A team of researchers led by astronomer L. Ilsedore Cleeves of the University of Michigan, Ann Arbor studying the origin of water in the solar system concluded that up to half of it formed before the sun was created(1). This result may suggest that water is more widespread in planetary systems than previously thought.
Our solar system has plenty of water. In addition to Earth, water is found on the moon, Mercury, Mars, comets, and the icy moons of gas giants such as Jupiter and Saturn. But where did this water come from? Water is known to form in the clouds of gas and dust of the interstellar medium (ISM) from which planetary systems are created. But is the water destroyed when newly formed stars start generating heat and light and formed again later, or does the original water survive and remain today?
An image of a region of interstellar dust in the Orion system.
Cleeves and colleagues sought to answer this question by focusing on deuterium, a heavy form of hydrogen created along with normal hydrogen during the big bang. In the universe, there are approximately 26 deuterium atoms for every million hydrogen atoms. But in the water found in the solar system, deuterium is six times as prevalent. Scientists conclude that when water was formed, the reaction creating deuterium-rich “heavy water” was faster than the one creating normal water, so the proportion of deuterium in water increased.
However, the enrichment of deuterium happens only under certain conditions: it has to be extremely cold (only a few tens of degrees Kelvin), and oxygen and some sort of ionizing radiation are required to start the reaction. All of these are available in the ISM, and astronomers have observed “heavy water” in the ISM. This medium, then, could be the source of the solar system’s water.
Nevertheless, there remains the question of whether this interstellar water could survive the sun’s birth. To find out, Cleeves and colleagues wanted to determine whether the same water-forming reactions could have occurred after the sun’s formation, in the protoplanetary disk of gas and dust from which planets form. The team constructed a model of the chemical processes creating water in a protoplanetary disk and found that the ionizing radiation present does not produce “heavy water” fast enough. As Cleeves reports, “we found that heavy water didn’t form in any abundance over a million years” (1).
The team estimates that up to 50% of the water now on Earth may have existed before the sun’s formation 4.5 billion years ago. This is good news for other planetary systems, in that water likely exists everywhere, waiting for planets to form(2). It is also good news for hunters of extraterrestrial life.
Sources:
1. Clery, D. (2014, September 25). Half of Earth’s water formed before the sun was born. Science/AAAS | News – Up to the minute news and features from Science. Retrieved September 28, 2014, from http://news.sciencemag.org/
2. Cleeves, L. I., et al. (2014). The ancient heritage of water ice in the solar system. Science, 345(6204), 1590-1593. Retrieved September 28, 2014, from http://www.sciencemag.org/content/345/6204/1590.short