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Titan's changing lakes
rlorenz
post Aug 23 2009, 06:26 PM
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QUOTE (HughFromAlice @ Aug 23 2009, 01:08 AM) *
To me this looks like a brilliant conclusion to the work of a highly multiskilled team! Congratulations.


It was Lauren who did all the work. One of those discoveries that starts with 'that's odd....'
(namely that the amplitude histogram of the echoes was nonGaussian. Essentially the surface is
so flat that the echo power is dominated by returns from a small area (almost a point target)
and the echoes are sufficiently in phase that the saddle-shaped histogram of the transmitted
chirp is retained.) Thus we can get information showing that few-hundred-meter-wide areas
on Ontario are flat at a fraction of a radar wavelength.)

The effort was complicated by the saturation of the signal, which was then lossy-compressed,
although Lauren managed to reverse-engineer the processing chain to recover some quantitative
backscatter numbers nonetheless.

This experience let us fine-tune the re-observation on T60 with stronger attenuator settings.
Unfortunately that data were lost due to the DSN outage.

So, the elevation profile (reported in my LPSC abstract) shows Ontario is flat to ~10m over
tens of km, and the echo histogram data show it is flat to ~3mm over ~100m scales.

Flat as a millpond, as they say
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nprev
post Aug 23 2009, 09:08 PM
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It's a fascinating result, Ralph, and clearly some impressive instrumentation detective work was involved. Congratulations to you & your associates!

Of course, this apparent extreme flatness begs a lot of questions. Can the surface winds of Titan really be that torpid over such a substantial surface area? You would think that at least a few ripples would be generated by (presumed) small-scale atmospheric convection due to the temperature differential between the liquid & the surrounding shore, unless the whole system is truly isothermal. Alternatively, could the fluid itself be extremely viscous due to the presence of complex organics/contaminants (like runoff sediments from rainstorms), or do we have an inadequate understanding of the gross physical behavior of low-temp methane/ethane/whatever mixtures?

As usual, major discoveries always produce many more interesting questions. I don't see some of these being resolved until we splash (or plop) a probe into one of these lakes.


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djellison
post Aug 24 2009, 08:01 AM
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QUOTE (rlorenz @ Aug 23 2009, 07:26 PM) *
Flat as a millpond, as they say


Minus the ducks. Their wake would have ruined the 3mm factor smile.gif
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AndyG
post Aug 24 2009, 08:26 AM
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Reading this made me think of the artwork by Richard Wilson. It's entitled 20-50 - essentially a room full of old sump oil, perfectly flat, very smelly, of "unknown" depth, and highly reflective at low angles. You can walk into it...



(Picture nabbed from the Saatchi Gallery site)
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ngunn
post Sep 1 2009, 11:24 AM
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BIG changes observed at Ontario Lacus in the 4th abstract here:

http://www.abstractsonline.com/plan/ViewSe...51-9adfcf8a8005

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titanicrivers
post Sep 1 2009, 12:40 PM
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QUOTE (ngunn @ Sep 1 2009, 05:24 AM) *
BIG changes observed at Ontario Lacus in the 4th abstract here:

http://www.abstractsonline.com/plan/ViewSe...51-9adfcf8a8005


Hmm... don't seem to be getting that link to work nigel. Any other link to follow?
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Hungry4info
post Sep 1 2009, 01:15 PM
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QUOTE (titanicrivers @ Sep 1 2009, 06:40 AM) *
Hmm... don't seem to be getting that link to work nigel.


I confirm.


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ngunn
post Sep 1 2009, 01:34 PM
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Strange, it works for me OK, although it does involve two steps - clicking on the title of the fourth paper in the session.

However, assuming you're not even getting to the Session programme try going the long way round from the link I just posted in 'conferences and publications'. Follow links to 'Titan Surface'.

Lots of other goodies there too.
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belleraphon1
post Sep 1 2009, 01:54 PM
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All..

I found I had to outside the forum and go directly to the DPS 41st web page to get these abstracts...

The one in question is in Session 21 Titan surface... I am copying the abstract here because others have had problems getting to this.

Yes, ngunn a lot of juicy abstracts on this site ....

"Title Further Constraints on the Smoothness of Ontario Lacus using Cassini RADAR Specular Reflection Data

Author Block Lauren Wye1, H. A. Zebker1, R. D. Lorenz2, J. I. Lunine3, Cassini RADAR Team
1Stanford Univ., 2Johns Hopkins University, Applied Physics Lab, 3University of Arizona, Lunar and Planetary Lab.

