[Atomic-S (OP)]

Consider this picture of Phobos:

Note that Phobos exhibits roundish craters. Note that it also exhibits long parallel grooves. Note that some of the long parallel grooves turn into long rows of craters, suggesting that grooves and craters are not distinct types of features, but actually the same type of feature.  However, if craters are formed by impacts, how can they form in long rows one-crater wide that are extensions of grooves?