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Before I tell you about Titan I should set the stage a little.

Voyager 2, as you probably know, went past Jupiter, Saturn, Uranus, and Neptune. All four - because they were lined up, and it was possible to fly past all of them - and this happens only once every couple hundred years; won't happen again in any of our lifetimes. We had this one shot. And we sent the two Voyager probes, both capable of making this grand tour.

But only one did. Why? Because we altered the trajectory of the other one - Voyager 1 - to fly close to Titan. That meant it had to give up Uranus and Neptune - both gas giant systems, all their moons and rings and mysteries, neither of which we'll visit again. Voyager 1 gave up all that, leaving them to its sister alone, just so its path through the Saturn system would take it close to Titan.

And twenty years later, when we launched Cassini towards the Saturn system, Cassini - unlike Galileo, which went to orbit Jupiter - carried a lander with it: the Huygens probe. The first thing Cassini did when it reached Saturn was drop the probe... so the probe could land on Titan, take pictures as it descended, and survive for a couple of minutes on the surface. We didn't bother to send a lander to any of Jupiter's moons. Titan's only the fifth thing we've landed anything on at all, after the Moon, Mars, Venus, and one near-Earth asteroid.

What makes Titan so special? Read more... )
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Like almost all of Saturn's moons, Enceladus keeps one face toward Saturn. (I just talked yesterday about the interesting exception.) That's for good reason - the closer you are to an object, the stronger the "slope" of its gravitational pull (and Enceladus orbits very close to Saturn: second closest world after Mimas). Because the planet's gravity is getting stronger quickly as you move towards it, the front side of a moon orbiting Saturn is pulled toward Saturn harder than the back of the moon. If it started out rotating, this tidal force slowed its rotation down - and given hundreds of millions or billions of years, the rotation stops, and the moon winds up facing the same side toward its planet.

But that happens to all the moons. (Our moon keeps one face toward the Earth for the same reason.) But because Enceladus is so close to Saturn, the tides have a much more interesting effect.

What is the effect? Well, how tides work, the front of the moon, being permanently closer to Saturn, "wants" to be in a lower orbit than the back of the moon - this creates a constant pressure to pull the moon apart. And when you have constant pressure like that on a world, the world creaks and groans. The parts shift against each other, and flex back and forth. In other words, it's seismically active. Read more... )
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Weird enough for you yet?

Hyperion is almost as big as Mimas - which has enough gravity to force itself into a sphere - yet is not remotely spherical - instead, it looks like this.



But that's not the weirdest thing about it. Read more... )
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Methone is a tiny, tiny little rock. It's only three miles long. Its orbit is between Mimas and Enceladus. Cassini found it about six years ago.

Why is Methone on this list? Simple. It's a freaking space pod left by aliens.



Read more... )
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The most common reaction I've heard when these two moons are explained to people (*) is "you have got to be shitting me."

(* People, as opposed to physicists.)

Like Prometheus and Pandora, these two aren't worlds (they aren't spheres); they're just rocks, and in fact are not far from that pair. Epimetheus and Janus orbit inside of Mimas, very close to the rings, but outside of the F ring that Prometheus and Pandora make.

Rocks aren't, by and large, all that interesting. But these two are not just interesting but dumbfounding, not because of what they are but because of what they do.

I'll let you figure out the problem. They're each about a hunded miles in diameter (on average; they're irregular rocks). Their orbits are perfectly circular, and they both orbit exactly in the plane of the rings. And the radii of their orbits - the paths traveled by the two moons' centers - differ by only thirty miles.

Think about that for a minute. I'll wait. Read more... )
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I originally considered making a list of the top six moons of Saturn, not the top ten, because the ones up till now - as interesting as they (hopefully) have been, are just not in the same class as what's coming. Iapetus is, to me, where things really start getting good.

When Cassini (the astronomer) discovered Iapetus, he had a problem: he could only see it on one side of Saturn and not the other; years later, he saw it on the other side, but he needed a bigger telescope: it was only one tenth as bright.

