Saturn's Coolest #3: Hyperion
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.
Hyperion is pocked by craters, like almost everything is. The craters on Hyperion, though, are all very deep, and the material at the bottom of every crater looks oddly dark. And the craters look almost like they want to slump inward, like Hyperion really is too big not to be a sphere. But that's not the weirdest thing about it.
Hyperion's orbit isn't circular. It's significantly eccentric, despite being in the ring plane barely further away than Titan, and not inclined at all, and it's the only gas giant moon with an eccentric, non-inclined orbit, and it's by far the closest eccentric moon to any gas giant. And you wouldn't expect to find a moon with an eccentric orbit this close in, when there's so much traffic; eccentric orbits don't play well with other nearby orbits in the same plane. But that's not the weirdest thing about it.
It's in a resonant orbit with the massive Titan - for every four orbits of Titan, Hyperion completes exactly three. And Hyperion's point where it's nearest to Saturn - and thus nearest to Titan's orbit - is also the point where it's nearest to Titan - that's where the resonance meets up. But that's not the weirdest thing about it.
Hyperion is one of the least dense objects in the Solar System. It appears to be made mostly of ice, but it's less dense than solid ice would be. It has to have massive caverns - void spaces - deep inside itself. But that's not the weirdest thing about it.
Every moon of every planet, except for the really, really distant ones - we're talking the ones five or ten million miles out - feels the effect of its parents' tides, and keeps one side pointed at the planet it orbits. Our moon does this, and Earth has really wimpy gravity compared with the gas giants. Titan does. Triton does, and it goes around Neptune backwards. Even Iapetus, orbiting two and a half times as far from Saturn as Hyperion, keeps one face toward the planet.
Hyperion doesn't. Okay, so neither do really distant moons like Phoebe - Phoebe takes two Earth years to go around Saturn and it rotates in two and a half Earth days. That Hyperion doesn't have a rotation equal to its revolution isn't the weird part.
Hyperion doesn't have a rotation period at all.
Hyperion tumbles chaotically. It does an end-over-end constant backflip with multiple axes of rotation, like an Olympic athlete going off the high dive.
And every sixty-three days - when it makes a close pass by Titan - the tumble changes. Standing on the surface of Hyperion, not only do Saturn and the stars an the Sun trace complicated patterns, curving in the sky, sometimes rising and setting on the same horizon, never repeating their pattern twice... and it can't ever settle down to a rhythm of any kind because it keeps getting respun, like a child repeatedly batting a spinning top every time its wobble might calm down, or like the high diver - on his infinitely-long high dive loop - bumping the next lower diving board and changing his complex maneuver, over and over.
There is so much about Hyperion that fascinates me. That tumble is to me as bizarre a gravitational wtf as Epimetheus and Janus swapping orbits. But there's still more to it.
Here I'm totally speculating, but there's a bunch of accumulated historical hints about the Saturn system, about something catastrophic happening in the very, very distant past, and to me it all seems to center on Hyperion.
Consider what Hyperion is. It's too big not to be a sphere, but it's not a sphere. That's because it's not dense enough - less than a third the density of most other sizeable moons, about half the density of water - suggesting it has huge voids inside it, since there's not much that's solid but less dense than ice. But it's way too big to have *formed* with large voids inside it, if it formed the same way as the other moons. And it's in an eccentric orbit, when everything else near it is (for good reasons) in circular orbits.
This suggests pretty firmly that Hyperion is what was left over when some larger moon, outside of Titan's orbit, was broken into pieces - probably from a massive collision, with a comet or Centaur or another, obliterated moon. And several pieces of that broken moon later settled back together, awkwardly but reasonably gently, to make Hyperion.
But there'd be other pieces from the collision that went elsewhere... and some may have fallen into the inner system and caused the massive craters on Mimas and Tethys. Probably it's why there are so many rocks around in there - the three moons between Mimas and Enceladus (Methone is one), the four companions of Tethys and Dione, the dozen or so small ones (Prometheus and Pandora and Epimetheus and Janus and several smaller, less-interesting neighbors) near, and even among, the rings. No other gas giant has as many tiny moonlets so close to it.
And perhaps some of the broken pieces of this old moon wound up falling all the way inward, getting trapped in very low orbit by interactions with the moons, and eventually breaking up ... to form Saturn's rings. Hyperion may be the relic left behind from the event that made Saturn into the beautiful ringed planet we know.
Or perhaps not. Maybe Mimas' Herschel and Tethys' Odysseus/Ithaca and the dozens of sizeable rocks in the inner system and Saturn's rings and Hyperion's voids and eccentricity and tumble all have totally separate causes. Maybe.
But I'd really like to know. I want us to explore the great ice caverns of Hyperion someday.
Still have two more moons to go - two moons more interesting than all of these. If you know Saturn you know which ones they are, but there's no question they belong at the top.
