Well, for those who really like RADAR SAR swaths, today is a good day for you. The T13 RADAR swath from April, THE ENTIRE SWATH, has been publically released on the Photojournal.

Titan (T13) Viewed by Cassini's Radar - April 30, 2006
http://photojournal.jpl.nasa.gov/catalog/PIA08552

PIA08604: Xanadu's Meandering Rivers
http://photojournal.jpl.nasa.gov/catalog/PIA08604

PIA08605: Titan's Geological Goldmine - Radar Movie
http://photojournal.jpl.nasa.gov/catalog/PIA08605

Wow! Thanks for the heads up!

They're slightly vague about the scale, would you be able to tell more exactly -- is this at 128 pix/degree? That would be closest to the "smallest details in this image are about 400 meters across" -- or is it actually 400 m/pixel?

--Emily

The pixel scale is approximately 234 m/pixel.

A few comments on some of the amazing features seen in this swath (my favorite thus far) as well as a comment on the movie.

First, regarding the movie: in the movie, they highlight a few of the intriguing features seen in the swath, particularly the Xanadu east and west "coasts". While these may be the margins in RADAR, they are not the margins at IR wavelengths. The west margin basically matches, though the upper portion of the margin in the swath does not match. Part of the IR-bright Xanadu terrain appears dark. The eastern margin doesn't match AT ALL. The IR margin is actually beyond the eastern end of the swath. There are no IR traces of the RADAR margin on the eastern side. Again, the one constant of broad IR dark terrain (not the small patches, which can appear hilly, smooth, rough, etc. in RADAR) is that they are covered in longitudinal dunes.

Now on to the swath itself (for the rest of this discussion, note that RADAR illumination is from above). One thing I love about this swath is that it has "real" topography. Sure, the others did as well, but this one has honest to goodness plateaus, and other really cool geomorphology. In this first image, below, you see some plateaus just east of the river systems in western Xanadu. The most prominent plateaus is along the top of the swath on the right side of this cutout. What is neat is that you can see several streams starting out at the bottoms of canyons along the plateau margin. You can also see this at a smooth topped plateau to the southwest. Throughout this area, you can see plateaus with canyons along their margin, which suggest that methane can erode into the surface and these rivers can form in much the same way that river do on earth.

Click to view attachment

In this next cutout, we see a patch of hills at the eastern end of the RADAR swath. This isolated hills are located within an area of smooth, bright terrain in eastern Xanadu. Emanating from the hills are three radiating channels (though the ones to the west and south may be the same channel diverting around the patch of hills.

Click to view attachment

This swath also appears to have a number of apparent impact craters. The third image below shows a few of them, though it can't be ruled out that some or all of these are actually cryovolcanoes. This view also reveals the smallest impact crater revealed thus far at around 15 km across (the bottom left one). The bottom right crateriform is Guabonito. The top right has an apparent central peak, but the extension of smooth material from its southwestern margin has led to the suggestion that it might be a cryovolcano. the crateriform at top right may be a viscously relaxed crater where only its rim is still visible.

Click to view attachment

Thanks, Jason. Interesting.

is there a composite with all radar swaths so far?

Cassini Reveals Titan's Xanadu Region To Be an Earth-like Land
http://saturn.jpl.nasa.gov/news/press-rele....cfm?newsID=675

I don't see that that extension would invalidate an impact origin. We know from terran impact craters that a lot of heat remains in the ground for a long time after the impact. Geothermal effects last for millenia and in the Sudbury structure in Canada the crater seems to have been filled by a lava pool for quite some time. It seems quite reasonable that there might be a lot of secondary melting and flooding after a large Titanian impact. This might also explain that the Titanian impact structures are so subdued, large impacts may be self-destroying.

tty

Wow... Two complete SAR swaths within the space of a week or so... Life is good.

Not wanting to be thought of as an ingrate, I have to wonder why T13 was released so soon? T7 over Mezzoramia took a whole year to come out, and now T13 is out before the lid has been lifted on T8 over Belet/Adiri?

Maybe NASA is planning to thrice bless the faithful with T8 release soon? Please?

It depends upon how much evidence of civilization they have to scrub out of the swaths before posting - part of the prime directive you know, not to antagonize the natives

Seriously, no complaints about how quickly the radar images are being released. I can understand why mission scientists would like more time to ponder what they are trying to interpret.

Read Emily's interpretation.

...which worked out to just about exactly 192 pixels/degree (i.e. 1.5 * 128), a number that made sense. Thanks, Jason. --Emily

Lunine waxes poetic:

QUOTE

"Although Titan gets far less sunlight and is much smaller and colder than Earth, Xanadu is no longer just a mere bright spot, but a land where rivers flow down to a sunless sea," Lunine said.

