Space Dandruff
I would think it's easy to determine if the cause is impacting meteorites, since debris would only be rising from specific spots at a time. If they rise from all over the place at the same time it must be something local and generalized which propels them, although it must be quite energetic to fling 10 cm rocks at 3 m/s. I'm no specialist but that sounds more like an explosion than the gentle push of random water vapor releases.
I guess the only way to know is to get a closer look?