FTS-NASA

Moderator: Amanda Hendrix
February 27, 2007
1:00 pm CT

Coordinator:	I would like to notify all parties that the call today is being recorded. If you do have any objections, you may disconnect at this time.

	Thank you.

Amanda Hendrix:	Okay. Hi everybody. Welcome to the February 2007 CHARM telecon.

	Today, we have Dr. Ralph Lorenz from Johns Hopkins University, the Applied Physics Lab with us, and he’s going to be telling us about dunes on Titan.

	So Dr. Lorenz, please go ahead.

Ralph Lorenz:	Well…

Amanda Hendrix:	Whenever you’re ready.

Ralph Lorenz:	Good morning, good afternoon.

	The first slide is me just to - what I look like. I’m standing on a sand dune. In fact, the right kind of sand dune in Northern Arizona; and in fact, until August of last year, I worked at the University of Arizona in Tucson, which is a rather more desert-like place than Maryland. But my enthusiasm for dunes remains.

	If we skip on to the next slide there, you enjoy a little bit about of advertising. The book out on spinning flight -- Frisbees, boomerangs, samaras, and skipping stones, there’s also stuff about spinning rockets and satellites and asteroids.

	And the Huygens probe, the Huygens probe was spun for stability before it hit Titan’s atmosphere and it has a set of little wings on it to spin it at a controlled rate during the parachute descent to pan around the field of view of the instruments and in particular, the camera. Interestingly, it sort of spun the wrong way, which is something that is still puzzling us.

	But some of that is discussed in this book, which you might find interesting, those of you especially involved in education, you know, getting people to think about how a Frisbee actually works and so on, it’s always a good example to visualize.

	And for the hard core space heads among you, it’s a rather encyclopedic text coming out in a month or so by Andrew Ball and myself and a couple of others which includes a lot of technical details on all the space probes and planetary landers. When I say “probes,” I mean specifically things that have gone in-situ that have been at or on a planetary body. And there are several detailed case studies and of course, Huygens is one of those.

	So with that advertising out of the way, going on to the third slide. This is a rather old one we’ve put together in 1995 - sorry, 1994 just when I moved from the University of Kent where I got my PhD working on part of the Huygens probe to the University of Arizona to work on HST imagery of Titan which had just started to improve as the Hubble space telescope was repaired.

	This is a set of four images. They’re not really images they’re a map re‑projected onto a globe viewed from four longitudes. Remember, Titan rotates synchronously so it always points the same face to Saturn.

	And the upper right is Titan’s leading face or the forward face if you like. And this crude map was made with HST images that 940 nanometers, that’s in the near infrared but is still of the wavelength that can be detected with CCD detectors, the kind you probably have in your video cameras.

	940 nanometers is - you can compare it with 670 which is sort of red and 430 which is blue, so it’s, you know, well beyond the red but - and beyond our ability to see. It’s the wavelength that TV remote controls operate at, if that helps at all.

	But anyway, as you go to longer wavelengths, Titan’s atmosphere is progressively more transparent and so you can see through the haze that basically blocks the view of the surface from Voyager’s cameras, which only went out to the red wavelength, and which is, you know, as you might expect ‑- bright areas and dark areas, not uniformly bright and dark and that immediately poses the question: well, why? What is it that makes some areas brighter than others? Are they being covered in something, or are they being, you know, are they having something washed off?

	As you may know, one of ideas about Titan -- an enduring idea and one that has been resurrected in the last few months -- is that there may be abundant hydrocarbon liquids on Titan’s surface. Methane and ethane are both liquids at Titan’s surface conditions. Methane has been detected in its atmosphere and so the idea of, you know, the atmospheric abundance being buffered by deposit from the surface came about.

	Also, methane is destroyed by solar ultraviolet light and one of the main products of that process is believed to be ethane, which is also a liquid. And so, there’s the whole idea that Titan might have seas of methane that may, over geologic time, get progressively enriched in ethane, and these would appear dark. And so, one idea was that maybe all these dark areas are seas. And it turns out that’s in fact what they are but not quite in the sense that we thought.

	Titan has proved to be much more interesting and complicated than we ever imagined back - even only just over a decade ago.

	Going on to the fourth slide, this is just a quick retrospective before Cassini’s arrival.

	It turns out if you know what to look for there is a hint of signal of Titan’s surface albedo; it’s reflectivity in bright and dark areas in Voyager data. The Voyager camera only went out to orange wavelength; it didn’t even go all the way into the red let alone the near infrared.

	It turns out there’s just a little bit of transparency in Titan’s atmosphere there such that if you try and make a map and send out - you have to throw away about 95% of the light because that’s all just scattered in the atmosphere.

	There is consistent pattern of bright and dark areas on the surface. But, as you can see on the bottom, sort of match up with both the map seen from Hubble space telescope.

	And just as a sort of aside, if you were in a spacesuit floating around Saturn, you would -- especially if you put on red sunglasses to isolate the longer wavelength and maybe even polarizing sunglasses to reduce the scattering a little -- you should see some bright and dark patterns.

	And of course the question is well, what are these patterns?

Amanda Hendrix:	So Ralph?

Ralph Lorenz:	Yeah?

Amanda Hendrix:	In the bottom of Page 4, the HST maps are the same as on Page 3 but different wavelengths then?

Ralph Lorenz:	The map on the right is exactly the same map as was projected on the four globes. The map on the extreme left is also a Hubble Space Telescope map but at 673 nanometers at red wavelength. So it’s got the same pixel scale, if you like, as the longer wavelength one.

Amanda Hendrix:	Okay.

Ralph Lorenz:	Really adequate map. But it’s a wavelength that’s comparable with the central image which is the Voyager map.

Amanda Hendrix:	Got you.

Ralph Lorenz:	I mean, as you can see, the Voyager coverage was not very good. I mean we just kind of whipped past.

	The sequence of observations that were used to make the Hubble map just for reference took, I think, 14 HST orbits. HST time is allocated in orbits which is roughly 90 minutes but spaced over two weeks, I mean, the time it had to rotate underneath the telescope to get the full range of longitude was Voyager just whipped pass.

	But, you know, the point is, is that sometimes the Voyager couldn’t see Titan’s surface, and I’ve even said that myself in my first Titan book. And if you have a dogged graduate student who has spent some time, then they can in fact prove you wrong, which is kind of a nice thing to pull out of the old data sets.

	And I’m sure that Cassini’s will be - being worked on for every bit as long as the Voyager stuff, 17 years after it was acquired.

	So going on to Slide 5, this is a cartoon. This particular variant was put together by (Jonathan Lunine). There were various cartoons like this, but much simpler, put together after Voyager and they sort of start really at the left of this part.

	You might have ethane and methane seas and slowly the methane comes out and replenishes the atmosphere. And at high altitudes, solar ultraviolet light breaks the methane down. The hydrogen that’s liberated escapes from Titan but heavier compounds are produced and these form the haze that makes Titan’s atmosphere so hard to see through, and they produce C2H6 which is ethane which is a liquid at Titan’s surface temperature and C2H2 which is acetylene which is solid at Titan’s surface temperature. And there’s a model about the relative amounts of these compounds that are produced.

	As we started to think more and more about what we might find on Titan, the diagram became progressively more complicated. You see a meteorite screaming in, labeled Bolide, and producing an impact crater, possibly inducing some surface chemistry.

	Then there’s - it was only in the late ‘90s and around the turn of the millennium that clouds were observed – methane-nitrogen clouds -- and it was observed to change over a short time scale, just a few hours which suggested that they were precipitating, so there was rain forming underneath them, rain or hail would be rain by time it gets to the ground.

