Half of Earth’s water formed before the sun was born

Good news for hunters of extraterrestrial life: Water may be more widespread in planetary systems than previously thought. A team of researchers studying the origin of the water in our solar system has concluded that up to half of it formed before the sun itself was born—that is, in the cloud of dust and gas that was the progenitor of our solar system. If water can form in abundance in such clouds, then it may be found everywhere.

Our solar system is awash with water. Apart from Earth, water is found on the moon, Mars, Mercury, comets, and the icy moons of the giant planets. But where did it come from? Water is known to form in the clouds of gas and dust of the interstellar medium (ISM) from which planetary systems coalesce, but is it destroyed when the newly formed sun starts pumping out heat and light, only to be formed again later? Or does that primordial water survive star formation and remain around us today?

To answer that question, a team led by astronomer L. Ilsedore Cleeves of the University of Michigan, Ann Arbor, focused on deuterium, a heavy form of hydrogen that was created in the big bang along with normal hydrogen. There are about 26 deuterium atoms for every million hydrogen atoms across the universe, but it is six times as prevalent in the water on Earth and in other solar system bodies. Scientists conclude that when the water formed, the reaction creating deuterium-rich “heavy water” was slightly faster than the one creating normal water, so the proportion of deuterium in water increased.

But that enrichment of deuterium happens only under certain conditions: It has to be very cold (only a few tens of degrees above absolute zero), plus you need oxygen and some sort of ionizing radiation to get the reaction going. All of those things are available in the ISM. The ionizing radiation there is cosmic rays, particles from distant sources that zip through space at high speed. And astronomers have observed water in the ISM that is highly enriched in deuterium, so that could be source of the solar system’s water.

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