[X] My Assistant

[ngunn]

We discussed this briefly before in the "Equatorial Sand Seas" thread, posts 252 to 257, so I will understand if there are few further comments, but drawing from some recent abstracts there is now more to say so I thought I'd start a separate thread. First the neutral assessment of the issue that concluded those earlier deliberations:

QUOTE (rlorenz @ Feb 9 2008, 02:59 PM)
I dont see a strong case yet for any large-scale re-orientation (don't see a case against it either)

To summarise the issue as I see it:

The surface of Titan (like some other moons) is thought to be mechanically decoupled from the interior by a subsurface ocean. Unlike the other 'ocean' moons, Titan has highly active surface processes including fluid flow and solid mass redistribution. Deposition of material from the atmosphere is thought to favour the polar regions. We observe regional slopes, large scale lake basins, mountain chains (with preferential E-W orientation) and possible faults that could in part be maintained by migration of the entire crust with respect to the rotation axis. A testable prediction can be made: Titan's surface formations will be found to include some that originated far from their present latitude.

Now, what's new?:

http://www.cosis.net/abstracts/EGU2008/101...008-A-10189.pdf
Lorenz, R.; Stiles, B.; Kirk, R.; Zebker, H.; Callahan, P.; Radarteam, T.C.
Geophysical Results at Titan from Cassini RADAR : Topography and Spin State Overview

Note the last paragraph. Even the wind could be 'torqeing the surface around'.

Also relevant though not specific to Titan:

http://www.cosis.net/abstracts/EGU2008/013...008-A-01318.pdf
Harada, Y.
True polar wander due to surface mass loading: Interaction between rotation and deformation through pole tide

I'll leave it there for now to see if others have comments.

[Juramike]

[Wikipedia to the rescue again...]

Earth's "pole tide" is primarily due to the Chandler wobble. (Found by Seth Chandler, it has a 20 m amplitude with a period of 433 days. It is the only tide that does not involve the influence of external bodies).

It's source was recently found to be due to changes in pressure at the bottom of Earth's oceans.

http://www.jpl.nasa.gov/releases/2000/chandlerwobble.html

-Mike

[ngunn]

Just to avoid confusion that 20 metres amplitude is for the wobble, not the tide. The amplitude of Earth's pole tide is 6mm, according to answers.com.

[Juramike]

Yup. The amplitude of the wobble is 15 arc seconds, which is 15 m at Earth's surface. But the tide itself is teeny-tiny.

Wikipedia/Chandler Wobble

Mike

[ngunn]

Returning to Titan, in place of that tiny wobble I'm imagining large secular shifts, possibly episodic or possibly continuous, with maybe also cyclical components superimposed. The 'tidal' stresses induced by adjustment to the new orientation of the ellipsoid would surely be large enough to cause tectonism. I'm guessing also that the freshest ridges produced in this way would exhibit some sort of systematic relation to the present rotation axis, while older ones would have more or less random orientations.

[ngunn]

In this abstract the authors refer to four possible processes for mountain building on Titan:

http://www.cosis.net/abstracts/EGU2008/101...008-A-10197.pdf
Mitri, G. ; Bland, M. ; Lopes, R. M.; Showman, A. P.
Mountain Building on Titan

Their preferred explanation is global shrinkage. I don't question that this could have played a major part, but I would note the following:

None of the four possible processes mentioned account directly for systematic alignment of tectonic ridges approximately parallel to the equator. They seem to be relying on post-formation erosional modification to produce this asymmetry in observed azimuths. Now when I look at (for example) the wavy but roughly E-W ridges in Adiri on the T8 SAR swath the very last thought that would occur to me is that these are the remnants of a swarm of once randomly-oriented ridges with all the N-S trending ones eroded away.

A further consideration is that global shrinkage is more or less a one-off process, whereas crustal migration is a 'renewable' resource driven ultimately by Titan's weather.

[Juramike]

More on the polar wander of Titan. Implications that the surface is decoupled from the core and slipping around on a subsurface ocean.
(Still no numbers on the size of this effect, however, but it is visible in RADAR swaths not lining up.)

Space.com article: http://www.space.com/scienceastronomy/0803...itan-ocean.html

From the article, Titan's crustal thickness: 50 - 150 km thick
Subsurface ocean thickness: 100 - 200 km thick


[Editorial aside: This puts Titan's subsurface ocean only twice as deep (and twice as far) as Europa's subsurface ocean.]

