[X] My Assistant

[Guest_BruceMoomaw_*]

http://www.planetary.org/blog/article/00000495/ :

(1) "Ralph Lorenz gave a talk about the dunes on Titan. Since much of the material from his talk is in a paper that is apparently in press in Science, it was a little better developed than I'd heard before. For example, in the past I have heard Ralph cite examples of dune-forms developed in snow in Antarctica where the dunes have almost no topographic expression but are visible to imaging techniques that are sensitive to ice grain size. Now, however, Ralph was reporting measurements of the heights of the Titan dunes -- they average 150 meters high, with a 2-kilometer spacing from crest to crest. He's now citing examples from the Namib desert. He showed some really beautiful Space Shuttle photography of those features."

Lorenz also says -- in both his LPSC abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1249.pdf ) and his similar EGU abstract ( http://www.cosis.net/abstracts/EGU06/09468/EGU06-J-09468.pdf ) -- that longitudinal dunes only occur in places where the wind speed fluctuates a lot, which says very interesting things about Titan's weather.

(2) "Guiseppe Mitri presented an interesting modeling study where he asked the question: Are the observations of atmospheric methane relative humidity and thunderstorms/cloud frequency consistent with a desert planet containing tiny fractional lake coverage? According to his calculations, he said, a 50% relative humidity of methane in Titan's atmosphere could result from lakes covering only a small fraction, 0.2 to 4 percent, of the surface. (This was assuming 'tropospheric overturning scales of 10 to 100 years' but I don't know what that means.) I also noted that his calculations implied that if such lakes exist, they evaporate at a rate of 3 to 10 meters of elevation per year."

Mitri added ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1962.pdf ) that "The rapid shoreline changes predicted here suggest that repeat Cassini observations of putative lake-like features should be performed at intervals of a year or more: any shoreline changes would provide evidence that candidate lakelike features are actually lakes." The prime target, presumably, should be Ontario Lacus. (I imagine that "tropospheric overturning scales" refer to the average time between a methane molecule evaporating from surface liquid into the air, and its raining back again to the surface as liquid. The longer that time is, the more you can combine high atmospheric humidity with a small amount of liquid actually on the surface. Titan looks more and more like an eerie cryogenic parody of the American Southwest, where its "sands" and its rare but violent rainstorms are concerned -- but its reasons for that storm pattern are somewhat different.)

[The Messenger]

http://www.planetary.org/blog/article/00000495/ :

Lorenz also says -- in both his LPSC abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1249.pdf ) and his similar EGU abstract ( http://www.cosis.net/abstracts/EGU06/09468/EGU06-J-09468.pdf ) -- that longitudinal dunes only occur in places where the wind speed fluctuates a lot, which says very interesting things about Titan's weather.

(

My geographer says that longitudinal dunes form when the wind is bi-directional, blowing almost exclusively in two almost-opposing vectors. I guess that is consistent with fluctuating winds, but definitely puts a different spin on it. Can someone clear this up?

[elakdawalla]

My geographer says that longitudinal dunes form when the wind is bi-directional, blowing almost exclusively in two almost-opposing vectors. I guess that is consistent with fluctuating winds, but definitely puts a different spin on it. Can someone clear this up?

Ralph said what you said, but I think that may be what Bruce meant by "fluctuating."

--Emily

[scalbers]

Mitri added ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1962.pdf ) that "The rapid shoreline changes predicted here suggest that repeat Cassini observations of putative lake-like features should be performed at intervals of a year or more: any shoreline changes would provide evidence that candidate lakelike features are actually lakes." The prime target, presumably, should be Ontario Lacus. (I imagine that "tropospheric overturning scales" refer to the average time between a methane molecule evaporating from surface liquid into the air, and its raining back again to the surface as liquid. The longer that time is, the more you can combine high atmospheric humidity with a small amount of liquid actually on the surface. Titan looks more and more like an eerie cryogenic parody of the American Southwest, where its "sands" and its rare but violent rainstorms are concerned -- but its reasons for that storm pattern are somewhat different.)

Interesting way indeed to characterize Titan's weather. Perhaps a complementary way to consider this is that the solar energy input is small compared to the energy needed to evaporate all that methane, hence the long overturning scale. Since the relative humidity is fairly high, then when some type of dynamical disturbance does materialize, then a lot of methane can be squeezed out resulting in infrequent and heavy rain episodes, similar in some ways to a desert on Earth.

