[X] My Assistant

[Guest_AlexBlackwell_*]

The June 1, 2006, issue of Nature has a new paper by Nimmo and Pappalardo entitled "Diapir-induced reorientation of Saturn's moon Enceladus" as well as an accompanying News and Views piece by Paul Hanlon. For the specific links, see the Editor's Summary.

[Guest_AlexBlackwell_*]

NASA-funded Study Says Saturn's Moon Enceladus Rolled Over
NASA/Jet Propulsion Laboratory
May 31, 2006

Saturn's moon Enceladus may have rolled over to put a hot spot at the pole

[Guest_AlexBlackwell_*]

The June 1, 2006, issue of Nature has a new paper by Nimmo and Pappalardo entitled "Diapir-induced reorientation of Saturn's moon Enceladus"...

I've read the paper and found it pretty interesting. I especially appreciate papers that offer ways to test the authors' hypotheses. For example, Nimmo and Pappalardo suggest "at least two additional tests. First, the distribution of impact craters—expected to show leading–trailing hemisphere asymmetry if the satellite orbits synchronously—will be affected by, and could constrain, reorientation. Second, Fig. 2 shows that a negative, long-wavelength gravity anomaly (with an amplitude of a few mGal at 200 km spacecraft altitude) will be associated with the anomalous south polar region. Regional gravity anomalies similar to this have been detected at Ganymede by the Galileo spacecraft." (References omitted).

For those without access to Nature, perhaps Nimmo may offer the paper on his publications page. Or the co-author, Bob Pappalardo, may have a suggestion

This post has been edited by AlexBlackwell: Jun 5 2006, 03:54 PM

[volcanopele]

I've read the paper and found it pretty interesting. I especially appreciate papers that offer ways to test the authors' hypotheses. For example, Nimmo and Pappalardo suggest "at least two additional tests. First, the distribution of impact craters—expected to show leading–trailing hemisphere asymmetry if the satellite orbits synchronously—will be affected by, and could constrain, reorientation.

How would such a test be effected by differences in surface age between the terrains at the center of the trailing and leading hemisphere. At the centers of each are regions of young terrain that may or may not have been formed at around the same time. It is possible that the leading hemisphere smooth plains may have been formed much later than the terrain on the trailing hemisphere (or vice versa), and such differences could skew the results of crater counts in both regions with respect to this test.

Second, Fig. 2 shows that a negative, long-wavelength gravity anomaly (with an amplitude of a few mGal at 200 km spacecraft altitude) will be associated with the anomalous south polar region. Regional gravity anomalies similar to this have been detected at Ganymede by the Galileo spacecraft." (References omitted).

Not quite sure we can accomplish that on E03, but hopefully that will be done during the extended mission. That would certainly be a much more definitive test than crater counts in the trailing and leading hemispheres.

[Rob Pinnegar]

Just as a sidenote: Looking up Saturn's orbital velocity about the Sun and Enceladus' orbital velocity about Saturn, it turns out that Enceladus is faster: about 12-13 km/s versus 9.6 for Saturn around the Sun. So Enceladus' angular velocity with respect to the Sun changes sign; it can move "backwards" as Saturn continues to move forwards.

This has got to cut down on the chances of Enceladus being "hit from behind" in a big way. When its velocity vector is parallel with Saturn's, could anything on a non-hyperbolic orbit around the Sun actually catch up with it? There's always the chance of being hit from the side by chance, of course, but statistically speaking the leading side really should take the brunt of the beating.

I always sort of felt that the idea of leading/trailing hemisphere asymmetry in crater counts was a bit of a pipe dream, due to the sheer number of craters involved, and "crater saturation" (though on Enceladus that may not be a huge issue). For outer moons like Iapetus where the moon's orbital velocity isn't a substantial fraction of Saturn's, this could be true... but in Enceladus' case one would think that it ought to be a noticeable effect (stressing here "ought to be")...

