Tag: rings

Saturn's shadow slices the rings

By Phil Plait | August 29, 2012 6:30 am

There is a whole lot of awesome in a picture of Saturn and its rings just released from the Cassini spacecraft. Check this out:

Mmmmm, ringalicious.

Cassini was about 2 million kilometers (1.2 million miles) from Saturn when it took this picture, so we’re seeing a decently wide-angle view. At the time, the spacecraft was below the plane of the rings, looking north (up, if you like). The Sun is off mostly to the left and up a bit.

The first cool thing is obviously the shadow of the planet itself cast on the rings. It cuts across like a black scythe! As I looked at the picture my eyes and brain kept trying to fill in the missing arc of rings, which was amplified by a slight afterimage as my eyes moved around. It’s a difficult illusion to ignore.

Second, I love how you can see all the different rings in the picture, including the thin, lumpy F-ring outside the main band. The big gap is called the Cassini Division; it’s not really an empty space since there are many faint thin rings inside it. They’re just hard to see here. The Cassini Division is fairly easy to spot even through a small telescope, looking from Earth like someone took a knife to the rings and sliced them.

Third, you can see the tiny moons Janus (below the rings on the left) and Epimetheus (above the rings on the left) as well. I wonder how hard it is to get a picture like this without seeing any moons in it? Saturn has quite the fleet of them.

Fourth, look to the left, just where the inner arc of the rings cuts across Saturn. You can see the planet right through the rings! The rings aren’t solid; they’re composed of gazillions of particles of nearly pure water ice. There are spaces between the particles, so we can partially see through them, like looking through a screened window.

Fifth, and perhaps most cool of all: the part of Saturn we’re seeing here is the night side, entirely unlit by the Sun. The bottom (southern) part of Saturn is only noticeable by its absence! But what’s that glow in the north?

That, my friends, is ringshine! Although this part of Saturn is in nighttime, the Sun is still shining on the rings (wherever you don’t see Saturn’s shadow across them). The ring particles are very bright and shiny. They reflect the sunlight, which then illuminates the northern hemisphere of Saturn. The southern half is still dark because the ice particles tend to reflect light back up, like a mirror. Since the Sun is coming from the north, that’s the way the light gets reflected. I’ll note that most of the light gets reflected away from Saturn, to the upper right in this picture, but enough is reflected back to make the cloud tops glow softly.

This happens on Earth too, when sunlight reflects off the Earth and illuminates the dark part of the Moon. This is called Earthshine, also poetically called "the old Moon in the new Moon’s arms." It’s quite lovely.

And it’s science! Which is lovely, too.

Image credit: NASA/JPL-Caltech/Space Science Institute

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The ups and downs of Saturn pictures

By Phil Plait | June 11, 2012 1:00 pm

When I look at Cassini images of Saturn — with its multitude of rings and fleet of moons — I am inspired, moved, and even awed.

And sometimes I laugh. When I saw this image, for example, I actually chuckled to myself. Why?

[Click to encronosenate.]

This gorgeous shot was taken on December 30, 2011 and released just today as the Cassini Image of the Week. It shows Saturn’s gorgeous rings seen nearly edge on, and the tiny moon Epimetheus, only 113 kilometers in diameter, next to them.

It’s a lovely image to be sure, and my very first thought was; I wonder if Epimetheus is closer to us than the rings, or farther away? If we’re looking down on the rings, from the north, then Epimetheus is closer to us. But if we’re looking up from underneath the rings, Epimetheus is on the other side of the rings. I could mentally switch my perspective back and forth, but I couldn’t tell which view is correct! This prompted my chuckle, as I wryly smiled at my brain’s confusion (I love optical illusions).

So take another look: are we looking down on the rings, or up? Hint: the Sun is shining from the north, down on the rings.

It’s a bit of a conundrum, isn’t it? Just by looking it’s almost impossible to figure out! If you’re familiar with Cassini pictures, the rings look subtly different if they are illuminated from above and you’re looking at them from underneath, and vice-versa. But it’s hard to tell. And to be honest, I wouldn’t have known without reading the caption for the image.

The answer is we’re looking up. The Sun is shining down on the top of the rings, and we’re looking up from underneath, putting wee Epimetheus about 1.5 million kilometers (900,000 miles) from Cassini when this picture was taken. If it helps, hold up something round like a DVD and look at it from underneath. As another helpful guide: in the image above, the part of the rings at the top of the picture are closest to you, the bottom farther away, and Epimetheus father still.

And I bet that even knowing that, some of you are having a hard time picturing it. Our brains are funny things, easily fooled when there’s symmetry in a picture, especially when that picture shows an unfamiliar object. I’m sure Carolyn Porco can just glance at something like this and figure out everything she needs to understand the geometry! I’m not so sure I could’ve.