Indeed. Here's the source of the poetry, for those who've forgotten their Coleridge. I like the fact that this poem has already been the source of a lunar geologic term.

The well-known phrase "down to a sunless sea" has been used, for example, as the title for a couple of books, including one of my favorites by David Poyer.


Careful, Emily, some of the nutcases may think you're referring to "dome[s]" and "walls and towers."

Seriously, though, does anyone (Phil Stooke?) happen to know why "rille" was used for the lunar trenches/valleys even though "rill" is listed as an alternate spelling in Webster's? I suspect that is was because "rill" is also used to denote "a little brook or rivulet" but I'm just guessing. BTW, the word is of Germanic origin.

Rille - well, glancing at ye olde OED confirms that rill and rille are both acceptable spellings of the same word, but it looks as if rille is the german spelling. In the 19th Century lunar studies were dominated by germans, especially Beer and Madler, but others as well, so my guess is that this comes to us courtesy of 19th Century german selenography.

Incidentally, according to OED, rill and rille are equally acceptable for terrestrial rills (small streams) but only rille is used for the lunar valleys.

Phil

Interesting. My version of Webster's (New World) has the opposite, i.e., both "rille" and "rill" acceptable for the lunar valleys; terrestrial rivulets and brooks using "rill" only.

I guess I need to put this in line behind "Hill 305" on my to-do list

"Interesting. My version of Webster's (New World)..."

Well, Noah was a bit of an eccentric!

I can honestly say I've never seen 'rill' used for the moon, as far as my somewhat capricious memory goes.

Phil

Neither have I, for what it's worth.

The additional possible lakes are of particular interest to me (as noted in the original Cassini news release I linked to already a few posts ago). I've also included links to Emily's article, the TPS Titan radar page and Jason's Titan radar page in my July 19 post about this on the MJ blog. Very interesting.

Despite its Germanic origin, "rille" looks more Latinish than "rill" does, so it might be a subtle stylistic choice to stick to a theme. A rille through a mare under a monte.

There's no helping "crater" though.

Interesting piece from New Scientist

http://www.newscientistspace.com/article/d...with-caves.html

That's fantastic, if true. But how do we know that these are the only mountains on Titan? There are parts that haven't even been imaged yet, and I thought that our ideas of the topography of the rest of Titan were pretty uncertain.

Caverns? That's pushing the interpretation a bit...

Phil

Not so much interpretation as imagination of what occasional, torrential methane rains could do on a steep, malleable terrain. How long would a balloon need to last to circumnavigate Titan. So many interesting places to visit...

Correct me if I'm wrong, but I don't think anyone had predicted that we'd find caverns on Titan (if indeed they exist! ).

So far we've been thinking of Titan, basically, as a 2-dimensional world to explore. If this interpretation is correct, Titan might have an inside as well as an outside worthy of investigation.

If the surface of Titan is close to isostatic balance it makes sense for the highest topography to consist of lower density material. How might a clathrate-type material behave in response to temperature and pressure changes? I imagine it foaming up and then freezing at the surface. Such a material might be highly susceptible to eroson by fluids, forming caverns etc., but a closed cell foam could still have the necessary dielectric properties even without eroded caverns (measureless to man or otherwise).

I have yet to come across a "startling new" interpretation of Titanian geology that (upon my asking him) Ralph Lorenz hasn't been able to point to as being predicted in some past book or paper of his. I think I'll him about the caverns thing and see what he has to say.

--Emily

All.... if I am not mistaken I believe Ralph Lorenz mentioned this in conjunction with an early Titan theory regaring karst terrain...... check his book 'Lifting Titan's Veil'.

Hope we get some on T16 soon.

Craig

I thought I remember reading that Ralph predicted that there would be no karst terrain and no caves on Titan...

Given all the predictions of an active methane/ethane "hydrologic" cycle on Titan and, in some models, putative subsurface liquid reservoirs, one would have to come up with a good explanation of why Titan would not have caves or caverns.

I recall there was a portion of the Huygens descent imagery showing some low cliffs or hills along the edge of the "shoreline" near the landing site which had some interesting looking "pits" or "hollows" in them. Cave-like openings or some other kind of erosional features? They were easily visible but not discussed much at the time and open to interpretation. Perhaps even just odd shadowing, I don't know. I'll find the image again, it's in my files.

Yes, that was my recollection as well. It had to do with the ability of liquid methane/ethane to erode water ice. There had been some graduate paper that suggested easy erosion. As it turned out that data was incorrect.

I do wonder if there is a lot of CO2 mixed in with the water ice on Titan? I wonder if that would change the result? If there are other ices mixed in that erode more easily, that might lead to karst type terrain and possibly caverns. Or would VIMS have determined that (composition mix) already?