	And sort of almost as NASA thought, you see in the corner here, dune. I wrote a paper in 1995 with some colleagues in Arizona speculating what we might find dune‑wise on Titan. The expectation was in fact that we wouldn’t find any basically because the winds would be too gentle even though it needs to move stuff around on Titan, and I’ll get into the details in a few moments.

	There isn’t very much sunlight to drive wind, so we might not expect a lot of Aeolian material. Also, given that there might be large bodies of liquid on the surface, as on the Earth, seas tend to catch them and stop it growing around.

	So dunes really were just kind of tucked away in the corner of the - well, we can’t say for sure there aren’t any, but probably not, we’ll put in there just because it looks pretty.

	But the interesting thing is, I guess, speculative as this chart was, basically everything on this chart has been discovered, has been seen.

	Titan has proved to be every bit as interesting and exotic as we might have imagined.

Amanda Hendrix:	And this is from what era again, this part?

Ralph Lorenz:	This part, I think this version was put together by Jonathan Lunine in 1995. I think it was published in a - was it the Optical Society of America, the SPIE conference proceedings, but it’s been used in great extent. But it is a good kind of summary.

	Okay, moving on to Slide 6. This is just a quick primer on dunes and ripples.

	As you apply wind stress to a surface of particulate material -- and the same, by analogy, applies in river beds actually -- the more energetic winds will actually lift particles above the surface. For a given wind speed, there are sort of turbulent fluctuations on top of that wind speed and some of those fluctuations will be upwards. If those upward fluctuations exceed the velocity at which a particle will fall through the air, then the particles get lifted up into the sky and this is, you know, like dust storms and dust devils sort of process happens.

	As you have very low fluid speeds, you just kind of roll particles along the ground.

	Intermediate between these two sets there’s a process called saltation from the Latin word for “to jump” and what happens is sort of enough energy to pick up a particle and accelerate it a short way but it comes back down to the ground in a short jump. And when it hits ground, it often kicks others into the air as well. So there’s a transient motion that the energy for the movement is coming from the wind. But any given sand particle won’t be given - won’t be moving continuously, it just kind of hops around.

	Now, if you imagine the wind near the surface, as you’re probably aware, if you, you know, have a picnic on the beach, the closer you get to the ground, the less the wind speed drops to in principle zero at the - infinitely close the ground. And so, if you imagine sculpting a progressively higher mountain, the wind will get stronger at the top.

	So as a sand dune grows, and you can imagine just starting this infinitesimal little barrier that catches some of the saltating particles such that they start to pile up, then as the pile grows the wind stress on top of it gets stronger and stronger. It’s a process that’s probably familiar to you. If you water the garden and sort of squeeze the flow with your thumb, you know, the flow will be accelerated.

	When I was an undergraduate in South Hampton in the UK, I got a graphic illustration of this process when I made my first soaring flight and hang glider. I was all set up to land back on the top of the hill with a soft landing. After months and months of hauling the damn glider back up the hill after a flight, and realized I wasn’t going forward anymore because the wind on the top of the hill was awfully faster than at the bottom and that I was landing more or less vertically within feet of some telephone cables, which is a little disconcerting but it all worked out.

	But that certainly made this process of compression and the acceleration of wind at the top of the mount very vivid in my memory; so I can certainly how dunes become self-limiting. But as a dune grows, the wind stress on the top increases to the point that the sand gets transported away as fast as it’s applied and therefore the dune stops growing.

	Now, the details of the shape of the dune is something we’ll get into a little bit later.

	The next slide, Slide 7 is a chart. It shows speed on a vertical axis -- actually not strictly speed, it’s actually a measure of wind stress so there’s a density factor in there -- versus the diameter of a particle, and the lines or rows of symbols are the speeds needed to lift a particle off the ground, to cause it to saltate. And you can see at the right of the graph that all the curves slope upwards.

	As you have a bigger particle, it takes more wind speed to lift it off the ground and that makes sense because area grows as diameter squared, but mass and, therefore, weight grows as diameter cubed. So as you increase size, it gets harder to lift stuff up.

	You can also see that this process is very dependent on atmospheric density. I’m going to get some background noise (unintelligible). People with the - trying to saltate particles into a vacuum cleaner, and APL seemed to have an uncanny knack of doing so just when I’m on the telephone.

	Yeah, you can see that there’s a strong dependence on atmospheric density. There’s also a gravity effect. But the thin atmosphere of Mars needs a higher speed to move stuff around than the terrestrial atmosphere or that of Titan which is the little row of asterisks.

	You’re also looking at possible (paleo)-Titans at different atmospheres in this exercise.

	Now, what you may notice and may be surprised about is that these curves aren’t monotonic; they don’t always increase towards the right, there is in fact a minimum, and then the threshold speed starts increasing again because of the smaller particle. And that’s a little bit empirical and the numbers may not be strictly correct for Titan because the materials are so different, but this isn’t a measure of the cohesion between particles.

	As particles get smaller, I mean, they’re easier to lift in the sense of generating aerodynamic forces large compared with their weight. The stickiness or the electrostatic adhesion between particles begins to get larger as well instead as you get to smaller sizes and the very small sizes this takes over. That’s why, you know, you can, for example, you can only pour granulated sugar out into little heaps, but if you get what Brits call icing sugar or confectioner’s sugar or powdered sugar, you can pour it out and cut a little vertical wall in it and the wall won’t collapse because the particles stick together enough and it, you know, it’s the same stuff, it’s the same sugar that’s in the larger grain sugar but because it’s so fine-grained, the particles stick together and can form a wall and similarly, they get progressively harder to lift up.

	There’s a sort of optimum particle size, a size at which the threshold speed is a minimum so there’s particles around and it’s kind of what typical sand is, not unsurprisingly, a quarter or a tenth of millimeter. And that minimum is a little bit larger on Titan than it is on Earth, maybe a factor or two or so, so one might think that, you know, sand is accumulated into dunes because it’s been transported in this way that the particles might be a little bit bigger than terrestrial sand, so imagine coffee grounds or something like that.

	I guess coffee grounds is a very bad analogy to use. It’s about - probably about right in terms of particle color and density, but I guess it would depend on whether it’s for your filter coffee machine or whether it’s for your espresso machine or whatever. But organic sand on Titan would probably not be unlike coffee grounds.

	So going on to - yeah, and just - there is a red line here and the red line is, I think, an estimate of what a thermally forced wind would be on Titan. That was done before - way before Huygens and we’ve just had some rough physical constraints like how much sunlight we think is getting down to Titan’s surface.

	And what you find when you do this calculation -- and this is the sort of standard threshold calculation done on Mars and elsewhere -- is that this average wind speed is less than the threshold and therefore sand dunes wouldn’t form, and that’s kind of the conclusion I got. As it turns out if you go through the same calculation for Earth you would sort of find that sand dunes are pretty marginal on Earth too and that basically is a statistical thing that the sand isn’t always moving, that it’s only occasionally, maybe 1% or 10% or whatever fraction of the time that the wind is above the threshold to move sand around. To make the dune, you only need it to be above the threshold sometimes.

	So that meant that perhaps this thermally driven conclusion of no dunes on Titan was maybe premature. It turns out that we may have been wrong about the principal source of near-surface winds on Titan, and I’ll come back to that in due course.

