Water has been fundamental to the rise of life on Earth. At the same time, it is a fundamental parameter for evaluating the possibility of life on other planets. Identifying the origin of water on Earth is the key to understanding how environments that favor life arise and how likely they are to be found elsewhere.

A new study, published in Science (ref.), by a team including Conel Alexander from Carnegie, has discovered that much of the water in our Solar System likely originated as ice that formed in interstellar space. In other words, Earth ‘s water is older than our star, the Sun.

Where does the water in the Solar System come from?

Water is found throughout our Solar System. Not just on Earth, but also on comets and icy moons, and in the shadowed basins of Mercury. It has also been found included in mineral samples from the Moon and Mars. Comets and asteroids, in particular, primitive objects, provide a natural time capsule of the conditions during the early days of our Solar System. Their ices tell of the ice that surrounded the Sun after its birth, whose origin was until now an unanswered question.

In its youth, the Solar System was surrounded by a protoplanetary disk, the so-called solar nebula, from which the planets were born. It was unclear to researchers whether the ice in this disk had originated from the interstellar molecular cloud or whether this interstellar water had reformed from the chemical reactions that took place in the solar atmosphere.

“Why is this important? If water in the early Solar System was primarily inherited as ice from interstellar space, then it is likely that similar ices, together with the prebiotic organic matter they contain, are abundant in most or all protoplanetary disks around forming stars” said Alexander. “But if the water in the early Solar System was largely the result of local chemical processes during the Sun’s birth, then the abundance of water may vary considerably in the formation of planetary systems. This would obviously have implications for the potential emergence of life elsewhere”.

Results from computer models

In studying the history of ices in our Solar System, the team, led by L. Ilsedore Cleeves of the University of Michigan, focused on hydrogen and its heavier isotope, deuterium. Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. The difference in mass between isotopes results in subtle differences in their behavior during chemical reactions.

The ratio of hydrogen to deuterium in water molecules can indicate to scientists the conditions under which the molecules formed. Interstellar water ice has a high ratio of deuterium to hydrogen because of the very low temperatures at which it forms. Until now, it was not known how much of this deuterium enrichment was lost during the Sun’s birth. At the same time, we did not know how much deuterium-rich ice was present in the newborn Solar System.

The team, therefore, created models that simulate a protoplanetary disk. The test verified whether the deuterium-to-hydrogen ratios found in meteorite samples, Earth’s water, and comets could be reached. Since the Solar System could not independently achieve the ratio between the two elements, some of the water on Earth originated in interstellar space and is older than the Sun. “Our findings show that a significant fraction of the water in our Solar System is older than the Sun. It means that abundant interstellar ices rich in organic substances should be found in all young planetary systems” says Alexander.

Stefano Gallotta
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