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Titan Review article
Juramike
post Dec 18 2007, 04:26 AM
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QUOTE (rlorenz @ Dec 17 2007, 07:24 PM) *
...why hydrolyzed tholins are fluorescent (see Icarus paper by Hodyss et al a couple of years ago..)


More pi system fun. When pi-systems get really extended they can allow multiple modes of accessible pi system excited states. So instead of the simple pi-->pi* from above post, you get pi-->pi*1-->pi*2 + hv. So instead of the same color light going off in a different direction, you get a different color light being emitted. So it will look brighter in that color. The new color corresponds to the energy gap between the excited states.

[Sometimes when we do UV based TLC described above, instead of a black dot against the phosphorescent background, we get a spot that "glows back at ya". Usually this is bluish, but other fun colors are often seen as well. IIRC, anthracene will glow back at ya blue when illuminated with a 254 nm UV lamp (or it might be at 210 nm - more things seem glowy at shorter wavelengths)]

Usually the reradiated wavelength is specific (and tunable) for the molecule. This trick is used to identify certain materials and uhhh, bodily fluids [How many time have we seen them whip out the tunable light source at a crime scene on "CSI"?]

[[A lot of ion channel assays use a really cool trick with fluorescence. Biologists use a fluorescent compound (call it a red glower) that floats on the outer surface of the cell membrane (it's got a lot of charged functional groups that won't let it pass through the membrane bilyer). Biologist then add another fluorescent compound that has an acidity close to pH of the cell environment. There is another fluorescent compound (call it a blue glower) that will protonate and lock up to one side of the membrane, which ever one has the lower pH. Under normal conditions of cell polarization the blue glower is on the inside membrane far from the red glower. You zap with a laser at the right wavelength that excites only the red glower and it glows "red". But when the membrane (via ion channels) is depolarized with an active compound, the ionic gradient shifts, the blue glowing free-floating compound switches to the same side of the membrane as the red glowing compound. Because of their proximity, the excited red molecule can now transfer fluoresence energy to the blue glower. So when you zap a depolarized membrane with the laser to excite the red molecule, it gets excited, tranfers energy to the blue glower and you get a "blue" glow instead of "red". So now you can figure the cells polarization state by measuring color red = normal, blue = depolarized. This nifty trick is called fluorescence resonance energy transfer (FRET) and is used all the time in biological and chemical assays. And the fluorescent dyes used are usually big aromatic compounds with heteroatoms liberally sprinkled in the ring system.

Check out (cool diagrams): http://en.wikipedia.org/wiki/FRET

Why do they go through all this effort? Because one of the most common problems with screening compounds with a single-flourescence assay is that you get all sorts of false positives.

It seems a whole bunch of polyaromatic compounds are out there in nature just waiting to glow back at ya and mess up your single-flourescence assay. The double-flourescence trick gets around these impostors.]]

So with all sorts of aromatics dripping down from the atmosphere, a UV light on Titan would be a really psychedelic experience.

-Mike


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ngunn
post Dec 18 2007, 11:40 AM
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Fascinating, Mike. But you've got me wondering if a UV light source is absolutely necessary. Could there be chemical reactions going on that produce the coloured lights directly? Would a cryovolcanic eruption produce a cold firework display? Would a violent rainstorm be accompanied by strange glows? I guess you'd have to be underneath the haze to observe such phenomena if they occur.
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rlorenz
post Dec 18 2007, 01:59 PM
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QUOTE (Mongo @ Dec 17 2007, 07:58 PM) *
.....
The other option would be some form of dirigible balloon, with ducted fan(?) for some degree of directional control, that mainly stays in the troposphere with occasional descents to the surface for samples. It would have to be powered by RTGs or perhaps a nuclear reactor
.......


OK. right there you took the thread away from discussing the next Flagship into 'someday, wouldnt it be
nice'

You can debate the readiness of an RTG dirigible, but reactors are not presently on the cards.

btw - ITAR doesnt *prevent* anything, it just necessitates paperwork. Clean interfaces help.
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vjkane
post Dec 18 2007, 05:05 PM
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QUOTE (Mongo @ Dec 18 2007, 12:58 AM) *
The other option would be some form of dirigible balloon, with ducted fan(?) for some degree of directional control, that mainly stays in the troposphere with occasional descents to the surface for samples. It would have to be powered by RTGs or perhaps a nuclear reactor, which should also allow enough power for a direct link to Earth, eliminating one link in the communication chain (although the bit rate may be higher if an orbiter can relay its transmissions).


Leaving aside the question of power source, designing a system that can touch down repeatedly with a high probability of survival is really hard. Winds could easily blow you around at low altitudes. When you start your descent, you will be over point x, but as you descend the winds may take you to very dangerous point y, and you are too far away for real time control from Earth.

I'm not an engineer, but it might be easier to have a balloon that drops small landers (although once they have to carry heavy instruments like a mass spectrometer, they may not be so small...) at interesting points.

In a side conversation with Ralph L, he pointed out that there are many, many mission options. The hard part is to nail down the science goals and establish the budget. Once that is done, the engineers can apply their creativity.

