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Posted by: ngunn Aug 26 2009, 09:36 AM

From 'Science Daily':

http://www.sciencedaily.com/releases/2009/08/090825163726.htm

Posted by: Juramike Aug 26 2009, 12:50 PM

Very interesting!

My own experience with a wide variety of organics shows that while a few pure materials are sometimes the ideal "free flowing powders", usually you are stuck with a sticky clingy mess. I can easily imagine that any airfall deposit or processed organic would be much more likely to be sticky and clingy rather than a nicely behaved crystalline powder.


Does this mean that the dunes grow forward and outward in a similar manner to the yukky electrostatic dust that accumulates on a plastic vent intake? (like rime frost) I guess this also implies that once the grain hits a dune, it wouldn't bounce away further or along the dune. Dunes are the terminal sinks for the grains.

Is there a key feature that could be determined from orbit that could nail it one way or the other?

(F'r instance: How would the thickness of a dune vary along it's length? How would it bifurcate? (would it bifurcate in the windward direction?))

Posted by: Juramike Aug 26 2009, 01:00 PM

Wait a minute!

The transverse dunes of T8 (just SW of Adiri).

That would imply either non-sticky saltating grains that find themselves in a unidirectional environment (entrance to Ching-Tu)
-OR-
They are sticky grains that are exposed to a windfield strong enough to overcome their stickiness, pluck them away and form transverse dunes.

Posted by: ngunn Aug 26 2009, 02:08 PM

I have a rather old pair of hi-fi speakers at home with black foam rubber covers that have started to perish. They leave little piles of dust and 'crumbs' on the surfaces they stand on. Each time I clear it up I imagine I'm handling Titan dune material. You can make it blow about (and go up the vacuum cleaner, fortunately) but it is definitely clumpier than an ideal powder.

I imagine the climate modellers will be happy with dune material that natually forms linear dunes whilst being less fussy about the wind regime required.

It will be interesting to learn, over time, if the reassessment of the many parameters and processes mentioned in the last paragraph of the article helps to make more sense of Titan's dunes/weather system as a whole. Glad to see you're already taking a crack at it Mike.

Posted by: marsbug Aug 26 2009, 02:30 PM

Exploring the exact mechanisms of adhesion at work with titan materials and under titan conditions would be fascinating. On earth most surface properties are actually determined by the layers of surface contaminants, of which water and organics are among the most common. Actuall direct contact between two materials will often result in an instant bond- hence why frictional forces tend to be so much higher in a vacuum. Low temperatures and direct contact will result in instant cold welds between asperities, or can result in much lower adhesion as the cold changes the properties of surface contaminants. So exactly how things work in titans ultra cold, organic-materials-and-cryo-liquids environment is compelling science. The biggest mystery is why things look so much like earth, and if that continues down to the finest scales?

Surface science is important to almost every industry, so we might learn some new tricks from titan?

Posted by: Juramike Aug 26 2009, 05:00 PM

From a practical standpoint, I've always wondered why (organic amine or heterocylic amine) free bases are so sticky and difficult to work with, yet when you acidify and make the corresponding acid salt (e.g. pyridinium hydrochloride) it behaves more like an ideal free flowing powder.

Under Titan conditions, any organic amines would likely form as the free base. Assuming the ammonia/water ocean and solid ices derived from that, the free base form of any organics would also be preferred.

So theoretically at least, from either a kinetic (formation) or thermodynamic (equilibrium with the environment) standpoint, the free base form on Titan should be preferred.


Why are free bases in general more sticky than the salt forms? Speculating wildly, it could be that the free base amines form relatively weak transient H-bond interactions between the molecules in the solid network. These sloppy intramolecular bonds make the intramolecular arrangments less ordered. But protonate them up under acidic conditions, and you force a local cationic charge that can set up a more defined and orderly network of cation-counterion <and solvent> couples. A more ordered network makes a more solid matrix.

But that's about the limit of my knowledge of atomic-level surface science....

In any case, knowing the exact composition of Titan dune sands is going to be key.

Posted by: belleraphon1 Aug 26 2009, 06:46 PM

And knowing how all these materials really behave under cryogenic conditions.... over eons. Only way to really know for sure is to go there

Craig

Posted by: marsbug Aug 26 2009, 07:56 PM

Thats well above my level of organic chemistry! But from working on putting down nitride and oxide coatings there will very likely be an intermediate layer between any two grains. In fact it's bloody hard to get rid of, even under high vacuum conditions most things have a layer of oxides that need ion etching off. Depending on the exact conditions on earth it's almost always water plus greater or lesser amounts of what else is in the atmosphere. From there a lot depends on how the surface reacts with water and the atmospheric gasses, so as a total shot in the dark I'd suggest that the free bases have a better affinity for atmospheric water, or more likely something they make as a layer on reacting with air ( oxygen content a likely suspect?) has a better affinity. On titan the grains would (I imagine) be most likely to condense monolayers of simple organics to start with. More wild speculation: Is there any chance polymers could form and each grain could have a tangle of polymers around it, acting like very tiny velcro?

Posted by: Juramike Aug 26 2009, 08:17 PM

Polymeric tangles - very likely.

Somewhere (forget where, but I think it was in a CHARRM presentation) they had identified the number of monomers in the haze particles.

