The January 4, 2007, issue of Nature has a paper by Stofan et al. and an accompanying News and Views piece by Christophe Sotin on Titan's lakes. See the Editor's Summary for a synopsis and links.

See also the related Space.com story.

In conjuction with this release, there is also some images releases:

Pseudo-color view of North Polar Lakes
http://photojournal.jpl.nasa.gov/catalog/PIA09102

Hopefully they will release a full-resolution version of Figure 1 from the paper.

More details are on the Cassini site now: http://saturn.jpl.nasa.gov/news/features/feature20070103.cfm

New Evidence of Liquid Methane on Saturn’s Moon
By JOHN NOBLE WILFORD
The New York Times
January 3, 2007

Emily has just posted a pretty informative blog entry: Titan's Lakes.

Moon River: Titan's Polar Surface Dotted with Lakes of Methane
By David Biello
Scientific American.com
January 3, 2007

JPL Podcast -- Lakes on Saturn's Biggest Moon
Interview with Ellen Stofan
January 3, 2007

Do you think the extremely low reflectivity of these features mentioned by Emily in her blog means that we should not expect specular reflections? These have often been said to be a smoking gun for bodies of liquid, but maybenot.

Jon

Perhaps but I suspect the lack of specular reflection detection is due mainly to the lakes' location (i.e., northern hemisphere) and the solar illumination and flyby geometries.

Aren't we talking about radar here, not solar radiation? There were some reports of specular radar reflections in Arecibo data some years back, but not repleatable and not consistent with any known surface features, as i recall.

Jon

Isn't it impossible for Cassini to see any radar specular reflection because
the radar causing it would have to come from a source other than Cassini?

As far as solar specular reflections are concerned, is there a forseeable time
in the future when the geometries will be proper between the sun, the "lakes",
and Cassini to see them?

Well the opposite has certainly been tried - bistatic radar with Cassini's radar signal being picked up by Earth receivers.

I thought the liquid surface was suposed to be transparent to RADAR, with the beam penetrating up to tens of metres depth - in which case I don't see how specular RADAR reflections could occur, except maybe at very low angles of incidence.

A small contribution from me ...

940 KB:

It is interesting that "dry lakes have margins or rims and a radar brightness similar to the rest of the surrounding terrain". That could mean that these lakes generally lack radar-dark organic deposits and even not very dark patches should be lakes (shallow ones).

Oh, okay. I was referring to the lack of a specular reflection detection in the ISS and VIMS imagery/spectra.

As for radar, yes, Arecibo observations of Titan [Campbell et al., 2003] did detect "a small specular component...present for about 75% of the subearth locations observed." [Emphasis added]. And Cassini RADAR has also detected specular components, for example, during the Ta flyby; see Fig. 3A of [Elachi et al., 2005], which shows a "[p]lot of backscatter cross section versus incidence angle [that] shows specular behavior at small incidence angles (blue) and dropoff at largest incidence angles (red)." [Elachi et al., 2005] note a little further down in their paper: "The [Cassini] SAR and scatterometer observations are consistent with previous Earth-based data, even though the wavelengths and the specific areas observed differ."

As for Stofan et al. [2006], though they do not mention specular components per se, they do report that for the T16 flyby swath the "backscatter of some of the dark patches is extremely low," and that "at Cassini SAR imaging incidence angles is consistent with that expected from a very smooth surface of any kind (for example liquid, rock, ice or organics) or even simply a non-reflecting, absorbing surface (for example a low-density surface smoothly matched into a non-scattering absorber such as fluffy soot or dirty snow overlying a uniform and electrically absorbing substrate)."

For the record, they also note that "[i]ncidence angles across the [T16] swath vary from 15 degrees to 35 degrees."

Just as an aside, note that apart from different observation geometries, Cassini RADAR and Arecibo also operate at different wavelengths: Ku-band wavelength 2.17-cm for the former; 13-cm for the latter.

These lakes are not even receiving a glimmer of daylight currently, so obviously, there is no geometry that has allowed specular reflections of sunlight since Cassini has arrived. Perhaps in the extended mission.

I would think that attaining the geometry for a specular radar glint would be somewhat difficult, because only the nadir groundtrack would fit the bill and with these lakes near the north pole, that would mean a flyby that seriously changes the inclination of Cassini's orbit. The specular daylight glint could be obtained with the craft at a greater distance from Titan, but would mean doubling the diffusion due to haze.

