As much as half of the water in Earth’s oceans could be older than the Sun, a study has found.
By reconstructing conditions in the disk of gas and dust in
which the Solar System formed, scientists have concluded that the Earth and
other planets must have inherited much of their water from the cloud of gas
from which the Sun was born 4.6 billion years ago, instead of forming later.
The authors say that such interstellar water would also be included in the
formation of most other stellar systems, and perhaps of other Earth-like
planets.
The dense interstellar clouds of gas and dust where stars
form contain abundant water, in the form of ice. When a star first lights up,
it heats up the cloud around it and floods it with radiation, vaporizing the
ice and breaking up some of the water molecules into oxygen and hydrogen.
Until now, researchers were unsure how much of the 'old'
water would be spared in this process. If most of the original water molecules
were broken up, water would have had to reform in the early Solar System. But
the conditions that made this possible could be specific to the Solar System,
in which case many stellar systems could be left dry, says Ilsedore Cleeves, an
astrochemist at the University of Michigan in Ann Arbor, who led the new study.
But if some of the water could survive the star-forming
process, and if the Solar System’s case is typical, it means that water “is
available as a universal ingredient during planet formation”, she says.
To find out, Cleeves and her colleagues modelled the
conditions soon after the Sun lit up. They calculated the amount of radiation
that would have hit the Solar System, both from the young star and from outer
space, and how far that radiation would have travelled through the cloud.
Those conditions determine how new water molecules form from
hydrogen and oxygen, and in particular the odds that the molecules include
deuterium, an isotope of hydrogen whose nucleus contains a neutron, in addition
to the usual single proton. The model predicted an abundance of
deuterium-containing water, also known as heavy water, that was lower than that
in the Solar System’s water today.
But the interstellar clouds where Sun-like stars are
currently forming — and thus, presumably, the material from which the Sun
formed — have a higher proportion of heavy water compared to the current Solar
System. This is because these clouds are subject to the continuous bombardment
of cosmic rays, which tend to favour the inclusion of deuterium. Therefore, the
authors concluded, the young Sun’s radiation was insufficient to account for
the amount of heavy water seen in the Solar System today, and some must have
existed before. They estimate that somewhere between 30% and 50% of the water
in Earth’s oceans must be older than the Sun.
“If the disk can’t do it, that means we must have inherited
some level of these very deuterium-enriched interstellar ices from the birth
environment of the Sun,” says Cleeves. The study was published in Science.
Ewine van Dishoeck, an astrochemist at the Leiden
Observatory in the Netherlands, says that the study’s conclusions are based on
good arguments but are still only theoretical. But confirmation could come next
year, she adds, when the Atacama Large Millimeter Array, a radio telescope in
Chile’s Atacama Desert, begins to study the chemical processes underlying the
proportion of heavy water in protoplanetary disks.
Even if the formation of typical stellar systems does not destroy all of the pre-existing water, it does not mean that water-drenched planets need to be the norm throughout the Universe. Venus and Mercury have no water, and Mars seems to have lost most of the water it once had — and it is still unclear what determines whether a planet gets to become wet and to stay that way, says Cecilia Ceccarelli, an astronomer at the Institute of Planetology and Astrophysics in Grenoble, France.
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