Not sure about oxide coatings. Titan's atmosphere is pretty much reducing. And I'm not sure how much hydrolysis would occur at low temperatures. There is a very small amount of water vapor in the atmosphere, but at those low temperatures it would be hard to hydrolyze a nitrile.

(It's not so easy on Earth either. You need to heat to 398 K (100 C) with KOH).

I think on Titan more fragile materials could exist that would quickly hydrolyze/oxidize/thermally modify under terrestrial conditions. It is very inert.

(This is something to remember when looking at chemical analysis of surface samples, it is very likely the sample preparation techniques will modify the surface sample to something different. A lot of techniques will be "destructive." So what you detect won't necessarily be what is there to start with....)

Titan would be a great environment for doing sensitive organometallic chemistry outdoors. (Except for the cyanide.)

Posted by: Juramike Aug 26 2009, 08:40 PM

Low temperature = long reaction time. (tough to get over those energy barrier mouintains with only a moped)
Most of the chemical reactions I'm used to dealing with don't happen at all much below -78 C (dry ice/acetone temperature) or 195 K. That is still 100 K above a balmy afternoon on Titan.

So the low temperatures would effectively insulate the molecules from further reaction. There might be all sorts of exotic things just waiting to react, if only they had a little push...or a long enough time...

So the low temperature + long reaction time (a few billion years) would imply that the low barrier reactions would eventually make it over.
[It'd be a neat exercise to calculate the time difference for a standard reaction between 95 K and 298 K. Someone must've done this somewhere...]

Ahh but what happens during impact events when the local temperature rises to "molten" levels? These would be episodic events, but likely when Titan chemistry would be more Earth-normal.

So...funky atmospheric chemistry making fragile weird molecules that don't do much most of the time, then an episodic reaction cycle during an impact that locally makes chemistry closer to "cold Earth normal stuff), then back to the deep-freeze of accumulating bizarro atmospheric products again.

That's what I'd predict on a molecular level, on an interaction level (surface interactions between molecules) it could be even wierder. Many of the "weaker" interactions on Earth could be relatively strong on Titan. Things like dispersion interactions, pi-stacking, stuff like that. Hydrogen bonds (relatively weak on Earth), could be hard as nails at Titan's low energy. (A 3 kcal H-bond could be a pretty tough barrier at 95 K, not so tough at 298 K). All those dispersion interactions (formerly known as Van der Waals inteactions) could make the molecules sticky at low temperatures. The sticky energy factors would be much higher than the molecular rebound factor (lower kinetic energy = slooooow moving molecules that don't bounce so much).

-Mike

Posted by: ngunn Aug 26 2009, 10:16 PM

I always enjoy a chemistry lesson here, even if I don't have the background to benefit fully from it. So thanks, and keep it coming. I had been wondering if stonger hydrogen bonding at low temperature might contribute to the cohesiveness of Titan's dune material. But then I thought: the hydrogen bonds are not actually any stronger, it's just that thermal agitation is weaker. What if the force opposing the cohesion is not thermal agitation but mechanical agitation by the wind? Surely the hydrogen bonds would have no special advantage against this foe?

All the same, slightly sticky dune sands on Titan seem highly plausible. It will be interesting to see whether the scientific consensus accepts cohesion as a new and distinct explanation for linear dunes. I don't have access to the full text of the paper but from the abstract it seems that some of the evidence is geomorphological/anecdotal rather than physics and chemistry based.

Posted by: belleraphon1 Aug 26 2009, 11:57 PM

I second ngunn here. Mike et al, fascinating to read your comments.

So much of our knowledge is derived from having to splash around in this one little pond we call Earth. So much of the universe is beyond our reach (for now).
What revelations await the first human hiker of Belet?

Our children will know wonders.

Craig

Posted by: Juramike Aug 27 2009, 04:00 AM

Kinetic energy is pretty much kinetic energy. Wind agitation results from gas molecules smacking into surface molecules and imparting kinetic energy into the surface molecule (which then translates to the next molecule, etc., etc.)

The key energy is the amount of molecular kinetic energies required to rip the weaker connections of one grain particle away from the next.
(Think of ripping apart velcro)

The particle-particle interface will be the aggregate sum of many, many, many individual molecular interactions at the surface. The individual-particle energy should be similar to the energy of dissolution and will be a result of the crystal packing forces. For a molecule's view of the whole process check out this Wikipedia article in http://en.wikipedia.org/wiki/Crystallization.

So whether it's from kinetic energy from wind or kinetic energy due to temperature, it's all the same: Can one grain particle pull away from the other?

(And thinking this through, normal SiO2 sand particles should get stickier at cryogenic temperatures as the intraparticle bonds would become proportionally stronger compared to the thermal kinetic background.)

Posted by: marsbug Aug 27 2009, 09:58 AM

Thanks for all the info mike, is http://www.alcor.org/Library/html/HowColdIsColdEnough.html the kind of article you were thinking of? There's a neat table about halfway down.

Posted by: Juramike Aug 27 2009, 02:27 PM

Thank you!!!

Posted by: Littlebit Aug 28 2009, 01:18 PM

Low gravity. Thick Atmosphere. Very low temperature. Low density grains. I wonder if we should be looking for similar dune-like structures in ocean floor strata.