[Guest_AlexBlackwell_*]

Ralph said what you said, but I think that may be what Bruce meant by "fluctuating."

Undoubtedly, though Lorenz's (and Messenger's geographer's) terminology is scientifically more precise. "Fluctuat[ion]" implies more variability than mere bi-directionality.

This post has been edited by AlexBlackwell: Mar 16 2006, 04:52 PM

[Guest_AlexBlackwell_*]

Undoubtedly, though Lorenz's (and Messenger's geographer's) terminology is scientifically more precise. "Fluctuat[ion]" implies more variability than mere bi-directionality.

My apologies to Bruce. I stand corrected. From Lorenz's T11-T14 TOST RADAR presentation:

"Winds must fluctuate (diurnally/tidally ? Maybe seasonally) in order to form longitudinal rather than transverse dunes." [Emphasis added]

This post has been edited by AlexBlackwell: Mar 16 2006, 09:10 PM

[Guest_BruceMoomaw_*]

Actually, in neither abstract did Lorenz say just what he meant by "fluctuating".

As for Titan's rainstorms, there has been a suspicion for a long time that the combination of high tropospheric methane humidity with infrequent rain clouds can be explained by the startling but true fact that Titan's atmosphere, smog or no smog, is on the whole much CLEANER of solid particles than Earth's is. The only reason Titan's atmosphere is opaque in the visible to outside observers is that, while its smog content is very low, it towers up to such an amazing height above the surface. Earth's air has far more dust in it -- not only picked up from dry land, but salt particles spattered off ocean-wave foam and then drying out in the air.

Thus, with fewer solid nuclei around which a saturated vapor can condense (water in Earth's case, methane in Titan's), Titan's methane tends to remain gaseous until it rises to a REALLY high humidity level in some local place -- at which point droplets finally start to condense out anyway, and themselves serve as the nuclei for very rapidly mushrooming big drops of methane that then thunder out of the sky in a brief but violent cloudburst.

[elakdawalla]

Thus, with fewer solid nuclei around which a saturated vapor can condense (water in Earth's case, methane in Titan's), Titan's methane tends to remain gaseous until it rises to a REALLY high humidity level in some local place -- at which point droplets finally start to condense out anyway, and themselves serve as the nuclei for very rapidly mushrooming big drops of methane that then thunder out of the sky in a brief but violent cloudburst.

...except that with larger size, and lower gravity, and higher atmospheric pressure, they must fall more slowly than Earth raindrops. I wonder if we'd be able to tell that if we were standing on Titan...

--Emily

[Rob Pinnegar]

...except that with larger size, and lower gravity, and higher atmospheric pressure, they must fall more slowly than Earth raindrops.

The lower gravity would probably be negated by the greater distance plummeted. But terminal velocity wipes out that distinction, anyways.

So, what determines terminal velocity: atmospheric pressure or atmospheric density? If it's the former, Titan's terminal velocity for falling objects shouldn't be *that* much less than Earth's. If it's the latter, things could change. Anyone know?

[Edit: I guess the density and surface tension of liquid methane have to come into play here. But let's not make it *too* complicated.]

[Guest_AlexBlackwell_*]

Actually, in neither abstract did Lorenz say just what he meant by "fluctuating".

I didn't read the two abstracts you referenced. I thought maybe you had seen his T11-T14 TOST RADAR presentation which, as I quoted, did use that specific term. I guess great minds think alike, huh?

In any event, his TOST presentation had the Shuttle-based imagery of the Namibia dunes. They are strikingly similar to the ones on Titan.

This post has been edited by AlexBlackwell: Mar 17 2006, 01:02 AM

[helvick]

The lower gravity would probably be negated by the greater distance plummeted. But terminal velocity wipes out that distinction, anyways.

So, what determines terminal velocity: atmospheric pressure or atmospheric density? If it's the former, Titan's terminal velocity for falling objects shouldn't be *that* much less than Earth's. If it's the latter, things could change. Anyone know?

[Edit: I guess the density and surface tension of liquid methane have to come into play here. But let's not make it *too* complicated.]

Vt = sqrt (2*weight/Coeff of drag*density*x-sectional area)

Note weight is proportional to local gravitation and density changes with altitude in an atmosphere so these are all somehat variable.
For titan Vtat the surface is about a third of what it is on earth for stuff that is reasonably solid (density 1kgm^3 +-0.3) and doesn't create significant aerofoil flows (Cd=~9+-0.2).

