Titan's changing lakes Options

[peter59]

"During T64 the RADAR team will be looking for surface changes within the north polar seas Punga Mare and Ligeia Mare during a ride-along observation at closest approach. These seas were also last seen two years ago. Since then, changes in the weather patterns both in the tropospheric methane clouds and higher altitude ethane clouds may have produced changes in the shoreline of these two large lakes. The T64 RADAR SAR swath will stretch from near the north pole south across Titan's anti-Saturn hemisphere down to just north of a bright region known as Adiri."

Looking ahead - Rev123 Dec 18'09 - Jan 03'10

[elakdawalla]

I have no doubt that the VIMS folks have done the work to show that this reflection couldn't come from anything but body of liquid methane or ethane, but keep in mind that a specular reflection can be generated by non-liquid surfaces, like the glassy surfaces of basaltic lava flows

Although you wouldn't expect basaltic flows on Titan, if there is cryovolcanism, then there should be ice flows. What will the surfaces of those flows look like?

Not that I'm suggesting that's what's going on at the north pole. We know what the topography looks like up there -- dissected plains, dark "lakes." This latest result just shows those "lakes" are smooth at a wavelength of 5 microns, which means they're really really smooth.

Jason Barnes, if you're reading this, I'm wondering if you can explain how the public-release image was produced. THe caption only mentioned one wavelength, 5 microns. Is this a colorized image from a single wavelength, or is it produced from more different bands, do you know?

[scalbers]

My guess on the ice floes or lava is that it would be a bit more diffuse and thus less intense than what we see for Titan, like in Emily's article showing Antarctica.

For fun and comparison here is a sun glint from Lake Erie:

http://upload.wikimedia.org/wikipedia/comm...ie_sunglint.JPG

Is there a possibility for Cassini to get a shot like this? The solar altitude in the above image may be ~10-15 degrees, seemingly similar to the Cassini shot. Viewing conditions for Cassini might improve as the solar declination increases, over a relatively lower latitude lake. What would the maximum possible solar altitude be over such a lake, maybe 40 degrees? This might be less given the duration of the Cassini mission.

Maybe my main question should be can a similar image be taken at closer range to Titan?

[Jason W Barnes]

Although you wouldn't expect basaltic flows on Titan, if there is cryovolcanism, then there should be ice flows. What will the surfaces of those flows look like?

We are well aware that you don't require liquid to have a specular reflection. Clean ice would have one, for instance. But the INTENSITY of the specular reflection of any solid just can't compare to that of a liquid surface. Jason Soderblom is writing a paper detailing the science behind the total intensity of the reflection and its implications for index of refraction of the material, the radius of Titan, size of the sun, distance between them, angle of incidence, and other factors. I'll post it here when it's out, since I'm a coauthor. But it will be a few months -- this is actually pretty hairy when you really get down to it . . .

Jason Barnes, if you're reading this, I'm wondering if you can explain how the public-release image was produced. THe caption only mentioned one wavelength, 5 microns. Is this a colorized image from a single wavelength, or is it produced from more different bands, do you know?

Right; it is colorized from one wavelength. Actually it is our 16 VIMS channels from 4.8-5.2 microns coadded together and then colorized to a pleasing Titanian shade of orange. I have IR color versions of the image that I've made, but I have to say that I'm not convinced that they necessarily add anything over the one that you see here. Remind me once the paper comes out and I can post them, if you like -- but until then you're stuck with the public image release, I'm afraid!

- Jason

[elakdawalla]

Thanks for the explanation! I'm happy to wait for the paper -- I was just wondering if the color was actually providing any information other than "this is supposed to be Titan"

--Emily

[scalbers]

I'll also be looking forward to Jason(s) et al's upcoming paper. Considering some of the factors of reflectance we can look at the Fresnel equations. As an example glass at normal incidence has about a 4% reflection. We can calculate the somewhat lower values for water, and similarly for methane (slightly less than for water) and ethane. This reflectance increases for grazing incidence as can be noticed by looking at the reflection of the sky in a lake at various angles.

Steve

[rlorenz]

Spectacular glint image and I agree with the "iconic" status that Bob Pappalardo gives this image. Now with northern spring we might anticipate seeing more of these coming up. I wonder if the size of the glint is constrained mostly by the size of the lake, size of the sun, or roughness of any waves? .....
VP, what type of non-liquid flat material would likely be on Titan? Could we expect basaltic lavas? What is the chance they would correlate in location with the purported lakes on Titan?