-Mike

[ngunn]

Great article, thanks for posting it Mike.

My latest thoughts . . . Longitudinal asymmetry in the north polar lake district: could a slow secular migration of the crust be linked to this? If the crust was moving over the rotation axis there would be an 'upstream' and a 'downstream' side. Maybe they would also be uphill (with sapping and sinkholes?) and a downhill (with large enclosed basins?) sides. I was kinda hoping someone with a better handle on the data could look at the southern hemisphere imagery to see if there is evidence of a similar regional gradient there 180 degrees out of phase with the north.

(I'll probably get over this whole thing sometime, but at the moment it just won't go away. )

[rlorenz]

Implications that the surface is decoupled from the core and slipping around on a subsurface ocean.
From the article, Titan's crustal thickness: 50 - 150 km thick
Subsurface ocean thickness: 100 - 200 km thick
-Mike

For the ocean thickness you'd need to look at the latest thermal models by Tobie etc.

As for the ice crust thickness, we measure the spin change, which is the ratio
delta_angular_momentum / moment_of_inertia

We dont really know what the deltaAM is - Tokano's model says X. If we adopt X, then you get
maybe 150km.

But - Tokano's model isnt right - e.g. it gets the dune orientations exactly in the wrong direction (as
does every other Titan GCM - maybe something fundamental about Titan like distribution of
mountains with latitude that we dont understand) so the deltaAM may be different. But even if you
assume the wind field measured by Huygens screeched to a sudden and total halt, the deltaAM is
only twice as large. It's really hard to avoid inferring a subsurface ocean (which we expected on the
basis of thermal models anyway..)

The calculation ignores any drag by the ocean on the bottom of the crust. Dunno what that might be,
but it would have the effect of making the motion less (and thus making us think the crust is thicker
than it is). Similarly, and maybe more important, the core may be nonspherical and may be
coupled gravitationally to the crust, which also effectively 'loads' the crust making us think it is
thicker than it is.

So this is just the beginning of a long story about Titan's rotation, which presumably will change
seasonally, and may be superposed on effects like polar precession (period of a few hundred years)
or maybe the libration that Noyelles has talked about.

Interesting times.

NB - I think the Tobie/Sotin News and Views may have an error in its figure - talks about 3 deg
obliquity. It should be 0.3 degrees

[ngunn]

Presumably it's even possible (at least temporarily) for the rotation axis of he crust to be different from that of the interior. Lots more to discover here, for sure. Titan, with active processes, could provide chronological control better than that availailable on other ocean moons. More reasons to go back.

[rlorenz]

More reasons to go back.

Quite.

As ever, 'our own' Emily Lakdawalla's coverage of the story is superlative. See

http://www.planetary.org/news/2008/0320_Wh..._Mountains.html

[nprev]

Indeed, that was an excellent article (and a terrific illustration, Doug, Emily, and Bob!)

Ralph, re the crust decoupling: Does this in any way place contraints on surface topography, or is this purely a function of crustal tensile strength? It seems to me that if a body like Titan ever had continents or an analog to Mars' Tharsis Bulge--or in fact any localized large crustal mass concentration--that the crustal shell might well rotate chaotically over time, with a very long damping period after equilibrium was restored by whatever means (presumably weathering or collapse of the unbalancing feature). It's probably far too soon to ask, but are there any signs of "ringing" from the distant past, or even from major impacts?

I just find it odd that the crustal rotation is apparently so stable, with such a small axial offset. The dampening of internal and external events might be much more rapid then I think.

[vjkane]

Does anyone know how far a radar in orbit around Titan could probe into the crust to detect the crust/ocean boundary?

I seem to remember that the orbiter would be too far away from Titan (because of the extended atmosphere) to probe deeply.

[elakdawalla]

Indeed, that was an excellent article (and a terrific illustration, Doug, Emily, and Bob!)

Thanks! The illustration is from Bob's excellent Planetary Report article on oceans in the outer solar system, from the July/August 2007 issue; Doug did all the real work on the 3D models, I just laid it out. Bob gave us the info on the latest geophysical models from various sources for the depths to the various boundaries.

--Emily

[ugordan]

Emily, great article (as usual), covering an amazing find from Cassini. I think you have a typo on a couple of places, IIRC the first SAR swath was acquired on October 2004, not 2005.

