Every weekend when I can I do an interactive live video chat on Google+ where people can ask me questions about space and astronomy. I call it Q&BA, and it’s always fun to hear what questions are on people’s minds.
Apropos of my recent post about Saturn’s moon Enceladus, I got this question: "Which moon has the best chance for life: Titan, Europa, or Enceladus?" This is a common question, and worth exploring! Here’s what I said:
Mars is still an interesting place to look for life, but those moons — all three — are very, very enticing. I’d love to see us launching future space probes with some icy targets in their sights.
[P.S. The aspect ratio of the video is stretchy for some reason; the video looked fine before I uploaded it to YouTube. I'll try to track this problem down.]
I have an archive of Q&BA links and videos. Take a look and see if there are other ones that tickle your imagination.
- Q&BA: Can we build a space habitat?
- Q&BA: The Science of Science Fiction
- Q&BA: How does a gravity slingshot work?
- Q&BA: Why spend money on NASA?
- Q&BA: What happens if you are exposed to the vacuum of space?
[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
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? Read More
Mike Brown is an astronomer, specifically one who studies Kuiper Belt Objects, those giant frozen iceballs that haunt the solar system out past Neptune.
In fact, Neptune’s biggest moon Triton has a lot of characteristics similar KBOs — it may be one captured by Neptune — so observing it gives an interesting opportunity for a compare-and-contrast study. So this past weekend Mike was using the Keck telescope in Hawaii to observe Triton along with its (adoptive?) parent planet, and took this fantastic image of the pair:
[Click to poseidenate.]
This false-color image shows the two worlds in the infrared, specifically at a wavelength of about 1.5 microns, twice what the human eye can see. Methane strongly absorbs this color of light, so where Neptune (in the upper left) looks dark you’re seeing lots of methane clouds, and where it’s bright there are clouds higher up, above the methane. Triton is in the lower right, and is bright because it’s covered in ice which is highly reflective.
Now this is all very pretty and interesting and sciencey, but if you know me at all you know there’s more to this story.
Mike tweeted about the image, and I oohed and ahhhed at it, of course. But then he tweeted again, saying he was also observing Jupiter’s moon Europa, but it was too bright to get good images using the monster 10-meter Keck telescope. It "saturated the detector" which is astronomer-speak for "overexposed".
That’s funny, I thought. Neptune looks fine in the image, and the random noisy grain in it makes it clear Mike wasn’t anywhere near saturating the image. Now I know Europa is closer to the Earth, so it should look brighter, but geez, it’s a moon, and a lot smaller than Neptune. How could it be too bright to image?
It turns out my all–too–human and all–too–miserable sense of scale has failed me again. Math to the rescue!
I love anaglyphs (3D pictures) and I love astronomy animations and I love Jupiter, so how much do you think I love this anaglyph animation of Jupiter?
[Note: the embedded version here shows it as two separate animations. Go to the YouTube page and you'll see a 3D label at the bottom of the player. Click that, and you can set the animation to be red/green or lots of other options. Currently, I can't seem to embed the video that way, so again I urge you to go to the YouTube page.]
This is from Chris Owen, an amateur astronomer equipped with a 25 cm (10″) Newtonian ‘scope (the same kind I had for about 20 years!). The animation shows Jupiter over the course of about 2.5 hours, with one exposure taken every five minutes. You can also watch Europa and Io, two of Jupiter’s big moons, orbiting the planet as well. He created the animation straight, then converted it to 3D. You can see the original on his DeviantArt page — that’s a 3 Mb image, which is why I didn’t embed it, but click it to see because it’s cool.
I like the 3D version; you really get a sense that Jupiter is a ball, and it’s nifty to be able to see the two moons as being farther away than the planet itself, proven positively by seeing Europa physically go behind Jupiter as it orbits. Note too that these observations were made last year, before the Southern Equatorial Belt disappeared.
While these animations are a bit of fun, I suspect they will actually give people more of a sense that these objects aren’t just points of light in the sky, but worlds. I’m a fan of things that give people a deeper connection to the Universe, so I really like these anaglyphs!
Four hundred years ago tonight, a man from Pisa, Italy took a newly-made telescope with a magnifying power of 33X, pointed it at one of the brighter lights in the sky, and changed mankind forever.
The man, of course, was Galileo, and the light he observed on January 7, 1610 was Jupiter. He spotted "three fixed stars" that were invisible to the eye near the planet, and a fourth a few days later.
Here is how he drew this, 400 years ago:
He noted the stars moved around Jupiter as they followed it across the sky, and so was the first to figure out that other planets had moons like our own. It wasn’t an easy observation; his telescope was still small, the field of view narrow (so not all the moons were visible at the same time), and the moons faint next to Jupiter’s brilliant glare. But Galileo persisted, and figured it out. We call these four the Galilean moons in his honor: Io, Europa, Ganymede, and Callisto.
Here’s how we see them today:
The image above [click to embiggen] is from the New Horizons spacecraft as it shot past Jupiter in early 2007, showing all four moons. Each is scaled to show its true relative size to the others. It’s impossible not to wonder what Galileo would have thought, knowing that just shy of 400 years after he made his first observations, we would fling our robotic proxies out into the solar system and get close up views of the objects he discovered.
Think of it! For all of time before, Jupiter was just a light in the sky. And then, forever after that night forty decades ago, it was a world, surrounded by more worlds.
[See more pictures of Jupiter and its moons in a gallery over at 80 Beats.]
Galileo went on to observe craters on the Moon, spots on the Sun, and the phases of Venus. It was that last that may have been his crowning achievement, because the way Venus showed phases meant it could not possibly orbit the Earth, and that it must orbit the Sun. The geocentric theory had held sway for over a thousand years, but Galileo proved it was wrong almost overnight. Of course, the Church wasn’t thrilled with this, though I suspect they might have rolled with it if Galileo hadn’t been such an arrogant jerk and published a manuscript insulting the Pope, a man who used to be his friend and supporter.
If there is a lesson in there, I leave it to my readers to suss it out.
Now, all these years later, a lot of legends exist over the man. He didn’t invent the telescope, he wasn’t the first to point it at the sky, and he wasn’t even the first to publish his drawings. But he was a merciless self-promoter, and because of that we do remember him now (again, any lessons learned here are up to you). And it’s not entirely unfair to do so; he was a tireless observer, a wonderful artist, a great inventor (he may not have been the first to build a telescope, but he made his far better than its predecessors) and a brilliant scientist who, even if he hadn’t done so much for astronomy, would still be remembered today for his other work.
Tonight, just after sunset, Jupiter will be a glowing white beacon in the southwest. I have a Galileoscope, an inexpensive telescope created as part of the International Year of Astronomy 2009, an effort to get as many people on Earth to look up as possible. I think perhaps it would be fitting if I brave the subzero temperature outside, maybe for just a few minutes, and take a look at the mighty planet. Tonight’s display is better than Galileo himself had it: all four moons will be perfectly arrayed, two on each side of Jupiter’s face.
I’m not a very religious man, nor am I a very spiritual man. But I know there will still be a sense of connection, a sense of wonder that I will have tonight that I will share with a man long dead, but whose life and achievements still echo through time.