Blogs / Bad Astronomy

On November 21, 2009, the Cassini spacecraft sliced past Saturn’s moon Enceladus, shaving the iceball at a distance of 1600 km (1000 miles). From that distance, the view was astonishing…

It’s been known for some time that the south pole of Enceladus is lousy with geysers, erupting water into space (though the ultimate source of the water is still a bit of a mystery). But this new pass shows 30 geysers, 20 more than were previously seen! One major geyser also appears to have waned a bit since the last pass, showing that not only is stuff going on, but things are changing, too.

This mosaic of the surface of Enceladus overlays a high-res optical image with thermal hot-spots. You can see that the hottest parts — which are actually at -90° C (-140° F), so I guess "hot" is in the (frozen) eye of the beholder — line up along a huge fracture in the moon’s surface. The fracture is called Baghdad Sulcus and is one of the places on the moon erupting water geysers. The fracture is about 500 meters (roughly 1/4 mile) deep, and this image shows about a 40 km (25 mile) swath along it. There’s evidence of particles from the geysers re-falling here, and also house-sized icy blocks that may be rubble that has been seismically shaken and settled downslope.

There’s a lot of science in those images, and in the others returned from that close pass. But I think my favorite from these is one that may also have scientific value, but, like almost everything Cassini sends back, is perhaps more striking for its artistry.

That’s a crescent Enceladus, replete with geysers, and its parent planet Saturn in the foreground. Wow, that’s pretty. I love how gray Enceladus looks and how much brighter Saturn is. I was thrown for a moment; Enceladus has a reflectivity of nearly 100%, meaning it reflects nearly all the light hitting it, while Saturn only reflects 30-50% of the light that hits it (depending on how you measure it). But this depends on the viewing geometry! Enceladus is thin crescent, so the light is hitting it at a very low angle. A lot of the light hitting the moon is sent straight back toward the source (the Sun), so not much of that light gets sent off in other directions. It’s not that Enceladus is intrinsically fainter than Saturn, it’s just that the light is reflected off in another direction, and not towards Cassini in this image.

As in life, sometimes what you see depends on how you look.

Cassini has been orbiting Saturn since 2004, and in all that time it has not disappointed. It continues to return a veritable bounty of information about Saturn and its fleet of moons. If you want to stay on top of Cassini news, subscribe to the email list, and follow imaging team leader Carolyn Porco on Twitter!

We live on a wonderful planet with a beautiful Moon. But I sometimes think we got gypped.

The view from Saturn can be sooooo much cooler:

That’s the moon Enceladus passing in front of (what we science-types call "transiting") Rhea. Enceladus is about 500 km (300 miles) across and Rhea is 3 times bigger. Rhea was about 2.7 million km (1.7 million miles) from Cassini when these shots were taken, and Enceladus a bit closer at 2.3 million km (1.4 million miles). Of course, by a bit closer I mean the same distance our Moon is from the Earth, so there was plenty of room for this mutual event.

Here’s a zoom of the middle frame:

Awesome. Note that these images were taken about one minute apart, so things were hopping. It’s amazing to me that not only can we put a probe around Saturn and get images of its moons, but our math and physics are so freaking accurate we can say, "Hey, you know what? On this date at this time if we turn Cassini that way we’ll see a moon over 2 million kilometers away pass in front of another one nearly 3 million kilometers away."

Every morning, I have a 50/50 chance of finding my keys. That kinda puts things in perspective.

Every year, this gets harder.

Not that deciding what pictures to use in 2006, 2007, or 2008 was all that easy! But astronomy is such a beautiful science. Of course it has scientific appeal: the biggest questions fall squarely into its lap. Where did this all begin? How will it end? How did we get here? People used to look to the stars asking those questions, and coincidentally, for the most part, that’s where the answers lie. And we’ll be asking them for a long time to come.

