Monday, 25 October 2010
Cascades of Cassini's wonders
Saturn at equinox, found on APOD, imaged by Cassini.
I first fell in love with Cassini back in early 2007 when Mark Leese, who works on the project, came to give us a talk at Sussex University. At the time, Huygens had comparatively recently dropped onto Titan's soil. I remember three things most clearly from the talk. One was the video of Huygens spinning down on its parachutes, one of its instruments going thud-thud-thud like my heartbeat. Another was when he asked if we wanted a break, and Tim Metham, our course tutor, replied: "No, this is riveting!" - he wanted to hear it all, right now! And finally, he took us on a tour through Saturn's rings . . . those little blocks of ice, once thought to be dust and rocks, but made of frozen water, so they gleam . . . many of which looked like little dots - but one was blue. Was it an anomalous blob? No - it's the Earth.
And it wasn't for another couple of years that I encountered the traditional Pale Blue Dot, which you can see and listen to here. But the lump in the throat was exactly the same.
Here it is again, an insignificant point of light, a tiny flicker against this backlit Saturn:
Do read its caption on APOD. Imaged by Cassini.
I'm a poor substitute for Carl, but nevertheless I tried to give a little of that sense of hugeness in my Tea with the Stars lecture the other night. I described to the audience how the rings had scattered the sunlight to brighten up Saturn from behind - and then I zoomed in on the Earth. I don't know if it came across. It's often too personal to say.
Back to more practical terms, I was thrilled to be asked to write a piece for Astronomy Now's yearbook on what Cassini will be up to next year, and Keith, the editor, is happy for me to blog about what I found out.
I e-mailed various Cassini scientists and was answered by two, Carl Murray and Joe Burns, both of whom agreed to my ringing them up and taking up lots of their time with asking occasionally silly questions. Although I really must invest in a dictaphone or something else to record what people are saying (at the time I just scribbled it down; they were very sympathetic about waiting!), it's definitely easier to get information out of people by talking to them than by e-mail. I could ask very general questions and let what they said lead up to specifics; often the specifics came by themselves, rolling on waves of enthusiasm. I love talking to people who are exhiliarated by what they're doing! I hope it goes without saying that neither they nor Keith are responsible for any errors I have made . . .
Meanwhile, Keith had kindly pointed me to Cassini's 2011 timetable. That took some dissecting - mostly drawing up tables of types of event. To summarise, it'll make 16 orbits, usually using Titan's gravity for the slingshot effect but making 30 course corrections. It'll look at the Sun and our pale blue dot 11 times. Cassini has an elliptical orbit, allowing it to view moons at different distances from Saturn, and also goes "through the ring plane", from north to south, 29 times! This isn't through one of the actual rings, obviously; it picks fairly empty areas. But even so, it'll need to "employ protective mesaures" half a dozen times or so. I asked Carl Murray what these were and he said mostly turning the instruments inward, except obviously the cosmic dust analyser which loves that sort of thing.
And what about Cassini's main job - the moons? Well, as you'll see, it heads past lots of those. Most passes are only distant ones, though these can be useful, Joe Burns explained to me, as they show you the whole moon rather than just a "patch" of it; this allows them to check general brightness, which in turn tells us about their atmospheres, temperatures and so on. But the important, nearby passes will be Rhea, Enceladus, and Titan. Those will be checking the moons in great detail.
Rhea and Janus from Cassini. NB I'm finding all these on APOD but (update, Feb 2011) have just been told off by the legendary Carolyn Porco for not making it clearer that Cassini took them. All the originals can be found at that link. I'm now updating the links wherever possible - it's not easy! I will however keep the APOD links alongside as they are friendly and informative.
Here's where Huygens landed:
Tethys behind Titan from Cassini.
(A friend remarked that whenever he talks about Cassini he feels he should just shut up and show the pictures. I know how he feels! You can easily waste half a day going to APOD Search and typing in "Cassini" . . .)
Titan is the only moon in the Solar System with a thick atmosphere. This atmosphere is actually denser than the Earth's, and is mostly methane. However, this methane is split apart by sunlight, and if Earth is anything to go by this generates free radicals, which would then react with other methane and anything else around such as nitrogen to generate quite large molecules. This makes the famous haze which we can't see through. Cassini can see through to some extent with radar and IR. Radar doesn't bounce off liquid, so these dark patches were lakes:
Found on APOD; taken by Cassini Radar Mapper.
Herewith some beauties that Huygens found when it detached itself from Cassini and dropped down into Titan's atmosphere . . .
The landing site (having trouble finding the original one here) . . .
From eight kilometres high (JPL/Cassini) . . .
About five kilometres high, a fisheye view . . .
Rivers and lakes, as predicted (Cassini) . . .
And rocks and sandiness, just like Earth, or Mars. (Cassini)
And an artist's impression of what it looked like there.