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.
Hyperion is pocked by craters, like almost everything is. The craters on Hyperion, though, are all very deep, and the material at the bottom of every crater looks oddly dark. And the craters look almost like they want to slump inward, like Hyperion really is too big not to be a sphere. But that's not the weirdest thing about it.
Hyperion's orbit isn't circular. It's significantly eccentric, despite being in the ring plane barely further away than Titan, and not inclined at all, and it's the only gas giant moon with an eccentric, non-inclined orbit, and it's by far the closest eccentric moon to any gas giant. And you wouldn't expect to find a moon with an eccentric orbit this close in, when there's so much traffic; eccentric orbits don't play well with other nearby orbits in the same plane. But that's not the weirdest thing about it.
It's in a resonant orbit with the massive Titan - for every four orbits of Titan, Hyperion completes exactly three. And Hyperion's point where it's nearest to Saturn - and thus nearest to Titan's orbit - is also the point where it's nearest to Titan - that's where the resonance meets up. But that's not the weirdest thing about it.
Hyperion is one of the least dense objects in the Solar System. It appears to be made mostly of ice, but it's less dense than solid ice would be. It has to have massive caverns - void spaces - deep inside itself. But that's not the weirdest thing about it.
Every moon of every planet, except for the really, really distant ones - we're talking the ones five or ten million miles out - feels the effect of its parents' tides, and keeps one side pointed at the planet it orbits. Our moon does this, and Earth has really wimpy gravity compared with the gas giants. Titan does. Triton does, and it goes around Neptune backwards. Even Iapetus, orbiting two and a half times as far from Saturn as Hyperion, keeps one face toward the planet.
Hyperion doesn't. Okay, so neither do really distant moons like Phoebe - Phoebe takes two Earth years to go around Saturn and it rotates in two and a half Earth days. That Hyperion doesn't have a rotation equal to its revolution isn't the weird part.
Hyperion doesn't have a rotation period at all.
Hyperion tumbles chaotically. It does an end-over-end constant backflip with multiple axes of rotation, like an Olympic athlete going off the high dive.
And every sixty-three days - when it makes a close pass by Titan - the tumble changes. Standing on the surface of Hyperion, not only do Saturn and the stars an the Sun trace complicated patterns, curving in the sky, sometimes rising and setting on the same horizon, never repeating their pattern twice... and it can't ever settle down to a rhythm of any kind because it keeps getting respun, like a child repeatedly batting a spinning top every time its wobble might calm down, or like the high diver - on his infinitely-long high dive loop - bumping the next lower diving board and changing his complex maneuver, over and over.
There is so much about Hyperion that fascinates me. That tumble is to me as bizarre a gravitational wtf as Epimetheus and Janus swapping orbits. But there's still more to it.
Here I'm totally speculating, but there's a bunch of accumulated historical hints about the Saturn system, about something catastrophic happening in the very, very distant past, and to me it all seems to center on Hyperion.
Consider what Hyperion is. It's too big not to be a sphere, but it's not a sphere. That's because it's not dense enough - less than a third the density of most other sizeable moons, about half the density of water - suggesting it has huge voids inside it, since there's not much that's solid but less dense than ice. But it's way too big to have *formed* with large voids inside it, if it formed the same way as the other moons. And it's in an eccentric orbit, when everything else near it is (for good reasons) in circular orbits.
This suggests pretty firmly that Hyperion is what was left over when some larger moon, outside of Titan's orbit, was broken into pieces - probably from a massive collision, with a comet or Centaur or another, obliterated moon. And several pieces of that broken moon later settled back together, awkwardly but reasonably gently, to make Hyperion.
But there'd be other pieces from the collision that went elsewhere... and some may have fallen into the inner system and caused the massive craters on Mimas and Tethys. Probably it's why there are so many rocks around in there - the three moons between Mimas and Enceladus (Methone is one), the four companions of Tethys and Dione, the dozen or so small ones (Prometheus and Pandora and Epimetheus and Janus and several smaller, less-interesting neighbors) near, and even among, the rings. No other gas giant has as many tiny moonlets so close to it.
And perhaps some of the broken pieces of this old moon wound up falling all the way inward, getting trapped in very low orbit by interactions with the moons, and eventually breaking up ... to form Saturn's rings. Hyperion may be the relic left behind from the event that made Saturn into the beautiful ringed planet we know.
Or perhaps not. Maybe Mimas' Herschel and Tethys' Odysseus/Ithaca and the dozens of sizeable rocks in the inner system and Saturn's rings and Hyperion's voids and eccentricity and tumble all have totally separate causes. Maybe.
But I'd really like to know. I want us to explore the great ice caverns of Hyperion someday.
Still have two more moons to go - two moons more interesting than all of these. If you know Saturn you know which ones they are, but there's no question they belong at the top.