On a side note, I see that all of the moons in that cutaway collage have liquid water layers at some depth. I remember a time when all the icy sats were regarded as dead, frozen bodies. When did this "paradigm shift" happen that all of these bodies are postulated to have liquid layers beneath the surface?

Europa used to be an exception, now it appers to be just one of many?

[ngunn]

Ralph, Emily mentions that you point out that a long-lived lander would be good for tracking these motions. I'm wondering if having a few passive radar reflectors dotted around the surface would be a good idea too? These could in principle function virtually indefinitely as position markers (unless moved around by floods!) available to all future missions.

[rlorenz]

Ralph, Emily mentions that you point out that a long-lived lander would be good for tracking these motions. I'm wondering if having a few passive radar reflectors dotted around the surface would be a good idea too? These could in principle function virtually indefinitely as position markers (unless moved around by floods!) available to all future missions.

In the limiting case, that is what we have now. Titan has thoughtfully made its surface non-featureless
so there are passive reflectors (like mountains) we can track.

A transponder on even a single lander lets you make a range and doppler measurement with equisite precision
(the 2007 Titan Flagship report cites some Mars work on the topic)
And you can use a seismometer to probe the thickness of the crust too

With a Titan orbiter you can of course repeat (with much higher cadence and precision) the exercise we have
done here - notably you could isolate any librations (with the orbital period) from the seasonal rotation changes.

[rlorenz]

Does anyone know how far a radar in orbit around Titan could probe into the crust to detect the crust/ocean boundary?

I seem to remember that the orbiter would be too far away from Titan (because of the extended atmosphere) to probe deeply.

Again, see the 2007 Flagship study report
http://www.lpi.usra.edu/opag/announcements.html

The orbital altitude of 1500km doesnt help (IIRC MARSIS has done sounding from 800km altitude) so
the range costs you 6-8dB. Not sure if radar sounding through more than 4km of ice has been demonstrated
at either Mars or the Earth yet.

Sounding through 100km of ice from that range would likely be an impossible challenge but I havent
ever seen an actual calculation to that effect. A balloon-borne instrument could do better than an orbital
one in principle, but on the other hand for that platform you'd likely want a shorter wavelength
operation (giving better vertical precision) to keep the antenna manageably small. More of a
sedimentology investigation (how thick are alluvial fans, sand seas, lakes etc.) than deep crustal sounding.

Induced magnetic field, seismic measurements, tidal distortion of the crust (and rotation) are the real handles
on the large-scale internal structure.

You can also look at geomorphology/topography/gravity - as nprev was asking. Are large crustal structures
compensated? Cassini just wont develop the global topo dataset to do a good job on that kind of question, although
I guess we might be able to poke at a few structures

[edstrick]

Based on simple modles of "geo"thermal heat-flow and temp/pressure phase diagrams of water, internal oceans were predicted in the icy Galillean in the 70's, maybe pre-Voyager.

Further modeling suggested that solid state convection of the warm mantle ice above internal oceans would suck enough heat out of the oceans fast enough to freeze them over the history of the solar system.

Continued modelling with the added model complexities of things like ammonia and salts and non-linear coupling between tidal heating rates, internal temps, convection rates, etc etc etc..... reveals how much we don't know that we need to know for the models to be anything close to definitive.

Then Galileo found compelling evidence for internal oceans, based on magnetic field interactions with highly conducting internal zones within the moons... presumably oceans.

Titan/Triton/Pluto/Enceladus/... models all feed off the work done on the Galillean sats.

[ugordan]

Continued modelling with the added model complexities of things like ammonia and salts and non-linear coupling between tidal heating rates, internal temps, convection rates, etc etc etc..... reveals how much we don't know that we need to know for the models to be anything close to definitive.

Thanks, edstrick. I must have missed it when this point dawned upon the science community. The scientists often have a tendency to be absolutely certain of things when they don't in fact realize the entire complexity of the issue.

[remcook]

Emily/Doug/Bob: I saw that figure used yesterday morning already in meeting for (future) Dutch planetary scientists, not even about this story cool!

[nprev]

Thanks, edstrick. I must have missed it when this point dawned upon the science community. The scientists often have a tendency to be absolutely certain of things when they don't in fact realize the entire complexity of the issue.

I don't know if it's certainty as much as a need to derive a hypothesis that best explains the observations, which, as you say, usually don't encompass the full scope of all effects. This is merely the seed, however; getting to the real truth takes time.