Density is the key variable from your question - drag is a result of momentum transfer betwwen the atmospheric particles and the moving object.

[Edit] The surface tension of methane will have some effect as it will inform the Cd of the methane drops. I'm in no position to estimate what that will be my gut instinct would be astounded if it reduced Cd and thus led to higher velocity precipitation.

Also worth noting that a Vt of 33% of earths leads to 10% the impact force for equivalent "rain drops" _and_ methane is only about 55% the density of water so titan rain or snow would be noticably less "forceful" than an earth water equivalent.

[ngunn]

Terminal velocity increases with r squared so if the drops are big this will quickly outweigh the effects of lower g, lower density fluid and higher atmospheric viscosity. I've done a rough calculation based on drop radii 10 times those typical for terrestrial rain and the velocities come out in tens of metres per second. So if they're that big Bruce is right - they would be thundering down and making substantial impacts.

[Guest_BruceMoomaw_*]

In any event, his TOST presentation had the Shuttle-based imagery of the Namibia dunes. They are strikingly similar to the ones on Titan.

They sure are. Take a look at the pictures in the LPSC abstract -- you literally cannot tell them apart.

[The Messenger]

They sure are. Take a look at the pictures in the LPSC abstract -- you literally cannot tell them apart.

If it looks like a duck...

[Guest_AlexBlackwell_*]

If it looks like a duck...

Nick Hoffman, who has pushed (variations of) his White Mars model for several years now, has an amusing quip: "For those who persist in saying that if it walks like a duck and has a beak like a duck, then it must be a duck, I suggest you read up about the platypus. These marsupials even lay eggs but they don't quack!"

[Guest_RGClark_*]

...
Thus, with fewer solid nuclei around which a saturated vapor can condense (water in Earth's case, methane in Titan's), Titan's methane tends to remain gaseous until it rises to a REALLY high humidity level in some local place -- at which point droplets finally start to condense out anyway, and themselves serve as the nuclei for very rapidly mushrooming big drops of methane that then thunder out of the sky in a brief but violent cloudburst.

It would be much more VIOLENT if there happened to be lightning!
Anyone calculations on the effects of this methane igniting?


- Bob

[helvick]

Anyone calculations on the effects of this methane igniting?

Igniting the methane and other hydrocarbons would require an oxidant - and there isn't any AFAIK.

[The Messenger]

Nick Hoffman, who has pushed (variations of) his White Mars model for several years now, has an amusing quip: "For those who persist in saying that if it walks like a duck and has a beak like a duck, then it must be a duck, I suggest you read up about the platypus. These marsupials even lay eggs but they don't quack!"

Inverted fonts give me headaches. So would living in Australia - (Holding on all the time, so I wouldn't fall off.)

From Ms Emily,

Chuck Wood gave a talk about craters on Titan, but he had little new to say because after nearly two years in Saturn orbit Cassini has seen only two unambiguous craters on Titan. He pointed out some very small features in the radar data that he said looked much like craters, but they were so small (5 to 10 kilometers diameter) as to be under the lower size limit that is imposed by the present density of the atmosphere (which would shield Titan from smaller impacts). He said that this could be evidence that either we aren't modeling the impactors coming in to the atmosphere correctly or maybe the current pressure of the atmosphere is transient...

There is another explanation I can see, because I am always looking at everything sideways:

We didn't see a bright streak from Kleck as Huygens entered the atmosphere, and likewise, the rather limited temperature sensors on Huygens did not record the dramatic increase in temperature expected during the entry. Likewise, the cooling of the probe was much slower than predicted. Call that what you will, I think it is a difficult thermodynamic puzzle, because we think we understand these things. Now factor in the rapid intial descent (as documented in the Doppler signature), and there is a possible trend.

Rewind just a little bit further: The fluid dynamics in the atmosphere of Jupiter is also a puzzle, from what I have read, because we should be seeing enough turbulence to break up the streaks into a more chaotic mix. Likewise, the MER's did not experience the expected plasma blackout during entry.

What I have postulated - speculated - is that amoung other things, we are seeing an increase in effective Reynolds numbers with increasing distance from the Sun. Granted, there is no currently accepted theoretical bases for this, but it is one solution to the puzzle: Smaller than expected craters are found because the lower-than-expected turbulence in Titan's atmosphere doesn't break them up.

