Oceans from the sky / David Jewitt and Edward D. Young

New findings are reigniting the debate over whether comets, asteroids, or completely different factors are the source of seawater on Earth

When you stand on the beach and look at the waves coming from the horizon, it is easy to think that the sea is an eternal thing. Our ancestors must have thought so. In many creation stories, the watery abyss was before the land and even before the light. Today we know that Earth's global ocean was not always there. Its waters, as well as every drop of rain, every gust of moist air and every sip from a glass, are a reminder of an event that happened eons and eons ago: when the sea fell from the sky.
All water in our solar system ultimately originates from a primordial cloud of gas and dust that collapsed to form the Sun and planets more than four and a half billion years ago. The cloud was rich in hydrogen and oxygen, the two basic components of water, H2O. This wealth is not surprising, since hydrogen and oxygen are among the most common elements in the universe in general: hydrogen is the most common and oxygen is in third place (helium, a noble gas that does not react in chemical reactions is the second). Most of the gas that was in the primordial cloud was absorbed by the sun and the giant gas planets, which formed before the rocky planets. A large part of the remaining oxygen binds to other elements, such as carbon and magnesium, but the remaining hydrogen and oxygen were able to react with each other and create an amount of water many times greater than the amount of rock in our solar system.

However, the observations show a different picture. Earth and its neighbors Mercury, Venus and Mars are rocky and non-water worlds. Their relative dryness is a result of the place and time in which they were created. When the cloud that was about to become our solar system collapsed, its angular momentum flattened the material into a swirling disk, where the planets formed. The formation of rocky worlds is considered a gradual process that occurs step by step, when relatively small objects in the disk collide with each other and stick together to form a larger object: microscopic grains aggregate into small stones and some into rocks and some into building blocks several kilometers in diameter from which planets are built. These building blocks are called "planetesimals". Many of the planetesimals that remained after the formation of the planets were the heavenly bodies we call today asteroids and comets.
In the innermost parts of the disk, near the sun, the gas became very hot as a result of friction and proximity to the sun and the hydrogen and other light elements evaporated. The planets closest to the Sun were thus formed from the relatively dry material that remained. While the dry, rocky sky bodies near the Sun grew farther away, roughly where Jupiter and the asteroid belt are today, the temperature of the disk was low enough to freeze the water and other volatiles. Astronomers call this threshold the "snow line". It is widely believed that most of the water on Earth arrived beyond this line, in showers of icy asteroids and comets that were possibly hurled into the inner regions of the solar system by the large outer planets in the final stages of the planet formation process.

Evidence for snowlines and late-stage interplanetary collisions has recently been obtained from observations of other solar systems whose planets are still forming. When you look through a telescope into the depths of interstellar space, you can see in the distance the same primordial processes that took place here in our solar system. Still, many aspects of the wondrous act of the formation of the Earth's oceans are still shrouded in fog and being vigorously explored. Even if Earth's oceans seem eternal and unchanging, new evidence brings us closer to answering the question of exactly when and how they formed, and whether it was comets, asteroids, or a completely different mechanism that brought all that water to our once-dry planet.

A dry-as-paper marine planet

Viewed from space, one might think that the most appropriate name for the Earth is actually the "Ocean Globe". Water covers more than two thirds of its surface and makes up many of the creatures that live on it. The oceans, which average four kilometers deep, contain enough water to fill a sphere with a diameter greater than 1,300 kilometers. Still, many are surprised to learn that all the water in the oceans occupies only 0.02% of the Earth's mass. That is, if the Earth were a Boeing 777 weighing 300 tons, then the mass of all the water in the oceans would be the same as one passenger. The fresh water, stored in the little ice at the poles, in the clouds, in the rivers, in the lakes, in the soil and in the animals and plants on the surface of the globe is a zero part of it.

There is more water deep below us, within the mantle of molten rock of the planet, which is about 3,000 kilometers deep, from below the solid crust to the bottom of the liquid iron core. The water there is not liquid. They are a percentage of the molecular structures of "hydrated" rocks and minerals that were dragged under the crust by tectonic processes. Even if some of this water, trapped in rocks, can break out of the mantle and return to the ground through volcanoes, most of it is buried there. Even deeper, there is the inner core, solid and heavy, made of an alloy of iron and nickel. It is possible that the core, whose mass is about 30% of the Earth's mass, contains even more water than the mantle, in the form of hydrogen which, if not for the great heat and pressure, would have bonded to oxygen.