	So Slide 8. The second radar flyby of Cassini T3 back in February 2005 just a month after the Huygens probe found some sort of bright often wiggly subparallel dark streaks on the surface and they looked like they kind of defined some sort of flow pattern but they didn’t have any particular shape to them, they were just streaks. Maybe they were liquids seeping out of something in the ground, couldn’t tell; maybe they were streaks of solids being moved around on the surface.

	It was hard to be sure. We didn’t have enough of these features to understand the relationship between the other things around them, the sort of bright pattern. We didn’t have a sense of how wet or otherwise the surface was.

	It was noticed that the dark streaks did sort of line up with dark regions seen optically by Cassini’s camera at 940 nanometers, and do I show that in the next slide? No.

	The next slide shows a radar image of, I guess, sand dunes in Turkmenistan and the idea was that while maybe these wiggly things on Titan are dunes. But we certainly couldn’t be sure because all we saw were dark streaks and there’s a lot of ways of making dark streaks on a planetary surface and yes, there’s some slow process almost certainly involved. But whether it’s a lot of liquid or it’s a lot of…

Man:	Okay.

Ralph Lorenz:	Hello? Somebody said okay? Maybe that…

((Crosstalk))

Man:	I just joined.

Ralph Lorenz:	All right, great.

	Well, moving on to Slide 10, as I was mentioning…

Amanda Hendrix:	Ralph, can you tell us what the scale on Slide 9 compared to 8?

Ralph Lorenz:	Yes, this would be sort of a zoom. I can’t remember the swath width. I think it’s something like 20 or 30 kilometers.

Amanda Hendrix:	Okay.

Ralph Lorenz:	Whereas, you know, the swath on the Cassini slide, Slide 8, is more like 200 so it’s like 10x zoom-in. So maybe these features aren’t too different in size, a factor of two different maybe.

	But, you know, there are many different features that would produce the same kind of pattern in an image. And in fact, we found some radar - colleagues who studied the Earth’s ice sheets, we found some sort of snow dune features which are - have the same bright and dark pattern in Antarctica in the radar images and when you go out there, there is absolutely no surface expression at all, I mean the ground is just flat and it’s just that some sort of process when the snow was being deposited causes the particle size to be different in a sort of stripy pattern.

	And so, there’s certainly no certainty that just because we saw these dark streaks that they were dunes in the sense of being positive topography. So, you know, we had to remain open at the time to alternative interpretations like, you know, just streaks of solid material without any surface topography or maybe even they were, you know, surface material property changes or some sort of liquid seep. They’re still ambiguous and that’s particularly a huge problem for Titan in the sense that Titan is the first icy satellite, or outer solar system object, studied with radar.

	So there’s a whole new set of physical processes and we’re looking at it in basically an entirely new way. We can’t make comparisons with optical observations of the icy satellite like Ganymede or Callisto. And because the materials are so different, one has to bear some caution comparing with terrestrial radar, but terrestrial radar is nonetheless a useful sanity check.

	Okay, moving on to Slide 10. This is the old HST map again with several of the radar swath shown on it and T3 sort of in the middle of the fourth square from the right. You can see that little lobster or scorpion-shaped pattern kind of pointing east where that’s the bit that we’ve seen in the radar two slides ago. And that same little lobster shape was seen in Cassini optical images.

	So we knew that some - at least some low latitude dark areas, optically dark were associated with these dark streaks, whatever they were. And we also knew that the trailing side of Titan, so longitude 270 or the west half of this image around the equator, were some of the darkest optical regions on Titan period. Certainly, they can be seen from the ground.

	And so, we had the expectation already by October ‘06 - ‘05, October ‘05 we already had the sort of ability to guess what we might find and we thought well, okay, the T3 dark lobster is full of streaks and we’re going to look at the darker stuff on T8, maybe we’ll find more of those streaks. So, and this sort of the most dark bit in the middle there is named Belet.

	The dark regions and bright regions kind of got named before anyone knew what they were, I mean we just had to refer to them as something more fixing than, you know, the big bright spot around 30°S and on the leading face because that stuff gets really old when you’ve got day-long meetings.

	So Slide 11 is the Huygens probe. The Huygens probe did not land in a dune field. As it turns out it wasn’t maybe that far off.

	This mosaic is a quite nice re-projection done by E. Karkoschka in Arizona. You can see the sort of bright triangular area, the sort of bright highlands, and you can see the dissection due to the river channels there filled with - perhaps filled with dark material. And the Huygens probe sort of landed in the foreground in that sort of ellipsoidal bright margin to an area, sort of at about 4 or 5 o’clock from the center.

	But up in the upper right, you can see two sort of linear streaks and those weren’t - not much was initially discussed or thought about for those features because in the individual frame from the camera, these are more or less horizontal and it’s very easy if you’re, you know, working with difficult flat fields and atmospheric effects to create artifacts that are purely horizontal in an array imager like a CCD.

	So we weren’t quite sure if there were real streaks or not. But certainly some amateur astronomers that’s pulling the images off the Web mosaiced them together and found they made this sort of streaky pattern.

	And what we found with the observation in radar in T8 is shown on the next slide. The sort of background rectangle is the segment of the T8 radar swath and the irregular pattern of polygons on it are individual frames from the camera on board Huygens. And sort of in the upper middle, you can see these two streaks, whichever two that you saw in the upper right of the mosaic.

	And if you click through the next couple of slides, you sort of progressively fade out the DISR mosaic and get left with the radar image and you can find that these two streaks match up.

	And in fact, the overall correlation, looking at the landing site area itself and the actual landing site is a little (unintelligible), the correlation is not great. I mean optical images and radar images show different things. Radar responds to surface composition in the sense that denser and more metallic materials are very reflective. Rough materials, that is rough on the scale of the radar wavelength of 2 centimeters or larger, appear very bright. And slopes facing towards the radar appear bright.

	So where an optical image basically just responds to the color of the surface that has been painted. If it’s covered in dark material, it’s dark, and if it’s bright material, it’s bright, without too much because of all the scattering on Titan’s atmosphere of shadowing and illumination angle.

	So in fact, it was these two dunes that provided the correlation that allowed us to locate the Huygens landing site in this radar image and thereby confirm the location that was actually fairly close deduced by tracking of the probe.

Amanda Hendrix:	And that’s - what the lat and long again?

Ralph Lorenz:	The latitude is - let me get this right, is it 10.2 or 10.3 south and the longitude is 192 point something west.

Amanda Hendrix:	Yeah. So in 12, 13, and 14 we’re looking at very zoomed in portion of the bigger swath that’s shown in 10?

Ralph Lorenz:	Absolutely. If you go back to Slide 10 where it says the word “equator,” that little mosaic, the radar image is about - probably about the size of the small “O” in equator.

Amanda Hendrix:	Okay, great.

Ralph Lorenz:	So yeah, this whole thing - yes, I should have put a scale on.

	The distance between the Huygens landing sites and those dunes is about 30 kilometers whereas the whole swath is, you know, close to 100 across there.

	So going on to Slide 15, this is kind of the image that made it all kind of crystallized.

	This is from the middle of that very dark area, Belet in the radar image. And what we see here is that there’s dark material all over it but that here we see these streaks and they have bright edges on top.

	Now, the radar is - imaging with radar is like seeing with a flashlight in the dark, the illumination is coming from the direction that you’re looking, so it’s very sensitive to surfaces facing directly towards you, you get a strong glint.

