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.

Like any scientist, I love a good mystery. Sometimes it’s fun when they are long, complicated, involve subtle and difficult layers, and require a vast effort to unravel.

And sometimes it’s cool when they are simply stated and simply solved. Like asking "Where does the water in Saturn’s upper atmosphere come from?" and finding out the answer is "It rains down from the moon Enceladus."

Water has been seen deep in Saturn’s atmosphere before, but a few years back it was detected in the upper atmosphere as well, and that’s a bit weird; there don’t appear to be any ways to get it from deep down in Saturn to the top parts of its clouds. So how did it get there?

Well, the tiny, icy moon Enceladus was discovered to have geysers at its south pole, actively spewing out quite a bit of water into space. Most of it goes into space and is gone forever. Some actually forms a ring around Saturn called the E-ring, and some no doubt hits other moons. Generally, when a moon blasts stuff into space (like Jupiter’s moon Io does with its sulfur volcanoes) the material forms a big donut-shaped region around the planet. It was figured that Enceladus was doing the same thing with water around Saturn, but even the Cassini spacecraft, which is right there, couldn’t detect it. It’s pretty hard to sample.

But astronomers used Herschel, an Earth-orbiting infrared observatory, to observe Saturn. They found a peculiar feature in the infrared spectrum of Saturn, and realized it’s from this Enceladusian water torus. Apparently, about 3-5% of the water from Enceladus’s geysers falls on Saturn, literally raining down in sufficient quantities to explain the presence of the water detected in the ringed planet’s upper atmosphere.

(more…)

[REMINDER: I'm guest hosting Dr. Kiki's Science Hour today at 4:00 Pacific time!]

Ya know, for a tiny ball of ice, Saturn’s moon Enceladus really knows how to pose for a picture:

Cassini snapped this shot from 34,000 km (20,000 miles) away, looking down on the northern hemisphere of Enceladus. Peeking just over the edge is a slice of Saturn’s rings, too.

Most of the action on Enceladus is at the south pole, where geysers of water are erupting. But up at the other end of the 500 km wide moon — for comparison, Colorado is 600 km across — it’s still pretty nifty. The reflective, icy surface is saturated with craters, including that interesting triple smackdown on the left. Something must have broken apart as it hit… though I’ll note the two big craters are elongated, indicating a very shallow angle of impact, while the third smaller one is round. It may only coincidentally line up with the other two. If that’s the case, maybe a binary asteroid hit here long, long ago.

Cracks snake their way across the surface too. Enceladus certainly has liquid water under its perpetually frozen exterior, though there’s some debate over whether it’s a global ocean or pockets of liquid. Still, the remarkable thing is that a moon smaller than some states and frozen to a temperature of -200°C could still have liquid water hidden beneath at all. I remember, as a kid, reading books by scientists wondering if we’d find water in space. Cripes. It’s everywhere!

Image credit: NASA/JPL/Space Science Institute