Remember: seeing isn’t always believing. It’s easy to fool our eyes and brain, but in the end the Universe knows what it’s doing.

Image credit: NASA/JPL-Caltech/Space Science Institute

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Ice moon, ghost moon

By Phil Plait | May 10, 2012 8:55 am

One of my favorite things to do is take a gorgeous astronomical image and pierce down into it, finding some detail not discussed in press releases and other articles.

On the other hand, sometimes I’ll post a picture because it’s so, so cool:

[Click to encronosenate.]

That’s a shot of Saturn’s rings and moons by the Cassini spacecraft, taken in mid-April 2012. Cassini was nearly in the same plane as the rings, so they look like a knife cutting across the image. The bright moon is Enceladus, tiny and icy, almost but not quite full as seen from this angle.

But the scene stealer is Titan, the moon as big as a planet — bigger than Mercury, actually — looming in the background, nearly invisible. This image, taken using a filter that only lets through green light, shows just how much darker Titan is than Enceladus. The bigger moon is shrouded in a thick, hazy atmosphere, and reflects about 1/5th of the sunlight that falls on it. Enceladus, on the other hand, is covered in ice, and reflects nearly all the light that falls on it. So the brightness ratio you see here is real: Titan really is far darker then Enceladus.

… and there you go. I drilled down a bit into the picture’s science anyway. I guess I had to. It’s in my nature; when it comes to science, I’m reflective too.

Image credit: NASA/JPL-Caltech/Space Science Institute

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Exoplanet news Part 2: Exosaturn!

By Phil Plait | January 12, 2012 10:03 am

[I’m trying to catch up with all the news that’s been released this week while I was off lecturing in Texas. This is Part 2 of a few articles just about exoplanets. Part 1 was posted earlier.]

Astronomers have found one of the most interesting exoplanets yet: one with a very extended ring system!

[That’s an artist’s impression of the system; click to encronosate.]

The planet was discovered with the SuperWASP (Wide Angle Search for Planets) telescopes — a UK project that employs low-magnification but very sensitive cameras which can observe large areas of the sky at the same time. It orbits a young star called 1SWASP J140747.93-394542.6, which is 420 light years away. The star’s youth — 16 million years — indicates that the rings are probably the leftover remnants from when the planet formed.

The planet and its rings were discovered using the transit method: looking for small dips in starlight as a planet passes directly between us and the star. This is how the vast majority of exoplanets are found. Usually, when you graph the brightness of the star over time, the dip in the plot as the planet transits the star starts suddenly, drops to some minimum, then jumps back up (see here for example). The whole thing is usually over in a matter of hours at most.

But this planet took nearly two months to transit the star! And the dip was weird: there were multiple times the star dimmed then got brighter again, at one point having 95% of its light blocked. Even though the planet wasn’t seen directly, the most obvious explanation is a ring system similar to Saturn’s (though much larger), blocking the light. It must have gaps in the rings, like Saturn’s do, to explain the starlight jumping up again over time. Overall, four rings were detected, and they stretch tens of millions kilometers in diameter!

Saturn’s ring are only about 300,000 km across, so clearly this planet must be much more massive than Saturn, and the rings denser. It may be a little unfair to compare it to Saturn at all; it’s more like a super-Jupiter still surrounded by primordial debris. Unfortunately, we don’t know how massive the planet itself is; you need Doppler data for that and none has been taken yet. The astronomers who discovered this system, of course, are looking into obtaining Doppler data. It’s even possible the object is so large it’s actually a brown dwarf and not a planet.

Perhaps most intriguing about all this are those gaps in the rings. The easiest way to explain them is that there are objects there, moons, sweeping out the material in the rings. Saturn’s rings have gaps for this reason. In fact, there are hundreds of gaps in Saturn’s rings! These are caused by resonances: if a ring particle orbits twice for every one time a moon orbits, for example, the moon’s gravity tugs on it every time it swings by, pulling it into a different orbit. Over time, all the particles in that orbit are gone, leaving behind a gap.

If the planet itself is big, how big are those moons? Could one be Earth-sized? It’s an idea that’s been around awhile, but none has ever been seen… yet. All these super-Jupiters being found have a lot of gravity, and it’s possible they have big moons. We’re also getting better at detecting smaller objects, so it wouldn’t surprise me if that announcement is made sometime relatively soon, too!

I’ll note that the idea of looking for rings and moons is more than an idea: the Hubble observations of the star HD 209458 I mentioned the other day were taken to look for moons and rings around that planet! None were seen, but astronomers will keep trying. There are a lot of planets out there, and one thing we’ve learned is that variety is the spice of nature.