Why? Well, obviously it was bright on one side and dim on the other... but we had to wait till Cassini (the orbiter) got to Saturn before we had any decent pictures that explained any further. (Arthur C. Clarke made this differential an important plot point of his novel 2001. Clarke may have been closer to the truth than he knew... but I'll get to that.) Read more... )
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At this point, when I go over this list, I notice a couple themes about Saturn and her moons: they've taken a lot of major-impact damage, there are a bunch of complicated gravitational relationships, and there sure are a lot of little rocks around. Oh, right, and there are these dazzlingly bright rings, which none of the other gas giants have.

Complicated gravitational couplings happen when many moderate-to-small things - small world and big rocks - are orbiting in fairly close quarters, and have been doing it for billions of years. The rest fall into the "what we still have to work out" part of Saturn. We don't know for sure why there are near-worldshattering impact craters on two of the inner moons; we don't know why there are so many little rocks around; we don't know the original source of the rings. I'm not done talking about any of those yet, because the speculations out there in the scientific community intrigue me, and I deliberately arranged this list to talk about the most significant pieces of those mysteries later on.

But there are lots of rocks. Read more... )
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Third world out from the gas giant, Tethys is an ice ball, cratered but slightly smoothed from old ice melts. Like Mimas, it has an enormous crater - Odysseus - that, while it's more smoothed out and eroded than Herschel, is as big - proportionally - as Herschel is. Since Tethys is twice the size of Mimas, that makes Odysseus twice the size of Herschel.

To explain what I find so interesting about this impact on Tethys, go look at a map of Mars for a minute. In the southern hemisphere of Mars, there's a ginormous ancient impact basin - Hellas. Same relative size - Hellas is about as big compared to Mars as Odysseus is compared to Tethys. Now, if you turn Mars over and look opposite Hellas, you see the largest dry-land chasm in the Solar System: Valles Marineris (three thousand miles long). When the Hellas impact happened on Mars, part of its crust split, like the skin of an orange, opposite the impact.

The same thing happened on Tethys. Ithaca Chasma is the second-largest dry-land chasm in the Solar System: it's about half the length of Valles Marineris. Except it's on a moon with a diameter of less than seven hundred miles... which means the fourteen hundred mile long chasm goes three quarters of the way around the world. It's sixty miles wide and three miles deep, and pretty much splits the entire moon in half. When something hit Tethys and made Odysseus Crater, it hit hard.

Tethys also has resonances with several other moons in the Saturn system. For every orbit of Tethys, Mimas orbits exactly twice. (This isn't so special; Enceladus and Dione have the same relationship, and around Jupiter, Ganymede-Europa-Io are all bound together the same way.) But Tethys also has two Trojan companions - that is, rocks a couple miles in diameter that share the same orbit as Tethys, one sixty degrees ahead, one sixty degrees behind. This is all in addition to the relationships Mimas has with Pandora and with Saturn's rings.

The impact basin and chasm, and the complicated gravitational effects combine to put Tethys on my list.

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Mimas is the innermost and smallest world of the Saturn system: about two hundred miles in diameter, it orbits just outside all the easily-visible rings.



One of the most iconic pictures from Voyager 1, as it flew through the Saturn system in 1980, was this picture of Mimas. Its resemblance to the Death Star, just after Empire Strikes Back was released, was a cool and almost spooky coincidence. It looks like it does because of the huge crater (Herschel) taking up a third of one side of the moon. That crater is interesting; Mimas isn't the only one of Saturn's moons to show clear evidence that large amounts of debris was flying around the system at some point in the very distant past. The crater is so big that some models say it should have shattered Mimas when it hit, so why it didn't is a good question.

It's not the only thing interesting about Mimas, though. Read more... )
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Until recently I wouldn't have put Rhea, or the similar moon Dione, into a top-ten list of Saturn's interesting moons. It's a moon. It has craters and mountains that make for a wispy appearance at a distance. It's otherwise pretty well-behaved and looks kind of generic as moons go. But the Cassini orbiter found out something weird about Rhea that we can't, at this point, explain. The whole Saturn system has a certain amount of charged particles around it, basically resulting from the planet's magnetic field. The charged particles follow a known and expected pattern around each moon.