The specular radar glints from Earth were based on 12.6 cm wavelength radar from Arecibo. Cassini's RADAR uses 2.17 cm wavelength radar. It seems to me that a lot of surfaces that are flat at 12.6 cm would be rough at 2.17 cm scales. On terrestrial beaches, anyway, water doesn't always seem flat to me at 2.17 cm scales. I guess there would be a specular component, but with a lot of noise due to the roughness of the surface.

http://images.art.com/images/products/regu...00/11863959.jpg

Note that the specular reflections obtained from Earth were in the bright landscape that we know NOT to be liquid, but rather icy. Better bring cramp-ons for your "moonwalk".

You can get specular reflections off a transparent surface - sun glint of a window is just this.

Jon

Possibly silly question, but here goes: Is the "specularity" of a liquid (whose surface is presumed to be essentially smooth, like a pane of glass) in any way dependent upon its composition? What I'm wondering is whether we might be able to garner some clues as to the composition of the lake material based on the lack of observed specular reflections to date, all things considered (esp. viewing geometry, of course). Perhaps the lake fluids are opaque (absorptive) at the observed frequencies?

Heck, let me go WAY off in left field...what if the lake solution has an optically polarized component, like liquid crystals? We really don't know too much about this chemistry set gone amok that is Titan...

I may be wrong, but I think that specular reflection will always occur at a zero-phase angle, and that you won't get the sort of effects you're hoping for. As I said, I may well be quite wrong here! However, you set me to wondering whether there are any refraction effects in the atmosphere of Titan which could reveal the composition of various layers? I'm thinking of the way that high-altitude ice crystals in our own atmosphere produce sundogs and the like. There was a Sky & Telescope article, IIRC, several years ago on this subject, and which also looked at things like the effects of frozen CO2 in the Martian atmosphere. Now, in the case of specular reflection we'd be looking down through the atmosphere at a bright point (indeed, more of a point than with a view looking up at the Sun) so I wonder whether there would be any effects visible against the surface haze. Another thought might be whether appropriate crystal sizes and geometries could act as a diffraction grating...


Bob Shaw

Hmm...great points, Bob, as usual. I think I remember that article as well, and there were some fairly bizarre possibilities as far as Martian sun dogs, etc. In fact, wasn't there some reference to apparent specular reflections observed at Solis Lacus from Earth that might be explained by CO2 cirrus?

Just goes to show yet again how alien Titan really is. We need to critically re-examine ALL preconceptions & analogies when evaluating this data.

Specular reflection occurs when the incidence and emission angles relative to the the normal (perpendicular) to the surface are equal *AND* are in the same plane as the normal to the surface. It's essentially a mirror reflection.

The strength of a specular reflection depends on the material: whether radiation penetrates into the surface and is absorbed or diffusely scattered from inside the surface vs a highly efficient metallic reflection. Compare a reflection from the surface of black glass, milk glass or a metal mirror.

For natural surfaces, the surface is not geometrically flat, but is usually "rumpled" due to surface roughness or waves on a liquid. That broadens the specular reflection from a shiny point to a diffuse brightspot, usually but not necessarily with an approximately gaussian brightness profile.

For natural surfaces, the surface may be multi-component, with rocks or rough spots mixed on meters or tens of meters scale with smooth, specularly reflecting spots. As an approximation, the overall reflection behavior is the sum of the individual components weighted with their area abundance: say 25% smooth ponds and 75% rocky terrain would have a specular reflection 25% as bright as a large lake.

True!!! But there is a big change in refractive index for light going into glass. By the 'surface being transparent' I meant hardly affecting the RADAR waves at all. I was thinking that when microwaves enter methane/ethane there would be much less velocity contrast than for light entering glass, due to the absence of polar molecules. I was picturing the radar being reflected mainly by any fine icy particles held in suspension, so that what the RADAR 'sees' would be more like a cloud of smoke than a glass-like surface.

Of course my visualisation may be way wide of the mark and badly in need of an update. Does anybody here happen to know the refractive indices of liquid methane and ethane at (a) RADAR and ( infra-red wavelengths under the conditions prevailing on Titan? If the answers for RADAR are substantially greater than 1.0 I'll be happy with the possibility of glass-like RADAR glints from the lakes.

strange - apparently a b in brackets means someone wearing sunglasses . . . .