But astronomy is so visually appealing as well! Colorful stars, wispy, ethereal nebulae, galactic vistas sprawling out across our telescopes… it’s art no matter how you look at it. And our techniques for viewing the heavens gets better every year; our telescopes get bigger, our cameras more sensitive, and our robotic probes visit distant realms, getting close-up shots that remind us that these are not just planets and moons; they’re worlds.

So every year the flood of imagery takes longer to sort through, and far longer to choose from. And the choices were really tough! This year leans a bit more toward planetary images than usual, but that’s not surprising given how many spacecraft we have out there these days.

I don’t pick all these images for their sheer beauty; I consider what they mean, what we’ve learned from them, and their impact as well. But have no doubts that they are all magnificent examples of the intersection of art and science. At the bottom of each post is a link to the original source and to my original post on the topic, if there is one. If you disagree with my picks, or think I’ve missed something, put a link in the comments! All the pictures have descriptions, and are clickable to bring you to (in most cases) much higher resolution version. So embiggen away!

And welcome to my annual Top Ten Astronomy Pictures post. Enjoy.

Just a quick note: Emily, at The Planetary Society Blog, just posted a way cool mosaic made up of four pictures from Cassini showing the Enceladus icescape. I love the perspective on it, and how you can tell you’re looking down on the tiny moon from an oblique angle. It’s quite lovely. Go look!

And in case you missed it, here are links to 3D red/green anaglyphs of Enceladus too. Awesome.

On November 20, 2009, the Cassini spacecraft buzzed the surface of Saturn’s icy moon Enceladus once again, returning dramatic images of its water geysers and wrinkled, ridged surface:

That raw image (which means it has not been processed to remove instrument/detector artifacts like bad pixels and such) was taken when Cassini was a mere 2000 km (1200 miles) above the moon’s surface. The features are beautiful and plentiful… and it looks like a great place to ski. Bonus: the low gravity would make the experience last longer!

Cassini got an overview of the geysers, too, when it was still more than 500,000 km away:

Remember, these are raw images; that bright "star" just above Enceladus is probably a cosmic ray hit on the detector and not an actual astronomical object.

Over at The Planetary Society blog Emily is, of course, having kittens over the pictures, and has made some stereoscopic pairs of them (though I’ll wait for the red/green anaglyphs; crossing my eyes at my monitor makes my tummy queasy). [Edited to add: in the comments below, BABloggee Alex links to anaglyphs he created. Very cool!]

Stay tuned, because as these images are processed things will only get cooler.

Today (as I write this), the Cassini spacecraft passed just a hair under 100 km (62 miles) from the surface of Enceladus, an icy moon of Saturn. This little moon is scientifically incredibly interesting; there are geysers at the south pole that are spewing out water! The images are just now coming in, and have not been calibrated or processed yet, but they are still breathtaking. I particularly like this one:

[Click to embiggen, as usual.]

That, me droogs, is high art. Enceladus was about 190,000 km (118,000 miles) away from Cassini when that shot was taken, a little under half the distance of the Earth to the Moon. From this angle, Enceladus is lit in a gorgeous thin crescent, but we can see detail on the dark side, I suspect due to light reflecting off Saturn onto the moon. You can see ridges in the surface; the moon has a thick crust of ice presumably floating on an undersurface ocean of water (though there have been arguments about that), so the surface is a bit of a mess, looking for all the world(s) like ice floes seen at our own north pole.

The geysers are obvious too, blasts of light at the top of the moon’s limb as the water erupting from the south pole is lit by the Sun. Thumbing through the raw images is a delight (once there, set the target for Enceladus, choose both narrow and wide angle, and put in dates of October 30 through November 3 to narrow the search). You’ll see dramatic images of the moon, its limb, the geysers, and everything.

Stunning, and wondrous. And there’s better to come: as Carolyn Porco herself mentions on Twitter, the primary purpose of this flyby was not to get images; November 21st is the imaging flyby where we’ll see lots of spectacular shots of the moon. So stay tuned!

Image credit: NASA/JPL/Space Science Institute