Current accepted theory of what caused crater chains.

The science communities explanation for the formation of crater chains is "when an asteroid or comet with low tensile strength is pulled apart by tides during a close approach to a planet" (Bottke et. al. 1997) and "comet P/Shoemaker-Levy 9 (henceforth "SL9") provided a striking demonstration of tidal disruption during its penultimate encounter with Jupiter in 1992" (Bottke et. al. 1997). 1

Simply stated that a comet broke up and impacted the surfaces of bodies in our solar system called the Tidally Disrupted Comet theory.  Tidally disrupted comets happen when a body like a comet which is made up of all kinds of bits and pieces of different materials comes close to a large enough body for it's gravity to effect the approaching object. Because of the different materials that make up the comet the gravity effects can pull the heavier density materials lose from the less dense materials thus breaking up the comet. Because of the varying densities the comet will break up further and in many sized pieces drifting rapidly apart 2 from each other, see W.F. Bottke et. al. 1997 3 for tidally disrupted comets.

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References:
1 http://www.astro.umd.edu/~dcr/Research/bottke_icarus126%2C470.pdf
2 http://photojournal.jpl.nasa.gov/catalog/PIA01264
3 http://www.astro.umd.edu/~dcr/Research/bottke_icarus126%2C470.pdf