No one knows how much water there is inside the earth. Because we cannot sample there and the estimates of the amount of water passing from the surface of the ground inward and vice versa are poor. It is likely that the mantle alone contains as much water as the ocean system, essentially doubling the global water supply. But even so, if we add this water to the surface of the ground, we only have 0.04% of the mass of the Earth, or two passengers in a loaded Boeing 777. It may sound strange, but the Earth is about 100 times drier than an old bone, which contains only a tiny amount of water. Still, the question of how the little water we have got here must be answered.

Comets or asteroids?
Since it is widely believed that at the beginning of its existence the earth was even drier than it is today, researchers dealing with the source of water in the world focused on the relatively late stages of its formation, after the moon was formed.

The surface of the young Earth, like the other rocky planets in the solar system, must have been still molten, at least in part, tens of millions of years or more after its birth. The melting was the result of the tremendous energy that flowed into the planet through swarms of mountain-sized planetesimals that fell upon it. There is indeed geochemical evidence that the Earth's ocean of molten rocks (magma) contained a certain amount of water, but molten and hot rock does not excel at preserving them, so most of the moisture of the ancient Earth and the planetesimals must have been released in the form of ionized gas and steam. Some of this material was lost in space, but some of it may have fallen back to Earth and become trapped again in rock before being swallowed deep into the mantle.

Later, more huge impacts changed the water supply on and near the surface of the ground. In particular, Earth appears to have collided with a Mars-sized object about 4.5 billion years ago, a collision that blew up a cloud of material that cooled, coalesced, and became the Moon. The energy of this global collision removed most of the atmosphere, boiling at once any ocean of water that existed and creating an ocean of magma hundreds of kilometers deep. If the earth was dry when it was formed and if it contained water, this fatal blow that created the moon drained it of all the water it originally had.

Based on this knowledge, scientists have long been trying to find a source of water that reached the Earth after the formation of the Earth-Moon system and its cooling. Since the 50s it has been known that comets are rich in ice. They enter the inner regions of the solar system from two large clusters located on the outskirts: the Kuiper Belt (starting near the current orbit of Pluto) and the Oort Cloud (starting far beyond the Kuiper Belt and reaching perhaps half way to the nearest planet to us). Perhaps, many researchers believed, comets are the main source of water on Earth.

But this idea ran into difficulties in the 80s and 90s, when researchers first measured the ratio of the two common isotopes of hydrogen in comets from the Oort cloud. The nucleus of normal hydrogen (H) has one proton, while the nucleus of the heavier isotope, deuterium (D), has one neutron in addition to the proton. The relative abundance of deuterium compared to normal hydrogen is a useful fingerprint that can be used to trace the history of a celestial element. If the Earth's ocean was made of comets that had melted, the ratio between deuterium and hydrogen (D/H) in the water molecules found in it should be close to that in the comets observed today. But in comets in the Oort cloud, ratios twice as large as those in normal seawater were found. It is therefore clear that the origin of most of the water on earth is different.

However, in recent years, D/H ratios have been measured in Kuiper belt comets with values ​​similar to those of the Earth's ocean, which confirmed the belief that comets brought the water to it. But now the pendulum is once again swinging away from the comets. At the end of 2014, findings from the "Rosetta" spacecraft launched by the European Space Agency showed that the D/H ratio in comet 67P/Churyumov-Grasimenko is three times greater than that of the ocean, a point for the argument that the extraterrestrial source of water is not comets. This result, together with arguments based on dynamic analyzes of the trajectories of objects falling on Earth from areas rich in comets, indicate that even if there were here and there comets that brought water to Earth, it is unlikely that this was the main mechanism of bringing them.
Asteroids are the obvious alternative, and today there is a broad consensus that they are the source of water on Earth. Like comets, asteroids are also pieces of small planetesimals from which the planets were made. The asteroids of the "main belt", the asteroids in their orbit between Mars and Jupiter, are much closer to the Earth than the Kuiper belt, and when they go out of orbit, their chances of hitting the country are much greater. The proof of this is found on the moon, which is covered with craters from ancient asteroid impacts. Meteorites, fragments of rock that reach the Earth's surface and originate from asteroids, also fill our nature museums as a poignant reminder that the Earth is still bombarded from space. The study of the rare pieces of asteroids provides a glimpse into their history and helps determine whether they are indeed the ones that filled the Earth's ocean with water. Studies of certain types of meteorites have already shown that their D/H ratios match that of seawater.

