Sometimes, the images from the Cassini Saturn probe are so cool it’s tempting just to post them and say, "Look at THAT!"

See what I mean? [Click to gigantesenate.]

But of course, I can’t just leave it at that. This image, taken on January 4, 2012, is a bit different than most. Sure, we see Saturn’s magnificent rings, nearly edge on from this perspective. And we’ve seen this icy moon Enceladus many, many times (see Related Posts below for tons more pictures). Look at the bottom of the moon: see those fuzzy streaks? Those are geysers of water spewing from cracks in the moon’s south pole! Cassini has been studying them intently ever since they were discovered; they are proof that liquid water exists under the surface of Enceladus, though it’s still being argued over whether it’s in pockets, like lakes, or the whole moon has an ocean of water under the surface.

Despite all that, I keep getting drawn to the crescent shape itself. We can never see that from Earth. Saturn is much farther out from the Sun than we are, and geometry demands that from home we always see these worlds nearly fully lit by the Sun. The only way to see them like this is to go there.

But also, that giant circular feature is really interesting. It’s big, maybe 200 km (over 100 miles) across, and a bit darker than the surrounding surface. I tried locating it on an atlas of Enceladus, but it wasn’t obvious at all. I thought it might be an impact basin, but a little scrounging online led me to a paper by Cassini imaging team leader Carolyn Porco, which says there are no large impact basins on Enceladus! So what is it?

Well, why not go to Dr. Porco herself? I sent her a note, and she kindly replied. That region is called Diyar Planitia, and it stands out among the surrounding terrain because it’s much smoother. It does have narrow surface features, but they’re too small to be seen at this resolution. At the low angle at which we’re seeing it here, it looks a little bit darker than the rougher terrain around it, so it’s easier to see (which is why on an atlas it’s harder to find). It is roughly circular, but that may simply be coincidence. Enceladus has been massively resurfaced, with some areas much older than others, due to various forces under the surface — looking this all up I learned a new one, called diapirism, where lower density material underneath higher density material can rise up and break through. That’s one process that’s helped change the surface of Enceladus over the eons.

That’s pretty nifty. And think about that! Today I learned of what is to me a new region of the solar system, one that has an interesting and complicated history, molded by vast forces over long-stretched times, one of which was also new to me. How wonderful to get all that from what’s otherwise just a pretty picture!

But of course, in science, there’s no such thing as just a pretty picture. Science is a tapestry, a vast complex fabric interwoven with countless threads. Each of those threads is amazing, each important, and each leads to another. And that’s where the true beauty of science lies.

As Cassini weaves its way around the multiple moons of Saturn, it’s not really a coincidence when one gets in the way of another. As a matter of fact, it’s a guarantee. These are called mutual events, and when Cassini dove past Dione, it saw this terrific view of Mimas peeking out from behind it:

Nifty, huh? [Click to encronosenate.]

Dione is nearly 3 times larger than Mimas (1100 versus 400 km wide), but Mimas was also more than 6 times farther away, making Dione loom nearly 20 times larger in this shot. I like how you can’t really see the unlit side of Dione, but Mimas marks it pretty well, sliced in half by the edge of the larger moon.

Funny, too: I was thinking to myself that if Cassini was in position to catch this shot, then it should have also caught Mimas when it was on the other side of Dione, the lit part. Well, seek and ye shall find: I searched the Cassini raw image archive and found it! I put a small version of it here; click to embiggen. You can just barely see a small segment of Saturn’s rings in the lower left corner, too.

Neat! I like it when stuff makes sense. While this alignment is rare to see from Earth — we’re a lot farther away, and the geometry has to be precise — we do see moons transiting across their parent planets, and, far less often moons in front of moons. But what’s rare to us is common to Cassini, with its front row seat to this amazing system of worlds.

One of these things is not like the others:

The Cassini spacecraft took this lovely image in December 2011, during a close pass of Saturn’s moon Dione. Ignoring Saturn’s rings slashing through the picture, we see, from left to right, the moons Dione, Prometheus, and Epimetheus. Which is the odd moon out?

Here’s a hint: Dione is 1100 km (700 miles) across, Prometheus 86 km (53 miles) along its longest axis, and Epimetheus 113 km (70 miles). Got it now?

Yeah, sure, Dione is far larger than the other two! But that’s not my point: Dione is round, while the other, smaller moons are lumpy and rather potato-shaped. Why?

Size matters. In this case, a bigger moon means more mass, and that means more gravity. In general, the force of gravity points toward the center of an object. As you add more mass to an object, gravity gets stronger. On a small moon, a big lump of rock like a mountain feels very little force downward, while on a more massive moon the force would be larger. If the moon has enough mass, and enough gravity, the force will be more than the internal strength of the rock itself, and the mountain crumbles.

So moons that are big and massive enough will tend to flatten their surface, or, more accurately, shape them into spheres. Dione is big enough to do that. Prometheus and Epimetheus are not. Dione is a big ball, the other two are spuds.

