One of the best known photographs of Earth, this image was taken by the crew of the final Apollo mission as the crew made its way to the Moon. Named the “Blue Marble,” the image highlights how much of the planet is covered by water — 71 percent of the surface. How this came to be remains an open scientific question.
(NASA)
Theories abound on how Earth got its water.
Most widely embraced is that asteroids, and maybe comets, crashed into our planet and released the water they held — in the form of ice or hydrated minerals in their crystal structures — and over time water became our oceans. The inflow was especially intense during what is called “the Late Heavy Bombardment,” some 4 billion years ago.
The isotopic composition of our water is comparable to water in asteroids in the outer asteroid belt, and so it makes sense that they could have delivered the water to Earth,
But there is also the view Earth formed with the components of water inside the planet and the H₂O was formed and came to the surface over time. Several hydrous minerals in our mantle store the necessary elements to create water and in this theory the pressure from hot magma rising up and cooler magma sinking down crushes this hydrous material and wrings them like a sponge. Water would then find its way to the surface through volcanoes and underwater vents.
Now a new model has been proposed and it has a novel interest because it originates in the discovery of thousands of exoplanets in the past quarter century.
This new approach, described by Anat Shahar of the Carnegie Institution for Science and colleagues from UCLA in the journal Nature, says that Earth’s water could have come from the interactions between of a very early and primarily hydrogen atmosphere and the scalding ocean of magma that covered the planet.
That the planet could have had a thick hydrogen atmosphere that wasn’t quickly destroyed is a new idea and it comes from the finding that many so-called “super-Earth” exoplanets have, or had, such an atmosphere. While super-Earths are larger and more massive than Earth, many are rocky, terrestrial planets and so share characteristics with our planet.
“Exoplanet discoveries have given us a much greater appreciation of how common it is for just-formed planets to be surrounded by atmospheres that are rich in molecular hydrogen, H2, during their first several million years of growth,” Shahar said. “Eventually these hydrogen envelopes dissipate, but they leave their fingerprints on the young planet’s composition.”
An illustration showing how some Earth’s signature features, such as its abundance of water and its overall oxidized state could potentially be attributable to interactions between the molecular hydrogen atmospheres and magma oceans on the planetary embryos that comprised Earth’s formative years. (Illustration by Edward Young/UCLA and Katherine Cain/Carnegie Institution for Science.)
As Shahar explained to me, under their new model our oceans would have been filled due to basic thermodynamic and chemical forces at work on baby Earth:
As part of the process by which our star was formed, a disk of leftover molecular hydrogen, helium and dust grains surrounded and began to orbit it.
The same dynamics are at play in exoplanet solar systems and scientists now have evidence that an “envelope” or atmosphere of hydrogen forms around some larger and some smaller super-Earth exoplanets early in their formation and can remain in place for millions of years. And for the planets of medium size, she said, the current thinking is that the hydrogen-rich atmosphere just dissipates too early to detect.
Anat Shahar is a staff scientist at the Earth and Planets Lab at the Carnegie Institution for Science. She specializes in the study of solid planets and their cores. (Carnegie Institution for Science.)
The question that Shahar and her colleagues asked is whether Earth also once had a hydrogen-rich atmosphere that remained for some time. If an early version of Earth had sufficient gravity to keep that atmosphere in place, then what role might it have played in defining some basic characteristics of the planet?
Their conclusion was that atmospheric hydrogen could and would have interacted with other gases in the hot ocean of magma that was then on the planet’s surface (and is hypothesized to exist on all newborn rocky planets before solid crusts can be formed.)
Atmospheric hydrogen can only dissolve into a liquid, and that’s precisely the phase of the liquid-hot magma ocean. Since the magma ocean would have been oxidized, that interaction would have brought together the hydrogen and oxides in the magma and forming mantle. Water would have been the inevitable result.
Shahar says that other water could well have been delivered later via asteroids, but the interaction between the molecular hydrogen early atmosphere and the magma ocean could have produced as much water as the planet needed. More than 70 percent of its surface is oceans, glaciers, lakes and water in other forms.
“Our model shows that at least one ocean of water could have been formed this way and with some tweaking it could be significantly more,” she said. “So no other water had to be delivered — though it may have been.”
What’s more, Shahar said, a hydrogen-rich atmosphere would act as a kind of thermal blanket, keeping the magma ocean from more quickly solidifying into a crust. That could in turn keep the hydrogen envelope in place for a longer time, allowing for the formation of more water.
About 4.5 billion years ago, in the early days of Earth’s existence, much of the surface was a bright hot ocean of magma, consisting of a mixture of molten or partially-molten rock, volatiles (dissolved gas and gas bubbles) and solids (suspended crystals). The magma behaves as a liquid, which means that the quantity of solids floating in the system is not enough for the solid particles to connect with each other. The photo was in taken during the eruption in 1954 of the Kilauea Volcano. (J. P. Eaton)
The hypothesis underlying Shahar and her colleague’s work — that a hydrogen atmosphere could have lasted around Earth for a significant period of time — has been generally discounted until recent years.
