Small pieces of the mineral opal have been found within a brecciated ureilite meteorite from Antarctica. According to a team of scientists led by Prof. Hilary Downes of Birkbeck College London, UK, this find demonstrates that meteorites delivered water ice to asteroids the early Solar System.

Opal is made up of silica with up to 30 percent water in its structure, and has not yet been identified on the surface of any asteroid.

Before the new work, opal had only once been found in a meteorite, as a handful of tiny crystals in a meteorite from Mars.

Prof. Downes and co-authors studied the meteorite, named EET 83309, an object made up of thousands and broken pieces of rock and minerals.

The researchers used different techniques to analyze the opal and check its composition.

They see convincing evidence that it is extra-terrestrial in origin, and did not form while the meteorite was sitting in the Antarctic ice.

“EET 83309 was derived from the surface regolith of an unknown differentiated parent asteroid,” the scientists said.

“The opal occurs as replacement of ureilitic olivine and suessite (Fe 3 Si), and as broken fragments among the clasts derived directly from the parent ureilite asteroid. It can be banded or unbanded, and could easily be mistaken for graphite in back-scattered electron images.”

“It contains considerable amounts of FeO and in some places is devitrifying to a SiO 2 phase.”

“The textures strongly suggest that the opal formed before the meteorite was delivered to Earth, as in places it is surrounded by a terrestrial weathering rim.”

“Thus it reflects the presence of water on or near the surface of the parent asteroid.”

They added: “ureilites are well-known for being completely anhydrous, so the presence of opal implies that the water which reacted with the ureilite minerals came from another source within the Solar System. We suggest that this may have been an impact of an icy planetesimal on the surface of the ureilite parent asteroid.”

“The pieces of opal we have found are either broken fragments or they are replacing other minerals,” Prof. Downes said.

“Our evidence shows that the opal formed before the meteorite was blasted off from the surface of the parent asteroid and sent into space, eventually to land on Earth in Antarctica.”

“This is more evidence that meteorites and asteroids can carry large amounts of water ice,” she said.

Although we rightly worry about the consequences of the impact of large asteroid, 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.”

Prof. Downes and her colleagues reported their results this week at the National Astronomy Meeting 2016 in Nottingham, UK.

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Hilary Downes et al. Delivery of water to asteroidal regolith revealed by meteoritic opal. NAM 2016