In the infrared, the refractive index of liquid methane is about 1.2-1.3 (Martonchik and Orton, 1994)

I (and all of us, I'm sure) have seen a lot of different liquids on Earth. I can't think of any true liquids that weren't specular, but a milkshake, which is actually an emulsion of oil droplets (of several different kinds) in water, has a pretty low specular component. And various kinds of black, opaque oil have a lower specular component.

As I mentioned, though, liquid is not assured of being specular at radar wavelengths. I think there's still a specular component, but much less so than with visible light. For what it's worth (and to me it's worth quite a lot), I often get to see moonlight reflecting off the San Francisco Bay, and last night was such an occasion. As my train ran north and the groundtrack of the Moon moved across the water, I was made strongly aware of the change in specularity from one location to another. Where water is choppy, there are lots of shadows. And, after all, you see the reflection of lights across bodies of water as a streak, because the orientations of "facets" of the water at a distance can deviate from the plane and reflect light in the "wrong" direction, which means the ideal observer of a specular reflection misses some and some offset location "gets" that light. Which, if you deform the surface enough, makes it fully Lambertian with no specular component.

All that mumbo jumbo aside, I recommend to anyone to get a chance to see the Moon reflect off of a body of water while a train moves you along.

correct me if I'm wrong, but it seems that the 'non-specular' components are very low for Titan's lakes (they are very black, at the level of the radar noise), so I would think that there would be a significant specular part. If there would be small specular reflection you would see more signal scattered back or not? Problem with Cassini RADAR is that you would have to look straight down.

It's not necessarily the case that if there is little non-specular component, then there must be a significant specular component. There is another possible fate for incoming lightrays, namely absorption. For example, black velvet has a low non-specular component, but also a low specular component. Obsidian has a low non-specular component and a high specular component. A glass of milk has a high non-specular component and a high specular component. Cotton balls have a high non-specular component and a low specular component. Every permutation exists.

"Every permutation exists" (JR)...and I think that's the core paradigm.

We really don't have any definitive idea what precisely is in those lakes, chemically speaking. Therefore, it may be wise to examine these things individually for subtle differences in radar reflection/IR absorption characteristics...seems like the best application of Cassini's instrumentation to derive significant ground truth data.

Thanks for that info. So IR 'glints' are a good possibility at least. But still they will be difficult to observe because of the high latitudes and consequent low viewing angles required.
Interesting discussion on the RADAR reflectance - definitely one to watch over coming months and (hopefully) years.

yes, I was thinking about that some time after I wrote the post, but then I thought: could a shallow lake absorb all radiation? Could be possible I guess. Maybe you need very deep lakes to do this, but Titan's lakes don't seem very deep, though you can't tell for sure of course.

The examples of the items with low specular components you mention al have very rough surfaces, making the scattering diffuse I imagine, hence the low specular components. I expect a very smooth surfaces (a level liquid) to have a relatively high specular component. That's why a radar-dark surface is consistent with a lake.

but then it can still be completely absorbing (close to 100%), but somehow I think a high specular component is the easiest option. Hopefully we will find out at some point!

edit - a nice example of high absorbing terrain are Mars' 'stealth' regions ( http://www.nrao.edu/imagegallery/php/level3.php?id=114 and some bit about it here http://marsrovers.nasa.gov/spotlight/merlanding02.html), which are something like dust or thick ash layers. I think a dust option was discussed in the Titan paper, but was disregarded because of the surrounding geology, but I can't remember exactly right now.

My intuitions aren't very good about what a lake, suspended silt, or a lakebed on Titan might do to radar! I agree that I think that there will be a specular component from the surface -- it's just that it's not a logical necessity.

I found some imagery online of SAR 6-cm wavelength views of waves in the Pacific. The image is of much higher resolution than Cassini will ever produce for Titan, and it's clear that RADAR has essentially no chance of showing the swoopy corduroy texture of individual waves on Titan. I'll attach an image that is scaled down to RADAR:Titan resolution (and is therefore rather small). Note that RADAR has about twice this wavelength. If waves on Titan have twice the spatial frequency (they are probably still more spread out), Titan waves would look to RADAR about as earthly waves look in this image.