The consensus view was that the hydrogen quickly burned off around all our solar system’s relatively slow-growing rocky planets, while remaining for a longer time around the gas and ice giants in the outer solar system. Without a longer-lasting early hydrogen atmosphere, the question of how the planet got so wet seemed to require an answer involving asteroids and the ice and hydrated minerals they would have carried.
In today’s exoplanet era, however, planetary scientists have learned that many solar systems have types of planets that we do not have, and that could shed some new light on that consensus view about hydrogen atmospheres and rocky planets.
These planets are the “super-Earths” mentioned above. They are larger and more massive than Earth but smaller and less massive than the planet next in size in our solar system — Neptune. They are abundant in the Milky Way.
Planets in this size range can be rocky or they can be gaseous, depending on how large they are. Observations over the past three decades of exoplanets have shown that hydrogen-rich atmospheres are likely present for some period of time around many super-Earths (and the larger, gaseous sub-Neptunes.)
Using those observations and insights from exoplanet study, Shahar and colleagues propose that Earth also could have had a period surrounded by a hydrogen-rich atmosphere, and that it might have lasted long enough to play a central role in the ultimate composition of the planet. All that was needed was for the proto-Earth to be large enough to hold onto the hydrogen atmosphere for a longer period, as newborn super-Earths can do.
She said that using knowledge from exoplanet science to help understand possible dynamics in our solar system is likely to become far more common in the future. That’s because as more is known about how the often wildly varied worlds of exoplanets and their solar systems evolve, that knowledge will be fed back into efforts to refine our understanding of how Earth and the other planets in our solar system became what they are now.
Hilke Schlichting, a UCLA professor of Earth, planetary, and space sciences and one of the researchers on the project put it succinctly in a release. “Our work,” she said, “shows that we can learn a surprising amount about Earth and our own past from the countless exoplanets that dot our galaxy.”
The team is now looking to go deeper. They next plan to explore the chemistry of exoplanet atmospheres in even greater detail, which could yield even more insights into our planet and the galaxy as a whole.
This image from the European Southern Observatory’s Atacama Large Millimeter Array shows the very early stages of the formation of the star HL Tauri and the protoplanetary disk surrounding it. Estimated to be less than one million years old, any planets that might be forming in the disk could be surrounded by a hydrogen atmosphere. The star is 240 light-years away. (ESO/NAOJ/NRAO)
To reach their conclusion, Shahar, lead author Edward Young of UCLA and colleagues started by collecting what has been learned in recent decades about rocky super-Earths and their early hydrogen atmospheres.
The researchers then used mathematical modeling to explore the exchange of materials between molecular hydrogen atmospheres and magma oceans by looking at 25 different compounds and 18 different types of reactions—complex enough to yield valuable data about Earth’s possible formative history, but simple enough to interpret fully.
Interactions between the magma ocean and the atmosphere in their simulated very young Earth resulted in the movement of large masses of hydrogen into the metallic core, the oxidation of the mantle, and the production of large quantities of water. This was all based on thermodynamics — the branch of physical science that deals with the relationships between all forms of energy.
In a release, Young said that if most of the water on Earth was created here and if many of the large Earth-like planets in the galaxy were formed under similar conditions, then it stands to reason that there might be a lot of Earth-like planets out there with sufficient water for life.
Earth’s protoplanets are often thought to have had the mass of Mars or smaller . Young et al. propose that one Earth protoplanet had sufficient gravity to retain a hydrogen atmosphere that would have interacted with the magma ocean on its surface. Hydrogen gas could have mixed into Earth’s mantle before it solidified, leading to the production of iron oxide, and also entered its metallic core. Incorporation of hydrogen into the core and the oxidation of hydrogen in the atmosphere (triggered by evaporation of oxides in the magma ocean and mantle) could have led to the production of a large fraction of Earth’s water. (Nature)
The model put forward in the Nature paper not only can explain the presence of water on Earth but can also answer another of the perplexing questions about the formation of our planet: Why is there less mass in the iron core than calculations say that there should be?
A few scientists had already suggested that the answer could be that light hydrogen made its way down to the core and bonded with the liquid iron and other elements. Shahar said that the group’s model offers a explanation of how hydrogen might make it down to the core, just as the model produces water and an oxidized surface.
“Many of the specific conclusions we reached have been put forward by others,” Shahar said. “But we are the first to put then all together and make the larger model that shows how a hydrogen-rich atmosphere and a magma ocean could have produced these signature features.”
In a commentary accompanying the Nature paper, Sean Raymond of the University of Bordeaux writes that “the sequence of events put forward by the authors is so intuitive that one might wonder whether it is, in fact, generic. And if the authors’ model can also be applied to the known exoplanets, there is hope that it could be tested.”
“Of course,” he wrote, “it’s worth keeping in mind that other solutions already exist for each of the problems that the authors attempt to solve….{And} the comprehensive nature of the authors’ model might prove to be a weakness.”
“For instance, evidence that hydrogen is not the light element responsible for the density deficit {in the core} would compromise the model, as would a revision to the critical proto-planet mass required for a long-lived magma ocean.”
“Despite these uncertainties, the authors have demonstrated that early interactions between magma oceans and atmospheres represent a key ingredient in future models of how Earth was shaped.”