Note that gravity’s not the only thing that can make objects spherical. Water has surface tension, for example, caused by the electrostatic attraction between water molecules. In space, without gravity, drops of water are spherical. Random processes can generate round objects too: I bet if we could get a super-duper close look at Saturn’s rings, we’d see the trillions of chunks of ice that make up the rings are round too. But that’s from collisions; there are enough of those bits of ice that they smack into each other. Since they spin and tumble, over time any part of a chunk will have gotten hit by some other chunk, and that will tend to make them round.

So how big does an object have to be before it starts to become round via gravity? That’s complicated, and depends on its composition — a ball of ice the same size as a ball of iron will have far less gravity since it’s so much less dense, and will have lower mass. But for a ball of ice and rock — like Dione — that size is clearly no bigger than 1100 km across. And if you’re wondering how this might play into our concept of what a planet is, then you might want to read this. I’m way ahead of you!

Carolyn Porco, the leader of the Cassini spacecraft imaging team, tweeted about this picture last night, and it’s simply overwhelming:

This is a stunning portrait of Saturn taken by the Cassini spacecraft in December. Its beauty and fantastic — in the literal sense of being like a fantasy — cloudscape are so overwhelming you might not even notice the moon Tethys hanging just under the knife-edged rings. To give you an idea of how immense Saturn is, "tiny" Tethys is over a thousand kilometers across.

This picture was taken using an infrared filter, where details in the clouds come out. That, plus the shadow of the rings on the southern hemisphere, make this one of my favorite pictures of Saturn I’ve seen in quite some time.

A few days ago, I posted an incredible picture of Saturn taken from Cassini, showing the partially-lit planet and two of its moons. I used this picture to point out the massive scale of Saturn and its moons, something it’s easy to forget when you’re scanning all the amazing images.

BA Bloggee Matt Andrews liked the post, but thought it needed more. He took the picture from Cassini and added a map of the United States to it. I thought it was pretty cool, and so just in case you were having a hard time grasping just how frigging huge Saturn is, this ought to blow you away:

Ye. GADS. Astonishing, isn’t it? That scale looks about right to me. I know how big the US is; I’ve driven across it several times, and it’s a heckuva trip. To see it dwarfed like this is simply incredible.

So when I post these pictures and talk about how mind-boggingly ginormous these objects are, even the familiar ones, keep this in mind: I’m not kidding. Space is big.

With the Cassini spacecraft orbiting Saturn and making frequent fly-bys of all the weird moons there, it’s easy to post one incredible close-up after another. But sometimes, you have to take a step back and get some context, see the bigger picture.

Cassini can do that, too. And when it does, the beauty and scale of the Saturn system is simply breathtaking:

[Click to encronosate.]

This image shows, of course, the ringed planet itself, with the rings seen edge-on and their shadow cast across the planet’s southern hemisphere cloud tops. But look to the left, just below the rings; see that half-lit disk? That’s Enceladus, an icy moon of Saturn. It’s about 500 km (310 miles) across, which may start to give you an idea of how much area this picture covers. Even though it’s as big as my home state of Colorado, it’s positively dwarfed by the looming presence of Saturn behind it… and we’re not even seeing very much of the planet here! Saturn is over 120,000 km (75,000 miles) across, nine times the diameter of Earth.

Saturn is big.

To pound this home, look even farther to the left of Enceladus. See that black speck? I’ve enlarged the picture and annotated it here; the arrow points to Epimetheus, a lumpy gray potato moon of Saturn. It’s about 113 km (70 miles) long. That’s small for a moon, perhaps, but on a human scale it’s a huge rock, more than ten times the height of Mt. Everest.

Yet it’s a speck in this picture, easily missed if you didn’t know it was there. But I guess that’s not surprising; Cassini was 1.2 million km from Saturn when it took this shot, three times the distance from the Earth to the Moon!

Sometimes people ask me, what’s the one thing you wish people understood better about the Universe? And if I had to pick just one only, it would be this: scale.

The Universe is huge, and we’ve barely dipped our toes into it.

Image credit: NASA/JPL-Caltech/Space Science Institute. Tip o’ the meterstick to Carolyn Porco.

Late last year, a huge storm erupted in the clouds of Saturn’s northern hemisphere. Huge even at the beginning, it grew even larger rapidly and eventually wound itself all the way around the planet! It grew to the incredible length of 300,000 km (180,000 miles) — end-to-end, it would’ve stretched 3/4 the way from the Earth to the Moon!

The folks at the Cassini mission have just released a treasure trove of new pictures of the storm, and they are very, very cool. Check this out:

[Click to encronosenate.]

It looks like it’s a ghost running from Pacman, doesn’t it? Hmmm… maybe it is.

Taken in late December 2010, this image shows Saturn in the infrared using a combination of 3 different filters. What you see here in this false-color as white and blue are actually clouds high in the atmosphere; yellow/green are mid-level clouds, and red/orange deeper material. The rings are edge-on right in the middle, and you can see the shadow they cast on the planet. From its reddish hue, this storm was deep in Saturn’s atmosphere. it apparently has two eyes — if ever that term were accurate, it’s here — with one being deeper in the clouds than the other. Clearly, the tail had just started to form.

But it grew. A lot. A few weeks later, in January 2011, Cassini took this image:
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