European Space Agency website
If you download these images and use an image viewer to "animate" them, there appears to be a pool of liquid behind the "flat white" object with fluctuating, circular ripples that look like those from rising gas bubbles.* I will make a note to observe gas bubbles rising the next time I take a bath, which shouldn't be too far off, now that spring is approaching.
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*You haven't been on the wacky backy, have you Kronk? Ed.