Except Rhea. When Cassini flew through the ring plane near Rhea - that is, through Rhea's orbit, above its equator - there was a drop-off in magnetic particles. Huh, the scientists said. This means there's probably a dim ring of uncharged particles around Rhea. That's pretty cool; let's take a picture the next time the orbiter goes by. So the orbiter went by again, they snapped pictures - no ring. Nothing. Just a gap in the charged particle density around Rhea.

Why? We don't know. Some kind of magnetic field around Rhea that repels the charged particles? Probably. Why and how did it get there? We don't know. But this mystery is enough to make Rhea - Saturn's second-largest moon - interesting, and earns her a spot in my top ten.

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I'm cheating here. These two aren't worlds, they're rocks. And there's two of them in this slot, but that's because it's the two of them working together that makes them interesting.

These two potato-shaped rocks (each about eighty or so miles across the long way) orbit just outside the main part of Saturn's rings; their orbits are only a few thousand miles apart. What makes them interesting is that between the two of them there's a very thin ring - the F ring - and it's the presence of these two moons that "traps" material in this ring; their gravity herds tiny particles of ring-material into an orbit between the two; they clear out particles in their own orbits and maintain a thin ring, looking almost threadlike, between them.

But the F ring isn't just a thread. Take a look - it has ripples. (When we first took pictures of it with Voyager, it looked braided.) Read more... )
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A few notes before I begin my list. This is boring history and dry info; skip it if you want.

Saturn is about ten times as far from the Sun as we are - that is, close to a billion miles away. It takes thirty years to go around the Sun. Its axis is tilted, like Earth's is, which is why we can see its rings. It's about nine times Earth's diameter - about seventy thousand miles across.

Galileo was the first to see Saturn's rings, but didn't know what they were. Christiaan Huygens figured out what the rings were, and also discovered Titan, Saturn's largest moon. Giovanni Cassini discovered four more of Saturn's moons, and was the first to see that the rings had a gap in them (the Cassini Division). All three of them did their work in the 17th century.

Better telescopes showed us four more moons over the next 250 years, but until 1977 they were points of light in telescopes - we knew their rough sizes and masses, and of course we could watch their distances from Saturn and how long they took to orbit it, but that was about it... until we actually sent something out there to start taking pictures. We sent three flyby missions during 1978-1981 - Pioneer 11, Voyagers 1 and 2. Then, in 2004, the Cassini orbiter, carrying the Huygens probe, reached Saturn, and it's still there, eight years later. (So when I talk about what "Cassini" or "Huygens" found out, it's about the 21st-century probes, not the 17th-century astronomers.)

Some of the moons that I'm going to talk about, but not all, are "world-sized" - that is, big enough for its own gravity to pull it into a sphere. Your typical Saturnian moon orbits the planet's equator in a circular (non-elliptical) path, and keeps one face toward the planet all the time (the way Earth's moon does toward Earth). Most of Saturn's moons have no seismic activity, magnetic field, or atmosphere.

That's the typical moon. There are exceptions to every one of those rules - that's why this is so interesting to me.

The world-sized moons of Saturn, in order of distance from the planet, are: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus. Except for Titan and Rhea being larger than Iapetus, that's also the order from smallest to largest. Two other moons are "borderline" whether they should get the same status: Hyperion (orbiting just outside of Titan) and Phoebe (extremely distant).

[livejournal.com profile] tavella mentioned that the number of Saturn's known moons has gone from about 20 last time she looked to something like 60 now. Yeah. My reaction is this. Saturn has, at least, billions of "moons" - meaning things orbiting it. The rings are made of "moons" - a mile or less in diameter, with no real division between "moons" and "ring material", except for "things big enough for Cassini's telescopes to discern them individually". It also has an undetermined but huge number of progressively smaller rocks orbiting it at extremely large distances - captured asteroids and centaurs and comets, going from barely-noticeable down to who-cares; each time telescope technology gets better we find a bunch more rocks.

They're rocks. Yeah, when we find them we give them names. But I'm not about memorizing names, not now that we know a lot more about things than their names. There are lots and lots of rocks orbiting Saturn. I'm mostly interested in the worlds - and only interested in a rock if it shows us something as interesting as a whole world.

