[Over the past few weeks, I've collected a metric ton of cool pictures to post, but somehow have never gotten around to actually posting them. Sometimes I was too busy, sometimes too lazy, sometimes they just fell by the wayside... but I decided my computer's desktop was getting cluttered, and I'll never clean it up without some sort of incentive. I've therefore made a pact with myself to post one of the pictures with an abbreviated description every day until they're gone, thus cleaning up my desktop, showing you neat and/or beautiful pictures, and making me feel better about my work habits. Enjoy.]

With planetary pictures, angle is everything. If you have your back to the Sun and face your target, it’s fully lit, and looks like a disk. But if you go around to the other side, and put your target between you and the Sun, it becomes a crescent. Get the angle just right, and that crescent gets very thin…

… which is a view of Saturn’s moon Enceladus we can never get from Earth, but one that the Cassini spacecraft gets all the time. And it’s way, way cool:

[Click to encronosenate.]

But there’s an added bonus here, one that makes this picture that much more amazing: that fuzz at the bottom? Those are enormous geysers, towering sprays of water blasting out of cracks in the surface of the moon and reaching upward for hundreds of kilometers!

We’ve seen the geysers before, and in fact Cassini has flown through them to find out what they’re made of (turns out water laced with lots of organic goodness like acetylene, formaldehyde, and much more). They’re very dim, but easy to see when backlit by the Sun like this.

So we know Enceladus must have liquid water under its surface, to feed these geysers. But is it local, like a subsurface lake, or is the ice of the moon floating on a global ocean? New studies of the cracks from which the geysers emanate seem to indicate the water is everywhere! The geysers are formed from gravitational stress when the moon nears Saturn in its orbit, and the size and shape of the cracks really make it look like the water source is a global ocean, like Jupiter’s moon Europa.

Isn’t that amazing? We can learn a lot about a tiny, icy, backlit world, just by tasting its water.

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

Because I love each and every one of you, here is a fantastic portrait of two worlds: Saturn and its ginormous moon Titan, courtesy of the Cassini spacecraft:

Isn’t that breathtaking? [Click to encronosenate.]

I love the panoply of shadows from the rings on the cloud tops of the gas giant planet, clearly showing Saturn has not one big ring, but thousands of thin ringlets. You can also see subtle patterns in the clouds as well. If you look very closely, you’ll see the shadow of the moon Prometheus on the left just below the ring shadows — the moon itself is the white speck just above the rings to the right, just to the right of Saturn’s limb — as well as the shadow of the moon Pandora on the right below the rings. Pandora itself is well outside the frame of this shot though.

Of course, fuzzy Titan looms of the planet’s edge on the right as well. Titan is huge, bigger than Mercury, and if Saturn weren’t there might be considered a planet in its own right. But definitions aside, Titan is a varied and complex place, worthy of intense study. It has weather, lakes of liquid methane, dunes blown and sculpted by wind, and boulders made of water ice harder then rock is on Earth.

Who wouldn’t want to take a closer look at a world like that?

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

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.