Opal, the national gemstone of Australia, is silica, the most common ingredient of sand, with a number of water molecules attached to it. On Earth Opal forms when water evaporates from a slurry of sand and water which is deposited repeatedly in a crack or fissure in a rock over a long period of time. The resulting opal may contain from 3 to 21% water by weight.
Professor Hilary Downes of Birkbeck College London headed up a team which discovered Opal in a meteorite which was found in Antartica. Previous to their work the only known extra-terrestrial Opal consisted of a few crystals which were found in a meteorite from Mars.
The Antarctic meteorite that Professor Downes and her team studied, EET 83309, is made up of thousands of small pieces of rocks and minerals. This space rock was created by collisions between objects from various parts of our solar system. Some of them are likely to have been carrying large amounts of water ice. The Downes team found solid evidence that the opal was formed before their sample left the surface of the parent asteroid, traveled through space, and was recovered by humans on the Antarctic ice sheet.
Professor Downes concludes "This is more evidence that meteorites and asteroids can carry large amounts of water ice. Although we rightly worry about the consequences of the impact of large asteroids, billions of years ago they may have brought the water to the Earth and helped it become the world teeming with life that we live in today."