Enceladus seemed to orbit with a thick clump of ring matter trailing behind it, almost as if it were dragging the material in its gravitational wake. What astronomers suspected instead â?? and what Voyager confirmed â?? was that Enceladus was not dragging matter but expelling it, chugging through its orbits like a locomotive and leaving a vapor trail behind it. What astronomers couldn't know for sure was what the exhaust was made of.

For more, check out this Time article.