Date: August 28, 2009
Title: Saturn’s Moon Enceladus – Past, Present, and Future
Podcaster: David Seal
Organization: None
Description: August 28th will be the 220th anniversary of William Herschel’s discovery of Saturn’s moon, Enceladus. Enceladus is rapidly becoming one of the most interesting bodies in the Solar System, due in large part to jets of water vapor emanating from it’s active south pole which hint at the possibility of subsurface water pockets or an ocean. This exciting object is currently under study by NASA/ESA’s Cassini spacecraft currently orbiting Saturn, and moving towards its second mission extension with more close flybys of Enceladus on its timetable.
Bio: This podcast covers the history, current understanding, and upcoming plans for Enceladus, and is moderated by David Seal, Cassini Mission Planner at NASA’s Jet Propulsion Laboratory, with participation from Cassini Project Scientist Dr. Robert Pappalardo, JPL Enceladus expert Dr. Amanda Hendrix, and Marek Kukula, Public Astronomer of London’s Greenwich Observatory, which is home to the only surviving component of Herschel’s telescope used to discover Saturn’s now famous moon.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Physics Department at Eastern Illinois University: “Caring faculty guiding students through teaching and research” at www.eiu.edu/~physics/.
Transcript:
Transcript for IYA “365 Days of Astronomy” podcast, August 28th.
Podcast by David Seal, Mission Planner, Cassini Project. NASA / Jet Propulsion Laboratory, California Institute of Technology.
David Seal: Hi, and welcome to the International Year of Astronomy’s podcast for August 28th. I think you’re in for a real treat today, as we mark the 220th anniversary of the discovery of Saturn’s fascinating moon, Enceladus, by the British astronomer William Herschel. Enceladus is rapidly rising to the top of the list of fascinating objects in the solar system. My name is David Seal, and I am the mission planner for NASA’s Cassini project, which has had a spacecraft in orbit around Saturn for over five years, and as your guide today, I’ll explain why this tiny moon is so interesting. We’ll be talking about Enceladus – it’s past, present, and future.
Now, earlier this summer I was fortunate to be among the group of NASA scientists and engineers who got to travel to London, England for one of Cassini’s annual science meetings. In the middle of a week – a really intense week of scientific discussions and strategic planning, we were really pleased to be invited to the Royal Observatory in nearby Greenwich. Now, this observatory is perched on a hill in the middle of a beautiful park overlooking London to the west, and so, it offers really stunning sunset views of the city in addition to hosting a superlative observatory and astronomical museum.
Some of you may be familiar with Greenwich, as the zero meridian of our longitude system passes through it and takes its name, but our travels today take us to a small courtyard on the edge of the observatory grounds which happily hosts a large, ten-foot section of William Herschel’s grand 40-foot telescope. Herschel almost certainly discovered Enceladus with this telescope, or at least observed it many times.
Standing next to this important historical antique, I had a chance to chat with Dr. Marek Kukula, a galaxy and quasar expert who holds the title of Royal Observatory Public Astronomer.
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David Seal: A wonderful acquisition to display for..
Marek Kukula: It was like rusting at the bottom of his garden for decades, or something…
DS: So he must have ground the mirror, or had it ground..
MK: Yeah, it must have been one of the biggest optical engineering jobs ever undertaken at the time.
DS: Not far from London, then?
MK: Yeah, it’s about 30 miles from here – 50 kilometers from Greenwich. When he lived there, it was a very nice country town.
DS: I did look it up on Google Earth, and there’s not a lot about Herschel in that town.
MK: It’s all sort of traffic circles and industrial estates and things.
DS: Well, it sort of enhances the idea that the some of the greatest discoveries were made in the simplest of places.
MK: It is a key piece in our collection. It has moved around over the years, and the setting it’s in at the moment is brand new. This garden’s been re-landscaped in the last few weeks. So I think it’s a really nice way to display it. But, I don’t think there was never any question really of having it inside.
DS: Giving you an idea of what it was like.