So anyway, that's the textbook stuff. Boring part over.
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I'm going to try something. Maybe it'll totally bore you. I don't really care. :) It's not about politics - so I should get some credit at least for that.

I'm going to talk about Saturn. That is, about the parts that interest me - the parts that most people who aren't space geeks don't know a thing about. If you're like most people, you know Saturn's the sixth planet from the sun and it has rings. If you're into astrology, you might know that this year it's in Virgo; you might even know it spends two and a half years in each sign (which means it takes about thirty years to go around the Sun). If you were at any point a nerd, you know Saturn is about nine times the diameter of the Earth and it would float if you threw it into a big enough bathtub. And that's it, for most people.

None of that is particularly interesting to me. Read more... )
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We all knew that Saturn's south pole looks like a whale's eye.

It turns out Saturn's north pole is a hexagon.

Freaky.
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People care so much about it. It's really less relevant than the argument over whether the millennium started on 1/1/2001 or 1/1/2000.

They followed precedent. Ceres was a planet for about two years after it was discovered (of course there was no IAB back then).

That said, James Nicoll, who is never wrong, is right about the problems with the definition. No other star but the Sun can have planets, nor (should we discover one) could we demonstrate that a new (say) Earth-sized body at 250 AU was or wasn't a planet, since we don't know whether its orbit region has been cleared... though perhaps we could prove mathematically that it must be. Perhaps the 'cleared its region' works as a mass limit.

It pisses people off -- cf. the metric system comment in Nicoll's thread -- but it was inevitable. The Kuyper Belt just has too much junk of too many different sizes; we are going through with KBOs what we went through two hundred years ago with asteroids.

But it bugs me a lot that the number of extrasolar planets has gone from 120+ to zero.
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Okay, so it really is a bigger deal than I thought, because of the weird geography -- the south pole is striped with areas a hundred degrees warmer than the rest of the moon.

So maybe the Enceladeans' power plants leaked heat into the surrounding rock, which melted and produced catastrophic geysers...
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Walking home after walking with Robert to school, I noticed the thin-crescent Moon (only because it happened from my vantage point to be brushing the tips of some trees as I walked, and my eyes picked up the relative movement).

Hey, cool. So I called Kate to come look.

Then I had a thought. Ducked my head inside to check my screensaver -- okay, about ten degrees north-north-east of the moon -- then went back out and peered till I found it. Fixed my eyes on it, lowered my head to be pressing ear-to-ear with Kate, and moved us till it was right at the tip of a convenient tree's top branch. She stared for a moment, then announced that she saw it, then bounced and bubbled with excitement when I told her what it was -- there are, after all, three, not two, solar system objects you can see in the sky during the day.
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In fourth through sixth grade, I spent an hour each week -- and did a summer camp -- at this place. That black addition in the back of the building that you can barely see over the roof? That's a planetarium. With a Spitz starball, the same model as the one at the Discovery Museum in Sacramento.

So you might say I have been into this stuff for a while.

Well, as it turns out, the Discovery Museum has need of a planetarium volunteer for Saturdays. And they'd of course love to have someone who knows the starball and the sky doing some of their shows.

The Saturday shows aren't rocket science: you have to gear it to be comprehensible to toddlers in the audience, so you mostly keep it super-simple, point out the north star, talk about a few constellations and their shapes, and that's about it. Still -- this is an opportunity I can't pass up... the Planetarium was a significant part of what made my childhood unique, and passing this on, in small doses, to another generation is irresistable.

The shows are at 1:00 and 3:00, so I'd be there for about three hours most Saturday afternoons. I talked it over with Robert and Kate; they are going to alternate Saturdays to come with me, so that I can spend some one-on-one time with each of them. (Kate in particular is in constant danger of being cheated out of 1:1 time since there's always someone around who's either less independent or more sophisticated, so this will hopefully work out to be positive for them as well.)
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I note the LA Times has some excellent commentary relating to Tycho Brahe.

Though I think "Brahean Blunder" is a bit awkward.

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