One of the kind of agonizing things I've learned so far in the engineering world is that you have to be ready to catch a lot of arrows in your chest during peer reviews when your ideas and proposals are critically examined; I'm pretty sure that pure science works the same way.

It's necessary, though; the perspectives and unique observations of others inevitably refine the work and if all goes well make it practical...if not, then fatal flaws are revealed. Either outcome serves the best interests of all involved, and if you walk away from the podium wiping blood off of your brand-new sport jacket, it's okay...at the end of the day, getting to the core truth is all that really matters.

[ngunn]

So this is just the beginning of a long story about Titan's rotation

I am genuinely astonished at how many whole new fields of scientific enquiry you have been in at the start of: 'cryo-aeronautics', alkane hydrology, aerosol radiocarbon - I've lost count - and now we have 'meteo-geophysics'. Who knows where that will lead?

- - -

On the more general discussion about open-mindedness in science versus the need to frame and test a specific hypothesis, there surely has to be a full spectrum of approaches depending on the object of study. Advances in astronomy and planetary science in particular have definitely shifted the balance in the direction of a less hubristic, more imaginative, rule-nothing-out approach. I would say that in this regard today's science community is a huge improvement on the one I remember as a student.

[Juramike]

RADAR strippin'
story so grippin'

water below ice
slides so nice

a lubed up layer
one strange player

slipping, slidin'
oceans hidin'

winds are blowin'
landmass goin'

surface shifting
polar drifting

back and forth
no true North

a possible reason
the change of season

breakin' the connection
crustal convection

hydrocarbon riches
smack my ridges

a whole new map
changes my rap

To J and the Crew
I got nothin against you

And no 'dis to the E
but put the lander on me

Zig-a zig-a ziga wiff-waff....

(fade)

[vjkane]

Ralph -

Congrats on a great paper. I think we all assumed the ocean was there, but this nicely clinches it.

I have come to decide that the next Flagship mission should go to Titan. Besides Titan's inherently interesting features, two factors finally convinced me: (1) a great many more questions can be answered if a long-lived lander is possible. Titan is cold, but easy to land on. (2) Jupiter missions can be done with solar power (albeit with some engineering challenges). Given the dwindling plutonium supply, we should send the next Flagship to a destination that requires nuclear power.

(The recent study of solar power for the outer solar system reported that a Europa orbiter would lose 30% of its power to radiation damage, although it also noted the need for a more thorough analysis.)

[scalbers]

Here is some coverage of the sub-surface ocean on the Cassini web page:

http://saturn.jpl.nasa.gov/news/press-rele....cfm?newsID=826

[nprev]

Just gotta give mad props to Juramike here for those phat rhymes, yo!

Y'all need to swing by my crib here in central LA sometime--I got the 40s--and discuss the geophysical and geochemical implications of Titanian crustal decoupling Cali-style. Aight?

[Julius]

So it seems that TiTAN exhibits some form of plate tectonics.Do we know whether titan crust moves as a single unit or like in the case of Earth,split up into more than one unit interacting with each other compressing and subducting as they move along.Is it possible to detect plate boundaries if any by radar operating on cassini?

[ngunn]

Almost certainly all one 'plate'. Even so, we'll need two independent coordinate systems for it, one astronomical and one that moves with the surface. I imagine there will be predictive 'tables' for the sperical coordinate transformation, rather like marine tide tables here on Earth.

[ngunn]

The attached article (via Jupiter List - thanks!) is about Europa, but it's relevance to this thread is obvious:

Attached File(s)

 _jupiter_list__Wandering_Poles_Left_Scars_on_Europa.doc ( 3.63K ) Number of downloads: 205

 

[DrShank]

The attached article (via Jupiter List - thanks!) is about Europa, but it's relevance to this thread is obvious:

The sitation listed is poorly written version that leaked out before it was edited. the better draft can be found at

http://www.lpi.usra.edu/features/europa/cropCircles/

cheers
paul

[rlorenz]

Interesting stuff! The headline 'Global mapping of unusual large circular features on the ice-covered ocean world of Europa has revealed that Jupiter’s curious icy moon is even more unstable than previously thought.' seems a
bit strong - makes it sound like the whole place is about to blow..... if only that were true

The sitation listed is poorly written version that leaked out before it was edited. the better draft can be found at

http://www.lpi.usra.edu/features/europa/cropCircles/

cheers
paul

[DrShank]

Interesting stuff! The headline 'Global mapping of unusual large circular features on the ice-covered ocean world of Europa has revealed that Jupiter’s curious icy moon is even more unstable than previously thought.' seems a
bit strong - makes it sound like the whole place is about to blow..... if only that were true

"look out, its about to erupt!"