That is the Pacific off of Point Reyes (rather near me, as it turns out). Land is the brighter stuff over to the right, mainly. The ocean has textured grays that are darker closer to shore, generally, with a sort of "glint" near the westernmost point of land because the waves are compressed at that point and show a tighter spatial frequency.

Notice the dark "gash" in the upper right. This is a bay which actually has on outlet to the Pacific, but the bay is extremely long for its width and for the purposes here, it is essentially a lake. It has the same salinity as the ocean, but lacking the wavemaking expanse of the open sea, it appears very dark here. I would say if you cropped the Pacific out of here, this looks a lot like the land/lake RADAR images of Titan.

And there's no doubt that the difference between the ocean portions and the dark bay consists entirely of the orientation of the surface to the spacecraft. There would undoubtedly be a heck of a specular glint off of the bay if the geometry were correct.

The key would be if Titan's liquid behaves the same way as saltwater. If RADAR shoots through it and into the goop, we might be seeing a dark lake bottom, not the surface at all. Methane lake bottoms on Titan may be highly-absorbing at 12.6 cm -- who knows?

I would bet that there's a specular component. But we can't be positive.

See today's release:
PIA09112: Titan (T16) Viewed by Cassini's Radar - July 22, 2006

Hopefully this is not OT, but here's an excellent interactive tutorial on specular reflection:

http://micro.magnet.fsu.edu/primer/java/re...ular/index.html

The root site has many other great presentations for other optical phenomena.

Approximate location of past T16,T18,T19 and future T28,T29 and T30 radar swaths in region north of 70 degrees NL. PIA01942, PIA01943, PIA08740, PIA08741 and PIA09112 images are shown. The scale is 2 km per pixel. There is some distortion to the edge of the map (up to 20 km at 70 degrees NL), because overall view is not flat, while radar images are.

Sources:
http://cassinicam.com/titanflybys/titantracks.html
http://photojournal.jpl.nasa.gov/catalog/PIA09035
http://saturn.jpl.nasa.gov/multimedia/prod...CHARM_RADAR.pdf

This is very impressive, Olvegg! Thank you for this great north polar view.

Yes, very nice.

That's very good coverage in the northernmost circle -- over 50%.

When you consider the fraction of Mars that MGS has been able to observe (with almost unlimited ability to choose targets), the RADAR imaging of Titan, with just a few more targeted observations in the extended mission, should be quite good for getting representative classes of everything on Titan.

A follow-on mission mapping topography and finishing the SAR map will eventually be essential. Topography is one thing that the other instruments don't help much with and that local representative samples don't tell you nearly enough about.

Yes, my first choice for a Titan follow-up mission would be a Titan orbiter, with as good a radar suite as can be fit into the mass/power budget. Plus the other remote sensing instruments too, of course.

I know that a surface/atmospheric probe would be 'sexier', but for a global view of the surface/subsurface of Titan, you need an orbiter. Perhaps a small piggyback surface probe or balloon could be added, but I would hope not at the expence of a significant downscope of the orbital package. I would rather save the large-scale surface exploration for a third Saturn/Titan mission.

Bill

Which your grandchildren, or, if you've already got them, their grandchildren, will no doubt enjoy. Unless somebody discovers unambiguous signs of life at Titan -- or unless new technology slashes the cost of a spacecraft launch -- I'm not confident that we'll see two missions devoted to Titan within my lifetime! I'll be very happy to see one. (Please?)

--Emily

Well, if we'll only see one, here's hoping it doesn't fail on arrival... If there's anything worse than waiting for decades, it's waiting for decades for nothing!

I get somewhat depressed about outer planets exploration, we see so few missions because of cost. As a result, and because there are so many targets, many of the places I want to see explored probably will not see a mission in my lifetime.

I've pretty much concluded that the odds are good that I will leave this planet before a Neptune orbiter does. And given the expense of the radiation hardened electronics required of a Jupiter orbiter, I'm hopeful that we will see a Europa orbiter eventually, but I doubt we will see much more than that, and places like Io may never get revisited.

But I'm a bit more hopeful on Titan missions. Titan has so many things going for it: benign radiation environment, exciting and interesting environment (which the general public can more or less understand), and that lovely atmosphere which is so handy for aerobraking, aerocapture, and general aeronautics (baloons, dirigibles, etc). Titan is the one outer plantes moon that can somewhat easily support a direct entry probe with no requirement for a heavy orbiter for breaking into Saturn and then Titan orbit.

