Our blue planet having water seems such a basic and obvious fact that the question of
why Earth has water at all seems unimportant.
However, the origin of this vital component of life has long been a source of contention.
For a while, it's believed that Earth's water did not originate from the planet's region
of the protoplanetary disk.
Instead, it was hypothesized water and other volatiles must have been delivered to Earth
from the outer Solar System later in its history.
According to models of Solar System formation, Earth, being an inner Solar System planet,
should contain little or no water.
Early models of planetary formation predicted that the nebular gas near our young Sun was
too hot to form ice.
As a result, water as vapor could not have easily been incorporated into the materials
that formed the inner rocky planets Mercury, Venus, Earth, and Mars.
Only the outer Solar System planets, such as Jupiter, have an abundance of water and
other volatiles because they developed outside the "snow line", the hypothetical boundary
between water vapor and water ice in the solar nebula.
These planets formed far enough away from the Sun to be able to incorporate frozen water
easily.
Earth not only contains water in its oceans and atmosphere, but it also has the equivalent
of multiple oceans of water trapped within its deep inner rocks.
Scientists can study special kinds of meteorites formed at the beginning of the Solar System
to help solve this puzzle.
These space rocks, known as chondrites, have not been chemically modified and incorporated
into a giant planet.
The isotopic makeup of Earth shows that enstatite chondrite–like materials are the primary
building blocks of rocky planets.
Nonetheless, Enstatite chondrites were thought to be too dry to account for Earth's water.
Instead, carbonaceous chondrite, a water-rich kind of chondrite, was believed to have brought
most of the planet's water.
On the other hand, carbonaceous chondrites are not ideal because they formed outside
of the snow line and have a considerably different isotopic composition than Earth.
To explain these seeming contradictions, carbonaceous chondrite–like material is claimed to have
been brought to Earth from beyond the snow line when the planet was already mostly formed.
The birth of Jupiter or the inner Solar System wandering is assumed to have disrupted the
orbits of asteroids, ejecting them from the outer section of the asteroid belt, causing
these chondrites to pummel the early Earth.
In addition, the composition of the water in meteorites doesn't precisely match that
of Earth's: the extra-terrestrial version contains more deuterium, a heavier form of
hydrogen, indicating that there must have been another source of the water on our planet.
Recently, an international team tried to unravel the missing source of Earth's water: the Sun.
Curtin University researchers have contributed to the long-standing mystery of the origin
of the Earth's water, revealing the Sun as a surprising potential source.
According to a team of international researchers led by the University of Glasgow, including
researchers from Curtin's Space Science and Technology Center (SSTC), the solar wind played
a role in forming water.
The solar wind, which is made up of charged particles from the Sun that are mostly hydrogen
ions, created water on the surface of dust grains carried on asteroids that smashed into
the Earth during the Solar System's early days.
The Earth has a lot of water compared to other rocky planets in the Solar System, with oceans
covering more than 70% of its surface, and scientists have been trying to figure out
where it all comes from for a long time.
This new solar wind theory is based on an atom-by-atom examination of tiny shards of
the Itokawa asteroid, an S-type near-Earth asteroid retrieved by the Japanese space probe
Hayabusa and returned to Earth in 2010.
The scientists used Curtin University's world-class atom probe tomography system to get an incredibly
detailed look inside the first 50 nanometres or so of the surface of Itokawa dust grains,
which they discovered contained enough water to amount to about 20 liters per cubic meter
of rock if scaled up.
Dr. Luke Daly, a Curtin graduate who now works at the University of Glasgow, said the findings
not only provide scientists with a unique insight into the ancient source of Earth's
water, but it could also aid future space missions.
One of the challenges of future space exploration is how astronauts would get enough water without
carrying supplies.
The same space weathering process that formed water on Itokawa most certainly occurred on
other airless planets, implying that astronauts may be able to process fresh supplies of water
directly from the dust on a planet's surface, such as
the Moon.