Europa - new evidence for plate tectonics on an alien world?

[sathearn]

Scientists suggest new evidence that zones of crustal compensation may exist on Europa. Otherwise the pervasive zones in which crustal dilation is evident, and geologically recent, would imply expansion of this moon of Jupiter.

See story here.

[Florian]

Michael Manga said “We see expansion at the surface so there must be contraction somewhere.”

Always the same bogus logic. They will go nowhere as long as they're stuck with that preconceived idea.
Well, I still believe that the trigger of the revolution will come from the observations of moons like Europa.

[sathearn]

(1) There is that bias in the way the question is being framed, but at least at this stage they're being pretty clear about the alternatives. The danger, of course, is that the big scientific question which the strong evidence for crustal dilation presents will not be squarely addressed, but will instead be "resolved" by models that only consider data in light of the assumption that dilation equals "subsumption".

(2) Secondly, if it is true that subsumption bands have actually been identified, there is a question, as also on Earth, as to what they mean in terms of crustal compensation - do they imply local compensation or global compensation?

(3) But it should also be considered that the dilational bands cannot exclusively represent expansion of this moon. If, as stated in the article, the surface of Europa (all of it?) is between 40 and 90 million years old while the moon itself is < 4 billion years old, then unless the moon had zero surface area ~100 million years ago (!), then resurfacing as well as (potentially) expansion must be taking place. That in turn raises the question, from the point of view of theory economy, of whether it is necessary to postulate expansion in addition to resurfacing.

Let's look at some additional quotations from the article in light of points (1)-(3):

Scientists believe new ice is being formed on Europa along linear features called dilational bands. There are thousands of kilometers of these bands on the planet, potentially creating significant amounts of new ice crust. The problem is that nobody knows where all the old crust is disappearing.

Here we have a frank statement of the problem. Note the assumption that all the old crust must be disappearing somewhere. It is not said that nobody knows whether old crust is disappearing at a rate equal to new crust production in the dilational bands. But given the reference to the young age of the surface in the text of the article just preceding the above quote, and given my point in (3) above, the writer could be assuming merely the requirement for resurfacing rather than simply assuming constant radius.

“Unless Europa has been expanding within the last 40 to 90 million years, there has to be some process on this icy moon that’s able to accommodate a large amount of new surface area being created at dilational bands.”

This is a quote from Simon Kattenhorn of the University of Idaho, who was involved in the research which was reported on December 13 at the American Geophysical Union meeting. Presumably, this research will eventually be the subject of a published paper with co-researcher Louise Prockter of Johns Hopkins.

Again, it is a fairly forthright statement of the problem.

But note that Kattenhorn doesn't say that "unless Europa had zero surface area 90 million years ago, which is absurd, there has to be some process that..."

Then am I correct in understanding that the characterization of the age of Europa's crust given in the article applies to the whole crust?

How, exactly, was the age of Europa's crust determined?

Was the method of age determination independent of study of the dilational bands?

Note that "the large amount of new surface area being created at dilational bands" is evidently less than the entire surface area of Europa.

They [Kattenhorn and Prockter] found features that had been separated and shifted by movement of the crust, and when they matched those features all up again, there was a large area of crust missing.

What this actually means isn't clear. Is there evidence that this large area of crust was there - e.g. truncated structures? Or is this large area of crust only "missing" when constant radius is assumed? We await details.

“Once the reconstruction is done, we have this big area here that is missing, 92 kilometers wide, that is alongside one of these zones that we refer to using the term subsumption bands,” Kattenhorn said in his presentation.

They say the crust in the area they mapped was pushed down underneath an overriding section of crust along a 23-kilometer-wide subsumption band....

It seems from the above quotations that there must have been some evidentary basis for the features identified as subsumption bands independent of the mere existence of a gap after reconstruction: "this big area here that is missing ... alongside one of these zones..."; "overriding ... along a 23-kilometer-wide subsumption band...."

Again, we await details.

Finally, some notes on the numbers mentioned in the article. The study was based on mapping imagery covering 106,000 km² of Europa's surface. Since Europa's total surface area is given (Wikipedia) as 3.09 x 10⁷ km², the mapped area used in this study represents not quite 1/300th of Europa's total surface. The article also reported that images of sufficient quality that they can be used in this kind of detailed reconstruction currently exist for only about 10% of Europa's surface. Thus the study used about 1/30 of the area potentially amenable to study in this fashion.

[Florian]

Note that "the large amount of new surface area being created at dilational bands" is evidently less than the entire surface area of Europa.

They [Kattenhorn and Prockter] found features that had been separated and shifted by movement of the crust, and when they matched those features all up again, there was a large area of crust missing.

What this actually means isn't clear. Is there evidence that this large area of crust was there - e.g. truncated structures? Or is this large area of crust only "missing" when constant radius is assumed? We await details.

Indeed, that point is critical and unclear. I suspect that they are describing a lateral offset of features?
That would mean that what they interpret as "subsumption bands" could actually be transform faults?
The chaotic band could be resulting from ice warming by friction along the fault.

Impatient to read that paper... Should not be too long.

By the way, I sent a short email to Michael Manga. I may do the same with Steve Vance, who is a geophysicist, a wrote a blog about Simon Kattenhorn's presentation at the AGU (See HERE).

[sathearn]

The page below has references and links to Dr. Kattenhorn's publications. Many of them look interesting. Anyone motivated to take up questions of the interpretation of other planets and moons would do well to become familiar with such work.

http://www.webpages.uidaho.edu/~simkat/ ... tions.html