Meteorites, like the miners of their quarry, the asteroids, are characterized by a variety of compositions and different water contents. Asteroids from the region closest to us in the main belt, whose distance from the Sun is approximately twice the distance from the Earth, produce most of the rocky, water-poor meteorites that we study on Earth. On the other hand, asteroids on the far side of the belt, more than halfway between us and Jupiter, contain a certain amount of water. These asteroids have a tendency to produce meteorites called carbonaceous chondrites, inclusions of hydrated minerals and carbons (carbonates), in which water can make up a few percent of the rock's mass. The history of the water in these rocks was at the heart of a study by one of us (Young), a study based on observations of water and its passage through rocks here on Earth. The water-rich minerals in the carbonaceous chondrites grew from reactions of rocks with liquid or gaseous water, reactions that occur at relatively low temperatures of a few hundred degrees Celsius. On Earth, such minerals form as water seeps through porous rock. The minerals in the meteorites are evidence of a time when ice melted and flowed in the matrix of the asteroid rock.

The source of heat that melted all the ice was most likely the radioactive isotope aluminum-26, which was abundant in the early days of the solar system. Aluminum-26 releases abundant energy over several million years until it decays and is converted to the isotope magnesium-26. In the cold fringes of the young solar system, beyond the snow line, the heat emitted by the decay of aluminum-26 was an important, if short-lived, factor that shaped the geology and hydrology of volatile-rich asteroids. For several million years after the Sun formed, the water in many asteroids was liquid, supporting hydrothermal circulation systems such as those found in volcanic craters along Earth's submarine ridges. Mammified minerals and coals may have formed when hot brine seeped through cracks and fissures in the asteroids as the radioactive isotope heated it. In the later stages of planet formation, the gravity of the giant planets, far from the Sun, scattered material throughout the young solar system. He flew asteroids that contained water inward, past the snow line, to Earth and the other rocky planets.

We find evidence of such mixing of substances in the later stages of the formation of the solar system in the chemistry of the Earth and that of Mars. One such piece of evidence comes from the platinum group elements known as "iron lovers", or siderophiles, because their chemical affinity for iron and other metals is stronger than for rocks. In the new, molten Earth after the formation of the Moon, these elements would have been dragged down in the compressed iron and nickel streams that sank to form the Earth's core. But in practice today there is an incredibly significant concentration of siderophilic elements in the earth's mantle and even in its crust, in quantities corresponding to chondrite-like material, i.e. material that arrived in meteorites beyond the snow line. These materials make up about one percent of the Earth's mass and since they did not sink into the core, they probably arrived after our planet cooled sufficiently for the core to form in its entirety. This "late plating" of objects that hit the country explains how we may have enough platinum available for wedding rings and catalytic converters in cars. It can also explain how we might have enough water to fill the ocean. Apparently, all the rocky worlds and not just Earth and Mars were showered with this influx of material from the asteroid belt in the last stages of planetary formation.

However, there seems to be one major flaw in this tidy picture, in which the asteroids provided almost all the water in the world. The problem becomes clear when the researchers look at gaseous elements such as xenon and argon, known as noble gases, because they do not react with almost any other element. Because of this chemical indifference, the noble gases can be used as a tool to trace physical processes, since they are relatively free from the confusing effects of chemistry. If there is a close relationship between the rocky planets and the asteroids, then the relative amounts of noble gases should be similar in them. But from examining the ratio between the amount of xenon and the amount of argon in meteorites and materials that fell to Earth from other planets, it appears that both Earth and Mars are poor in these gases relative to meteorites.

In recent years, many possible answers to the riddle of the missing gases have been put forward, including some that may tip the scales again in the direction of crowning the comets as the actual suppliers of water and other volatile substances. As of the time of writing this article, the researchers are eagerly awaiting the first measurements of noble gases in the comet, which should come from the research being done by "Rosetta" on 67P/Churyumov-Grasimenko. These measurements may help us finally reach a definitive conclusion about the origin of Earth's ocean, but if we can learn anything from past trends, they may only raise more difficult questions that will keep the controversy going for a few more decades.

A simulated dichotomy?