So send your checks to NASA and letters to Congress.


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Mongo
post Dec 18 2007, 05:55 PM
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QUOTE (vjkane @ Dec 18 2007, 05:05 PM) *
So send your checks to NASA and letters to Congress.


So personal cheques directly to Ralph Lorenz are out? rolleyes.gif
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dvandorn
post Dec 18 2007, 06:31 PM
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QUOTE (rlorenz @ Dec 18 2007, 07:59 AM) *
You can debate the readiness of an RTG dirigible, but reactors are not presently on the cards.

And remember, folks, this comes from someone who was a lot closer to the JIMO debacle than most of us.

-the other Doug


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vjkane
post Dec 18 2007, 07:42 PM
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QUOTE (Mongo @ Dec 18 2007, 05:55 PM) *
So personal cheques directly to Ralph Lorenz are out?


Beer leaves fewer auditable tracks, and regardless of the outcome, you had a beer and good company. rolleyes.gif


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rlorenz
post Dec 18 2007, 08:13 PM
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QUOTE (Mongo @ Dec 18 2007, 12:55 PM) *
So personal cheques directly to Ralph Lorenz are out? rolleyes.gif


You could buy 'Titan Unveiled' when it comes out in April. I think my
royalties work out at about the price of a beer per copy.
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rlorenz
post Dec 18 2007, 08:16 PM
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QUOTE (dvandorn @ Dec 18 2007, 01:31 PM) *
And remember, folks, this comes from someone who was a lot closer to the JIMO debacle than most of us.


Actually I stayed well clear of that one (thankfully). I was, however, on the NRC panel that
contemplated it and other such missions

http://www.nap.edu/catalog.php?record_id=11432
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dvandorn
post Dec 18 2007, 08:42 PM
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QUOTE (rlorenz @ Dec 18 2007, 02:16 PM) *
Actually I stayed well clear of that one (JIMO) (thankfully). I was, however, on the NRC panel that
contemplated it and other such missions.

That still makes you closer and more knowledgeable than most (if not all) of the rest of us about the specific issue of flying full-scale nuclear reactors on outer planet probes, Ralph.

Sure, it's possible. The technical challenges and risks are just a little higher than can be overcome at the moment, I think.

-the other Doug


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Mongo
post Dec 18 2007, 08:59 PM
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QUOTE (rlorenz @ Dec 18 2007, 08:13 PM) *
You could buy 'Titan Unveiled' when it comes out in April. I think my
royalties work out at about the price of a beer per copy.

Oh, I intend to. I had bought "Lifting Titan's Veil" as soon as it came out in HC, so that's one beer worth of royalties already.
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tty
post Dec 18 2007, 10:09 PM
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QUOTE (vjkane @ Dec 18 2007, 06:05 PM) *
Leaving aside the question of power source, designing a system that can touch down repeatedly with a high probability of survival is really hard. Winds could easily blow you around at low altitudes. When you start your descent, you will be over point x, but as you descend the winds may take you to very dangerous point y, and you are too far away for real time control from Earth.


I couldn't agree more. Landing a dirigible in any sort of wind is quite tricky even with a live crew, real time control and a landing team on the ground.

Lowering an instrument package on a cable might be barely feasible in flat terrain. If the wind is reasonably steady and not too strong an autopilot could probably hold the dirigible more or less still. However if the probe got stuck the only option would be to cut the wire.
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ngunn
post Dec 18 2007, 10:24 PM
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QUOTE (rlorenz @ Dec 18 2007, 08:13 PM) *
You could buy 'Titan Unveiled' when it comes out in April. I think my
royalties work out at about the price of a beer per copy.


April. And there was I hoping that 'later this year' might still happen. (I see it's already up on Amazon.) Never mind, you can count on an April beer from me. And the rest of us here would definitely be too much for one night, so we'd better stagger those purchases. . .
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vjkane
post Dec 18 2007, 11:58 PM
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QUOTE (rlorenz @ Dec 18 2007, 08:13 PM) *
You could buy 'Titan Unveiled' when it comes out in April. I think my
royalties work out at about the price of a beer per copy.


I just pre-ordered my copy. I hope it publishes on time so I can bring it with me on my vacation.


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NMRguy
post Dec 19 2007, 01:14 PM
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QUOTE (rlorenz @ Dec 18 2007, 02:59 PM) *
OK. right there you took the thread away from discussing the next Flagship into 'someday, wouldn't it be nice'

So perhaps I can ask a question about the currently orbiting flagship?

In your very nice review, you state the following (below). I have heard a number of proposals for an XXM and this option would certainly allow for the longest observation window in the Saturn system. I know we can only have limited discussion about the possible XXM when we haven't yet finished the nominal mission, but is the "low-maintenance cycler orbit" a long term Cassini mission goal or YOUR preferred orbit evolution?

QUOTE (Titan_lorenz.pdf, p.142)
At the end of a 2-year mission extension, Cassini could easily be introduced into a low-maintenance cycler orbit between Titan and Enceladus. With minimal intervention, it would make repeated flybys of both bodies, yielding an efficient science return as long as its systems, propellant, and ground support hold out.
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