[Guest_BruceMoomaw_*]

One piece of supporting evidence absolutely jumps out at you when you read Carolyn Porco's "Science" paper on Cassini's photos of Enceladus: the apparent surface age, as judged by the crater count, indicates that the spot on Enceladus' surface where craters are being obliterated has slowly migrated south during the moon's lifetime. See Table 1 in http://pubs.giss.nasa.gov/docs/2006/2006_Porco_etal.pdf (pg. 1397).

This, however, points toward the version in which Enceladus' rocky core, with its hot diapir, is sliding around beneath its icy shell, with a lubricating layer of liquid water or very soft ice in between -- rather than the one in which the entire moon has rolled over as a single unit.

[Stephen]

NASA-funded Study Says Saturn's Moon Enceladus Rolled Over
NASA/Jet Propulsion Laboratory
May 31, 2006

Saturn's moon Enceladus may have rolled over to put a hot spot at the pole

Did it roll or was it pushed?

======
Stephen

[Guest_BruceMoomaw_*]

It rolled over on its own, because it was suffering from Diapir Rash.

[Guest_BruceMoomaw_*]

What the hell am I thinking?! As the "Nature" abstract says flatly (and Bob's earlier abstracts, whose content I had forgotten, also said: "If the diapir is in the silicate core, then Enceladus cannot possess a global subsurface ocean, because the core must be coupled to the overlying ice for reorientation to occur." I don't quite understand why this should be so, though; I'll have to reread the abstracts. At first glance, I'd think that the rocky core could indeed roll underneath an icy shell which remained in its initial orientation. And if it didn't, we neeed another explanation for why the heated region has unquestionably migrated across Enceladus' surface.

[JRehling]

Just as a sidenote: Looking up Saturn's orbital velocity about the Sun and Enceladus' orbital velocity about Saturn, it turns out that Enceladus is faster: about 12-13 km/s versus 9.6 for Saturn around the Sun. So Enceladus' angular velocity with respect to the Sun changes sign; it can move "backwards" as Saturn continues to move forwards.

This has got to cut down on the chances of Enceladus being "hit from behind" in a big way. When its velocity vector is parallel with Saturn's, could anything on a non-hyperbolic orbit around the Sun actually catch up with it?

Remember that something coming into the vicinity would be greatly accelerated by Saturn's gravitational field -- for it to come from "Saturn infinity" down to Enceladus's orbit, it would have a great component of velocity due to Saturn than Enceladus's orbital velocity (after all, the thing itself will end up flying on, while Enceladus is trapped because it's not moving fast enough). An object coming from Saturn-infinity to kiss Enceladus's orbit would have a velocity of about 17.5 km/s simply due to the acceleration on the way into Saturn, summed with its initial velocity in its sunward orbit. Any object in solar orbit with a semimajor axis greater than (or equal to or slightly less than) Saturn's and passing Enceladus's way on the antisun side of Saturn *would* have a greater velocity than Enceladus, and could overtake it. Of course, the probabilities remain much greater of a collision on the leading side -- this is simply to say that collisions on the trailing side are not hard to imagine.

The other factor is that Saturn's moons may have received significant bombardment from objects in Saturn orbit. There have probably been some catastrophic events afoot. For objects originating in Saturn orbit but with a greater semimajor axis than Enceladus, there would be no problem hitting Enceladus's backside at all. Since the rings are inside Enceladus's orbit, maybe the main source of jetsom has been from the "inside" (favoring leading side collisions), but given the mysteries of Hyperion, Iapetus, Phoebe, and the outer, captured moons, it's hard to rule out significant "outside" sources.

[Guest_BruceMoomaw_*]

Pappalardo and Nimmo's LPSC abstract on the idea is at http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2113.pdf , and a related one on the apparent compression/extension stress patterns in Enceladus' crust around the area (which may well be important evidence supporting the theory) is at http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2182.pdf . I don't, however, quite know whether or not this theory meshes easily with Porco's observation that the center of thermal activity has been slowly migrating southward across Enceladus' ice surface over the eons. Comments, Bob?

