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Originally Posted by JohnD
All,
Those 'straight-line' objects on Dione and Rhea. Ages ago, I posted about them on the Asterisk to get others' views and didn't. Just today, craterchain has suggested that such objects are the result of the impact of tidally fragmented objects. Googling for "crater chains" gets several clear examples, on the Moon, Callisto and even possibly the Earth, though some refer to "extraterrestrial strafing runs". I suppose that explanation IS about right, if you take out the "intelligence"!
Still, it worries me that the features that originally caught our attention, Travis' Fissure etc., are remarkabaly smooth sided for crater chains, or is that the result of 'image manipulation'?
John
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If you look at those straight features on Dione, Rhea and other moons, they are not really straight, but follow the curve and topology of the surface (I'm not talking about fissures, or "cracks", those can be formed differently). "Crater chains" is a correct description; upon closer examination the straight grooves are made up of overlapping craters. This means that different kinds of crater chains only differ in the space between the craters, and the width of the craters.
All of them are thought to be caused by fragmented impactors, which is clearly a major problem, because how can a fragmented object impact in a straight line, following the curve of a moon's surface and still be so closely spaced as to form overlapping craters. This raises the interesting question if we know how crater are formed at all. On large bodies like Mars, there is an alternative explanation: collapse pits. That explanation assumes that below the surface some erosional process causes empty spaces that subsequently collapse. The problem with that explanation is that it doesn't work for small moons and another problem is that the collapsed "roof" would leave a lot of debris inside the pits, which is clearly absent.
Cheers.