Date: December 9, 2010
Title: Titan and the Saturnian System
Podcaster: The Ordinary Guy from the Brains Matter podcast
Organization: Brains Matter – http://www.brainsmatter.com
Description: In today’s episode of 365 Days of Astronomy, Dr Andrew Prentice from Monash University explains some of his theories which he proposed initially in the 70s, and how they relate to the formation of the Saturnian system.
Bio: The Brains Matter podcast has been producing and communicating science stories and interviews since September 2006. The show is based out of Melbourne, Australia, and takes an everyday person’s perspective of science in easy-to-understand language.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2010, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
Hello everyone, and welcome to todays 365 DAys of Astronomy podcast. I’m the Ordinary Guy from the Brains Matter podcast – the show on science, curiosities and general knowledge. You can find that show at www.brainsmatter.com.
On today’s episode, I talk to Dr Andrew Prentice from Monash University. Andrew has been the centre of a bit of controversy over his career, with his view of the modern laplacian theory of solar system formation and the theory of supersonic turbulence. He has had some pretty amazing predictions that have turned out to be true, and you can find some of these on wikipedia. He also has a bit of a cult following at the university for his turn of phrase.
I spoke to him on some of his views of the formation of the Saturnian system.
Ordinary Guy: Do we know where Titan came from?
Andrew Prentice: That’s a very important question. I think Titan formed as a secondary planetary embryo in the same orbit of Saturn. Saturn, maybe once, had a core of about 15 Earth masses of… it was a pod, and then it captured it’s atmosphere of about a thousand Earth masses, meaning – it probably only captured a tenth, that say… 80 Earth masses then it was captured by the fifteen Earth mass core – the planet today weighs 95 Earth masses. So that pod would have – the way that great rocky core formed, because in the beginning, Saturn itself was spread around its orbit, there was a gas ring there, mostly hydrogen and helium. The gas cooled down, and tiny little grains settle out of the gas to form a concentrated stream, these little grains settle out of the gas to form a slurry on the mean orbit of the gas ring. And when this sort of concentrated stream of solids became sufficiently concentrated, although these grains themselves are very small, once you’ve got a great slurry of them there, they can start to feel their own self-gravity and then they start to clump together around the orbit. You more or less get this clumping process occurring early in the piece and you might even get a runaway and a whole lot of these things would form. Then you’d have another secondary swarm as well.
But the process of planetary accretion is very efficient and most of it would congeal to form a single planet, but it is possible you might have an isolated embryo – and Titan weighs about a ten thousandth of the mass of Saturn and so about a thousandth of the mass of this rocky core, and so it is possible that Titan tried to form its own planetary centre to capture gas, but it wasn’t fast enough and the main planetary core formed from which Saturn then once that happened captured its atmosphere and left Titan stranded and feeling a bit miffed around in the orbit. And eventually after years and years in the Saturn orbit, Saturn would have become aware there was this sort of impostor on the same orbit, and a failed sort of giant planet and it would have caught up with it. Now Titan at that stage was so big that it could have easily – even if Saturn had attracted to it, the great atmosphere of Saturn was a huge thing and very, very light – Saturn would have swept through that atmosphere and not been worried about it.
This could have happened a few times and eventually Saturn would have contracted inwards, and eventually when Saturn formed its moons, then it would have been a different story and I think when Titan, on one of its close flirtations with Saturn, didn’t realise it was heading straight towards a huge moon, like a companion of Rhea – Rhea has a radius of 765 – you know, it’s 1300 kilometres across, a huge distance – Saturn probably… Titan during its flirtative pass of Saturn, not looking where it was going and making a rather rude gesture at Saturn ran head on into this moon of which Hyperion is a remnant, so Titan was actually captured. It formed in the same orbit as Saturn, and I’ve actually used my theory to predict it was made of 50% rock and 50% water ice. Titan ran into one of these moons which slowed it down, and then it went into a very long, very eccentric orbit – a captured orbit around Saturn.
And even today, Titan still has an eccentricity in its orbit. It’s about three per cent and people are intrigued where it came from. But in the beginning, it fits in with my theory in fact; the eccentricity was much bigger. TItan’s eccentricity is slowly being eroded and eventually the moon will adopt a circular orbit so I think that Titan is an interloper. All of the physical characteristics of it point towards it. They still haven’t measured how much rock inside there is and how much ice, its’ very complicated and they’ve got to make a number of gravitational passes, but I have worked out what the internal structure should be as well, and that’s going to be tested over the next little while.
It’s a complex beast, Titan. Its gravitational field has a primary gravitational field due to its nearly spherical shape but it also has what I call mass anomalies on its mantle which I had predicted. The rough estimate I had of the moment of inertia is about 0.317 – it does seem to fall short of their current best estimate, but their current best estimate hasn’t been confused by.. hasn’t been able to be resolved by these gravitational anomalies – that has to be properly ironed out.