[vexgizmo]

What the hell am I thinking?! As the "Nature" abstract says flatly (and Bob's earlier abstracts, whose content I had forgotten, also said: "If the diapir is in the silicate core, then Enceladus cannot possess a global subsurface ocean, because the core must be coupled to the overlying ice for reorientation to occur." I don't quite understand why this should be so, though; I'll have to reread the abstracts. At first glance, I'd think that the rocky core could indeed roll underneath an icy shell which remained in its initial orientation. And if it didn't, we neeed another explanation for why the heated region has unquestionably migrated across Enceladus' surface.

A good point, but probably not, because the surface tectonic pattern, i.e. that in the ice shell, as reported in the Porco et al. Science paper suggests that the ice shell itself did reorient and was not simply deformed from below. In retrospect, the reorientation paper should have said this explicitly....

As for the cratering record, the counts don't have to be near the leading & trailing points--just noting distance from the apex vs. the center of the counting area will do. But resurfacing really might confound this. It was done successfully by Plescia on Miranda, but there are plenty of unresurfaced terrains there.

Here is a fun one from the Wikipedia entry on Enceladus mythology. Coincidence? I think not.

"In Greek mythology, Enceladus was one of the Gigantes, the enormous children of Gaia (Earth). During the battle between the Gigantes and the Olympian gods, Enceladus was disabled by a spear thrown by the goddess Athena. He was buried on the island of Sicily, under Mount Etna. The volcanic fires of Etna were believed to be the breath of Enceladus, and its tremors to be caused by him rolling his injured side beneath the mountain...."

[Guest_BruceMoomaw_*]

Hmmm. Is it possible that the ice shell swivelled south before the underlying rocky core and its own magma diapir did? (Your LPSC abstract indicates that the force encouraging such swivelling would be stronger for the ice shell than for the underlying rocky core, even if the latter contained a magma diapir which in turn produced a warm-ice diapir in the overlying ice mantle.)

In that case, after the underlying magma diapir warmed up the overlying ice and produced an accompanying warm-ice diapir (along with a lubricating layer of liquid water or soft ice between the rock core and the ice mantle), the initial warm spot in the ice mantle would have swivelled toward the south pole first -- causing the relatively stationary magma diapir to soften up a streak of overlying ice stretching northward towards the ice mantle's new equator -- and then the rocky core itself swivelled south more slowly, retracing its former route and thus heating that streak in the ice mantle a second time before itself finally also settling at the south pole and subjecting the ice there to a second, really prolonged warming. (The compression and extension tectonic marks on the ice's surface, as you say, would have been mainly the result of the ice mantle itself initially migrating south, rather than the rocky core later rolling underneath the ice to join it.)

[The Messenger]

How strong is Saturn's magnetic field at the orbit of Enceladus? I can imagine an object with a heavy iron core striking Enceladus at a relatively low velocity - trailing orbit kind of thing. This asymetric engine would be extracting rotational energy from Saturn via induced eddy currents, both creating a hot spot and sliding the hot spot slowly deeper below the icy surface: Energy and migration. This scenario would require the migration to proceed in the same orientation as the 'braking force' inherent in Eddy current transfer.

[volcanopele]

I don't, however, quite know whether or not this theory meshes easily with Porco's observation that the center of thermal activity has been slowly migrating southward across Enceladus' ice surface over the eons. Comments, Bob?

I have reread the relevant sections of our paper and I don't see any reference to the idea that the thermal activity has been migrating southwards. I think Squyres et al. 1983 might, in the Voyager 2 observations of cratered terrain in the northern and sub-Saturnian hemispheres, in that craters counts get progressively lower as you go south...in the sub-saturnian cratered terrains. I don't see where this is implied or discussed in Porco et al. 2006. From the looks of it, we mention three sites of thermal activity in the past: the trailing hemisphere as seen in Sarandib and Diyar Planitiae, the leading hemisphere (55 South-45 North; 45-130 West), and the south polar terrain.

You may be refering to a table in Porco et al. 2006 showing craterings statistics and calculated surface ages for 4 geologic units and the ultra high-res WAC. I should note that the latitude range for Unit III (Samarkand Sulci) is mislabeled (I think that should read 55-65 North, or there abouts), it shouldn't say south, or at least not that far south.