Craig

Perron et al. have a paper in press with JGR-Planets entitled "Valley formation and methane precipitation rates on Titan" (407 Kb PDF preprint). They don't seem to think the Huygens DISR imagery shows any caverns, etc. Here's the relevant passage:

"In terrestrial environments where dissolution causes significant erosion, there is often morphologic evidence of subsurface erosion. Karst terrain, in which surface runoff and groundwater have eroded carbonate bedrock, contains caverns, closed depressions, and discontinuous surface streams [Summerfield, 1991]. Although the images obtained during the Huygens descent are probably a very limited sample of the erosion features on Titan’s surface, it is notable that there are no obvious karst-like features in the dendritic networks near the Huygens landing site."

As everyone shares their thoughts on the images of Titan, it might be interesting to note that we almost lost the T13 data. We took at Cosmic Ray hit on the SSPS to the USO causing a loss of the downlink signal from the spacecraft. The SAR data was scheduled to be overwritten during the following observation block and my team had to scramble to zero-out data policing and reset the pointers on the SSRs to recover the data. I am glad we were successful as T13 has been described by Jonathan Lunine and others as the "key to understanding Titan".

Glad you guys are enjoying the RADAR SAR swath from T13.

-Douglas

Welcome to UMSF, Douglas

Thanks. Look like a good site.

Right you are, Emily. On p. 155 of his book he discusses karst terrain in which - on Earth - water dissolves limestone and creates caves, arches, and sinkholes. The idea was that methane and other hydrocarbons would dissolve the water ice on Titan. "The idea was neatly self-consistent in that liquid methane would create the cave systems in which it would hide". Unfortunately, it was based on the assumption that methane dissolved water - a conclusion borne out in only one lab experiment and then retracted and discredited.

This doesn't mean there are no caves on Titan - just that they're not created by this meechanism.



- John Sheff
Cambridge, MA

I think that Lorenz and Lunine [1996] found that there were scenarios where ammonia hydrates or other ammonia-rich ices could be eroded to form karst-type landscapes on Titan, though on Kyr timescales. This is pre-Cassini, though. And pre-Lifting Titan's Veil.

That is not a strong argument. Try using Google Earth randomly and see how long it takes before you hit a spot with recognizable karst features in the river network.
As a matter of fact many of the largest karst areas (e. g. Yucatan peninsula, Nullarbor plain) has no dendritic pattern since all water movement is underground. All you see is the occasional dolin or cenote.

There might well be other karstification mechanisms on Titan than methane dissolving ice. Even here on Earth there are at least three kinds, ordinary karst where water and CO2 dissolves limestone, thermokarst where water dissolves ice and halokarst where water dissolves salt. The latter two are rare though.

tty

You just gotta love their wording. The authors concede that Huygens DISR returned a "very limited sample of the erosion features on Titan’s surface" but nevertheless they found it "notable" that no caves, caverns, sinkholes, etc. were seen. I guess it all depends on one's definition of "notable," though maybe the passage is a result of trying to get the paper past the referees.

While the radar passes show that the channels on Titan are by no means rare, they certainly don't cover a particulalry large percentage of the moon. Huygens got very, very lucky, to come down in such a geologically active spot. Not to mention that it was geologic activity that the average guy on the street could fairly easily understand.

Mariner 4's flyby of Mars in 1965 comes to mind. Lotsa craters. Dull terrain. Based on that mission, and even the follow on Mariner 6 and 7, there was no sign of channels, volcanoes, or little bitty features like Valles Marineras.

Very risky to draw generalized conclusions based on a few descent images. Risky, hmmm. Maybe I should have said 'reckless'.

..."Mariner 4's flyby of Mars in 1965 comes to mind. Lotsa craters. Dull terrain. Based on that mission, and even the follow on Mariner 6 and 7, there was no sign of channels, volcanoes, or little bitty features like Valles Marineras. "

As I've pointed out previously, Mariner 6 identified "Chaotic terrain" as karst-like collapse features, possibly "thermokarst", and recognized the depressed valley terrain of eastern Valles Marineris. Mariner 7 saw complex modified polar terrains and part of the layered polar deposits as enigmatic curvilinear features, together with "featureless terrain" on the floor of Hellas. Cratered terrain dominated the best pictures at low latitude, but Mariner 69 did see that there were going to be major regional geologic features to be revealed and studied with global mapping. See the Mariner 69 issue of the Journal of Geophysical Research from 1970 (i think) for the full analysis.

