Polar wander/reorientation of Titan, possible mechanisms and implications Options

[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?