I note you didn't mention the Hephaestus Fossae image. Which explains nicely your skipping impactor images...

Hephaestus Fossae (Released 3 July 2002)

"Off the western flank of Elysium are the Hephaestus Fossae, including linear arrangements of small, round pits. These features are commonly called 'pit chains' and most likely represent the collapse of lava tubes. Lava tubes allow molten rock to move long distances underground. When the lava drains out it leaves unsupported tunnels, which can collapse and form pits. These particular pit chains are unusual because they change direction abruptly. In the lower portion of the image, pits have collapsed at the bends and allow us to observe the sharp, nearly right angle corners. These direction changes are most likely due to some sort of structural control during the emplacement of the lava tubes."

http://themis.la.asu.edu/zoom-20020703a.html