I would say that "large channels.... don't cover a particularly large percentage of [Titan]". The channels Huygens imaged are too small for Cassini to resolve, by radar or otherwise. Huygens might have been lucky; or it might be the case that Huygens-size channelling covers all of the non-desert terrain. Without a vehicle capable of making images of a large part of Titan from under the haze layer, I don't see how we're going to know if Huygens was lucky or not.

A "Titan Polar Orbiter" mission is likely to have a primary mission requirement of mapping the surface with enough resolution and signal-to-noise-ratio to be able to really do a good job on global geology. An "unfortunate" fact is that the more geologically active a planet/moon is, the higher the resolution you need to see the stuff that's "important". Compare Callisto and Europa, or Rhea and Enceladus.

Clearly, we can't see the surface worth <expletive deleted> in visible/near-IR data. We'll have to do a middle infrared imaging system at VIMS wavelengths, possibly the longer wavelengths to minimize scattering, but where we're running out of sunlight as we get out near 5 micrometers. We probably need global mapping at some 25 meter/pixel resolution, monochrome, two-look-angles for 3-D. I doubt 50 meter's good enough. Long enough wavelengths may give direct-enough illumination for reasonably good shadows and relief visibility.

Radar data may be most useful if we get multi-wavelength/multi-polarization/multi-look-angle scatterometry, which will give global maps of surface physical properties/structure and probe sub-surface. Radar is so <@#$@#$> speckly, it's hard to get clear surface morphology on small features. It may be more useful to do radar interferometry for precise global topography, and searching for regional surface deformation, than to try for high radar resolution.

Either way, just a dedicated imaging and radar orbiter mission isn't going to be cheap, but the resolution and complexity of the surface will eventually demand it, in one or two missions.

I found the Huygens image with the odd "hollows" in the hills along the "shoreline":

http://esamultimedia.esa.int/images/cassin...nd/Picture3.jpg

Left side of the image. Cropped zoom:



Could these tentatively be caves or some other kind of large hollows from erosional processes (such as the methane rains)? I thought I also remembered that these hills were thought to be icy, like Xanadu, but not sure.

From the July 19 Cassini news release:

"This land is heavily tortured, convoluted and filled with hills and mountains," said Steve Wall, the Cassini radar team's deputy leader at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "There appear to be faults, deeply cut channels and valleys. Also, it appears to be the only vast area not covered by organic dirt. Xanadu has been washed clean. What is left underneath looks like very porous water ice, maybe filled with caverns."

http://saturn.jpl.nasa.gov/news/press-rele....cfm?newsID=675

Oh yes...I remember good old Caput Campus...

OK, here's the result of my queries, which I just posted in the blog:

Ralph replied that "I think [Wall] is referring to the porosity that may be indicated in the radiometry polarization data, which isn't published yet." He added that he had indeed talked about caverns on Titan -- in the form of karst terrain -- in his book Lifting Titan's Veil, and that the idea had "originated with Jonathan Lunine in the late 1980s, but that was based on some lab data that subsequently proved to be wrong. We discussed solution erosion on Titan (and showed it to be tiny) in "Erosion on Titan: Past and Present" (Lorenz and Lunine, 1995, PDF format), although we didn't mention caverns specifically."

Ralph had copied Steve Wall and Jonathan Lunine. Jonathan added that the lab data he'd based his original karst terrain speculation on weren't his, they were "somebody else's published in the refereed literature." He also pointed out that another researcher, David Stevenson, "argued in the early '90s for a porous crust to 'hide' an ethane-methane ocean. It was purely theoretical, and he had one piece of gray [unpublished] literature on this in which he at least speculated about big caverns."

Steve Wall commented that "I confess the language is loose. The reference to porosity is (as Ralph suggests) based on radiometry. Size of the 'pores', and if they extend to anything anyone would call caverns, is debatable."

--Emily

Thanks for the clarification Emily......

regarding the data the says methane/ethane liquids cannot significantly erode water ice....... we know that the surface on Titan is not just pure water ice....... aside from the hydrocarbon gook, the bedrock is probably a mixture of ices. I note that there are hints of CO2 in the Huygens GCMS surface data. Wonder if any studies have been done with the effect of these liquids on a mixture of ices?????? I can envision a water ice stratum with inclusions of CO2 and ammonia. If these other two materials erode more easily, than you have a recipe for karst terrains and caverns.

Again.... where are the VIMS people???? Have they managed to get a grip on their data and remove the haze contribution to get spectra that are from the surface?

Craig

It's almost to the point where one could speculate that anything interesting found on any extraterrestrial world in the solar system can also be found on Titan. I've suggested that the Saturn systems is more interesting than the Jupiter system, but it's arguable that Titan does that all by itself. It's like someone fused Venus, Mars, Io, and Europa into one place and added rain, rivers, and lakes for good measure.