Liftoff. Jets of gas launch dust from comet Wild 2 in this artist's conception. The Stardust spacecraft swept up such dust and returned it to Earth.

Stardust Reveals Comets Were Once Wet

Dick writes about Earth and planetary science for Science magazine.

Planetary scientists once thought icy comets were pristine treasure troves that held nothing but the unaltered building blocks of the solar system. Strike another blow to that idea. Researchers picking through microscopic bits of comet Wild 2 have found minerals that must have formed when warm liquid water completely altered some of the primordial building blocks of comets.

The discovery comes from new analyses of Wild 2 dust collected near the comet by the Stardust spacecraft and returned to Earth in 2006. Later that year, Stardust researchers reported that Wild 2 dust included minerals that formed at the blazingly hot temperatures near the nascent sun. Solar heat had melted primordial dust, they presumed, which was then blown out to the cold outer reaches of the solar system and incorporated in the comets forming there.

After a closer look at Stardust samples, planetary scientists Eve Berger of the University of Arizona in Tucson and her colleagues have found a new kind of altered primordial matter. In an upcoming issue of Geochimica et Cosmochimica Acta, the group describes a variety of sulfur-containing minerals, including a form of cubanite, in Wild 2 particles. That form of the mineral is only created in liquid water below a temperature of 210˚C, they note. The group concludes that the watery alteration most likely occurred in the comet when heat from either an impact or radioactive decay melted pockets of ice, which then quickly refroze.

The newly recognized alteration "certainly puts a ding in comets' reputation as repositories of primordial solar system material," says meteoriticist Michael Zolensky of NASA's Johnson Space Center in Houston, Texas. "Wild 2 definitely went through the mill," he says. Primordial material may yet remain, but scientists are having trouble finding it. That only stokes researchers' desire to get their hands on a chunk of comet. The Rosetta spacecraft will place an instrument-laden lander on comet Churyumov-Gerasimenko in 2014, but no comet sample return mission is as yet planned.

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