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??

[ngunn]

Personal anecdote: perhaps not many people remember their 57th birthday as one of the best. I do. Not only did we have this VIMS observation of northern lakeshine, but also the Ontario SAR:

Ontario Lacus......... at last

The SAR of Ontario Lacus was long expected and advertised, but the VIMS Kraken Mare specular reflection was not mentioned in the July 8 Mission Description or in the 'Looking Ahead'. I wonder if it also was expected, or purely serendipitous?

[Jason W Barnes]

The SAR of Ontario Lacus was long expected and advertised, but the VIMS Kraken Mare specular reflection was not mentioned in the July 8 Mission Description or in the 'Looking Ahead'. I wonder if it also was expected, or purely serendipitous?

I guess if those are the only two options, then it's "serendipitous". We have been looking for specular reflections all the time, but haven't seen any -- the reason of course is that the Sun hasn't been shining on the wet places (Ontario excepted). So while we look at the images and keep specular in mind, we haven't before designed a sequence around it. Now that we've found one and see how totally cool it is, though, and what great science can be done with it, we're looking for opportunities in the future to do a planned specular campaign. It all depends on the spacecraft geometry, though, so we pretty much just have to wait for the right time.

- Jason

[rlorenz]

Now that we've found one and see how totally cool it is, though, and what great science can be done with it, we're looking for opportunities in the future to do a planned specular campaign.
- Jason

As I've remarked to you in person, it is totally cool. But what is the great science ? Since the specular point
is just that at any given instant, the geometry varies with time (i.e. the angle varies as the point tracks across
the surface, so you don't vary angle and position independently [this is also a problem in the radio equivalent -
the bistatic scattering experiment, results of which from T12 years ago have yet to be published] - maybe it's
not too much variation, I guess may depend on the specifics of a given observation.) If you
can resolve the brightness distribution around the specular point, then it is an interesting measure of roughness
across an assumed uniform structure like a lake, although is it any better than a SAR image of the same thing?
But a single pixel specular reflection is of limited utility, I think....

Not to be a (shiny) wet blanket, and I repeat, it is cool, but by itself it isnt telling us a lot about Titan unless I am
mistaken.

[Jason W Barnes]

As I've remarked to you in person, it is totally cool. But what is the great science ? Since the specular point
is just that at any given instant, the geometry varies with time (i.e. the angle varies as the point tracks across
the surface, so you don't vary angle and position independently [this is also a problem in the radio equivalent -
the bistatic scattering experiment, results of which from T12 years ago have yet to be published] - maybe it's
not too much variation, I guess may depend on the specifics of a given observation.) If you
can resolve the brightness distribution around the specular point, then it is an interesting measure of roughness
across an assumed uniform structure like a lake, although is it any better than a SAR image of the same thing?
But a single pixel specular reflection is of limited utility, I think....

Not to be a (shiny) wet blanket, and I repeat, it is cool, but by itself it isnt telling us a lot about Titan unless I am
mistaken.

You are mistaken.

Saying that there's no information to be had from a single pixel would imply that, for instance, transiting extrasolar planets would tell us nothing, since they're just one pixel. In fact this is an apt analogy. I approach the Titan problem from the exact same standpoint -- that of a lightcurve. I fit the lightcurve using various critical parameters, from which I get the science. For instance, the lightcurve tells you the path that the specular reflection takes (using the RADAR basemap), from which I can infer the triaxial shape of the equipotential surface, along with other cool things like wave properties and the composition (okay, index of refraction) of the fluid. Stand by for the paper, it will probably be a few months yet with my twin babies arriving soon, but I think that by the end you'll agree that your above statement is one-minus-correct, perhaps not unlike your 1996 no-sand-dunes-on-Titan paper!

- Jason

[rlorenz]

You are mistaken.

Wot, are you a graduate from the Roger Yelle school of diplomacy or something..?

I fit the lightcurve using various critical parameters, from which I get the science. For instance, the lightcurve tells you the path that the specular reflection takes (using the RADAR basemap), from which I can infer the triaxial shape of the equipotential surface, along with other cool things like wave properties and the composition (okay, index of refraction) of the fluid.

Hmm, well, I'll stand by for the paper. But I still don't see how you can get wave properties and
composition independently for each point on your lightcurve : I can see how you might derive one
value for each if you assume the properties are spatially uniform along the specular track,
which they may or may not be (as casual inspection of a resolved image of sunglint on a terrestrial
lake or sea will tell you)

In any case, I hope this is just the first of many cool VIMS lakes results in coming years as the sun
rises over Titan's north. Hopefully the sunshine won't kick up too many clouds that you cant see anything..

[Jason W Barnes]

Hmm, well, I'll stand by for the paper. But I still don't see how you can get wave properties and
composition independently for each point on your lightcurve : I can see how you might derive one
value for each if you assume the properties are spatially uniform along the specular track,
which they may or may not be (as casual inspection of a resolved image of sunglint on a terrestrial
lake or sea will tell you)

Agreed that there can't be composition and wave gradient distributions for each datapoint. But the composition of the lake should be the same for the whole lightcurve from mixing presumably, and if you assume a lower-order fit to the waves as a function of position, then you should be able to pull out some variations. Not from the present 4-point T58 lightcurve, mind you, but from potential future, tighter observations.

Also note that I would say that this technique has an advantage over the Wye et al. technique in that it does not require valuable closest-approach time -- the observations in question were a few hours after C/A IIRC.

- Jason

Edit -- Not to knock the Wye et al. work, which I think is awesome! Just that watching lakes looking for variations in waves would be expensive in terms of C/A time using that method.