Image credit: Michael Osadciw/University of Rochester


Enceladus fires on Alderaan

By Phil Plait | November 8, 2011 6:50 am

After 7+ years of orbiting Saturn, the Cassini spacecraft is still knockin’ ’em out of the park. On October 19 it swung by the icy moon Enceladus, taking dozens of amazing images of the little world. When it was 65,000 km (40,000 miles) away, it took this shot, right out of Star Wars:

[Click to DeathStarenate.]

That’s not a laser blasting out from the moon; it’s actually Saturn’s rings seen in the background! At the moment this was taken, Cassini was nearly in the same plane as the huge ring system circling Saturn, and so they’re seen edge-on, looking like a bright line. The Sun is off to the right, illuminating Enceladus, which from this angle makes it a lovely crescent. Normally, Enceladus looks really bright since its surface is icy and very reflective, but in this case the short exposure downplays its shininess.

The rings, though, are made of small particles, and when illuminated tend to throw that light forward (much like a wet road surface reflects oncoming headlights very strongly toward you). The sunlight scatters off the rings and gets bounced at the camera, making them look very bright.

As Cassini orbits Saturn its position relative to the moons and rings changes, and so within a short period of time of the show above it got this one, inset here, where the geometry is slightly different. I think we’re looking up from underneath the rings, so the bottom line is the far side of the rings, the upper line the near side, and then Enceladus. Both arcs and the moon are in the same plane, so if you picture it this is the only way this lineup makes sense! This image also shows a different view.

I found those images going through the Cassini raw images archive. I found something else, too… but that’ll have to wait for the next post. Stay Tuned!

Image Credit: NASA/JPL/Space Science Institute

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A panoply of moons and rings

By Phil Plait | October 24, 2011 4:43 pm

Take four moons, some rings, a schoolbus-sized spacecraft, and mix them together. What do you get?


That stunning shot is from the Cassini spacecraft orbiting Saturn. The big moon is Titan, and by big, I mean bigger than the planet Mercury. Big enough to have a thick nitrogen atmosphere, clearly visible in this picture. The bright moon superposed right on top of Titan is Dione, its icy surface shiny and white.

On the right, just outside the rings, is tiny, flying saucer-shaped Pandora. And the fourth moon? That’s Pan, the tiny white spot in the gap in the rings on the left, barely visible in this shot. But that’s understandable, since Pan is less than 30 km (18 miles) across, and this was taken from a distance of nearly 2 million kilometers (1.2 million miles) away!

I love pictures like this; they remind me that even after 7 years of Cassini touring around Saturn, there’s still much to see and much beauty to behold there.

Image credit: NASA/JPL-Caltech/SSI

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Saturn's rings cast long shadows

By Phil Plait | October 3, 2011 3:51 pm

On Earth, one way to watch the march of the seasons is to look for lengthening shadows as winter approaches. As the Sun gets lower in the sky every day, objects like trees and buildings cast longer shadows.

On Saturn, there are no trees and buildings. There’s not even a surface! So what do you do?

Look to the rings:

[Click to encronosenate.]

This image, taken by the Cassini spacecraft on August 22, 2011, shows the shadows of Saturn’s magnificent ring system on the tops of the planet’s cloud layers. Right now, Saturn is tipping its north pole toward the Sun, so summer is on its way there. But that means winter’s approaching in the southern hemisphere, and shadows get longer.

Saturn’s rings aren’t solid, but composed of countless particles of ice, probably no bigger than a few meters across at most, and the vast majority much smaller. There’s also not just one big ring, but hundreds of thinner ones; from Earth, using small telescopes, they blur into what look like a handful of single rings. Big telescopes hint at the truth, but when you send an actual space probe there you see all those rings in their individual glory.

Here, Cassini was almost in the plane of the rings, so direct details are scarce. But with Sun shining down on them, the rings’ shadows make it very clear that when it comes to Saturn, there’s just no substitute for being there.

Image credit: NASA/JPL-Caltech/SSI

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MORE ABOUT: Cassini, rings, Saturn

Saturn gets edgy

By Phil Plait | September 16, 2011 6:31 am

Observing Saturn through even a small telescope is amazing. The rings are so obvious and clear that sometimes, when I would show people the planet through my own ‘scope, they thought I was faking the view! But it really is that easy to see them.

Well, usually. Saturn, like the Earth, is tilted. That is, if you imagine Saturn orbiting the Sun, the north pole doesn’t point straight up, perpendicular to the orbit. Instead it’s tilted by about 27° (Earth is tilted by about 23° for comparison). What that means is that as Saturn circles the Sun we get a different viewing angle on the rings; sometimes we look down on them, sometimes up at them, and sometimes they are perfectly edge on.