So there we are. Titan is a solid world - but its surface is shaped like the Earth's, because of weather. That implies a cycle. Not a water cycle, for all water is frozen hard as rock there: it is, according to our best guesses, a methane cycle. It seems that those lakes and rivers are liquid methane, and methane rain falls from those hazy clouds. What we don't know is what drives the cycle: when does it evaporate or liquify? Is it the Sun - far less powerful out here, nearly ten times as far from Saturn as from the Earth (and those studying basic physics will know that means nearly 100 times less radiation, if I remember correctly) - or is it something else; volcanic activity perhaps? Can Titan hold onto its envelope of gas, or is it slowly losing it, as Mars probably lost most of anything it had lighter than carbon dioxide? The team may have found evidence of a changing coastline, but it's hard to tell, since these things take a long time to occur. There are still a few corners here and there of Titan unmapped (good old Huygens only lasted there an hour and a half), which is one of the tasks set for Cassini to do in its remaining estimated 7 years of life.
There is also some argument about wind on Titan. Dunes appear to point against the wind; this may be because only storms which occur rarely and go the opposite way from normal are strong enough to shift the sand. All in all, there are lots of delicious mysteries to go.
Besides its moons and rings, Saturn has a very complicated magnetosphere. Any planet with a molten core (that is, Earth plus the gas giants) has one of those. Do play around with it in this animation. Magnetic fields give off radio waves, which allows Cassini to study it, and it's got quite a few objectives on its (so to speak) hands. What's especially interesting about Saturn's magnetosphere is its interaction with its moons. Some moons have actually been found by local drops in the charged particles, which the moons take up - come to think of it I'm not sure if that's by gravity or by something else, sorry folks and anyone who can enlighten me and my readers, please do. (Good job this is a blog and not a news article. Of course, it may be that nobody yet knows why moons take up the charged particles . . .)
The effect of Saturn's aurora at its poles, from the VIMS probe, found at guess where. Sometimes these charged particle storms "punch through" Saturn's atmosphere, or indeed, drag it upwards. There are still a lot of mysteries and surprises - it was due to studying this magnetosphere that the moon Rhea has been hypothesised to have some kind of rings!
When I asked Carl Murray about this, he laughed wryly and said, "Well, that depends on who you believe!" He sent me a paper which describes their repeated efforts to go back to Rhea and find these rings again - which failed. Something is causing a local drop in charged particles; as yet, we know not what . . .
Rhea, the second-largest moon, whose surface is patchy and worn.
And, for me, the biggest surprise of all? Guess what's contributing not only to the very diffuse e-ring (the faint, outer one you see in that gorgeous backlit picture), but to the magnetosphere? The moon Enceladus.
Here it is, right in the middle.
These fountains are water. To be exact, they are salty water, indicating a rocky presence below, and they are ejected from the south pole by geyser activity. But why? Joe Burns remarked to me that something as small as Enceladus shouldn't still be hot; that it still is "calls into question our understanding of how things work" (always an exciting sort of sentence in science). It may be because it's in resonance with Dione, as Ganymede, Europa and Io are around Jupiter. This may be causing tidal shifts and heating.
Ultraviolet light from nearby stars is dimmed by these fountains, indicating that there's plenty of material there. Out in space, of course, with little or zero pressure but plenty of radiation flying around, much of it won't stay as water, but will break up into the charged particles that make up the magnetosphere.
Incidentally, the moons within the E-ring reflect more light than Saturn's other moons, indicating that they get blasted by these charged particles. I suppose to say that Enceladus "washes" them is going a little too far, but the thought made me smile. Actually, it's because such particles move pretty fast, and that melts their icy surfaces and keeps them smooth.
Another view of Enceladus's vents - if you click the first link, you'll see it's upside down! It makes for a terrific video too.
Enceladus looks, at first, like a pretty placid sort of world in comparison to all this trouble it's making - until you notice those tiger stripes. They indicate regions where the ice has melted. The stripes have a different temperature and composition to the rest of the moon, but as far as I know that's all we can say at the moment.
So, besides a very strange magnetosphere, unique rings, and the only moon in the Solar System with a dense atmosphere (Triton would have a similar one if it was warmer - on Triton, the methane is frozen, much as water is on Titan), Saturn has two moons where liquids are present. Could life exist in Titan's methane lakes, or under Enceladus's ice? Organic compounds and a liquid medium are present in both. I doubt it would be life like ours. I won't be personally disappointed if there isn't any. When people ask me if I believe in life on other worlds, I tell them that I accept my own ignorance on the matter and am simply waiting to see - which many people do not consider an acceptable answer! But life or no life, there's plenty for Cassini to do in its probably six remaining years.
Yes, it's due to last until about 2017. In fact, all flybys are planned until then. It may change, of course. But that's about how long the remaining rocket fuel should last. Once that time comes near, Cassini will head out a long way, 10,000km beyond the F-ring, to survey Saturn and all its moons from afar; then it will head in again - right into Saturn's atmosphere. While it's still transmitting, we might get our first glimpses of what it's like under that giant planet's visible surface. Further and further in it will head . . . until that's the end of the mission. And what an extraordinary mission it will have been.
Thanks to Keith at Astronomy Now, and Joe Burns and Carl Murray of the Cassini team for all their information, guidance and encouragement.
This was my last slide at my recent Cassini talk.