MK: In terms of collecting area is not particularly impressive compared to modern telescopes, but it was 40 feet long, which is still pretty big. Here in Greenwich we’ve just had – two years ago – a massive renovation which has allowed us to present modern astronomy as well as the history of astronomy. So we now have London’s only planetarium, we’ve got fantastic interactive galleries that present modern science as well, so we can tell the whole story of astronomy from the 17th century right through into the 21st century, when all this exciting stuff is happening. So it’s a really great time for us to be talking about astronomy.
DS: I wonder what Herschel would think today, it was just a speck of light to him, and it’s a whole world to us now.
MK: I think Herschel would have been really excited to be an astronomer today.
DS: Almost everyone who works at JPL has seen this drawing of this enormous structure.
MK: Yes. Well there’s another really nice story about it, actually, and that’s before he was an astronomer, Herschel trained to be a musician, and he was a composer, he was a very good musician in terms of playing as well, and his sister also was very musically talented. Of course Caroline Herschel, extremely important astronomer in her own right. And the story is, that the composer Haydn, when he was on one of his visits to London, actually came out to see Herschel and his telescope and looked through it and was so inspired by what he saw that he went away, and this was one of the things that inspired him to write the very famous chorus The Heavens are Telling the Glory of God, from his Oratory of the Creation. So it’s a nice example of how science and art kind of combine together.
DS: The most magical, interesting and culturally significant things seem to happen when the two get together.
MK: I think that’s very true. It’s interesting to think that when the Royal Observatory was founded in 1675 by Charles II, any educated cultured person would be expected to know about science and astronomy just as much as they’d be expected to know about literature and art and music, and that was just part of being a cultured person. It would be nice to think that we could move back towards that kind of model that the arts and the sciences weren’t seen as being quite such separate things.
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David Seal: Well, Marek and his colleagues certainly were excellent hosts to us that evening, it was a really great time. You know, if you look close, you can actually pick out Herschel’s tube just using Google Earth or Google maps, if you are sharp enough to find the Royal Observatory near London. Look for a rectangular structure kind of near the main observatory buildings. I’ll leave the proof of this as an exercise for the listener.
Now, one of the most exciting astronomical discoveries of this decade has got to be the discovery of plumes of water vapor emanating from the south pole of Enceladus. Keep in mind that this is a really tiny moon that is ten times as far from the Sun as the Earth. Now, with the Sun’s light expanding in all directions, side to side and up and down, that means that Enceladus really only gets one percent as much light, and warmth, and energy from the Sun as we do here on Earth. Therefore, the environment in orbit around Saturn is really bitter cold. To find a geologically active body in such an environment is stunning. Just the possibility that liquid water could exist beneath the surface of Enceladus, and be the source of the plumes, has shocked a generation of scientists, and will certainly keep the Cassini spacecraft and project busy for years to come.
To talk more about Enceladus’ plumes, and some upcoming observations that Cassini will make, I took some time with Dr. Robert Pappalardo, Cassini project scientist, and Dr. Amanda Hendrix, who is the co-lead for Cassini icy satellite planning.
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David Seal: So what do you remember from when you first heard about the plumes?
Amanda Hendrix: So I remember where I was before even before we knew that there was a plume at the south pole, but when we were first getting clues.
DS: It came in steps, right?
AH: Yeah, it was first in April 2005, and we were here at the lab having an icy satellites workshop. So we were over in building 183, and I remember very clearly Michele Dougherty giving her presentation about her magnetometer measurements, and saying something funny is going on – I don’t know if Enceladus has an atmosphere, or what.
DS: Because the magnetic field lines were bending in a strange way…
AH: They were getting draped around the body, like a non-inert body. So something funny was going on. And so she made a very compelling argument to make the next flyby, which was going to be in July of ’05 much closer than we were planning it. That was really interesting, and kind of exciting, because nobody knew quite what was going on, but something was.
DS: So it came in steps, over about a year, didn’t it, the clues were coming in piece by piece from the different instruments.
AH: Yeah, so the magnetometer got clues in February and March of ’05, those two flybys, and then, of course we lowered the July 2005 flyby, and that was the smoking gun, and we still hadn’t even seen the plumes at that point.
DS: With images, or anything like that.
AH: With images. That came later on in November.
