphnis, that’s the Keeler Gap, a narrow gap carved out the A ring by the gentle gravitational tug of war between Saturn and the tiny moon.

Daphnis was actually discovered by the Cassini probe in 2005, but its existence was suspected long before, when the Keeler Gap was itself discovered in images taken by the twin Voyager probes.  Voyager had already discovered the moons Prometheus and Pandora, which the Voyager team dubbed “shepherd moons” for the way their push and tug confined a group of particles to the very narrow F-ring.  So it was surmised that the other gaps would turn out to have moons creating them as well. Mimas seems to be responsible, at least in part, for both the Cassini Division and the Huygens Gap, and of course Prometheus and Pandora constrain the F ring.  Pan, discovered in 1990 from old Voyager data, is likely responsible for the Encke Gap.  And Daphnis is the sculptor of the Keeler Gap.

Or, at least, the main sculptor.  Saturn’s rings are very complex, and serve as a fascinating natural laboratory for studying gravitational interactions, and particularly the sort hypothesized to have created the solar system as we know it.  And Daphnis, like other shepherd moons, does not orbit perfectly neatly.  Its orbit is slightly inclined relative to the ringplane, and slightly elliptical as well.  Thus, it doesn’t produce a nice tidy circle, but carves out waves as it passes — waves both ahead and behind, and, as this recent Cassini image shows, sometimes it pulls off delicate tendrils of ring particles (look verrrrry closely, or just click to enlarge – you’ll see a thin wisp of material echoing the shape of the nearby wave in the ring):

That’s the closest image ever taken of Daphnis, a tiny moon roughly the same size as Mount Everest.  It appears to have striations running down its length, probably the result of accumulated ring particles — sometimes, even a tiny moon like this will manage to capture something and pull it down.  But if you want a more dramatic image of this effect, you will have to look at Daphnis near Saturn’s equinox, when the shadows are at their longest.  Then you can see what is hidden in this image: the waves aren’t just flat features.  They stand surprisingly tall.

By studying this process, scientists hope to better understand planetary formation.  Indeed, they’ve even found a few spots in Saturn’s rings where it appears that moonlets may be in the process of forming, clumping together at random until eventually one clump reaches a critical mass and begins to dominate the particles around it, gradually growing until it exhausts its immediate surroundings, carving out another gap.  Daphis itself shows signs that it may be accumulating material.  Saturn’s rings are an astonishingly and fascinatingly dynamic place.

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Cassini’s ring dives have begun

With its last flyby of the giant moon Titan, Cassini grabbed a gravity assist and slipped into an orbit that ventures closer to Saturn than any previous orbit.  This is the final phase of the Cassini mission, focusing on Saturn’s rings and the enormous gas giant itself.  This final mission begins with 22 orbits dipping almost to the F ring.  These orbits are much smaller than previous orbits, so they go much more quickly; the orbit will be adjusted again in April to set up another 22 orbits, these dipping inside the rings.  Those orbits will wrap up in September of 2017, when Cassini will plunge into Saturn itself.  It’s hard to believe it, but by that time, Cassini will have been in space for nearly 20 years.

Some of the first images from Cassini’s latest, closest periapsis (perikrone?) have been released.  Here’s the mysterious Hexagon from Saturn’s north polar region:

And here’s one of many raw ring images sent back, this one acquired December 5:

Stay tuned for spectacular new images as Cassini moves ever closer!

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Watching the Methane Clouds Go By – on Titan

I know here on Earth there’s an election, and I spoke yesterday about how important it is to vote.  Well, today I want to get away from all that.  So here’s something oddly peaceful, with a rather lovely soundtrack.  It’s a repeating clip of methane clouds drifting idly by on Titan.

I find something strangely pleasing, knowing that we have the ability to film things like this today.  We have a presence in the outer solar system.  Whatever else may happen, we can say that <i>we have seen amazing things</i>.

