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VanderL
2005-Feb-15, 09:11 AM
Hi All,

This APOD image (http://antwrp.gsfc.nasa.gov/apod/ap050215.html) shows Saturn's moon Rhea and the accompanying text expresses wonder about the light colored wispy streaks visible on this moon.
There is also something very strange that isn't mentioned, at the bottom there is a straight line visible. It is only straight because of the angle of viewing, it actually follows a curve. It is a real feature it was imaged before during on of the Voyager (not sure about this) flybys.
What could have created such a feature?

Cheers.

Guest
2005-Feb-15, 12:25 PM
Originally posted by VanderL@Feb 15 2005, 09:11 AM
Hi All,

This APOD image (http://antwrp.gsfc.nasa.gov/apod/ap050215.html) shows Saturn's moon Rhea and the accompanying text expresses wonder about the light colored wispy streaks visible on this moon.

Atmosphere? :huh:

Seems too much of coincidence that satellite angle of view shows "long straight line", could it just be a defect in the photo? I'd like to see an earlier (Voyager?) photo of this too.

VanderL
2005-Feb-15, 03:30 PM
Hi Guest,

Rhea has no atmosphere, and how could an atmosphere create such a feature?

This (http://www.solarviews.com/raw/sat/rhea1.gif) is the Voyager1 image, you can see that the straight line is actually curved and is a real feature.

This (http://photojournal.jpl.nasa.gov/jpeg/PIA06163.jpg) close-up of Dione also shows such a linear feature (a shorter one though), it is running across other features, including craters. This image also shows the linear feature in some places is composed of craters. Which would mean that the explanation I think is most likely is that it is a "crater chain" where the craters overlap to form a straight groove.

Cheers.

antoniseb
2005-Feb-15, 03:55 PM
These moons are in the range of size that is near the limit where gravity eventually makes them round. It's going to interesting to learn about the processes that created these surfaces.

VanderL
2005-Feb-15, 04:11 PM
Well, maybe these image are already sufficient to exclude some of the processes. The Dione image seems to exclude some sort of rift or crack. If Rhea's surface was formed similarly some close-ups will show if indeed it is a long straight series of overlapping craters.

Cheers.

antoniseb
2005-Feb-15, 08:15 PM
Originally posted by VanderL@Feb 15 2005, 04:11 PM
If Rhea's surface was formed similarly some close-ups will show if indeed it is a long straight series of overlapping craters.
I really don't know, It could be that the feature is a spray of ice-sand excavated from a blast in the surface. It could be some large rift. It could be a lot of things with no parallel on Earth or moon, because they are big, and not made of ice.

VanderL
2005-Feb-15, 08:53 PM
I really don't know

No problem, I'm not certain either, I'm merely looking for patterns in those strange new images. I hope the Cassini mission wil provide us with more detailed images and see whether the crater chain notion holds up.
Here are some more examples, Callisto (http://www.lpi.usra.edu/resources/outerp/cc1.gif), Ganymede (http://www.thunderbolts.info/tpod/2004/images/040825crater.jpg), and the Moon (http://www.boulder.swri.edu/~bottke/crater_chain/davy_chain.gif).

Closer inspection of the "straight" features on Rhea and Dione will show whether overlapping craters are the correct interpretation.

Cheers.

Guest
2005-Feb-16, 02:36 PM
I was only joking about the atmosphere. But OK, you proved to me it wasn't a defect in the photographic plates. Thanks! What can cause a straight-line fissure like this? Interesting, hmmm... (Not to mention the "wispy" geological features!)

VanderL
2005-Feb-17, 05:33 PM
I was only joking about the atmosphere.

Ok, missed it, but indeed what makes such linear features, I'm surprised there is no explanation offered of how these features could be created, I didn't even see them mentioned in the captions made by the imaging teams. I'm hoping for more details, and as I said I think they will resolve into crater chains that only differ in spacing and size of the craters. We see them all over the solar system so we could compare their characteristics and differences (like surface material, for example).
Explaining the "wispies" could prove to be more difficult, because they seem to be more of a thin layer or lack of a layer (although I'm nor sure about this).

Cheers.

Guest_wjwduke
2005-Feb-18, 01:14 AM
Could these linear lines be the result of a extremely close "near miss" respectably sized asteroid which tangentially skimmed the surface and continued on its journey w/o a surface impact clue? A simple calculation based on the moons diameter and the length of the line may provide some insight. I understand the lack of atmosphere however, could it's "close" encounter result in the friction necessary to "cut" these lines? Whatchathink :blink:

VanderL
2005-Feb-20, 10:38 AM
Hi wjwduke,

I think there is a catch in the situation you describe; the impactor would have to follow a path along the surface, and the surface is curved. It must have an appreciable velocity to make any impact at all and this speed would make sure that it wouldn't follow the curve of the surface instantly.
On top of that, how would it stay intact long enough, I think it would explode at first contact, or careen away from the surface (and maybe the rest will hit in another place), but in any case it wouldn't form these linear grooves, and certainly not anything that long, in my opinion.

Cheers.

sg_hi
2005-Feb-23, 06:50 AM
I can see their shadows for the 2nd image VanderL posted.