There seems to be no simple solution to the debate over whether asteroids or comets are the source of Earth's oceans. And maybe the problem is not in nature but in the questions we ask about it. The dichotomy between asteroids and comets may not be as sharp as previously thought. One of us (Jewitt) recently discovered, with Henry Hsieh from the Institute of Astronomy and Astrophysics of the "Academia Sinica" in Taiwan, comets in the main belt: the holiday objects in the asteroid belt but periodically sprinkle dust throughout the coffee like a comet. These objects, it turns out surprisingly, store ice even though their orbits are in the sunny, volatile-poor region bounded by the snow line. Moreover, as we have shown, the real question may not be why there is so much water on Earth, but why there is so little. There are countless ways in which the relatively small amount of water found on Earth could have reached Earth, ways that depend heavily on the details of the planet's history, the objects that collided with it, and the initial conditions of their formation. The multitude of possibilities allows for other, more exotic scenarios of the arrival of the water, scenarios that, even if they are unlikely, cannot be ruled out with certainty for the time being.
For example, theoretically, most of the water on Earth may have been there almost from the beginning. A new study suggests the possibility that hydrogen ions from the solar wind accumulated into hydrated minerals on the shapeless surfaces of interstellar dust particles, and these brought this watery material to the planets and to the building blocks from which they were assembled early in the formation process. Even so, it's hard to explain exactly how this primordial reservoir was preserved in the mantle and seeped upward only after the major collisions that shaped the planet ended.

Recently, celestial bodies larger than most comets and asteroids are also attracting interest. The dwarf planet Ceres, for example, which is 900 kilometers across, is the largest asteroid in our solar system. It is believed that about half the mass of Keres is water. In early 2014, researchers observed what appeared to be steam erupting from the dwarf planet at a rate of about 20,000 kilograms per hour, conclusive evidence that Keres is rich in water. The mass of Earth is about 6,000 times greater than that of Ceres. If, as quite a few researchers speculate, half of the mass of a hook is water, then the total amount of water on Earth, on the surface of the ground and below it, is approximately equal to the amount of water stored in only five hook-like objects.

Such objects were much more common in the young and chaotic solar system than they are today, and it is not difficult to imagine that some bodies like Keres found their way to its inner regions and hit the country. Just one of them would have been enough to give our planet the death of the ocean without needing additional showers of small asteroids and comets. NASA's Dawn mission began orbiting Ceres in early 2015, and it should provide us with a new, up-close look at its ice and gases emitted from it, and undoubtedly also a whole new crop of surprises concerning the history of water on our planet and beyond.

About the authors
David Jewitt has been interested in astronomy since a spectacular meteor shower in the London sky impressed him when he was 7 years old. He is a member of the American National Academy of Sciences and a professor at the University of California, Los Angeles. Jewitt drinks a lot of water (but only when it's in the form of coffee).
Edward D. Young (Young) is a professor of geochemistry and cosmochemistry and a member of the "Institute for Planets and Extrasolar Planets" of the University of California, Los Angeles. He looks for clues about the origin of the solar system through the study of the chemistry of meteorites, the interstellar medium and other stars with the help of sophisticated laboratory instruments and with the help of the world's largest telescopes.

in brief
Intense heat and light in the vicinity of the young Sun confined almost all water during the formation of the planets to the outer fringes of the Solar System, leaving the inner worlds comparatively drier.
The water on Earth probably arrived later in asteroid or comet showers. But the data we have leaves room for other hypotheses.
The exact way in which the water got here may remain an unsolved mystery for some time. This depends on whether and when we can begin a more extensive exploration of the rest of the solar system.
A single solution to the question of the source of water on Earth may never be found.

More on the subject
A Population of Comets in the Main Asteroid Belt. Henry H. Hsieh and David Jewitt in Science, Vol. 312, pages 561–563; March 23, 2006.
Water and Astrobiology. Michael J. Mottl, Brian T. Glazer, Ralf I. Kaiser and Karen J. Meech in Chemie der Erde–Geochemistry, Vol. 67, no. 4, pages 253-282; December 2007.
Detection of Solar Wind–Produced Water in Irradiated Rims on Silicate Minerals. John P. Bardley et al. in Proceedings of the National Academy of Sciences USA, Vol. 111, no. 5, pages 1732–1735; February 4, 2014.

67P/Churyumov-Gerasimenko, a Jupiter Family Comet with a High D/H Ratio. K. Altwegg et al. in Science. Published online December 10, 2014.

Pluto and Beyond, Michael D. LeMonique, Scientific American Israel, February-March 2015.
The article was published with the permission of Scientific American Israel

More of the topic in Hayadan:

• The water on Earth does not comet
• Water from a comet or an asteroid?
• Water and organic matter on the asteroid Themis