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