Abstract Cassini RADAR altimetry data collected on the 49th flyby of Titan (T49; 2008 December 21) over Ontario Lacus in Titan’s south polar region shows evidence for intense mirror-like specular reflections. Analysis of the strength of the specular return, which is expected to decline exponentially with increasing surface height variance, reveals that the surface is extremely smooth, with less than 3 mm rms surface height variation over the 100m-wide Fresnel zone (“Smoothness of Titan’s Ontario Lacus: Constraints from Cassini RADAR specular reflection data”, GRL 2009). The T49 echoes were stronger than expected, severely saturating the receiver and inhibiting an accurate estimation of the signal strength and, consequently, the rms surface height. While we developed a method to partially correct the echoes for the distortion incurred, our height estimate is only an upper limit. Further altimetry data over Ontario Lacus is expected in the T60 sequence on August 9th, 2009, where the receiver attenuation will be set high enough over the lake to avoid saturation, and quantization effects will also be minimized. In this presentation, we will report our latest estimates on the smoothness of Ontario Lacus’ surface and what they might suggest for limitations on the wind speeds or surface material characteristics.
This work was conducted under contract with the Cassini Project and was partially supported by NASA headquarters under the NASA Earth and Space Science Fellowship Program. "

Craig
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belleraphon1
post Sep 1 2009, 01:57 PM
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And, of course, I copy the wrong abstract... sorry admins...

"Title Evidence for Liquid in Ontario Lacus (Titan) from Cassini-Observed Changes

Author Block Jonathan I. Lunine1, A. Hayes2, O. Aharonson2, G. Mitri3, R. Lorenz4, E. Stofan5, S. Wall3, C. Elachi3, Cassini RADAR Team
1Univ. of Arizona, 2Caltech, 3Jet Propulsion Laboratory, 4Applied Physics Laboratory, 5Proxemy Research.

Abstract The first SAR observations of Ontario Lacus were made by the Cassini RADAR on passes T57 (June 22, 2009) and T58 (July 8, 2009), providing a nearly complete microwave view of a large lake first seen in ISS images (McEwen et al., BAAS 37, 739, 2005.) Subsequent Cassini VIMS observations of Ontario Lacus indicated the presence of liquid ethane in the lake (Brown et al., Nature, 454, 607, 2008). Comparison of the ISS and RADAR images, taken about 4 Earth years apart, seem to show that the extent of the liquid region--interpreted to be the sharp light-dark boundary at each wavelength--has shrunk. Assuming a topographic slope no larger than 0.1% based on altimetry from the T49 pass of adjacent areas, the shrinkage yields a change in the volume of the liquid of about 15 cu.km.-- an upper limit because the RADAR sees more deeply into the lake than does the ISS. We seek to determine the cause of the shrinkage. The seasonal phase of Titan between 2005 and 2009 permits the hypothesis that the evaporation of methane or ethane from the lake has been responsible. The evaporation of methane will be energy-limited thanks to its large vapor pressure at the southern near-polar temperature of about 92 K (Jennings et al., ApJ, 69, L105, 2009). The maximum evaporative flux at the summer pole is roughly 2 W/sq.meter (Mitchell, JGR, 113, E08015, 2008), leading to a loss over the four years between ISS and RADAR observations of about 20 cu.km of liquid methane. A second approach, assuming advective transport of warm and dry air over the lakes, yields a value several times larger. Ontario Lacus has changed in a way consistent with the hypothesis that it is filled with methane/ethane liquid.
This work is supported by the Cassini Project."

Craig
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remcook
post Sep 1 2009, 02:36 PM
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How well do the ISS and RADAR 'shorelines' correlate for the northern lakes?
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ngunn
post Sep 1 2009, 03:09 PM
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I have to admit I'm baffled by the numbers right now. Area 20 000 sq.km. and volume change 15 cu.km. imply a height change of just 0.75m. They quote an upper limit of 0.1 percent for the bottom gradient - that would translate to a minimum horizontal shrinkage of only 750 metres, surely unobservable in IR. Perhaps it's a lot wider than that, with even shallower gradients. I think we must wait for the full presentation to find out what's really been observed.

Still 15 cubic kilometres is a lot of liquid - enough to fill Loch Ness twice.
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Guest_Sunspot_*
post Sep 1 2009, 03:40 PM
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So when will us common folk get to see the T57/T58 SAR RADAR results?
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titanicrivers
post Sep 1 2009, 05:20 PM
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QUOTE (ngunn @ Sep 1 2009, 07:34 AM) *
Strange, it works for me OK, although it does involve two steps - clicking on the title of the fourth paper in the session.

However, assuming you're not even getting to the Session programme try going the long way round from the link I just posted in 'conferences and publications'. Follow links to Surface'.

Lots of other goodies there too.


See if I can get this link to the goodies to work!
(nope, will try another)

http://www.abstractsonline.com/plan/start....08CED373A512%7D

That one seems to work.
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ngunn
post Sep 1 2009, 06:18 PM
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I've edited my previous post 42 to correct a numerical error.

When will we see the SAR? I'd guess around conference time.
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