[Guest_BruceMoomaw_*]

Igniting the methane and other hydrocarbons would require an oxidant - and there isn't any AFAIK.

Yep -- if there was any way that Titan's atmosphere could be ignited, the whole world would have gone up in flames the moment the first meteoroid plunged into it (which is to say that Titan would never have had any methane in its atmosphere from the start).

One interesting corollary of this involves the fact that Titan is the only world in the Solar System besides our own where people could walk around without full-blown spacesuits. All you need is an oxygen mask and a (very) good insulated and heated coverall -- and the heating for the latter could, at least theoretically, be provided by water-weave underwear (like that used to cool the wearers of current space suits) hooked up to a burner fueled by the same oxygen supply. In this case, though, the flame would be an "inside-out" gas jet: the oxygen would be ignited as it streamed out into an atmosphere of methane, rather than vice versa.

[Rob Pinnegar]

One interesting corollary of this involves the fact that Titan is the only world in the Solar System besides our own where people could walk around without full-blown spacesuits.

Man, you'd need a good sealant around the edges of the mask. I'm no chemist but am guessing that "cyanogen" is close enough to "cyanide" to be worrisome here.

[scalbers]

Terminal velocity increases with r squared so if the drops are big this will quickly outweigh the effects of lower g, lower density fluid and higher atmospheric viscosity. I've done a rough calculation based on drop radii 10 times those typical for terrestrial rain and the velocities come out in tens of metres per second. So if they're that big Bruce is right - they would be thundering down and making substantial impacts.

My understanding from some of the microphysical modeling that's been done is that the Titan methane rain drops could be as large as 9mm, compared with 5-6mm on Earth. So the drops would be large yet still falling relatively slowly, kind of a otherworldly fantasyland to watch them fall (reminds me of blowing bubbles or something).

[David]

One interesting corollary of this involves the fact that Titan is the only world in the Solar System besides our own where people could walk around without full-blown spacesuits. All you need is an oxygen mask and a (very) good insulated and heated coverall -- and the heating for the latter could, at least theoretically, be provided by water-weave underwear (like that used to cool the wearers of current space suits) hooked up to a burner fueled by the same oxygen supply.

But what are you going to do about the wind chill?

[Bob Shaw]

But what are you going to do about the wind chill?

Cosmic String vests?

Bob Shaw

[Guest_BruceMoomaw_*]

Man, you'd need a good sealant around the edges of the mask. I'm no chemist but am guessing that "cyanogen" is close enough to "cyanide" to be worrisome here.

Oh, Huygens established that Titan has all sorts of lovely stuff on its surface. Cyanogen, benzene, maybe hydrogen cyanide -- yummy! (Cyanogen is indeed highly toxic, although in a different way from HCN.) But virtually all of this stuff is frozen solid on the surface (cyanogen has a melting point only modestly lower than that of water ice) and so none of this stuff exists in anything more than infinitesimal traces in the air. The real reason for wanting to be damned sure your suit is sealed is simply that otherwise you'll end up flash-frozen into a Popsicle stick. (Back in 1966 James Blish wrote a story called "How Beautiful With Banners" about a visitor to Titan shielded from the environment by a "living" space suit made from an artificially engineered virus -- which gets her into rather serious trouble when one of the local life forms decides to mate with it...)

[nprev]

Oh, Huygens established that Titan has all sorts of lovely stuff on its surface. Cyanogen, benzene, maybe hydrogen cyanide -- yummy! (Cyanogen is indeed highly toxic, although in a different way from HCN.) But virtually all of this stuff is frozen solid on the surface (cyanogen has a melting point only modestly lower than that of water ice) and so none of this stuff exists in anything more than infinitesimal traces in the air. The real reason for wanting to be damned sure your suit is sealed is simply that otherwise you'll end up flash-frozen into a Popsicle stick. (Back in 1966 James Blish wrote a story called "How Beautiful With Banners" about a visitor to Titan shielded from the environment by a "living" space suit made from an artificially engineered virus -- which gets her into rather serious trouble when one of the local life forms decides to mate with it...)

I remember that story...Blish (and everyone else) had the atmospheric density far too low, though...I remember that she could see Saturn from the surface.