Unfortunately I think this particular image is going to be more iconic than useful, in that the image does
not resolve the structure of the glint (i.e. you don't see the sun's image, or a pattern of speckles
about where the sun image would be - you just see a big square pixel that contains the integrated light
from the pattern). It is a good proof of concept, though, and is prompting the VIMS team to get
their analytical tools together for future opportunities.

The Cassini radio science team also does 'bistatic scattering' experiments, which are essentially the same
thing (but shine radio light from Cassini, observe on Earth). So far they havent published anything on
these experiments over low-latitude surfaces, but some are planned over northern lakes in the
proposed solstice mission.

On the radar team we'd actually considered whether we might see radio sunglint some years ago
(actually an occasional problem for terrestrial orbiting radiometers) - Bartolo Ventura in Bari, Italy
did a good part of his PhD thesis on it. But as for this particular VIMS observation, the spatial resolution
of the real-aperture radiometer doesnt usually allow you to resolve the glint pattern.

On the subject of non-liquid surfaces that can glint, I am reminded of my own commentary in 2003
on the groundbased radar work of Campbell et al which showed striking specular reflections -
see http://www.lpl.arizona.edu/~rlorenz and scroll down to 'Glitter of Distant Seas' for free
link to the Science article. At the time everyone** interpreted these to suggest liquids, but we now
know that the low latitudes on Titan don't seem to have persistent liquids. The question came up
at the time, of course, whether nonliquid surfaces could provide the specular reflections observed.

The answer was that such surfaces would have to be 'flat as parking lots' and they were 20km or
more across, which seemed improbable given what I knew about icy satellite surfaces at the time.
My guess now - and I am now a bit better field-educated on how some real-world sedimentary
surfaces can be that flat, see e.g. Australia and Tunisia pictures also on web page above - would
be that these were flat interdunes (which may well have been liquid-covered in the past)


**including me. No shame in that - simplest explanation at the time. Now we know better - Titan
isnt simple, all the liquids are now at high latitude.

[nprev]

Very informative and interesting, Ralph, thanks!

Just out of curiosity, is the monsoonal model for filling the polar lakes still the working hypothesis? Seems like there's an awful lot of seasonal fluid transfer going on, and it's a bit mystifying to me where all the energy to run the cycle is coming from given the opacity of Titan's atmosphere to so many bands.

All I can think of is that the upper atmosphere must be actively involved in energy absorption & re-emission somehow, but no obvious mechanism jumps out.

[scalbers]

Interesting summary and to hear that the VIMS team is considering future opportunities. I wonder if we might be able to speculate on the specular reflection opportunities with a tool like Celestia? Celestia I believe supports specular reflections so one could in theory watch when they materialize using an updated map.

[titanicrivers]

Just curious about the actual location of the glint based on data in the Photojournal image PIA 12481. http://photojournal.jpl.nasa.gov/catalog/PIA12481 The coordinates given are 71deg N and 337deg W. Using VPs north polar map again and using some protractor and caliper based interpolation puts the glint (just barely) in the southern part (referenced to 340W longitude) of sun-glint lake. This lake was also featured in Photojournal image PIA01942. http://photojournal.jpl.nasa.gov/catalog/PIA01942 The lake is notable for its association with many channels (probable inflowing rivers) and possibly is a bit larger than when imaged in Oct. 2006 (about 260 km in length then). The next Titan flyby T64 http://ciclops.org/view/6082/Rev123 probably will not get SAR imaging to see if shoreline changes have occurred. This might be anticipated as the poleward end of the lake appears shallow, showing channel structure within the lake outline.