Of all the moons out there, it's the one place I think we will see 2, and possibly more, missions in the next 30 years.

I want to see lots of exploration too, but I think we have to view outer solar system exploration largely as a series of games of twenty questions with the goal of asking the fewest questions to get to a given answer.

Imagine for a second that we had had to choose between Cassini's exploration of Titan and Huygens. There's no doubt that Huygens was a hundred times "sexier". And it certainly provided great science return. But Huygens without Cassini (not counting the fact that it would have had no radio link) would have been a snapshot of an amazing place, but a snapshot out of context. Did it land 20 km from a sea? It gave us the same temperature and pressure readings Voyager already identified. Given just Huygens and Cassini, I think you'd have to choose Cassini.

But the next questions are different ones. We have some context. I think an aerobot will provide close-ups AND its own context.

In the lastest “Cassini News from Saturn” (http://www.jpl.nasa.gov/videos/cassini/cassini200702/) is a short sequence of a SAR RADAR swath shown, covering the north polar land of lakes. I guess the upper (northern) part of the sequence was acquired during the T19 flyby. It shows more of the interesting lakes with steep margins and distinct edges - indicating some kind of topographic rim – as seen earlier in T16. The two lakes to upper right appear to be connected by a radar-dark channel. My cut-and-paste image shown below is centered somewhere around 240°W and 75°N, as reference I have added the T16 RADAR swath. Have fun!

Blink? (please)

Wow.....

Looking at these lakes is such a joy. Cannot wait to see the effects of seasonal change on Titan.

We have come a long way now since the Voyager 1 flyby. Still remember reading a great pre-encounter article on that (I think it was in the now defunct Star & Sky magazine) and what Voyager might find at Titan. Was the thin methane or thick nitrogen atmosphere model correct? If the thick model, how thick and could the surface be warm enough for liquid water/ammonia seas? Would there be any breaks in the clouds?

Titan has not disappointed and even though the methane oceans are not there (now), I find the real Titan to be even more interesting than that old Earth-centric model. Now we see a desert world (currently) with hints of ammoniated lavas, great hilly terrains, hydrocarbon dune seas, chains of ice mountains under methane clouds, ethane hoods and methane monsoons..... marvelous!!!!

My daughter was born the year following Voyager 1 as the sister craft swooped past Saturn in 1981. Last year Rachel had her own first children, twin boys Aaron and Dylan. May they see wonders beyond our current imaginations.....

And might I add, all you UMSFers do incredible work.... keep it up (what a joy it is that I found UMSF).

Wow again!!!!

Craig

A large(132mb) zoom movie is up on photojournal of the T16, T18, and T19 swaths:

Titan Lakes Movie

Here is a manual stitch of forty screencaps from the quicktime movie in an attempt to show as much of the lake district as possible. Rectangular gaps are due to areas missed in the movie's zig-zag simulated flyover. The T18 swath was not shown in detail and is excluded.

Click to view attachment


EDIT: T19 swath patched with familliar PIA releases. Images and movie courtesy NASA/JPL.

Some fun with perspective views:

Click to view attachment

Click to view attachment

Cool to the nth power...thanks, E!

Notice the lakes in the center of the top-most perspective view...there's a very linear raised ridge that is interrupted by a couple of lakes! Obviously, then, the lakes formed after the ridge, but how? They don't look like filled impact craters...are they collapse pits from the local "aquifer"? If so, they may be much more permanent than currently thought...

EDIT: Circled the area I'm talking about on Exploitcorporation's image:

It certainly does seem like there's a lot of visible topography in this area, but with these RADAR images, I'm never sure. Something about 600m walls around these lakes in another thread...really looking forward to increased coverage of this region on future passes, in addition to eventual ISS imagery. I love this moon, and especially love the puzzle-piece buildup of data, one flyby at a time. Reminds me of those goofy Advent calendars I had as a kid...or that life-size cardboard cutout striptease in either Bull Durham or Major League...I can't remember which. I think I may have just reduced my apparent IQ...

EDIT:nprev, are you referring to the vertical line (in this view) in your highlighted area? If so, I think that's an artifact of the imaging process. If it's the horizontal feature, I think I see what you mean.

Great work, Exploitcorporations!
I use your image to update my previous map. This time without future swaths, 0 degrees WL is to the right: