Scientists dont know why the storm became so gigantic. Richard Zurek, of NASAs Jet Propulsion Laboratory in Pasadena, California, said, "We dont understand the details." In June, the storm apparently grew from a smaller duster that appeared in a plain in the Hellas basin. Sequential images show that dust from that basin spread north and east, eventually covering the whole planet in a reddish-orange cloud.
http://www.wired.com/news/photo/0,1860,47511,00.html