This may be a naive question, but one thing that has puzzled me is that the few Huygens results I've seen don't seem to mention any truly complex hydrocarbons...and logic suggests that at least some must be present. Was the GCMS incapable of detecting these, or is it assumed (or just a fact of life) that high-weight molecular species decompose during the analysis to the point of being undetectable?

[Guest_BruceMoomaw_*]

The "Science" report on the latest interpretation of the GCMS findings said that there were some indications of more complex hydrocarbons, but those results are still being analyzed. There certainly seems to be something of the sort when you look at the actual GCMS spectra -- but keep in mind that the heavier such hydrocarbons are, the harder it would have been for the warmth of the heated vent on the instrument (which wasn't touching the surface) and the warmth of Huygens itself to vaporize them out of the soil to where the GCMS could reach them.

The fact that the instrument DID definitely seem to detect significant quantities of vaporized cyanogen and benzene fascnates me. Neither of those compounds has any kind of low vaporization temperature. According to Wikipedia, cyanogen melts at -28 deg C and boils at -20 deg C, while benzene doesn't melt until +5.5 deg C and doesn't boil until fully +80 deg C! If the GCMS really was picking up whiffs of these, there must be one hell of a high concentration of them on Titan's surface -- far more than was expected. (Some CO2 was picked up, too -- suggesting a pretty high concentration of that too, since its sublimation point, although a lot lower than that of the above compounds, is still far higher than Titan's surface temperature.) By contrast, theories predicted a lot of frozen acetylene and HCN (which both vaporize at low temperatures) accumulated on Titan's surface -- but there seems to be no firm confirmation yet of HCN, and there's no evidence for lots of acetylene (although the team's abstract for the upcoming EGU meeting mentions in passing that it was detected. Except for that, there's nothing new in that abstract.)

So there may be some quite surprising and complex organic reactions taking place on the surface, AFTER the original smog particles (consisting mostly of ethane, acetylene and HCN, according to theory) land there. Given the extremely slow rate at which all these compounds accumulate on Titan's surface (only 100 meters or so each of acetylene and HCN over all the eons of Titan's existence), is it possible that Titan's cryovolcanism has recycled virtually all of them downward through the crust and into the zone of liquid water or water/ammonia, causing quite complex reactions to occur before the products are spat back up to the surface? (Benzene is supposed to be one of the commonest polymers of acetylene.) Yet again, I regret like hell the fact that Huygens didn't carry a heated core tube hooked up to its GCMS, rather than most of those [naughty word] British surface sensors.

[nprev]

Ah! As usual, you have enlightened me, Bruce...thank you very much!

Man...you just have to wonder if there are DNA-analogs sitting underneath Huygens right now, busily duplicating themselves. I am sadly convinced that at least the surface of Titan is too hostile for actual life, but the sheer variety of possible complex organics on the surface is justification enough for a dozen missions.

We are far from truly understanding just how many ways you can braid carbon, hydrogen, oxygen, and nitrogen together...I am certain that Titan holds the key to a revolution in organic chemistry, if nothing else.

[Guest_BruceMoomaw_*]

One footnote: the same theories of atmospheric reactions that predicted relatively large amounts of ethane, acetylene and HCN in Titan's smog particles predicted virtually no benzene or CO2, and only a tiny trace of cyanogen. So, once again, there's something else going on down there.

[edstrick]

"Double, double, toil and trouble...
"Bunsen burn and beaker bubble!

[centsworth_II]

Yet again, I regret like hell the fact that Huygens didn't carry a heated core tube hooked up to its GCMS, rather than most of those [naughty word] British surface sensors.

You almost make it sound like it would be worthwhile to send a "quick, cheap" stationary lander to Titan while waiting for the full-blown balloon mission to take form.

[The Messenger]

The fact that the instrument DID definitely seem to detect significant quantities of vaporized cyanogen and benzene fascnates me.

This is strange...during the descent, on the surface, or both?

[Guest_BruceMoomaw_*]

It seems to have been after landing -- although that "Science" article was really pretty vague on the subject, and I can't interpret the mass spectral graphs published in it worth a damn. Certainly there was a lot MORE of them all on the surface, and it is flatly stated that they were evaporated out of the surface by Huygens' warmth.

[Guest_BruceMoomaw_*]

Minor footnote: it turns out that cyanogen really DOES poison you in the same way as most cyanides -- in fact, it is actually "cyanide cyanide" (CNCN).