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.

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.

We’ve known for years that Jupiter’s moon Europa almost certainly has an ocean of liquid water deep under its frozen surface. For one thing, the surface is almost all water ice. We also know that it’s covered in thousands of cracks that look very much like the type we see in ice floes floating on liquid water here on Earth. And we have a heating mechanism: tides from Jupiter as well as from the other moons flex Europa, causing its interior to warm up.

A nagging question has been how thick is the solid ice shell over that ocean: is it many kilometers thick, or much thinner? Evidence supports both arguments, which is maddening. However, that problem may now be solved: astronomers studying Europa’s terrain think the ice shell is generally very thick, but — and this is the cool part — may have vast underground lakes of water!

This picture is from observations of Europa made by the Galileo spacecraft, which orbited Jupiter for many years. It’s a combination of optical images and photoclinometry — using pictures to measure the heights of surface features. Purple and red is elevated terrain, and you can see that this looks like a depression in the surface. It’s filled with what’s called "chaotic terrain" for obvious reasons. Most of the surface of Europa has larger scale structure, and is more organized, as you might expect from a thick shell of ice. But these smaller regions are a mess, and it looks like this is from pockets of liquid water under the surface, giant lakes the size of North America’s Great Lakes, completely buried in the ice.

This artist’s view shows how this works; the lake is completely embedded in the ice shell. In general, the ice is very thick, explaining the usual look of Europa’s surface. But in some spots, just below the ice, the ice has melted. The ice above this underground lake is much thinner, perhaps only 3 km (about 2 miles) thick, explaining the chaotic surface in those localized spots.

That’s pretty nifty, but why is this so important? (more…)

This morning, I wrote about some pictures of Saturn’s moon Enceladus I found rummaging through NASA’s Cassini raw images archive. Enceladus is a small icy moon that may have an ocean of liquid water under its surface. It’s a fascinating world, and is one of those objects that cannot seem to take a bad picture; every shot of it is dramatic and intriguing.

Even so, as I clicked through the raw images from the distant spacecraft, I got a jolt when I stumbled on a series of pictures depicting the tiny disk of Enceladus with the gigantic visage of Titan sliding past! I quickly grabbed the images and made a short animation showing the scene, with a description:

[It helps to watch full-screen and in hi-res; I recorded it in 1080p. The images from Cassini look pretty good that way.]

Nifty, eh? I’ll note that in between some of the frames of the animation Cassini was programmed to change filters. That’s most obvious by looking at Titan itself; when the blue filter was used the atmospheric layers become more obvious — an upper level haze layer is dark in blue colors. Here’s one of those images using the blue filter:

You might wonder why the picture isn’t blue if a blue filter was used. (more…)

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

The other day I posted a great picture of Saturn and its rings taken by Cassini. While digging around in my archives looking for other posts about the rings, I found one from earlier this year that had a picture of the icy moon Enceladus with the rings in the background. When I saw the picture, I got a jolt: there was a crater chain on the surface that looks just like the one on the asteroid Vesta!

Here’s a side-by-side comparison:

Enceladus is on the left, Vesta on the right (click those links for higher-res shots). Pretty cool, huh? You can see both have two big overlapping craters of roughly the same size, and a smaller third one roughly aligned on top. The set on Vesta is nicknamed — for obvious reasons — "Snowman".

Craters like this form when the impacting object is not a single body; for example, many asteroids are known to be binaries, with both objects about the same size. Getting hit by that would leave two craters either very close together or overlapping, depending on the sizes, distances, and velocities of the impacting bodies.

(more…)

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.