[rlorenz]

Just out of curiosity, is the monsoonal model for filling the polar lakes still the working hypothesis? Seems like there's an awful lot of seasonal fluid transfer going on, and it's a bit mystifying to me where all the energy to run the cycle is coming from

Umm 'still the working hypothesis' ? That hasnt been my view for a year or two - my take is
that the clouds (and rain) are at the poles because of the insolation and circulation (and
maybe it helps that the lakes are there). It is net accumulation (precipitation minus
evaporation) that allows lakes to persist - and I think the precipitation part of the equation
is less important than the evaporation. (The clouds and the lakes may be there for the
same underlying reasons, but the clouds don't cause the seas in the short-term sense, except
for the small transient features noted by Hayes et al and Turtle et al )

You are right, the energetics (my canonical (Science, 2000; even hints in Icarus 1996)
energetic limit of 1cm per earth year on a long-term planetwide average I think still stands,
however rapid (m per earth year) evaporation can locally be on a temporary basis.

Even if you could have 1m per year, my empirical relation for lake volume (GRL, 2008)
of horizontal dimension in km equals depth in m, says Kraken and Ligeia are hundreds of
meters deep, so the present north-south asymmetry in lake distribution must reflect
>centuries and cannot be due to seasonal transfer (hence the longer-term cycle
advocated in the Aharonson article.)

There may be seasonal changes we can observe in these seas (it may be harder to
detect in the northern seas if their margins are steep - the very shallow slopes around
Ontario make the level drop easier to detect as a shoreline migration) but the seas
did not form in a season.

[nprev]

Thanks for the re-baselining, Ralph!

So...interesting implications. The dry lakes in the South might be worthy targets for investigation someday; presumably they hold sediments (chemically modified?) from runoff from higher surrounding terrain. Is there evidence at all for post-evap aeolian deposition on the lakebeds?

[Jason W Barnes]

Unfortunately I think this particular image is going to be more iconic than useful, in that the image does
not resolve the structure of the glint (i.e. you don't see the sun's image, or a pattern of speckles
about where the sun image would be - you just see a big square pixel that contains the integrated light
from the pattern).

Hey, man; I thought that you'd agreed to wait until the detailed papers come out before complaining any more. You've changed your mind, evidently.

Yes, the specular view is unresolved. But we have amazing information about the structure of the glint anyway! Let me try to spell it out so that it makes sense.

By your criterion, signal not spatially resolved, transits of extrasolar planets are useless. The planet is not spatially resolved in any sense, all we have from transits is a big fat pixel, resolved in TIME, that results in a lightcurve. But the lightcurves are spectacularly useful in revealing information about the spatial structure of the planet -- oblateness, ring systems, winds, orbital inclination, orbital eccentricity . . .

The specular glint is useful in precisely the same way. Because the glint is resolved in TIME, and has a whopping signal, forward-modeling with a chi-squared minimization can pull out much of the same information that you could get from a single, spatially resolved observation.

So I would recommend that you revert to your previous policy of waiting for the paper before $#!+ting all over every non-RADAR discovery by knee-jerk.

- Jason

[AndyG]

Even if you could have 1m per year, my empirical relation for lake volume (GRL, 2008)
of horizontal dimension in km equals depth in m, says Kraken and Ligeia are hundreds of
meters deep...

Can you point me at more information which lies behind this empirical relationship - and can any relationship, presumably based on Earth examples, be valid for Titan?

Andy

[rlorenz]

More iconic than useful..

Hey, man; I thought that you'd agreed to wait until the detailed papers come out before complaining any more. You've changed your mind, evidently.

No offense intended - it wasn't a complaint. Merely an observation that I think this picture is iconic: the
thoughts it provokes are not themselves detailed in the image.
Nor was the remark meant to impugn lightcurve measurements in general.

By your criterion, signal not spatially resolved, transits of extrasolar planets are useless.

Not at all. Well, first, we don't have radar images of extrasolar planets as we do of Titan, so the
incremental knowledge from a lightcurve of an exoplanet is dramatic ;-). Second, unless my understanding
of the problem has fallen far behind the state of the art since I cross-examined you during your PhD
defense some years ago, even your ingenious modeling would be hard-pressed to unambiguously yield
the insights you list from a lightcurve of 4 data points (which is what this Titan observation is). Maybe future VIMS
lightcurves will be like those from Kepler and we'll be able to extract all that you hope for, but a lightcurve
plot is not as iconic as this image even though such a plot may actually tell us more about
Titan than this pretty crescent - that's what I was getting at.

So I would recommend that you revert to your previous policy of waiting for the paper before $#!+ting all over every non-RADAR discovery by knee-jerk.

It isn't a discovery. It's a confirmation (the image release - which is what prompted the discussion - even says that).