DS: What do you remember about hearing about it?
Robert Pappalardo: Well I wasn’t part of the Cassini project yet at that time, I was a professor at the University of Colorado, and the first I really heard an announcement was an email from John Spencer, that said OK, now the story can be told because the Science articles were out. He send an email to a bunch of us, and attached the press release, and I was bowled over. I kind of remember thinking, “Oh yeah, activity, well, let’s see what the evidence is,” and went to the site showing the images, and it was spectacular. I could not believe that there was measurable heat coming off of Enceladus besides the bizarre geology itself. I was absolutely blown away that we were seeing an icy satellite in action. And what’s wonderful is how Enceladus sheds light on other moons. It’s like a dream come true to be able to watch what’s going on, when it’s going on, and then be able to go back to the other moons and say, oh maybe that’s what happened here, trying to put the pieces together in looking at other satellites like Europa or Miranda, and here it’s going on at Enceladus.
DS: Yeah, most of the time we look at things that aren’t dynamically varying, they’re not in action. Things happened millions of years ago, and we’re just seeing the aftermath.
RP: Right, and Io is the obvious exception, but it’s a rocky guy, and here finally was an icy satellite doing its thing. But it seems like the evidence is pretty good that there is liquid water somewhere down there, I’m not convinced that it’s connected to the surface. But that’s what the research is all about.
AH: But you know, liquid water under the surfaces of icy satellites is, sort of, not infrequent these days.
RP: Except, again that Enceladus is so small. And so now we’ve been surprised again. Could there be water at Enceladus? Maybe it’s really easy to get liquid water.
DS: What do you think Herschel would think about Enceladus today since his discovery. I mean he just saw a tiny white light.
AH: He did. And I think at the time, he didn’t know how exceptional it was. Because he had no idea then how big it was, or small. So he couldn’t attach a size or give it an albedo or a brightness. But I think he would think it was pretty cool.
RP: I’d bet he’d wonder how the heck it could happen. He’d probably want to learn all about tidal heating, and what makes a moon like Enceladus tick.
DS: It’s almost a pity that he had no idea how exceptional a satellite it was that he discovered.
AH: People had suggested it was active before [now], but nobody could make it work.
RP: And the degree of activity, how extreme it is.
AH: And the other funny thing though is the south polar location, still. We have ideas for why it’s there, but it’s a little bit different than anybody expected.
DS: Why there, why just that pole.
RP: And only there. Concentrated there.
DS: Cassini’s fortunate enough to have a couple of flybys coming up in the next month or two. We’re numbering them, and we’re calling them E7 and E8, and they’re both in November, right?
AH: Right. So E7, which is the seventh targeted flyby, is on November 2nd, and E8 is on November 21st, and these’ll be really great flybys. On E7, actually, we are going to be the closest to the surface in the plume that we’ve ever been. So we’ll be in the densest part of the plume that we’ve ever been. So this’ll be really exciting. So the fields and particles instruments will get to make some really great measurements, and kind of taste and sniff the plume, and really see what it’s made out of, and how dense it is, and what kind of gasses and particles are there. And on E8, this one is a little bit farther out at 1600 km from the surface, which is ideal for taking pictures, and doing compositional measurements, and temperature measurements. And that’ll be the key thing of this flyby, to get high resolution complete temperature maps of the south pole. So we need to finish doing that, but also there are some indications of variability of temperature along these tiger stripes, so that’s really interesting too.
RP: And looking farther beyond, to doing gravity science at Enceladus, and revealing what’s going on in the interior. Is there something different below the south pole compared to the rest of the satellite.
DS: Great. So people who are interested in Enceladus, and are as excited about it as we are should stay tuned for even more exciting and interesting papers and articles coming out for it.
RP: Definitely.
DS: Thanks very much.
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David Seal: Well I really hope you enjoyed this podcast, and will take some time to explore some of the things we talked about today. You can learn more about William Herschel and the Royal Observatory on the web at http://nmm.ac.uk and you can learn more about Enceladus, Saturn, and the Cassini mission at http://saturn.jpl.nasa.gov
Thanks very much for listening.
End of podcast:
365 Days of Astronomy
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