	And that’s what happening here that the - as the illumination is coming from above, the faces of the dunes, these long linear dunes are sort of broad-side on the radar and so they reflect the radar imaging directly back. And then on the down range, the lower side, the side further away from the radar, it’s dark either because it’s actually in shadow, which can happen, or because the radar energy is hitting it at such a glancing angle that it gets reflected away from the radar and not back towards it.

	So this image shows that these streaks, these particular streaks have a topographic expression that they’re linear bumps.

	And because there were so many more of them than we had seen before in T3 and they interacted much more visibly with the surrounding topography, it became clear that these were sand dunes. And that gets much easier to understand when you find the equivalent on the Earth, going on to Slide 16, and these are linear dunes in the Namib desert just off the - just on the coast of Namibia, just further north of South Africa here. I think I’ve actually seen these from the airplane once; I wish I was more awake, I mean, they just almost looked too big to be dunes.

	These are - these dunes are maybe 2 or 3 kilometers apart, many of tens of kilometers long, and up to a couple of hundred meters high in fact in some places. These are not common in the Americas actually; it needs a particular wind pattern to from linear dunes, and I’ll explain that in a second. But you can sort of see just toggling between the time radar image on 15 and the shuttle - digital camera image on 16 that, you know, these are pretty much the same thing, and remarkably similar these things in essentially every respect.

	Going on to Slide 17, you can obviously see these from the space shuttle. That’s where the picture used is from the astronaut - Story Musgrave made a little poem about some of the things he saw looking out the window and he’s got this nice poem that I guess I won’t bother reading for you just describing the striking dunes and how they look like ocean waves. So…

Amanda Hendrix:	Ralph, you say that the shape of the dunes is due to the wind patterns, is that also - does that also account for the scale, largely then?

Ralph Lorenz:	Yeah, that’s a good question. Nobody actually really knows what controls the scale of sand dunes. There is a theory about the minimum size of a dune but not really about the large scale size. So in that sense it is kind of surprising and remarkable that ones on Titan seem to be of such similar scale to the ones we see on the Earth.

	On the other hand, if they were much smaller and maybe there are, you know, billions of them much smaller, we wouldn’t see them with Cassini. And if they were much bigger that would be really quite impressive because, you know, the dunes do become self-limiting in size and a bigger dune needs more sand and the one question which we’ll come back to is, where did all the sand come from?

	Anyway, I better pick up the pace here and go on to Slide 18. This is a little chart showing the different kinds of sand dunes you can get and how the combination of variation in wind direction and how much sand there is controls the dune form. So as you see on the bottom of the chart, you get dome dunes and (intelligible) barchans, these sort of little isolated piles of sand. And when the wind always blows one way, you get barchans; and when it’s somewhat variable, you get domes.

	If the wind blows in a constant direction where you have lots of sand, you form transverse dunes and that’s maybe the kind you’re most familiar with. And strictly speaking, ripples are slightly different process in fact - but they end up - and the form is rather similar in that the axis of the dunes is orthogonal to the direction of sand transport and wind.

	And that’s not - that’s in fact what we find most on Mars. There’s lots of barchans and lots of transverse dunes and some star dunes. You get star dunes where the wind just veers in all kinds of directions.

	But maybe the most interesting one, certainly for Titan, are the linear dunes which you get when you have plenty of sand but that the wind varies but not an awful lot, that it’s - it gets sort of say two dominant wind directions and the dunes kind of line up sort of in the vector sum of the two. You know, if you imagine that sometimes the sand is being moved in the direction of arrow and then gets moved in the direction of the other arrow, you can sort of imagine the sand getting sort of funneled into these lines along the mean wind direction.

	So that requires us, in a way, explain where that kind of variability comes sometimes because it looks, and I’ll show some more evidence in a moment, that Titan’s dunes are just linear type not the transverse type.

	The linear dunes are actually about the most common dune type on Earth. But in the Arabian, Sahara, Namib, the Australian desert like the Simpson, not much in the Americas, which I guess must be because of the very - the wind regime. The wind direction change for the Namib and Arabian dunes is monsoonal, seasonal change in wind direction, maybe something different on Titan.

	I did find some linear dunes in the Moenkopi plateau in Arizona and was standing on one of those that would be the first slide in the presentation. Here actually sand is blowing up a cliff, up through gullies in this cliff side and it seems that the dunes sort of form streaks down wind of those gullies. A little holiday field trip snap for me there on Slide 19.

	Going on to Slide 20, this is a commercial image from TerraServer of the Arabian Desert so if you like a slightly different perspective, slightly different area from the Namibian one from the space shuttle. But, again, you see a beautiful sort of flow pattern defined by the linear dunes and you can see, as they come up from the bottom left, there’s sort of a shadow behind this mountain ridge in the center before the dunes pick up again and the dunes divert a little bit around the mountain.

	Going on to Slide 21, that’s just what we see on Titan. We see this pattern of lines and there’s a bright area in this radar image which suggests it’s rough or possibly higher than the surrounding terrain.

	And there’s not a sort of wake behind it. You could imagine there might be a wake going up sort of 11 o’clock. If these were transverse dunes, there would be a shadow behind the mountain there.

	But what you see instead is this little tail that lines up with the dunes so the sand is being transported along the crest of the dunes. These are linear or sometimes called longitudinal dunes. And that reinforces this interpretation.

	So not only were sand dunes sort of not really expected, but nobody imagined finding linear dunes specifically.

	Next slide, T17, some other observations just bring home this kind of pervasive pattern of tails behind bright mountains and the general flow kind of defined by the dunes almost making a streamline pattern. I mean these things almost look like little mountains kind of swimming through, you know, a sea of dunes. There’s really quite a lot of picturesque imagination you can apply in interpreting how these things might look if you were standing there.

	Next slide is Slide 23. And this is a chart of Titan’s orbit around Saturn. It takes two weeks or 16 days rather to go around Saturn. Titan always has the same face pointing towards Saturn or very nearly; Saturn actually vibrates back and forth by 3 degrees because of the eccentricity of the orbit.

	And there was a tide due to Saturn’s gravity on Titan. Because the distance to Titan changes by 3% throughout its orbit, the tidal force changes by about 10%. And if there were a global ocean on Titan, that tide would be a bulge about 100 meters in amplitude. You can compare that with about the 18 centimeters that the Moon produces in the oceans on the Earth.

	Now, the tides that we see on Earth are often much larger than that because there are resonant effects in ocean bases, so resonant ducts like the Bay of Fundy and the Bristol Channel or, you know, particular exceptional example, the mechanics of waves on Titan is actually we wouldn’t expect any resonance like that.

	But it’s important to bear in mind and actually Isaac Newton was the first to point this out, and I even quoted him on this Toulouse paper back in 1991 which was my first ever sort of scientific paper rather than an engineering one where I point out that, you know, there should be big tide in Titan’s atmosphere.

	If you were to walk around Titan’s surface - if you can go on to the next slide, Slide 24. This is the chart of pressure versus latitude and longitude in the upper left.

	If you were to walk around Titan with just a wristwatch barometer and you can, you know, get little digital barometers on wristwatches, you’d be able to see the millibar difference that this tide induces, you know, just as there’s a bulge in the ocean caused by the tide, so there’s a bulge in the Earth’s atmosphere and in Titan’s atmosphere.

	On Earth, the bulge is tiny compared with all the thermally driven activity or the strong sunlight and winds generated thereby that we see. But on Titan, the tidal bulge is bigger and the heat, the solar forcing is much less, and so it turns out that tidal currents that go along with this bulge as it changes in amplitude throughout the elliptical orbit.