(anybody out there old enuf to recall the B-52s, who are still touring . . . ?)

of course, stability is not always a physical property, as any therapist will tell you.
so consider the author

[ngunn]

Thanks for posting the better links. No doubt Europophiles will want to discuss this in a Europa thread, but to steer back to Titan I'll ask the obvious question. If Titan had undergone similar reorientation presumably any surface manifestations of the process would be very different from those visible on a naked ice world. What might they be? I made some guesses before but I'd like to know what others think.

[ugordan]

"The troughs and depressions are up to 500 meters (almost 1 mile) deep"

Luckily, this time there's no inbound orbiter that would rely on conversions such as this one.

[Juramike]

Here is another paper of interest in the May 2008 Icarus (Matsuyama is also co-author in this):

Matsuyama and Nimmo Icarus 195 (2008) 459-473. "Tectonic patterns on reoriented and despun planetary bodies". Pay for article, abstract here.

[I downloaded it yesterday, but I'll confess I haven't read it yet, there are a lot of Greek letters. Shapes (linked hexagons, pentagons, squares) and reaction arrows I'm cool with (I'm currently into squares), Greek letters with mathematical operators scare me.]


Looking at the figures showing predicted tectonic stress fields, I can see a direct relevance to Titan. This might be an explanation to the roughly parallel (and orthogonal) tectonic/graben pattern observed in the equatorial belt.

Titan appears to have (in the equatorial zones where things are "clearer"):

• Global roughly EW parallel tectonic ridge pattern (T8 Adiri, T25 and T28 Quivira and landmass N of Fensal, T13 Xanadu, T23 E Quivira)
• Major Graben structures lying also roughly EW parallel (T8 Adiri, T25 and T28 Quivira, T25 Tseghi and ISS Tseghi)
• minor graben structures or gentle terrain undulations running NS (T8 Adiri, T25 and T28 Quivira, T13 Xanadu, T23 E Quivira)
• Similar checkerboard patterns (major/sharp EW with minor/gentle NS undulations), with can be seen in T8 W Belet and in the ISS Senkyo/Belet basin

A detailed mapping of the Titans tectonic features (outside the Equatorial zones) and detailed modeling of tidal deformation and polar reorientation scenarios (like in Fig. 4 of the article) might just find a possible fit.

-Mike

[Juramike]

Freely available article on Polar reorientation due to Diapirism

Nimmo and Pappalardo, Nature 441 (2006) 614-616. "Diapir-induced reorientation of Saturn's moon Enceladus."

A low density diapir could cause polar reorientation. (Could a big diapir (say, Xanadu) reorient Titan?)

From the abstract "We predict that the distribution of impact crates on the surface will not show the usual leading hemisphere - trailing hemisphere asymmetry."

Even though the overall numbers of craters on Titan are low, the leading/trailing assymetry is not observed on Titan.


Freely available abstract on Polar reorientation due to Large Impact Basin:

Nimmo and Matsuyama, LPS 38 (2007) Abstract 1237. "Reorientation of icy Satellites due to Impact basins."

A big impact crater basin can also cause polar reorientation, with significant shifts (tens of degrees). Slow rotating satellites (Titan counts) are more prone to reorientation.

-Mike

[Juramike]

[Arms flailing wildly, a crazy, trance-like look in his eyes, Mike lunges to the whiteboard and starts scribbling in freneticly...]

1) Titan forms
2) Titan differentiates (the first time)
3) Titan gets smacked around by many impactors
4) Core overturn (differentiation #2)
5) Major resurfacing
6) Diapirism and/or last really big impact
7) Polar reorientation
8) Last major Tectonic patterns (and graben) set due to reorientation/despinning
9) Mid-size trailing impacts taper off

(then after a while or during 7-8-9)

10) atmospheric collapse.
11) Big nitrogen oceans time + nitrogen rains
12) Large waves during impacts/tectonic shifts/impacts carve channels in graben and oriented low lying areas
13) Titan "warms" up: nitrogen evaporates
14) Methane seas in basins; methane rain in equatorial zones
15) Last few impacts
16) Titan "warms" up; methane evaporates (mostly)

(then since a long time ago...)