OG: Okay, the second question I had about Saturn was the rings – how do hey fit in with the theory…
AP: Yes, the rings are very intriguing. There was Cassini and I think.. yeah.. notice that, and of course many observers have looked at the rings of Saturn. They’re almost – they’re very reflective. They’re so bright, they’re really pure water ice and if my theory was right, and the rings had formed from a former moon of Saturn, you’d expect as you got that close to Saturn you’d expect to see a lot of rock, so why aren’t the rings – why don’t they have a lot of rock inside them? That’s always been a worry for me and in fact I think I did predict there’d be a ring of Saturn called the F ring which would be made of rock particles but that’s not quite true, I mean that’s a prediction that did become unstuck. I did correctly predict the position of the ring, but it is made of ice, not rock, and that’s been a really big challenge and what I discovered was, as I mentioned earlier, the outer moons of Saturn have densities that fit in with this idea that they gradually increase as you go towards the planet, but then you come in within the orbit of Dione and then all of a sudden the density of Dione is something like 1.48 or 1.5; you come inwards towards Tethys and instead of the density going up, it actually goes down!
The density of Tethys is only about 1 – about the same as that of water. This shouldn’t happen according to my theory. And then if you go to Enceladus it’s also I think the next moon in actually, Enceladus the density goes up again to 1.6 so there’s somehow a dip in the densities that cannot be explained by my theory in its original state. And so what I realised was that as this great cloud of Saturn contracted onto this pod, eventually the stage would be reached – we have an 80-Earth masses of gas crimping down on this central core of rock and ice there it’s going to get so hot that it’s going to get so hot that much of the ice from the interior of the planet may be evicted. And about three years ago I published my theory for Rhea. I pointed out that when Saturn cloud had contracted inside the orbit of Dione maybe the interior, the core of Saturn had liberated a lot of water vapour and a lot of water had come out and drenched the atmosphere and the reason why the inner moons and particularly why the rings are so icy is that you had all this additional water that was evicted from the interior of Saturn, and in fact, that would explain why Tethys’s density drops, you know, down to 1, but then of course the densities increase again up to 1.6 but that fits in with the fact that the temperature is now starts to rise up again and so.. it’s all very well to make these crazy ideas, but I made the prediction that Enceladus formed at such a high temperature, in such a steamy environment, it actually formed as a watery moon.
It has a core of rock and then it would have – it has a radius of a great ocean of water around it – the whole moon only has a radius of about 250 kilometres but there would have been a rocky core of about 170 and a great watery ocean and I would have called it like a rain moon and it formed, it literally formed a rock and then a lot of water that rained out onto that to form a mantle. And sure enough, Enceladus has got a pure – a pure ice mantle. It’s just pure water ice and this can only be explained in this way if it once was actually a moon that had an ocean that gradually froze. And that fits in well, and then you come into Mimas, it’s actually, the whole of Saturn is cooling down and then it would – although Mimas would have started out as an ocean moon, it is also pure ice, it would have frozen – but then you come inwards again and you’d expect to see, maybe you’d expect to see smaller bodies that were.. that didn’t enjoy the ability to form oceans but you know.. the rocky objects and of course Saturn’s rings aren’t rocky, they are made of ice, so what I proposed were that the innermost moons of Saturn, interior to the orbit of Mimas – um… Pandora, Epimetheus, Prometheus, all these inner moons, they’re only about a hundred kilometres across, they did form as rain moons with little rocky cores, and then they froze, and they’re all close to each other and then over the great aeons of time that have elapsed since Saturn formed, and due to the ti.. the chaotic motions of Titan, these.. these inner moons would have been… struck one another and they do have co-orbital moons, and moons that do collide, and therefore the rings of Saturn are a fairly recent formation… they the rings of Saturn are shards of pure ice that have come from the mantles of brushing against each other of these tiny inner Pan and all these inner moons that have formed and then Atlas and Tethys and they’ve hit one another and brushed against one another and tiny shards of pure ice have come off and these tiny shards then will spiral in towards the planet, and that does seem to fit in with the chemistry of the model.
But it can only be achieved if you adopt this idea that the innermost moons of Saturn actually had formed in a liquid state and that’s being tested at the moment. So I think the bottom line is the following:
The Saturn system would have liked to have been like a miniature Galilean, like a Galilean system. Titan unfortunately came along and thwarted that plan. It destroyed some moons, and also the moons of Saturn hug that planet much closer than the moons of Jupiter and Saturn is three times smaller. And those inner moons… those inner gas rings of Saturn, they were still quite cold as I said, they were able to be cold enough so you could form ice and also closer in, water ice. If you look at the same position at Jupiter and the reason why Jupiter doesn’t have icy moons is that everything was so much hotter. And you see Io is mostly made of rock and iron and if you go inwards from the orbit of… I predicted there should be a rocky ring around Jupiter and that was in fact confirmed – a bit closer than I expected, so Jupiter was never able to enjoy the privilege of ice that Saturn enjoyed. Jupiter was too big and brash, made too much noise – a bit like me! Saturn went about its job a bit more quietly and finished up with a very nice result at the end. A very spectacular ring system. But it has this villain called Titan that’s still in orbit who’s done so much damage and even today, the navigation people at JPL say they still have trouble trying to understand the orbits of the Saturn moons – Titan is just introducing so much chaos – moons aren’t exactly where you want them to be, they try to do photographs, they have to slew the cameras and it – he’s such naughty fellow and … uh .. years of rehabilitation will probably be required and.. but I mustn’t enter politics now, but I am tempted to .. thinking back on one of our former Prime Ministers … but anyway, I think a detention centre would be the correct place for Titan.
Naughty Titan indeed! If you want to hear more entertaining and interesting stories on astronomy, and science in general – or want to hear more from Dr Prentice, head over to www.brainsmatter.com.
Bye for now!
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.