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Posted by: Mariner9 Nov 6 2005, 06:49 PM

The rule of thumb being bandied about by the project scientists and engineers is that the visual infrared imagary of Titan is degraded by the haze, and the effective resolution is roughly a factor of five higher than the listed "per pixel" value.

So, most of the imagary I've seen so far seems to be effectively above 2-3KM per pixel.

Cassini passes fairly close to Titan, I would think we'd be getting images in the 50-100 meter ranges, but I'm not seeing any images like that.

I'm assuming this has a lot to do with approach geometry and my impression that the best images are taken much closer to areas corresponding to Titan noon, than say Titan 6:00PM. So far the closest approaches were probably closer to the terminator than the center of the dayside, so we're getting much lower resolution. I also imagine that the time-share situtation with the instrument pointing (God I miss the Voyager and Galileo scan platforms) has something to do with this also.

My question is this, later in the tour are we going to get any of the visual imagary at that higher resolution, so we can make out features smaller than the current sets?

And any idea just how high resolution we can hope for?

Posted by: BruceMoomaw Nov 6 2005, 09:29 PM

The figure mentioned by Elizabeth Turtle at the COMPLEX meeting was about 1 km -- but that may have been for the longer-wavelength VIMS images, which are much clearer.

Posted by: JRehling Nov 7 2005, 01:42 AM

To resolve the horrible ambiguity, I assume that she meant 1 km resolution (500 m/pix)... but perhaps she meant 1 km/pix?

It seems to me that ISS definitely provides subkilometer pixels of effective resolution, and the VIMS people have been claiming better.

Posted by: edstrick Nov 7 2005, 07:03 AM

The problem is that light reflected off the surface is almost entirely scattered once or twice by the haze in the atmosphere before it escapes into space.

Take a sheet of waxed paper and a couple or so of wooden matches and put the matches down flat on a sheet of printed paper. Put the waxed paper down on the matched and you may well be able to read the text under the waxed paper, even though it's pretty fuzzy. Put a couple pens or pencils under the waxed paper to move it 1/2 or 2/3 cm above the text and you probably won't be able to read a 10 or 12 point font but you'll be able to read 20 point headlines (or some such text size)

The Titan haze is butterscotch colored: very absorbing in blue/violet/ultraviolet, nearly completely reflective in the far red and near infrared. The dust is "forward scattering", which means that most of the light coming up from the surface and scattered by the haze still travels in *ABOUT* the original direction it was going, but has been scattered a degree or five, but mostly not 10 or 30 or 90 or 120 degrees (which would be back down).

Looking straight down, this means that the surface view is blurred by the forward scattering but not totally wiped out. The smaller the feature, the more contrast it loses. A 30 km feature may (I'm faking real numbers here) have it's contrast reduced by 80%, a 3 km feature may have contrast reduced by 98%, and a 0.3 km feature may have contrast reduced by 99.8%, rendering the latter totally invisible. Looking partly toward the horizon, through more haze AND with much of the haze further from the surface you're viewing obliquely, the surface blurs very rapidly and contrast goes to hell in a hurry.

At longer wavelengths than the camera can see but VIMS can, the atmosphere's haze particles are much smaller than the infrared light's wavelength, and most of the forward scattering by the haze probably turns into much weaker and more omnidirectional "Rayleigh" scattering which is the same scattering that gas molecules do, resulting in Earth's blue sky. It decreases rapidly with wavelength and interferes little with seeing the surface. The problem with VIMS is that it inherently has much lower resolution than the regular camera, and the amount of sunlight rapidly decreases as wavelength increases (and the atmosphere gets clearer).

Posted by: BruceMoomaw Nov 7 2005, 12:34 PM

On reexamining my notes, I find that Turtle actually specified that that 1-km best resolution was "about five pixels" -- and she was definitely referring to the ISS, not VIMS. Which raises the question of what VIMS' best resolution really is.

Posted by: jmknapp Nov 8 2005, 01:00 PM

How many of the Titan closest approaches have been devoted to radar (precluding ISS)?

Posted by: BruceMoomaw Nov 8 2005, 03:02 PM

Well, they don't actually preclude ISS (or the other optical instruments) -- they just mean that those instruments can't make observations during the brief period of closest approach. So far, SAR has been done on four of the eight Titan flybys -- and that 50-50 division is supposed to be maintained during the rest of the mission. However, the science schedule is only worked out for the first ten or so Titan flybys in advance, and there is considerable talk of increasing radar coverage. For instance, one of the passes currently planned for gravity science (which rules out ALL other science observations), would allow either ISS-VIMS coverage of "Lake Ontario" in the south polar region, or radar altimetry of it (which might pick up a specular reflection off any liquid), or SAR mapping of some neighboring regions. It may well be reassigned for one of those goals.

Posted by: The Messenger Nov 8 2005, 03:26 PM

Not a good choice, IMO, understanding the surface will ultimately require a pretty good window on the mass distribution. There needs to be at least a few gravity runs early in the mission, just in case we lose Cassini.

I hope future (new) mission planers are looking at 'tethered' gravity instruments that will not have to compete with a full platform of instruments for science time.

Posted by: jmknapp Nov 8 2005, 04:10 PM

Thanks. I note that for T8, radar started from 25,000 km out (scatterometry) and continued for 25,000 km on the other side, meaning the NAC was on the sidelines for that entire time.

Posted by: edstrick Nov 9 2005, 07:58 AM

At a purely arm-waving level, I'd expect a reduction in the data-storage and targeting maneuvers allocated to closest-approach optical/near-ir imaging and increased allocation to maximum resolution and coverage VIMS data.

Posted by: BruceMoomaw Nov 9 2005, 10:53 AM

There are supposed to be four gravity-science Titan flybys during the main mission -- T11, T22, T33 and T38 between February 2006 and December 2007. I believe it's the very first of these that may be replaced by an examination of Lake Ontario -- at any rate, I know there will be a meeting within 2 weeks to discuss the possibility of that replacement, so I presume it's for the first gravity flyby.

Posted by: The Messenger Nov 9 2005, 04:05 PM

Tough call - especially since a gravity pass near the lake would also be exciting - to some of us. I guess if I were on the gravity team, I would support the change, if they would thrown me another bone, such as a gravity pass of one of the other moons in the very near future.

We only had a few gravity passes during the Galileo mission, and it took years to isolate the Ganymede anomaly. Early gravity passes are necessary - not only to help characterize surface feature, but to have enough time to analyse the data and figure out where future gravity passes should be injected into the mission.

Posted by: volcanopele Nov 9 2005, 05:33 PM

T38 may be replaced with an altimetry pass over Ontario Lacus.

Posted by: BruceMoomaw Nov 10 2005, 12:42 AM

Aha! So it's one of the LATE gravity passes, not the very first one, that would be cancelled. I'd think this would make it easier to plug another gravity pass into the early part of the extended mission as a replacement. In any case, examination of that possible lake is important enough that I think such a replacement justified.

Posted by: tedstryk Nov 10 2005, 02:18 AM

Assuming there will be an extended mission, I would rather see gravity passes there. If precedent is an indicator, the team will be much reduced, leaving much less funding and personel to plan. Better to do these relatively simple flybys then, than when the fully staffed team can plan observations. I also wonder how much gravity data can be obtained, given the problem of atmospheric drag, that, for example, damanged observations during the close non-targeted Enceladus flyby.