	May be the principal source of near-surface wind. Because there’s so little sunlight getting down to Titan’s surface, even though the winds at high altitude are quite strong, near the surface they are very weak, but in fact, you can get half a meter per second or more after this tidal effect.

	And if you were to factor in the density, probably into the slide back - was it in Slide 9 with the graph of saltation threshold, we find that, you know, half to one meter per second is enough to move sand around - if it’s dry and not sticky.

	There’s also the interesting feature of the wind field due to the tide, but if you average it out over a tidal period, you know, if you average out these fluctuations -- and remember, it’s fluctuating winds that give you linear dunes -- you get a net transport towards the equator and it’s at low latitudes that we see all the dunes it seems. There’s actually also a little bit of transport towards the pole, but the speeds associated with that are much, much less.

	Further, the tidal winds may be instrumental in making these dunes. First in making them happen at all because they’re the main source of near-surface wind, but there are aspects of the pattern that seem to be reproduced in the pattern of dunes that we see.

	So the next slide is just a repeat of the earlier one, reinforcing this issue about fluctuating winds needed for linear dunes.

	Next going on to 26.

	There is maybe one spot where we found a deviation in pattern away from the linear type. You can see this big, bright spot - now part of the region called Adiri and you can see just sort of to the south of it, the bottom of this chart, there’s some sort of vertical streaking, sort of vertical dunes and then right next to the sort of horizontally east-west linear one. And what I think may be going on here is that generally on Titan the tide is kind of wagging the wind vector around and the fluctuating wind field gives you linear dunes.

	But next to this big (unintelligible) is a bright and probably elevated area. The wind has sort of spun all around it and it doesn’t care that the tide is fluctuating the winds back and forth. The winds got no choice but to just get around the obstacle. And so, the wind regime there is probably rather unidirectional or constant and that’s when you would get transverse dunes.

	So you can see from the dune pattern that it’s consistent with this idea that the wind is being diverted around the topography and is basically being straightened by this feature and leading to a slight different dune pattern. But that’s about the only example of that kind of dune that we found so far.

	Next slide is Slide 27, it’s a terrestrial radar image from the shuttle imaging radar actually and it’s also the Namibian dunes as seen before, I guess, technically if you rotate this part by 150 degrees to align with the shuttle image shown earlier. But it’s the same dune field, same lovely linear dune pattern.

	And, you know, if you imagine you’ve been looking outside of a space shuttle from 200 kilometers up, you can imagine that one side of the image is being viewed at a steeper angle and in fact, that the right side is being viewed at a steeper angle than the far side, the left side. You can see that side is brighter as a result, it’s brighter on the right because the incidence angle is deeper and, you know, that effect pervades across this image, but it also applies in detail within the image.

	And you can use - if you know this function of brightness versus angle, you can use that to back out what the slope of an individual facet in the image is. So you can extract the slope profile from the brightness profile in a radar image if you assume that it’s all made of the same stuff, that you don’t have patches of particularly reflective material, particularly dark material.

	So if you do that on the Titan dunes -- next slide, Slide 28 -- you can generate a topographic profile and that’s shown in the solid line and that shows that the dune heights you would recover are 150 meters or thereabouts in the deepest area.

	The dotted line or dashed line rather is some shuttle radar topography measurements of the Namibian dune field. And, you know, those are maybe 100-meter in that particular bit that I found than there are isolated dunes there that are higher than that. So you can see both in sort of wavelength and in amplitude. These are dead ringers for the dunes we find, albeit in the rather remote parts of the Earth.

	Next slide, Slide 29 is an altimetry trace. Both radius or altitude are at the Y‑axis and the long track distance in kilometers is the X-axis. And this is an altimetry profile over some of the dune areas and the color is the representation of the intensity of the echo. And you can see they’re basically dead flat, I mean, there’s no more than maybe a few tens of meters of elevation change in the several-hundred-kilometer stretch of dune area.

	So it’s literally a sand sea, I mean that is the technical name for these large expanses of deep sand where you basically don’t have exposed rock between the dunes, there’s sand everywhere but the dunes are kind of lined up like waves.

	The next slide, Slide 30, is just to show if you take the Cassini radar image on the top left, zoom in on it to a 30 x 20 kilometer scale and then take the same scale piece of an Earth radar image at high resolution, you can sort of see A, that they are very similar in appearance and scale but also you get an idea of what you would see on Titan if you could see better than Cassini allowed us to. And you can see a lot of little isolated glints from basically individual facets of little dunes perched on top of the big dunes.

	The next slide, 31, Cassini images on the right and Earth on the left. Well, I have to actually think about that.

	We saw on T3 some of these sort of tapered dunes that just kind of fizzle out. And you see the same effect in areas of the Namib sand sea and other sand seas. You can sort of see the bedrock appearing from underneath. This is sort of an indication that the sand is not as abundant, the sand is running out.

	And if we go to the next slide, 32, this is a close-up of that sort of river mouth, dried-up river mouth in Namibia. And you can see this tapering but in much higher detail and you can see that they’re actually break down into little star dunes that, I guess, in this river mouth that perhaps because of it the wind regime is much more variable.

	There’s a lot more detail that is probably replicated in some sense on Titan that we don’t have the resolution to see with Cassini. But again, reminds us that we can - we’re seeing very terrestrial processes on Titan -- different materials, different rates, different speeds, but same process and exactly the same land form results. And so, we can go out to these places on the Earth and study the process to figure out what Titan might be like up close.

	Next, Slide 33 is a sort of map putting some of these sound observations back on the HST Map. You could see that the darkest areas have the biggest and thickest dunes and then Area C seems to have - which is at high latitude, seems to have the sand sort of fizzling out. So maybe more sand at low latitudes hence the darkest contrast. We know of course now that the dark - optically dark areas at high latitude are not dunes but are lakes instead, but that’s a whole other story.

	Slide 34 is a histogram of radar coverage up to about T19 so just before - a little before Christmas. This is apart from (Jamie Ratterbar)’s LPSC abstract, if anyone’s going to that meeting in a couple of weeks.

	The dashed curve is Titan’s surface. You know, there’s more area per five‑degree latitude bin at low latitudes than at high latitudes. So the dashed line defines Titan’s surface area.

	The dotted line is the area we’ve covered with radar up to this point. We’ve had a lot of northern polar coverage, which is what found the lakes in fact, and a little bit of equatorial coverage.

	And the solid line is the number of five-degree square bins that are filled with dunes or partially filled with dunes.

	And you could see all the dunes are within the tropics, less than 30 degrees latitude north or south, and that they cover about 40% over the tropic areas we’ve observed. The big, bright area, Xanadu is not dune-covered, it’s bright because it’s mountainous or something.

	So making the extrapolation from what we’ve observed to what we might observe if we could see the rest of the low latitudes, maybe 20% of Titan’s total surface is covered in these dunes. But then the question is well, why only in the tropics?

	The next Slide 35 is a radar image of the whole Earth. This is a scatterometry mosaic, so looking at the Earth’s surface at 40 degrees. So a flat surface like the Earth’s ocean is very black. In fact, scatterometry is used to measure ocean wind - sorry, wind speed over the ocean because the wave’s roughness gives you back scatter and the wave roughness can be correlated to wind speed.

	So you can see the polar caps have a particularly distinct signatures, some mountain ranges are visible. But you can also see some very, very dark areas that are stealth terrain on Earth where basically there’s no radar echo and that’s because sand when it’s dry is fairly radar-transparent, and so the surface tends to sort of soak up the radar image and not give a good echo.