 

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Saturn’s Hexagon has changed color

One of the most remarkable advantages of having an orbiter at Saturn is that we are able to record and study changes in it over time.  Back during the Pioneer and Voyager programs, it was all too tempting to think that these fleeting glimpses we got of the gas giants were representative of them always.  But of course it wasn’t.  Even a barren world like Mars changes over human timescales; a gas giant is essentially *made* of weather and so should change constantly.  Perhaps it’s because Saturn looks like such a smooth set of bands from Earth, or perhaps we have misled ourselves by thinking extremely long-term storms like the Great Red Spot are common.  But just as Galileo and the Hubble Space Telescope and increasingly acute ground-based observatories have shown dramatic change in the GRS and the rest of Jupiter’s storms, Cassini is now doing the same for Saturn.  Saturn has an axial tilt more like Earth, so it experiences significant seasonal variation, and this has created intriguing variety over time.  One of the things only Cassini can tell us about is the Hexagon, a vast structure across Saturn’s north pole.  From Earth, we cannot see structures at high latitudes; we just don’t have the right viewing angle.  But Voyager 1 saw the hexagon, and now Cassini has been watching it.  And although it’s clearly stable over long timescales, it isn’t static.  In fact, it’s color has changed rather dramatically:

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No one yet knows why, although going theories involve a shift in the distribution of methane due to changing levels of sunlight.  (Sunlight accelerates the breakdown of methane.)  We will not have Cassini around to watch the Hexagon for much longer, unfortunately, so scientists are working to gather whatever they can during the remainder of Cassini’s mission.

The next interesting thing to look for will be Juno: it’s the first mission ever to have a good viewing angle on Jupiter’s polar regions.  What will it find?  So far, we know Jupiter does not have a hexagon, and that the polar regions look very different from the mid-latitudes.  Less stripey, for one thing.  But that’s all we know so far.

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Being very busy with work, enjoy this view of Saturn

Work is very very busy right now; I’m burning the midnight oil.  But I don’t want to leave this blog unattended as long as last time, so here: enjoy this sublimely mysterious view of the nightside of Saturn, the brilliantly lit but narrow rings, and Enceladus.  The Cassini team titled this one “Dark and Arc”, and it was released just over a month ago.

Dark and Arc

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11 years of Saturn photography

The Wall Street Journal downloaded the freely available raw image files of every picture Cassini has taken while at Saturn and collated them together into one gigantic video.  The pictures flicker past extremely fast, but even so, Cassini has taken a heck of a lot of pictures; the whole video is a whopping three hours and forty-eight minutes long!  This spacecraft’s-eye view really drives home how long this spacecraft has been plugging away out there.

And the mission isn’t even over yet!  They’ll have to put out a “part two” when the mission is complete.  😉  So go ahead and watch this video, or more realistically, flick through it for a bit.  You notice a few interesting things.  WARNING: the flickering as the images change is fairly intense at times; if you have epilepsy, consider carefully before viewing.

  1. All the images are black and white, but they change tone dramatically, and flicker in a very predictable pattern.  Cassini produces color images by taking the same image with its black-and-white camera several times, but with different color filters in front of it (not all of which are discernible to the human eye; Cassini has a broader range than we do).
  2. Over time, the shadow on the rings changes.  Cassini arrived near a solstice, so the rings appeared “open” from our perspective; like Earth, Saturn has a significant tilt to its rotational axis, and this means that at solstice, Saturn’s shadow doesn’t stretch all the way across the rings.  Then watch as the years go by, and the shadow lengthens, and the shadow of the rings on the face of Saturn slips further towards the equator and thins until, at solstice, the rings appear to have a chunk missing and the ringshadow is a razor-thin line across Saturn’s equator.  This is the equinox.  Then, the pattern reverses, as the other pole moves towards summer solstice and the other slips into darkness, and the shadow of Saturn recedes across the ringplane.
  3. You get an idea of the sorts of observational priorities they go through, and also get a glimpse at how Cassini’s orbit has changed through the years, as it shifts from target to target, and you can see when there are shorter or longer exposures (the longer exposures produce star trails, as Cassini pivots to keep its target firmly in focus).  Saturn grows and shrinks, though note that it does shift between the narrow-image and wide-image cameras.  You can see moons grow closer and become the focus of intense scrutiny during the flybys.  It’s an interesting look at Cassini’s eleven years in Saturn orbit.

And now, if you’ll excuse me, I have some Doctor Who to catch up on….

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