17) Schizzle accumulates in temperate/polar regions
18) Sand seas get blown around in equatorial zone
19) Methane lakes/clouds/rains only in the polar regions.
20) Huygens probe lands.

[His frenzy over, Mike collapses at the board and is carried out by his colleagues to go get pizza.]

-Mike

[ngunn]

Nice, Mike, but you may need to add another reoriention (or a few) after number 17. I'm guessing Titan's weather redistributes mass to the poles faster than ice is redistributed on Europa.

[climber]

Luckily, this time there's no inbound orbiter that would rely on conversions such as this one.

Your remark drive me nervous since a Polar lander was about to land at this time
Anyway, 500m = almost 1 mile is as bad as my mental conversion between °C & °F

[Juramike]

Nice, Mike, but you may need to add another reoriention (or a few) after number 17. I'm guessing Titan's weather redistributes mass to the poles faster than ice is redistributed on Europa.

I had to do some mental gymnastics, but I think polar deposition will help just a teeny bit to keep the poles at their present position.

The estimated global organic deposition rate is 9.5E16 g Myr-1. Assuming it falls at polyacetylene density, this corresponds to 0.015 m Myr-1 averaged globally. (see post 270, Equatorial Sand Seas thread).

Assuming all the atmospheric organic deposition on Titan occurs at the poles (at least above 30 latitude), you end up with a rate above 0.015 m Myr-1. Assuming the North and South polar temperate regions together make up about half the surface area, but getting the whole amount of global deposition, you end up with about 0.03 m Myr-1. So this would only work out to about 60 m of stuff over a 2 billion year history.

And that's assuming it falls as flakes of polyacetylene (density 0.63 g/cm3). If it comes down as little poofballs of organic stuff that doesn't compact well, the density could be quite low indeed. For example: Ice has density 0.95 g/cm3; snow in the form of "Aspen champagne powder" can be as light as 0.1 g/cm3 or as heavy as "Cascade concrete" at 0.3 g/cm3.

So the amount of mass put on the poles by deposition would be overall pretty small. And it would be pretty fluffy at that.


[Now hopefully I understood the Diapir abstract correctly..]

The material deposited on the surface of the ice crust will be of lower density materials (fluffier than 0.63 g/cm3). Assuming some crustal bouyancy and lower rigidity, the extra weight will cause the higher density ice crust to sink a little. The net effect will be to make a surface mass deficit (lower density stuff up higher) and an interior mass excess at the polar regions. This would be a polar negative gravity anomaly. Overall Titan would be mass-bulgy at the equator, and thinner on the top (just like the typical middle-age male, sigh...).

So even if the amount of organic deposited on the pole was significant, the density factor would tend to maintain the current polar zones.


From some of the topographical data in the northern polar areas (see the Flagship mission document) it looks like there is an overall broad depression in the north polar zone of Titan. Using a similar argument, that north polar depression would tend to stabilize the Titanian poles right where they are.

-Mike

[Juramike]

Polar reorientation due to Large Impact Basin:
Nimmo and Matsuyama, LPS 38 (2007) Abstract 1237. "Reorientation of icy Satellites due to Impact basins."
A big impact crater basin can also cause polar reorientation, with significant shifts (tens of degrees). Slow rotating satellites (Titan counts) are more prone to reorientation.

Impact basins are lower density zones on the surface. They would want to be located at the poles (bulge in the middle, thin on top).

What would a big impact like Menrva do to Titan?

Page 2 of the abstract has a hypothetical basin on Pluto at colatitude 45, and W longitude 90 with a 602 km diameter and flat-floored depth of 2 km. (Why 602 km and not 600 km?). From this it was estimated to have a poleward reorientation amount (potential?) of 23.4 degrees.

Menrva is located at colatitude 70 and W longitude 90 and is 400 km diameter (depth unknown).

Titan has a bigger radius than Pluto (2576 km for Titan vs. 1195 km for Pluto) and so from equation 2 in the abstract the Quantity Q will be lower (2.5 times) for Titan given everything else constant. This would lower the expected poleward drift a little. Titan rotates slower than Pluto. Since Q is also inversely dependent on the square of the rotation frequency, the fact that Titan rotates 2x slower than Pluto would make Q 4 times bigger for Titan. The angle subtended by Menrva basin on bigger Titan compared to the hypothetical basin on smaller Pluto will be much, much smaller for Titan.

