Titan Review article Options

[rlorenz]

This just out. Not earth-shattering, but colorful - maybe handy as an up-to-date
Titan intro

http://www.jhuapl.edu/techdigest/td2702/lorenz.pdf

[Webscientist]

I bought in 2004 "Lifting Titan's Veil".It's of course the reference for Titan and I will reread it very soon to compare with what we know now ( presented in Titan revealed).

I'm fascinated by the radar images of the lakes in your Titan review.Unfortunately, the radar images don't give any indication on the appearance of the liquid.Does it appear dark, orange, blue... from a human eye?

Some dark and uniform patches located on the "white snow" of Iapetus made me think they were pools of hydrocarbons, similar to what we might find on Titan. Do you think that the idea is relevant?

[rlorenz]

I'm fascinated by the radar images of the lakes in your Titan review.Unfortunately, the radar images don't give any indication on the appearance of the liquid.Does it appear dark, orange, blue... from a human eye?

Some dark and uniform patches located on the "white snow" of Iapetus made me think they were pools of hydrocarbons, similar to what we might find on Titan. Do you think that the idea is relevant?

Lakes - get asked this a lot. Dunno. Probably like one of those 'Random_City at night' postcards - black.
Since the lakes are at the poles, its often nighttime. Sun and saturnshine is always low on the horizon, never
high in the sky, and only red light filters down to the ground. If you brought your own white light with you,
depends. Pure methane would look blueish - like Neptune - because of the methane absorptions in red. But
if there is a lot of reddish tholin suspended in it, maybe brownish (wine-dark sea?). So mostly black

White snow - even stuff like benzene (for example) at liquid nitrogen temperatures is white. I think
maybe anthracene is yellow (maybe Juramike can explain how things get dark/colored?). Soot of course is
black. I don't think we can rule out any of these of Titan (or Iapetus, for that matter..)

[Juramike]

White snow - even stuff like benzene (for example) at liquid nitrogen temperatures is white. I think
maybe anthracene is yellow (maybe Juramike can explain how things get dark/colored?).

Sure - I'll take a stab at it.

For organic molecules, things with molecular pi-orbital systems will absorb UV light. The electrons in the pi-systems get pushed up to an excited state. The UV photon goes in and excites the pi-cloud, then goes zipping off in another direction. Other photons just pass right through. Net result: UV gets absorbed.

The more extended and conjugated the pi-system, the lower the energy UV photons that can get absorbed.

Benzene has a UV peak absorbance at 210 nm. It looks white to our eyes, but is really absorbing some UV light. Put a bit of benzene on a phosphorescent silica background, and hit it with a UV light at 210 nm, and you'll see the black spot where the light didn't get through to the phosphorescent background. (At 254 nm the absorbance is kinda weak.)

(Chemists use this trick every day when monitoring reactions by TLC (thin layer chromatography). The bulk of compounds synthesized have extended aromatic or heteroaromatic rings. When there's no UV absorbance, like in aliphatic molecules, then chemists have to "do the dip" in order to stain the TLC using a reactive stain. [Still other chemists inject reaction crudes directly into the LCMS and clog up the instrument for everybody esle - these are bad chemists])

The more extended the pi-system, the lower the energy gap between the occupied and unoccupied pi-orbitals. Fusing aromatic rings together, or sticking certain functional groups in conjugation with the aromatic pi-system, all cause a shift to longer wavelengths. (Carboxyl, alkene, oxy, thio, halo - stuff like that), So things like napthyl, and anthracene (more and more benzenes in a line) make the maximum aborbance longer.

If you shift the UV absorbance into longer wavelengths, eventually you start absorbing in the visible spectrum. Remove blue light, and things look more yellow.

So the more extended the pi-system in a molecule, the yellower it looks.

Aside from the wavelength shift, there is also the effect of changing the extinction coefficient with certain functional groups, this can really amplify the absorbance exponentially. Check out the bathochromic shift (longer wavelength) and extinction coefficient jump for anthracene:

Benzene - lamba max = 255 nm (extinction coeff = 230) [much bigger absorbance hump near 210]
Naphthalene - lambda max = 314 (extinction coeff = 250)
Anthracene - lambda max = 380 (extinction coeff = 9000)

[In my advisor's group in graduate school, there was a guy in the next lab making large molecules resembling C60. As the aromatic system got larger, the compounds went from yellow, to an intense brick red. The guy's name was Rudiger Faust, and I strongly recommend his book "World Records in Chemistry" as a gift for anyone with even a slight hint of chem nerd in them.]

It does NOT take very much polymeric aromatic impurity to make things look highly colored. (Extreme case being black).

Most reactions always give a little black or highly colored aromatic goo that needs to be purified away. In my experience most reaction mixtures or slightly impure products (when things go good) always seem yellow. It's a rare and special day when someone gets a blue or green color in their reaction or product. (And we usually stand around and go "Pretty!")

Titan's surface and lakes are most likely highly colored. (Remember that black is a color).

-Mike

[rlorenz]

Sure - I'll take a stab at it.
<snip>

Mike - that was great. For your next assignment, explain why hydrolyzed tholins are
fluorescent (see Icarus paper by Hodyss et al a couple of years ago..)

[Juramike]

...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