	You can see the Arabian sand seas called out and the Sahara. The Namibian one doesn’t show up terribly well on this plot.

	But one thing we noticed with the dunes and deserts more generally on Earth is that they’re all around about 30 degrees latitude and that’s because that’s the latitude at which there’s down-welling in the atmospheric circulation, you get sort of big towering updraft, big cumulus clouds and thunderstorms at the intertropical convergence zone around the equator and that flow dries the air in the stratosphere or of troposphere and then the dry air comes down at 30 degrees latitude and so, these areas tend to get dried out at least where they’re not ocean and connected to liquid elsewhere.

	And so, you know, the Sahara and the Arabian deserts and the, you know, North American deserts in Mexico and Arizona tend to be at this sort of latitude, similarly the Australian and Namib and I have to comment that it’s 30 south.

	But that pattern might be different on Titan because the circulation is different. On Titan, there’s probably less symmetric circulation that you tend to have upwelling over one pole and then down-welling over the other, but the net effect may be to dry out low latitude, and in fact there’s a model predicting exactly that.

	So the extent to which we find sand seas only at low latitudes might be due to the availability of sand and perhaps something to do with the near-surface winds kind of moving sand across the surface to low latitudes, or it may be due to how dry the sediment is that there’s sand everywhere but it’s only around the equator that it’s dry enough to be sculpted into dunes. That’s the sort of big picture environmental kind of question that we don’t yet have a data to answer.

	Cassini is going to keep observing in a lot of different ways Titan’s surface and its atmosphere, and we’ll need to observe more to really have a good handle on why it is that the dunes are only in the low latitude areas.

	You should get familiar, on Slide 36, with the names of some of these sand seas. They were just named for being dark areas not because we knew they were sand seas when they were named. But they all get named after sort of after-life places, and some of them have really rather picturesque names -- Belet and Shangri-la and Fensal and there’s a few others.

	But hopefully, all these names will become as familiar to you, all of the (Mare Trivium) on Meridiani Planum on Mars. We’re going to be exploring these in much more detail in the coming decade, I hope.

	Next slide, just - is just underscoring a point I was just making about understanding Titan’s meteorology. We’ve seen some cloud patterns, there were a lot of clouds over the South Pole during southern summer so presumably sort of conductively driven summer thunderstorms. But those clouds have since disappeared as the seasons have changed. We’re now moving towards late fall, late fall in the south that is, and there are predictions of how much cloudiness and rain there should be it different latitudes.

	And these models also, as I mentioned, seem to suggest that methane moisture should dry out at least in the long term at low latitudes. Clearly, even the low latitude precipitation does occur because we saw those fluvial channels and in fact measured dampness at the Huygens landing site. So all that this requires is that it’s dry sometimes for the dunes to move around and at least occasionally, it’s wet to form river valleys.

	So we’re not getting a simple picture of Titan’s surface and meteorology, but we’re certainly getting an interesting one. I mean it’s just like the Earth.

	You know, I was at the meeting in Noordwijk in the Netherlands with the European Space Agency last week for the Titan’s Surface Workshop and there were dunes, you know, on the beach - at the beach and these dunes are sculpted by wind and they require the sand to be dry for the sand to be moved. But anyone who has been to the Netherlands will say it is not a dry place by any means. So you have this same sort of juxtaposition of damp and dry landscape being formed that just basically tells you that it’s an interesting place weather-wise.

	So next slide. What is the sand? We don’t know.

	We know it’s radar dark. We know it’s emissive of radar wavelengths as well and that’s consistent with a low dielectric constant material so probably not silicate rock; we can’t rule it out absolutely, but probably not - certainly not iron filings. I don’t think anyone expected that, but we can rule that out on (unintelligible).

	Maybe it’s ice sand. Maybe it’s made of organic.

	We know from the correlation with optical data that the sand is optically dark. That probably just favors an organic composition, but if you make little mixtures of ice and soot, for example, it doesn’t take a lot of soot to make snow really dirty. It’s the same reason that it’s hard to sort of paint over dark markings on white wall with white paint; the dark predominates.

	And in fact, I guess there’ll be later this year some findings from the VIMS instrument on Cassini of – they see - but not just bright and dark sort of colors. And it seems that the areas that have dunes in them have a particular color, but I can’t say more about that right now.

	Somehow this material has to form sand-sized grains, either it’s broken down from bedrock, you know, if it’s ice bedrock, maybe the boulder is being tumbled around and river channels would break down into sand. I mean that’s how boulders got rounded at the Huygens landing site. Or maybe it’s this haze in the atmosphere, this organic stuff that somehow sticks into sand-sized aggregates that, you know, becomes sand-sized and don’t grow much bigger than that because as they stop moving around they, you know, bits break off or something. And that process is not understood, but I guess the smart money right now would be on organic-rich composition for the dunes, but exactly what we don’t know yet.

	Next slide is sand budget. We know that some of these dune fields are huge, I mean, as I said, it might well cover 20% of Titan’s surface area. Some of the dunes are up to 100 meters. So you already have to figure out that those, you know, maybe 10s, maybe a 100,000s cubic kilometers of sand and the question is how is that sand made?

	Venus actually has very, very few sand dunes and what little sand there is was probably made of debris from impact cratering. Given the craters we’ve seen on Titan are very few even if it had the full complement we expect, and it doesn’t, that wouldn’t create this much sand. So some sort of erosive process, maybe in the river channels like I mentioned, or photochemical production seems to be the most likely ideas about where the sand would come from.

	If we’re right about the wind speeds required to move sand around and what the wind speeds actually are, it would take a few thousand years perhaps to form some of these sand dunes if you just started with a flat sheet of sand. If you were to move - try to move sand from high latitudes to low latitudes that might take millions of years. But you know, the extent to which these processes have happened, we don’t know because there are inhomogeneities in how material is generated on Titan.

	I mean we don’t know where - why the lakes are only at high latitudes, let alone the dunes at low latitudes, but somehow these global budgets of transported material needs to be figured out.

Amanda Hendrix:	And did you point out a place where you said it looked like the sand was running out?

Ralph Lorenz:	Yeah. If we want to go back to Slide 33 Panel C, you can see some tapering dunes. And Slide 32 shows the terrestrial example that on the large scale looks very similar and you can see the tapering dunes become smaller star dunes and the wind pattern is more complicated. But fundamentally, it’s probably that you’re just running out of sand.

	So these streaks that you see in Panel C are probably not 100 meters high. I mean these are probably just streaks. They don’t have the same glints on them that the higher sand dunes do. But as it turns out that…

Amanda Hendrix:	Area where there’s kind of less sand overall maybe.

Ralph Lorenz:	Exactly. You can see the - and you can see the brighter bedrock kind of poking through it. You see the dark streaks on something that’s less dark, whereas in the middle of Belet what you generally see is that everything is dark but the up-range slopes are brighter because of their orientation.

Amanda Hendrix:	Does that suggest maybe a younger area?

Ralph Lorenz:	Depends on what you mean by young. I mean, you know, the last sand grains are probably moving today, so everywhere probably has the same age in that definition. You know, I don’t quite know how to answer that one. But let’s finish off and maybe take a few questions.

	Slide 40 is the a bit of the T13 radar swath that is mostly of Xanadu. You can see Xanadu at the right. You can see that the dunes here are not so much east-west; it’s plunging towards the south, in fact also around this ring‑shaped feature Guabanito. And that suggests maybe that on the large scale, Xanadu is acting as a sort of obstacle to the wind, an obstacle to the dune.