Impact basins at colatitude 45 degrees will have maximal effect, compared to Impact basins closer to the equator. So this argues for a smaller effect for the more equatorial Menrva.

The depth is debatable. Menrva probably underwent viscous relaxation and infilling by lower density organic/ice sediments. (Delta structures in the interior of the crater as well as breaches on the W side.) The wierd looking structure on the SW corner would also decrease the effect of the lower density crater void, assuming it piped up stuff from down deep.

If this were a multiple choice test, I'd guess that Menrva on Titan would have a poleward orientation potential somewhere between 10 and 20 degrees. Similar to, but overall less than the hypothetical impact basin on Pluto.

-Mike

[ngunn]

[quote name='Juramike' date='May 16 2008, 11:15 PM' post='113761']


The estimated global organic deposition rate is 9.5E16 g Myr-1. Assuming it falls at polyacetylene density, this corresponds to 0.015 m Myr-1 averaged globally. (see post 270, Equatorial Sand Seas thread). END QUOTE

That's just the stuff that polymerises whilst airborne. What about methane rain that turns into sticky stuff once on the ground? Rivers flowing toward the northern lakes (or away from the southern highlands, but getting no further than Mezzoramia)?

I agree, though, the other mechanisms you discuss are interesting too.

Let's not forget either that at least in the case of Europa we could be talking about a fairly recent event, not a primordial one. IIRC, Europa's present ice crust is only tens of millions of years old.

[centsworth_II]

...IIRC, Europa's present ice crust is only tens of millions of years old.

I would expect that Europa has had an icy crust since it's creation.
Could a liquid water surface ever have existed on Europa? How constant
the thickness of that crust has been over the ages as it recycles, new ice
replacing old, is the question.

[ngunn]

I would expect that Europa has had an icy crust since it's creation.
Could a liquid water surface ever have existed on Europa?

I'd say not necessarily - and yes (or just very thin ice) during episodes of greater tidal heating than it experiences today. But whatever the case may be the present crust is not ancient so neither are the 'stretch marks'. We have to consider the possibility that crustal reorientations are almost routine on floating iceworlds, or at least the more active ones.

[DrShank]

I'd say not necessarily - and yes (or just very thin ice) during episodes of greater tidal heating than it experiences today. But whatever the case may be the present crust is not ancient so neither are the 'stretch marks'. We have to consider the possibility that crustal reorientations are almost routine on floating iceworlds, or at least the more active ones.

I am inclined to agree with these sentiments. We cannot go back any further than 60 myr or so into the past. the crust could be lots older or brand new. the geophysicists tend to think that its difficult to melt the ice once it freezes over but i suspect it depends on the tidal heating phase you are in.

i also suspect that it could do the "polar flop" quite a few times. that would take a painstacking rigorous unreavelling of the geologic record to search for periods of ancient TPW. we can partly do that but we have only 20% of the surface so imaged. a duanting task in an ideal world.....

paolo

[ngunn]

I fancy Ganymede as the one place where it might just prove possible to reconstruct more than one such event through remote observation alone. On Titan, however, the evidence should be there in the sedimentary record in some shape or form.

[rlorenz]

I fancy Ganymede as the one place where it might just prove possible to reconstruct more than one such event through remote observation alone. On Titan, however, the evidence should be there in the sedimentary record in some shape or form.

Right, trivially you could make some argument like 'Oh, I see a big lake basin at a low
latitude location, and we do not see lakes there today, so this location was once at
higher latitude'. However, that isnt a unique explanation, since methane delivery by
cryovolcanism could perhaps have led to a much wetter regime (to say nothing of
the faint early sun, which might have permitted N2 condensation on the surface).

So Ganymede may be the only place simple/inactive enough to interpret
remote-sensed geological clues to re-orientation.

[ngunn]

Do you think there is anything that the planned balloon or landers could look for, or perhaps discover by chance, that would provide a tell-tale clue, or are there too many variables on Titan even for that to be likely?

[rlorenz]

Do you think there is anything that the planned balloon or landers could look for, or perhaps discover by chance, that would provide a tell-tale clue, or are there too many variables on Titan even for that to be likely?

Not sure that there is a cute simple answer here, since everything is so connected.

