article_snippet: Scroll down for a video animation showing the effect .
article_snippet: This ultimately gives rise to this
article_snippet: The shape is defined with dark regions, produced by an unknown compound that absorbs ultraviolet radiation.
article_snippet: the feature has been a mystery for decades.
article_snippet: Many candidates have been proposed: hydrobromic acid, amorphous sulfur, disulphur monoxide or even living sources such as bacteria.'