Earth's water older than the sun, came from interstellar ice

The water in our solar system -- including up to half of Earth's supply -- has actually been here since before the birth of the sun, according to new research.

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Illustration depicting water in the solar system. Bill Saxton, NSF/AUI/NRAO

Somewhere between 30 and 50 percent of the water in the solar system -- including the water on Earth as well as the ice in comets, the discs around Saturn, meteorites and other planets -- was around before the birth of the sun, according to research conducted by astronomers at the University of Michigan.

In a paper published in the journal Science, the team explains that the water originated in the molecular cloud -- the nebula -- that gave birth to the sun, and predates the solar system by around a million years.

In the way of stars, the sun was born from a nebula, a cloud of dust and gas floating in space. As the nebula increased in density, its gravity would have caused it to collapse in on itself, forming a rotating ball of gas. As this ball cools, it becomes denser and its spin increases, and the gas and matter of the nebula around it form a flattened disc of material that swirls into the star's gravitational pull.

This disc is called the accretion disc -- or the protoplanetary disc. As the gas in the centre of the disc stabilises into a fully grown star, so too does the disc stabilise and coalesce into discrete planets and asteroids. A delicate balance of gravitational force (the sun's gravity attracting objects towards the sun) and centripetal force (the resisting force or the planet's spin around the sun -- think of spinning a ball on a rope) hold these objects in the solar system's orbit.

What the researchers sought to discover was whether the water was already extant in the sun's parental nebula, or whether the birth of the solar system also birthed the water within it.

"Why this is important? If water in the early Solar System was primarily inherited as ice from interstellar space, then it is likely that similar ices, along with the prebiotic organic matter that they contain, are abundant in most or all protoplanetary disks around forming stars," said Carnegie Institution for Science's Conel Alexander, who contributed to the research.

"But if the early Solar System's water was largely the result of local chemical processing during the Sun's birth, then it is possible that the abundance of water varies considerably in forming planetary systems, which would obviously have implications for the potential for the emergence of life elsewhere.

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sunrise image by Thangaraj Kumaravel, CC BY 2.0

To figure out where the water originated, the research team simulated the chemistry of the forming solar system -- a model that contained two kinds of water, regular water containing hydrogen, and "heavy" water containing the isotope deuterium, which has a different number of neutrons from hydrogen. Heavy water can be found on comets and in the oceans of Earth, and interstellar water has a high ratio of deuterium because of the cold conditions under which it is formed.

They went back to the formation of the solar system and simulated a protoplanetary disc without frozen heavy water, meaning that the solar system has to create heavy water from scratch. They discovered that the system could not achieve the ratios of deuterium found in samples of water from comets, meteorites and Earth's oceans.

"The implications of these findings are pretty exciting," said study co-author Ilse Cleeves. "If water formation had been a local process that occurs in individual stellar systems, the amount of water and other important chemical ingredients necessary for the formation of life might vary from system to system. But because some of the chemically rich ices from the molecular cloud are directly inherited, young planetary systems have access to these important ingredients."

"Our findings show that a significant fraction of our Solar System's water, the most-fundamental ingredient to fostering life, is older than the Sun, which indicates that abundant, organic-rich interstellar ices should probably be found in all young planetary systems," Alexander said.

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