Lander or balloon noble gas isotope measurements might constrain the methane delivery history better.
A full topo map will let us understand the methane aquifer system somewhat (and thus how much
near-subsurface methane there is). A sounding radar a la Marsis will contribute here, maybe discover a
load more craters under the dunes which might help understand the atmospheric history (i.e. small-crater
cutoff gives information on atmosphere density) etc etc. Also, until we've observed at several parts in the
seasonal cycle, we don't really know how much it can rain 'normally', to allow us to flag anomalously-wet
periods in Titan's history. etc.

Bottom line, we need to understand Titan as a system.

[nprev]

Ralph (and hopefully not too far OT given a systemic viewpoint), are there any tentative predictions for when the next monsoon season will begin (if it does?)

[ngunn]

Not sure that there is a cute simple answer here, since everything is so connected.

Bottom line, we need to understand Titan as a system.

Ralph, thanks for having a go at what was, in retrospect, an impossible question! Reorientation would presumably trigger climate change (and maybe even the reverse is conceivable?).

Nick, you've hit the nail on the head. We'll never make sense of it until we can sort out the various timescales - dating the processes is the key, and that's going to be hard. EDIT: If we're lucky the interval between equatorial floods at a given location will fall within the range accessible to radiocarbon measurement.

[Juramike]

I'm no expert, but I'll take a stab at it:

A Phoenix-style lander at a few select locations on Titan just might be able to tease out the fluvial/sedimentary history.
To really figure it out, the lander would need an oven and a mass spec (just like Phoenix), but a really impressively deep drilling rig (unlike Phoenix).

If what I'm thinkin' is correct, the catastrophic floods would leave large deep deposits or organic sediment in certain places. Trapped atmospheric gases in vesicles in these sediments might be key. You might be able to use the nitrogen (and carbon?) isotope record to tease out the timing.

I'm assuming that the lighter isotopes of nitrogen have been selectively leaping out of the atmosphere since initial formation. So older trapped atmospheric gases would have a lighter mix of isotopes than the current atmosphere. (It would need to be demonstrated that some of Titan's "rock" formations would be capable of trapping and keeping isolated from exchange trace amounts of ancient atmospheric nitrogen).

If the ancient episodic catastrophic flood speculation is correct, one would see sudden shifts in the nitrogen isotope ratio as you drill down through each layer (going lighter as you go back in time). Each individual deep layer would be pretty uniform however. The "ancient" part of the speculation would predict a large change between the current atmospheric nitrogen istope ratio and the nitrogen isotope ratio in the trapped sediment bubbles .


The monsoon history in delta sediments would work the same way, just a little more subtle, and possibly more recent (probably location dependent - might've been a really looooong while in the Equatorial Sand Seas, might've been just last season in the Polar region). Nitrogen isotopes might work, and maybe perhaps methane isotopes as well for the most recent stuff.


The isotopic carbon ratio will be more difficult to tease out. Methane might have been released from the interior on a spasmodic basis. So the isotope ratio in the sedimentary record might shift around depending on the (unknown) methane flux history. [You'd probably need an isotope ratio (like nitrogen) to help reference the carbon ratio].

Any complex organic solids (i.e. ethane and higher) on the ground would be expected to slowly become isotopically heavier over after each methane burp. This could confound the carbon isotope sediment record, since you wouldn't know how long the sediments were sitting on the surface before they all got swept together in a layer.

The recent record might be decipherable by finding a non-windy location at the poles or temperate region and digging gently through each layer of the schizzle and measuring the carbon isotope ratio of several deposited organic species. [If I understand correctly, the deuterium ratio might work out also, since the isotope effect should favor deposition of more heavily ( ! ) deuterated molecules. It should be much harder to break the C-D bond compared to a C-H bond when making ethane]. This record might be very amenable to a Phoenix style lander on Titan (as long as it doesn't ploof through all the layers on landing).

-Mike

[ngunn]

I've got a question for Paul too. Do we have any idea how long it takes one of these 'polar flops' to complete, once started? Even an order of magnitude estimate?

EDIT
Hey - synchronicity Mike - no action for 6 hours then we both post at once!
Interesting dating suggestions you have there. I've been wondering about potassium in the ices ??

[ngunn]

Aha! See end of second paragraph. I haven't checked the references yet. Anyone got newer info on this?
http://www.lpi.usra.edu/meetings/gold2001/pdf/3718.pdf