Mar 14 2007, 12:18 AM
There's been a fair amount of speculation that Iapetus' despinning, from its initial rapidly-rotating configuration to its current tidally-locked one, might have taken a long time, and that Iapetus might be the only major Saturnian moon that froze *before* it was despun. This has been presented as a possible explanation for Iapetus' strange shape.
Well, there are at least two huge basins on Iapetus that, while rugged, have relatively flat floors: the big one in Cassini Regio that faces Saturn, and the one in the southern part of Roncevaux Terra. Has anyone checked to see whether the "best fits" to the shapes of these basin floors match the curvatures of surrounding parts of the moon? If either of those basins formed before Iapetus was completely despun, their shapes might match some ancient Iapetean geoid, not the current one (some allowance for "slump" would be necessary here).
Given the bashed-up state of the basins, Iapetus' crazy shape, and our very incomplete knowledge of its gravitational field, I don't know whether this would even be possible. It might be worth a look, though.
Mar 14 2007, 03:06 AM
Excellent idea, I hope it can be implemented.
However, (you knew I would have one)
A possible complication; let's say both basins were formed by relatively similar 50 km diameter objects. After the impact of the first one, presumably we have a molten (water/CO2/ammonia slush?) crater floor that soon freezes into the current shape imposed by the current gravitational field, approximately a section of a sphere, and probably distorted per your idea from rotationally derived oblateness.
Now, when the second one hits (let's say millions of years later) it's mass and volume are incorporated into Iapetus. The crater floor will again freeze, but now it will primarily be a section of a larger sphere, modified, by (presumably) lesser rotationally derived oblateness.
(I am neglecting average elevation differences prior to both impacts, this would be another unknown, along with exact volume and mass of the impactor, incoming velocity, % of ejecta not landing back on the crater floor, angle of impactors incoming trajectory, and existence of possible subsurface cavities and fractures that might allow 'melt' to escape)
So, detailed precise surveys of both crater floors might tell us the order in which they formed, and the epochs when the impacts occured.
Another technique that might yield similar (confirming) data, (and probably one that would be exceedingly difficult to perform) would be to identify secondary craters that formed on the Iapetan surface from the ejecta blasted out of the primary craters.
These secondary craters may possess an interesting characteristic relative to their distance from the primary crater. If Iapetus was rotating, for example, in 10 hours at the time of the impact, for the furthest debris, flight times might exceed one hour. Iapetus will rotate over 30 degrees during the debris' ballistic flight, and possibly, the craters will be oblique from the low angle of the debris (it is practically in orbit, after all) and the long axes of the craters will point further and further askew from the direction to the primary crater proportionally to their distance from it. By measuring the distance and the skew angle, we can derive a rotation period for Iapetus at the time of the impact, and perhaps by knowing when Iapetus achieved tide lock to Saturn, determine a fairly precise date of the impact. It would be interesting to compare the ages of craters determined by the oblateness measurements, and the secondary crater skew technique.
Mar 14 2007, 03:21 AM
As noted in the new Enceladus thread (and other places), aluminum 26 is believed to have played an important part of the thermal evolution of various bodies around the solar system.
It seems Iapetus might be uniquely low in its' susceptibility to this heating effect. At Iapetus' current (and presumably past) distance from Saturn, its' orbital velocity is a 'pokey' 5500 kph. So the average orbital velocity of all the 'proto' Iapetan materials was also 5500kph. Iapetus took relatively longer to accrete than virtually any other solar system object. And all the while it was accreting,the Al26 heat was being radiated to space from the enormous combined surface area of all the proto Iapetan objects.
Iapetus might be the closest approximation to a 'cold formed' object in its' size range that we migh hope to ever study.
Also, the distance from Saturn, and its' therefore slow despinning, allowed the tidal heating to dissipate to space with little effect in the Iapetan interior.
This cold formation might certainly help preserve in the body of Iapetus itself geologic evidence of the Iapetan geoid, and perhaps, evidence from several epochs.
Mar 14 2007, 04:04 AM
Hmmm. That's an interesting point about Iapetus' rotation and debris spreading. It's hard to speculate on what sort of effect that would have without doing the calculations. One would think that at least Coriolis forces would be something to take into account.
If it is a real effect, it should happen on any rotating body. First guess: Probably the debris falls too quickly for it to be noticeable most of the time. Do we see it on Phobos? If not, don't look for it on Iapetus.
As for the two basins interfering with each other, I doubt there is too much chance of that -- mostly because those two basins are on opposite sides of the moon. Besides that, even a 50-km-diameter impactor wouldn't add all *that* much to the shape of a moon 1450 km across.
In other news, this Al-26 idea seems to want to stick around. Personally, I still don't much care for it, but that's based on gut reaction rather than any real understanding of the subject.
(There's room here for a God-awful pun along the lines of "warming to the idea", but I'm not touching it. I just can't duck and run that fast any more.)
Mar 14 2007, 03:22 PM
I am not sure I would look for the secondary crater skew effect on Phobos. The low escape velocity implies secondary craters on Phobos are formed by material that has been in Martian orbit before re-encountering Phobos. I am not sure about the Coriolis effects, my understanding is the ejecta particles would all be on ballistic trajectories, and their paths from the parent crater would be unaffected.
My understanding of the crater skew effect is that it occured on earth during the Yucatan impact, and if it happened in that event, it would be feasible to look for the effect on Iapetus as I have seen estimates of up to 1 billion years for the duration of time it took to achieve tide lock. Assuming the Iapetan surface records impacts from over 4 billion years ago, some of those impacts may have occured during an epoch when Iapetus rotated on its' axis much faster.
The hard part is determining which secondary crater (and there appears to be millions of them) goes with which primary crater. Perhaps computer software can be developed to analyze images for patterns in the craters that might be evident. Naturally, a complete high resolution series of images from Cassini will be required . . .
Mar 15 2007, 04:08 AM
Need to add a clarification.
Debris blasted from the Iapetan surface by an impact that is destined to recontact the surface in ~ an hour will be traveling at less than 1000 mph.
I am not sure the size range the debris chunks will exhibit, and I am not sure of their mechanical strength.
When the debris contacts the surface, will it crater? Or will it just skid to a stop?
For cryogenically frozen blocks of mostly water ice, these pieces could be pretty sturdy. Seems I remember seeing Mr. Wizard hammering a nail with a banana dipped in LN2. These pieces, contacting a disconsolidated surface of icy grains might simply skid to a stop. Would 'skid marks' be evident after so many eons? Would the 'skids' point back at the primary crater (with skewing for rotation) or would local slope conditions govern skid direction? Is the surface of Iapetus going to be covered with ~meter sized chunks like Mars? How do we pick out the ones we are interested in?
Might mess up what I am trying to discern here . . .
This is a "lo-fi" version of our main content. To view the full version with more information, formatting and images, please click here