Titan's changing lakes Options

[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.

[ngunn]

Excuse a very basic question: would that be one pixel at one wavelength or do you get an IR spectrum for that pixel?

[Juramike]

Excuse a very basic question: would that be one pixel at one wavelength or do you get an IR spectrum for that pixel?

Each pixel is being observed at the VIMS IR wavelengths. So, in theory, you'd get an IR spectrum for each pixel location. In practice, the methane absorptions limit how many wavelengths you could observe, then atmospheric scattering makes some of those wavelengths (esp. the shorter ones) difficult as well.

Somebody's gonna have fun applying all the atmospheric and haze corrections to get the corrected spectra.

It'll only be close to a "perfect mirror" at a few select wavelengths due to the intervening atmosphere. Kind of fun to imagine a funhouse mirror that would only reflect "blue" and "orange" but nothing else.

[ngunn]

applying all the atmospheric and haze corrections

I was thinking this kind of observation could be a powerful means of quantifying those things, thereby sharpening up the interpretation of other VIMS data, especially the remainder of the same image.

[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

[Jason W Barnes]

Excuse a very basic question: would that be one pixel at one wavelength or do you get an IR spectrum for that pixel?

Well, there's a spectrum all right, but as the AGU abstract states, the specular reflection has no effect on the wavelengths shortward of 5um. So there's a spectrum within the 5um window, and upper limits below that.

- 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..

[volcanopele]

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

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)

Well, that presumes a single image of sunglint. Multiple images showing how the reflection changes with phase angle would help.

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..

Titan's like Kansas. The skies are never cloudy all day.

[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.

[ngunn]

Article on seasonal and longer term change:
http://www.sciencedaily.com/releases/2009/...91129153401.htm

Source paper:
http://www.nature.com/ngeo/journal/vaop/nc...bs/ngeo698.html

[volcanopele]

We shall see. I still think its seasonal.

[Jason W Barnes]

We shall see. I still think its seasonal.

But then where are the empty south polar lakes? Does the whole south pole fill up in southern winter? This is a problem with both the seasonal and longer-term migration of volatiles. I think that the total volume of methane/ethane transported between poles over seasonal and longer timescales is small, and that Kraken stays pretty much the way it is year-round.

Maybe.

- Jason

[volcanopele]

Where are the south polar dry lakes? What do you think those low, flat areas are in the RADAR sar data that match up with ISS dark areas? That being said, the north polar region seems to have more dedicated lake basins while the south pole has mostly opportunistic playas (though there are a few of those up north too).

As for Kraken Mare, again, I think it is still plausible that Mezzoramia is the south polar version of that sea.

[ngunn]

I don't have access to the full paper, but from what I have seen I like the idea of seasons superimposed on 'superseasons'. Nevertheless there must be other major factors involved as well, regional topography being an obvious one. For me the fact that there is still legitimate room for widely differing opinions on such a major matter is humbling and quite wonderful in itself.

[Jason W Barnes]

Where are the south polar dry lakes? What do you think those low, flat areas are in the RADAR sar data that match up with ISS dark areas?

I think they're dark areas. They could be anything.

If the ethane content of Kraken Mare is substantial, then there's just no way to move it around on seasonal timescales. Oded's Milankovic timescales, maybe.

- Jason

PS -- I can't even get a copy of this paper -- I guess we're not subscribed to Nature Geoscience here for some reason?