	And if we go on to the next slide, 41, that’s from T21 just a couple of months ago. You can see, again, some bright recently elevated terrain blocking the dunes and diverting them. You really get a sense of the dunes sort of this light defining streamline and that the wind gets diverted around the little mountains.

	And if we go on to Slide 42, you can start to map out these large scale patterns and you can see that things sort of generally slightly to the north of eastward. Evidently, Xanadu gets in the way and does something. It’s kind of a laborious process.

	And Jani Radebaugh at Brigham Young University used to be at the University of Arizona has been generating the statistics to do this exercise and has measured something like 8000-plus dunes and there’s a bunch more from the last swath to map out.

	But the nice thing is with getting this sort of map is it’s an independent constraint on near-surface winds. It doesn’t tell you the wind at any one time, but it gives you some sort of vector mean perhaps. And that becomes important in fine tuning these global circulation models, the wind models, which if you look on the next slide, may have application in the future for figuring out where a balloon might go.  Because Titan has this low gravity and thick atmosphere, it’s a great place to fly, any way of flying, but particularly hot air balloons.

	And so, maybe in the future, we can get this sort of airplane window Huygens eye view of Titan’s surface, not only a close-up view, but over all kinds of different terrains as the wind blows the balloon over sand seas and Xanadu and other places.

	And one nice thing is that the tidal wind is an astronomical cause and is therefore, in some sense, predictable. And knowing what the wind is likely to be, if you can modulate the altitude of the balloon, which you can do easily in a hot air balloon, maybe we can exploit the knowledge of the tidal wind fields to drift in particular directions, say drift to the north or, you know, avoid Xanadu or something like that, which is kind of an exciting prospect.

	And the next slide just shows some model balloon trajectories given the present state of tidal wind modeling. The wind models are not yet able to reproduce the dune pattern but are going to. You know, I mean if you saw some data at a bunch of modelers, the modelers will sooner or later find a way of fitting it. And the fact that the models don’t fit the dune pattern right now shows us that the dunes are telling us something we don’t know yet.

	But, you know, this is in some sense a unique way of generating winds and the resultant landforms are particularly exotic but, in many ways, similar to corresponding ones we see on Earth, albeit for different reasons. So it might be that, you know, the general art of wind modeling can be improved by trying to match what we see on Titan.

	So, just to conclude, this is, you know, brings to 3 or 4 the number of bodies on which Aeolian features sand dunes are found. So it’s a new laboratory with different parameters, different gravity and all that for studying sand dune formation processes.

	Those implications on where the sand comes from and how sand sticks together, which probably is related to dampness and the wind, so these dunes can teach us about more general properties of the Titan environment.

	So far, we’ve only found lots of dunes in the low latitudes. There might be one or two isolated examples elsewhere. We don’t know whether that’s because of the winds or whether there’s just no sand or whether it’s damp. That needs to be figured out.

	The dunes were instrumental in finding the Huygens Probe in the radar imagery.

	And last couple of points. It might be that because, you know, the material in dunes is sorted that you only have the fine grain sand, that it’s a useful sort of calibration target for radar remote sensing. There are issues with the scattering from boulders of different sizes that make it difficult to disentangle surface - various surface properties from a single radar image.

	But by having a target area whose sort of characteristics we can guess at because it looks like a large area of consistent dunes, we can maybe fine tune our interpretations of the radar data better. Maybe we can figure out the wind pattern from these dunes, which might facilitate future missions to Titan.

	And it might be also that the dunes are a manageable place to land. We know what slopes sand can accommodate before it gives way. There aren’t going to be many gullies or boulders in sand dunes because the sand has been sorted and that makes these properties easy to define, that doesn’t mean it’s easy to land on it, but it’s easy to be sure what you’re likely to encounter.

	So that’s the dune story, one that was completely unanticipated before Cassini arrived, but one that’s doubtless going to be the subject of much future study.

	Thanks for your attention.

Amanda Hendrix:	Well, thank you very much. That was very interesting.

	Who has questions?

(David Torn):	I have one.

Amanda Hendrix:	Go ahead.

Ralph Lorenz:	Uh-huh.

(David Torn):	This is (David Torn) of the (Topper Planetarium). I’m a Solar System Ambassador and also Cassini Outreach.

	I was just wondering if there might be other factors to consider, although they probably play a more minor role than wind, in terms of the distribution of materials - distribution and re-distribution of materials like sand grains on Titan and that has made possibly phase changes, specifically maybe sublimation and refreezing, assuming these materials are volatile or frozen.

Ralph Lorenz:	This is actually, I mean, Titan poses all kinds of challenges for the nomenclature. I mean, you know, we had this sort of issue with the Huygens landing sites and, you know, what are those rocks just in front of the Huygens probe. Well, they’re probably not actually made of rock, they’re made of ice. But maybe they’re not even made of water ice, they may be just some solid organic material.

	And whether you call a solid organic material that would be liquid at terrestrial temperatures an ice because it’s solid at Titan’s temperatures is sort of a debatable matter. I mean I would say that it’s not an ice, it just happens to be, you know, a solid.

	So there’s no indication that the material that makes the sand dunes has undergone any kind of phase change. In fact, there are very few materials that would melt at Titan’s surface conditions. I mean methane and ethane are basically the only liquids and the surface is too warm for them to freeze.

	So whatever gets to the ground, I mean if it’s coming from the atmosphere, if it’s organic, whatever gets to the ground as a solid probably stays as a solid as far as we understand the chemistry.

	Now the chemist - the chemical models may have been wrong to some extent and that is that they predicted a lot of ethane enough to maybe over the age of the solar system produce several hundred meters worth of, you know, global ocean of ethane and some hundred meters or something of acetylene solid material.

	We haven’t seen evidence for nearly as much liquid as that. But maybe what is happening is the chemistry is different and basically more solid was produced in the atmosphere and less liquid.

	But as I say, there’s no indication that there’s been phase changes of the surface material. That doesn’t rule it out and certainly not under different conditions in the atmosphere if the atmosphere was thinner or something like that. But there’s certainly no evidence of that.

(David Torn):	Thank you.

Ralph Lorenz:	Uh-huh.

Amanda Hendrix:	Other questions?

	I have one more question; I know I’ve been asking a lot, but…

Ralph Lorenz:	Uh-huh.

Amanda Hendrix:	And maybe you’ve kind of already said this already. But in terms of the dampness of the sand or the dampness of the surface and the relative location of the dunes…

Ralph Lorenz:	Uh-huh.

Amanda Hendrix:	Do you expect seasons to affect that? In other words, maybe there would be more dunes in the south, even though you haven’t seen them yet, because it has been warm - relatively warmer down there?

Ralph Lorenz:	That’s a good question.

	There’s a couple of complicating factors. One is that dampness measured at the Huygens landing site was, you know, in the surface materials so just a couple of centimeters beneath the surface. Nobody’s done the calculation of how quickly ground would dry out, whether it could dry out on a seasonal timescale.

	If you have exposed bodies of liquid, you know, lake of pure methane, it would dry out at a rate of maybe 10 meters - several meters per terrestrial year. But if you have ground soaked in methane, because the methane has to sort of diffuse through the little pores between the grains, it gets slowed down. And people have done a lot of this kind of calculation for Mars, understanding how the ice consists in the (unintelligible).

	So nobody knows how long it would take the ground to dry out under Titan’s conditions.

