Titan's changing lakes Options

[ngunn]

Today's big news?
http://ciclops.org/view/5471/CASSINI_FINDS...ILL_TITAN_LAKES

Changes in the south polar region were announced late last year. Is there more to this story now??

[scalbers]

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? Potential glint expansion due to roughness might be more in the "up/down" direction than sideways. Over what range of phase angles will it be possible to observe glints? Looking at the reflectivity of the surface knowing the phase angle could yield the refractive index and thus information about composition.

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?

[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.

[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

[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).