"Amateur" astronomer Alan Friedman — who has taken some incredible pictures of the Sun that have graced this blog — took a series of images of Saturn over several years, and put them together in a very cool animation that shows our changing view of the ringed planet:

Pretty nifty [click to encronosenate]! The images come from observations Alan made over the course of 2004 to fall 2009, when the orbits of Saturn and Earth lined up to bring us through the ring plane. Saturn’s rings are incredibly thin, so they appear as an almost invisible line. Here are the individual images from the animation arranged in a montage:

[Again, click to embiggen.] You can really see some great details, including banding of the clouds on the planet, and the Cassini Division in the rings, a gap carved out by gravitational interactions with Saturn’s Death Star moon Mimas.

Right now, as the dance of the planets continues, Saturn is getting closer to the Sun in the sky, making it difficult to observe. In a few months though it’ll reappear on the other side of the Sun, rising in the early morning. But that’s OK, because in the meantime Jupiter is positioning itself in the east shortly after sunset, and is up all night for your perusal. Even a pair of binoculars will reveal moons and stripes of clouds on its surface. And while it takes patience to get a series of images of Saturn like Alan’s above, the moons of Jupiter can be seen to move in just a few hours. If you have clear skies, go take a look! There’s always something to see.

Image credit: Alan Friedman, used by permission.

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A trillion and five moons

By Phil Plait | September 13, 2011 9:45 am

On July 29, 2011, the unending dance of Saturn’s moons lined them up perfectly for a stunning view by the Cassini spacecraft: five moons arrayed for your pleasure:

[Click to embiggen.]

From left to right that’s Janus, Pandora (in the rings), Enceladus, Mimas, and Rhea. Perspective plays a role here; Rhea is three times bigger than Enceladus, but was much closer to Cassini when this picture was taken, so it looks even bigger.

But the moons themselves are so different from each other! Janus is a lump, too small to have enough gravity to crush itself into a sphere. Enceladus is mostly ice, so it appears very bright in this image compared to its rocky siblings. You can just barely see part of the monster crater Herschel peeking out of the dark side of Mimas, while Rhea is peppered with smaller craters. And Pandora orbits inside Saturn’s rings themselves, its meager gravity enough to entrain the particles in the thin F ring and keep it in place.

And, of course, the rings themselves, composed of countless tiny ice crystals. Over millions of years, collisions have ground them into pieces ranging in size from barely big enough to see to perhaps 10 meters across, the volume of a roomy two-car garage.

Amazing. And this vista was taken just a couple of weeks after Cassini’s seventh anniversary in orbit around Saturn. Even after all that time, and tens of thousands of images, it still has the capability to take our breath away.

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Titanic slice

By Phil Plait | June 28, 2011 11:00 am

A quirk of physics can lead to some real drama. Two quirks of physics can lead to very dramatic pictures.

"Why Phil, what could you possibly mean?", I hear you thinking*.

This is what I mean: Saturn’s moon Titan, sliced in twain by the planet’s rings:

[Click to enchronosenate.]

Due to a quirk of physics (aha!) all the moons and rings of Saturn orbit the big planet in the same plane. There are two reasons for this: one is that they almost all formed out of the same disk of material orbiting the Sun. As the pieces clumped together, they naturally all stuck to the same plane. A second reason is that any object that tries to stray out of that plane (or that gets captured by Saturn like some of its bizarre outer moons) feels a torque on it, forcing it back down. That torque is provided by Saturn itself, which has tides that tend to circularize and flatten all the orbits of the moons, confining them to the planet’s equatorial plane. The physics of this is a little hairy, but I do have a simplified explanation using the Earth and Moon as an example.

The Cassini spacecraft orbits Saturn, but it has rockets on board that can push it around into any orbit the engineers and scientists back home want. That usually means a tilted path, plunging it through the equatorial plane twice each orbit. When it is precisely in that plane it sees the rings edge-on, and if the geometry is right it can spot one or more moons.

In this case, the gigantic moon Titan was in its sights, and the angle was such that the rings slice right across it.

Titan is so big it has a substantial atmosphere, murky and thick, which hides its surface. So why do we see details on the moon in this picture?

Due to a second quirk of physics (aha aha) infrared light can penetrate Titan’s soupy air. Different surface features (like lakes of liquid methane and ethane!) emit and reflect different amounts of IR light, and that goes up through the clouds and into Cassini’s detectors, which were cleverly designed to detect that specific wavelength of light.

And that, you see, is how some nifty physics can provide you with a very dramatic picture. Well, that and a few hundred million dollars, a team of very smart people, and a decade or two to design, build, and get your spacecraft to its target.

… which, again, involves an awful lot of physics. So really, if you want to see just how spectacular and awesome the universe is, science is the way to go. We deliver.

Image credit: Credit: NASA/JPL/Space Science Institute

* All professional bloggers can read minds. It’s how we know exactly what to say to tick off the largest fraction of our readers.

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