	There’s also the effect that ethane was present and is probably present in most liquids on Titan’s surface. And it has the effect of - it has low vapor pressure, it doesn’t evaporate very quickly. And it has the effect when it’s mixed with methane of reducing the methane vapor pressure and stopping the methane evaporating.

	The best analogy I know of is very amply evident in Tucson. If you spill some water in Tucson on the floor, it evaporates very quickly because the air is very dry. But add some sugar to water, make a syrup, and as you probably know, (unintelligible) basically stays sticky and wet forever because there’s so much sugar in it that the amount of water that the water vapor pressure is reduced and so it doesn’t evaporate quickly.

	Again, that effect may be happening on liquids on Titan if you have ethane with them then stuff stays wet, not necessarily so wet with methane compared to the total methane plus ethane, but it would have the effect of keeping stuff damp, you know, for geologic of time.

	That said, we don’t know whether at a given location on Titan, particularly at low latitudes, it rains every Titan year. In all probability, it doesn’t.

	So all that the proximity of damp ground and river channels at the Huygens landing site and sand dunes 30 kilometers away only tells us for sure that if it’s damp at the Huygens landing site and clearly it rains heavily sometimes -- sometimes in geologic time -- and that at least sometimes the sand gets dry enough to move around.

	And again, you know, terrestrial examples are perfectly good because, you know, the ocean right next to the sand dunes in Noordwijk, the sand when I was there was damp and it was bucketing down with rain in fact, but there were still dunes. Sometimes the sand gets dry enough to move around.

	So unfortunately, it’s not a very constraining event. It would be great to be able to say oh look, there’s one millimeter of, you know, moisture at the Huygens landing site so it rained, you know, last year, but we literally can’t say that.

Amanda Hendrix:	Does anybody else have any other questions?

David DelMonte:	Hi. I have a question. David DelMonte, Solar System Ambassador.

	What would be the process to determine what the composition of this material is? Is this something that Cassini can tell us?

Ralph Lorenz:	Well, Cassini, the way we would do it would be with spectroscopy with the VIMS instrument.

	The difficulty is that Titan’s atmosphere, which is, of course, responsible for forming these dunes in the first place and making all these juicy organics on the ground, only let light through a specific wavelength, 940 nanometers being one of them, and there are about five or six windows where the light can get through in the near-infrared where you can make some spectral identification.

	And so, it’s not clear if there’s really enough information in those narrow windows given Cassini’s instrumentation, which is, you know, not just designed for Titan, it’s kind of a Swiss army knife kind of instrumentation or multipurpose.

	It’s not clear it will be able to do more than tell us that one or two compounds are present. It won’t tell us a lot about what compounds do not have signatures in those small spectral windows. But to - we may get a sense of what some components are, but we won’t know that they’re all the stuff that’s in the dunes and it might be that the most interesting stuff cannot be identified by Cassini.

	That would require a future mission of some sort to do an in-situ analysis to grab some of the stuff and put it in a gas chromatograph or a liquid chromatograph or do Raman spectroscopy or, you know, all kinds of organic techniques can be applied, but they need to be applied in-situ.

David DelMonte:	Okay. Thanks. Maybe another sort of rover.

Ralph Lorenz:	A rover? Well, I mean the Mars rover’s advanced as they are. They’ve only driven a few kilometers that would get you from one sand dune to the next. So if you care specifically about the dunes, you might not need a rover. One could imagine just a simple lander or one could even try something slick like a balloon that has a little grab thing that comes down from it or whatever.

	I mean there are other areas of Titan that we know have different infrared appearances and therefore different composition. It’s not clear that the dunes are the most interesting compositionally, but they’re probably the easiest to do in terms of landing and sampling.

David DelMonte:	Okay.

	Thanks for a very interesting lecture. I’ll let others ask questions.

Ralph Lorenz:	Great.

David DelMonte:	Thanks very much.

(Pete Goldie):	I have a question.

Ralph Lorenz:	Yeah. I’ll need to sign off in a couple of minutes, but let’s take this last question.

(Pete Goldie):	Thank you.

	What instrument - my name is (Pete Goldie). What instruments on Cassini can measure particles that are suspended in the atmosphere if at all?

Ralph Lorenz:	Well, particles suspended in the atmosphere more generally can mean, you know, the haze and clouds and…

(Pete Goldie):	I suppose I mean heavier than the atmosphere particles that are being sort of lost by the motion of the atmosphere.

Ralph Lorenz:	Well, I mean, this is actually an interesting paradox that, you know, you look at the cloud and you see it “floating” but really every drop, and there’s tons and tons of drops, are being pulled down by gravity. So, you know, it’s kind of a transient sort of thing.

	If you’re referring to material being saltated by material moving along the surface in the dune-making process, there’s nothing on Cassini that can really see that. It’s very difficult to discriminate any kind of suspended material close to the surface from material that’s on the surface. Only if you basically see it change can you guess that it’s likely to be on the surface, especially given that there may be liquids on the surface that can cause changes of their own, that gets hard to tell.

	I mean we can tell whether the clouds or whatever suspend higher in the atmosphere with imaging camera ISS and then with VIMS and you can get a little bit of information on the haze from the CIRS infrared spectrometer, far infrared spectrometer CIRS.

	But for a future mission, you know, you would probably equip a lander with what are called saltation sensors that will kind of impact gauges, microphone that basically hear the sand grains, you know, clinking onto the surface as they’re blown along.

	Does that answer the question?

(Pete Goldie):	I was also wondering in the higher detached haze of the atmosphere if you’re able to detect any of the material you suspect is making up the sand.

Ralph Lorenz:	Yeah. Higher up - I did omit one instrument. It’s also possible to see the haze and the ultraviolet when you get high enough in the atmosphere that the nitrogen gas isn’t blocking it and one can look at sunlight or starlight being blocked by haze in the atmosphere in the ultraviolet.

	There’s even evidence that there are heavy molecules, I mean, benzene and maybe much heavier than that, even at the altitude that Cassini flies through when it flies by Titan and takes radar images for example.

	So the haze creation process, which might well be, you know, the first step in the sand creation process, already begins at, you know, in the ionosphere, you know, well above the surface, a thousand kilometers up. And there are several of Cassini’s plasma instrumentation - several Cassini’s plasma instruments and ion neutral mass spectrometer are able to sample them directly as well as the remote methods I mentioned earlier.

(Pete Goldie):	Thank you.

Amanda Hendrix:	Okay.

	Well, Ralph, thanks again very much for a really interesting talk and we’ll let you go now. I know you have to run.

Ralph Lorenz:	Sure. Thank you and thanks for your attention. And don’t forget everyone, go out and buy the book.

	There is a - I did have a book out in 2002 called Lifting Titan’s Veil which repeats the incorrect suggestions that we probably wouldn’t find dunes. That is rectified in a book to come out in about 12 months called Titan Unveiled, its sequel. But I’m sure we’ll have another CHARM between now and then when I’ll get to plug that. It wouldn’t be a total surprise if we end up having a “Lakes of Titan” CHARM at some point in the future.

Amanda Hendrix:	That sounds good.

Ralph Lorenz:	Okay. Thank you all.

Amanda Hendrix:	Okay. Thank you.

Ralph Lorenz:	Good afternoon. Bye.

Amanda Hendrix:	And everybody else, we’re going to have the next CHARM telecon on March 27. We’ll send out an email about what the topic is going to be.

	Thanks everybody. We’ll talk to you later.

Man:	Bye.

Man:	Bye.

Amanda Hendrix:	Bye.

END