Did Huygens or Cassini follow up on this theory at all?

I don't think so...

__________________
Rovers forever! - ToSeek
"Carl Sagan sent a message to ET,
Neil Armstrong walked in the Sea of Tranquility
Steve Squyers built Spirit and Opportunity
Dan Haylen upchucked in zero gravity." -Brent Simon, The Space Camp Song
'Evolution and science are one thing, but you don’t mess with Yoko Ono. Everybody knows that. ' - 386sx

Huygens couldn't follow up. It's battery ran out shortly after it landed on the surface, and no new information was relayed by it. I don't think that Cassini has much in the way of instruments that could tell directly, though there do seem to be one or more cryovolcanoes on Titan which supports there being at least pools of something liquid not far below the surface.

__________________
Forming opinions as we speak

IIRC, Huygens picked up an ELF reflection during descent that might have been from a deep solid-liquid interface. I don't recall hearing any more about that, though.
The way Cassini deflects as it flies by Titan will give some information about the radial density gradient within Titan, which will in turn set physical limits on what is or is not plausible about its internal structure. But I haven't heard any results from that, either.

Grant Hutchison

What's "radial density"?

__________________
Fitting a three-parameter curve of uncertain form to ten points with three exceptions certainly brings one to the far edge of the known world. -- Bradley Ephron

Radial density gradient. The "radial" modifies the "density gradient". Signifying, "how rapidly the density changes along a radius"; in this case, a Titan radius.

Grant Hutchison

Gotcha. (And by "ELF" I assume you mean extremely low frequency radio waves.) But the density probably changes in a discontinuous, step-wise fashion. Most simple models of such moons assume a three layer structure: a rocky core surrounded by a silicate mantle, that is in turn surrounded by an "ice" shell ("ice" is a catchall word in planetary science referring to heavier volatiles like water, methane, and ammonia--as opposed to H₂ or He--that doesn't necessarily imply solidity).

Titan is probably typical in that regard, so it's almost certain that Titan has an icy shell--the question is whether it is liquid or not. And while gravitational measurements can say whether Titan has an icy shell or not, such gravitational measurements cannot settle the question of whether that icy shell exists in a partially liquid state--as they cannot for Europa.

__________________
Fitting a three-parameter curve of uncertain form to ten points with three exceptions certainly brings one to the far edge of the known world. -- Bradley Ephron

But liquid and solid have different densities, and a discontinuity at a point which supports theoretical models would go some way towards confirming theory. Hence my cautious suggestion that it would "set physical limits on what is or is not plausible about [Titan's] internal structure".
But the Cassini team seem to be more upbeat than me. At the Cassini imaging site they record::(My bold.)

Grant Hutchison

Oh.
Probably worth pointing out how, in my quote above, the Cassini team are using several flybys for these measurements. If you catch Titan at different distances from Saturn, you get a measure of how it deforms under tidal strain: that not only gives you density-gradient information, but an idea of the viscoelastic properties, too.
That second modality is also helpful in differentiating liquid from solid.

Grant Hutchison

They'll be lucky to get a good estimate of the J₂ gravitational moment (which is affected by how uniformly the density distribution is--the lower the J₂ is, the more uniform the density is). So while that definitely constrains the search space of plausible models, there is still a huge family of plausible models that can fit the data. The fact is we don't know the exact composition of the core--and that alone introduces enough uncertainty to swamp out any differences in density between frozen and liquid ice.

I'm not saying there isn't a liquid ocean--I bet there is. And there is some other evidence that's suggestive in that regard: the surface doesn't have a lot of cratering; apparently there's cryovolcanism; lots of nitrogen implies lots of ammonia that has a low freezing point; and if there really are liquid lakes on the surface, it stands to reason there is liquid beneath the surface as well.

But a definitive answer to the OP question would require simultaneous measurements of gravitational and topographical amplitudes of the semidiurnal tides, and to do that will require a dedicated orbiting spacecraft.

__________________
Fitting a three-parameter curve of uncertain form to ten points with three exceptions certainly brings one to the far edge of the known world. -- Bradley Ephron

Well, the plan (384 kb pdf) has always been to do just that:(This is the very last paragraph of the linked 1997 Icarus article from JPL.)

And as of last month, with three flybys under their belts, the same team were predicting a >90% chance of distinguishing between "ocean" and "no ocean" using measured quadrupole moments to dig out a precise Love number.

Grant Hutchison

__________________
Fitting a three-parameter curve of uncertain form to ten points with three exceptions certainly brings one to the far edge of the known world. -- Bradley Ephron

I knew you'd be pleased.

Grant Hutchison

So we still don't know for certain whether there is an ocean inside Europa? I thought that was a borderline-certainty by now.

Sorry, I misread your post and originally replied as if you were asking about Titan.
We were talking above about the measurement of various gravitational parameters using flybys: in Europa's case, the moment of inertia measured from such flybys supports a three-layer model, but doesn't tell us if any of the water layer is liquid, as Warren Platts has described.
But in Europa's case we also have magnetometer data showing that Europa is generating a magnetic field, suggesting it has a conducting layer somewhere. From modelling, it seems most likely that the conducting layer is a briny liquid.

(From the Cassini magnetometer, Titan doesn't seem to have its own magnetic field. So no hints there about Titan's subsurface ocean.)

Grant Hutchison

Unfortunately, Titan is not in the Jovian system (just to point out something obvious) and the magnetic field of Saturn is to a very high degree an aligned dipole. This means that the neat tricks we (I was on the magnetometer team) have done for Europa and Ganymede and Callisto, cannot be done for Titan.
For the three Galilean satellites we have used the time varying background field of Jupiter to find the induced magnetic field. Christophe Zimmer did a very nifty calculation to find out the exact parameter regime in which the oceans are located.
Titan does, however, cross the magnetopause of Saturn, and enters the solar wind during every orbit. But I do not think that the Cassini team can hope for anything similar by e.g. using the solar wind. However, I am sure that the Cassini team will try something.

__________________
************************************************** *************************
Optimism does not change the laws of physics. (T'Pol)
A good scientist has freed himself of concepts and keeps his mind open to what is. (Dao De Jing 27)
************************************************** *************************
Martin ( http://www.geocities.com/DrMartinV )

So (just to check my understanding): Titan doesn't encounter magnetic field variations during its orbit of the magnitude Europa does around Jupiter, and we therefore can't expect an induced magnetic field in Titan of anything like the same strength as the one detected at Europa, even if Titan has a similar "conducting layer" somewhere inside. Yes?

Grant Hutchison

Correct. In the case of Jupiter, there is the dipole tilt and therefore the magnetic variation around Europa is roughly +/- 200 nT (the radial component of the magnetic field), so there is a significant field driving the induction.

Now at Titan, there is basically no time varying magnetic field because there is basically no dipole tilt. Here is a picture of the orbit of Titan. Now, this is for quiet times in the solar wind. If the pressure increases, the orbit can cross the magnetopause and the bowshock and thus enter into the solar wind. But the variations in magnetic field in this case is rather small, and thus will be very difficult to measure, if there is an inductive effect at all. Also, in the solar wind you will get the same effect as at Venus and Mars in which draping of the magnetic field will create an induced magnetosphere, which will complicate things even more.

__________________
************************************************** *************************
Optimism does not change the laws of physics. (T'Pol)
A good scientist has freed himself of concepts and keeps his mind open to what is. (Dao De Jing 27)
************************************************** *************************
Martin ( http://www.geocities.com/DrMartinV )