Stardust reveals comets were once wet

Science : Comets were once thought to be pristine repositories of the building blocks of our solar system. The most recent challenge to that belief comes from new analyses of the comet Wild 2. Planetary scientist Eve Berger of the University of Arizona and her colleagues have found several different sulfur-containing minerals in microscopic pieces of the comet, including a form of cubanite that is created only in liquid water below 210 °C. They conclude that the alteration most likely occurred when heat—from an impact or radioactive decay—melted pockets of ice within the comet, which then refroze. Primordial material left over from the formation of the solar system may still be out there, but scientists are having some difficulty finding it. That challenge only increases their desire to obtain a piece of comet to analyze.