One of the enduring puzzles of our planet is why it is so wet.
Since Earth formed relatively close to the sun, planetary scientists have generally held that any of the water in the building blocks of early-forming Earth was baked out and so was unavailable to make oceans or our atmosphere.
That led to theories explaining the oceans and wet atmosphere of Earth as a later addition, brought to us by meteorites and comets formed beyond the solar system’s so-called “snow line,” where volatile compounds such as water can begin to condense into ice.
This snow line is a general area between Mars and Jupiter, and that means under this theory that our water would have had to come from awfully far away. Further complicating this view is that the isotopic makeup of that distant water ice is somewhat different from much of the water on Earth.
Now, a new paper in the journal Science from Laurette Piani of the Université de Lorraine and colleagues, argues that Earth’s water was simply acquired like most other of our materials, through accretion when the planet formed in the inner solar nebula.
To reach that conclusion, the group re-examined 13 meteorites of the parched type formed between Earth and the sun, and they found more than of enough hydrogen present to explain how Earth got so wet (wet for our solar system, that is.)
In fact, they extrapolated from their data that enough water was available in the nebular cloud that accompanied the formation of our sun and formed those early meteorites — called enstatite chondrites — to create three times as much water as our oceans hold.
“Our discovery shows that the Earth’s building blocks might have significantly contributed to the Earth’s water and that hydrogen bearing material was present in the inner solar system at the time of the Earth and rocky planet formation, even though the temperatures were too high for water to condense,'” Piani told me.
“The material that formed Earth in the inner solar system did contain hydrogen in its minerals and could have greatly contributed to the planet’s water budget.” That often invoked contribution to the Earth’s water budget from comets or hydrated asteroids from the outer solar system “might have been very limited.”
The meteorites they studied are chondrites, stony bodies that were formed as the sun and solar system came into being and have not been modified,by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early solar system accreted –the process by which solids agglomerate to form larger and larger objects — to form primitive asteroids.
Enstatite chondrites from the inner solar system that have fallen to Earth are known as among the “driest” rocks known — with vanishingly little of the “water” that can be found locked inside the molecular structure of other rocks. (Geological “water” is actually hydrogen that, when in the presence of oxygen, can become water.)
Piani said that because of the assumed depletion of hydrogen in enstatite chrondrites, this class of meteorites has not be well studied for the presence of hydrogen, and thus tended to be ignored as a source for the Earth’s water.
“Some measurements were nonetheless performed in the past but the results were very inconsistent — from 0.07 to 5 weight % of water were reported,” she said. “This is because only a few pristine ECs that were not altered on their asteroid nor on Earth exist.”
Piani is referring to the additional problem of potential contamination of the meteorite once it lands on Earth. Our planet is, after all, a wet place and it is difficult to tease out what signatures of water are age old and which were picked up on arrival to our planet. Piani said that in her study scientists used special analytical procedures to avoid being biased by the presence of water from Earth.
Following further analyses involving modeling of Earth’s formation that involved mixing of chondrite-like materials, the authors came up with that estimate that the EC-like materials that coalesced during the planet’s early formation could have delivered enough hydrogen to the growing proto-Earth to provide at least three times the amount of water in Earth’s present-day oceans.
Piani and colleagues also measured deuterium/hydrogen (D/H) ratio of the meteorites — an isotopic marker of “lighter” or “heavier” water. They found that enstatite chondrites carried a hydrogen isotopic signature closely aligned with that found in the Earth’s mantle, and in that sense a more plausible water source than those outer solar system comets and meteorites with a different D/H ratio.
This additionally suggests, she says, that the origin of Earth’s water lay within the rocks from which the planet was built — the materials that coalesced during the planet’s early formation.
E-type chondrites are also among the most oxygen deprived rocks known, with most of their iron taking the form of metal or sulfide rather than an oxide. But Piani said that there were times on early Earth when some oxygen was present from other sources, so the addition of hydrogen in inner solar system asteroids made a wet world possible.
Enstatite chondrites contain substantial amounts of the mineral enstatite (MgSiO3), from which they derive their name.
Based on spectral analysis, it has been suggested that the asteroid Psyche — the “metal” meteorite that was the subject of a recent Many Worlds column — may contain substantial amounts of the mineral and thus may be a source of this type of meteorite.
The Piani paper was accompanied by a commentary by Anne Peslier of NASA’s Johnson Space Center.
She found the analysis to be convincing, especially since it avoids the explanation that water came from afar via comets an asteroids.
“A water-rich type of chondrite called carbonaceous chondrite was suggested to have brought most of the planet’s water. Carbonaceous chondrites, however, are not ideal because they have a very different isotopic composition than Earth and formed outside of the snow line ”
“To reconcile these apparent inconsistencies, carbonaceous chondrite–like material was proposed to have been added to Earth from beyond the snow line when the planet was already mostly formed. For these chondrites to pelt the early Earth, either the formation of Jupiter or its inner solar system wandering has been suggested to have disturbed the orbits of asteroids, ejecting them from the outer part of the asteroid belt.”
“Piani et al. offer a simpler explanation,” she wrote. “The authors’ work brings a crucial and elegant element to this puzzle” of how the planet became wet, “Earth’s water may simply have come from the nebular material from which the planet accreted.”