Liquid Acquisition

Two new scenarios ramp up debate over how Earth got its water

Water is the life of the party on Earth. From shallow creeks to cascading waterfalls and raging rivers, it’s the primal heartbeat of the planet, nurturing a wealth of biological systems from the very simple to the amazingly complex. But no one knows for sure how Earth got this most precious of fluids.

Credit: NASA/GSFC Image by Reto Stöckli

Some researchers think that the Earth could have been born wet. Magnesium (green) in grains of the mineral olivine (molecular structure shown) could have held tight to water despite searing temperatures. Perditax/Wikimedia Commons
The super-Earth GJ 1214b (artist’s illustration) may either host a water-rich steamy atmosphere or a thick haze that masks its atmospheric composition. Paul A. Kempton
FLUID DELIVERY | A proposed scenario suggests that Jupiter, the largest planet in the solar system, could be responsible for water on Earth. B. Rakouskas

Some researchers contend that the Earth was born wet. Others assert that the planet only later acquired the liquid, ferried in from distant reaches of the solar system. That long-simmering debate has now reached the boiling point.

Two new ideas for supplying water to the early Earth have come to the forefront in the past few months. In thrashing out which scenario is more likely, researchers hope to develop a guide for finding water-rich and possibly habitable planets beyond the solar system.

During the Earth’s formation, 4.5 billion years ago, temperatures in the inner solar system exceeded 400° Celsius — hot enough to melt lead. So scientists had assumed that any water would vaporize: Earth would have been born bone-dry and would have acquired its water from elsewhere. For a while, icy comets seemed a suitable delivery source, but during the last decade scientists have found that comet water doesn’t chemically match the recipe for Earth’s oceans. Most researchers have turned to asteroids because these space rocks have water with a better chemical match, although they too have problems.

One of the new proposals circumvents the need for asteroids, offering not only new ideas about the origin of terrestrial water but also about the early days of the inner solar system. Alessandro Morbidelli of the C´te d’Azur Observatory in Nice, France, and collaborators suggest that the formation of the inner, terrestrial planets was accompanied by a rowdy encounter with the solar system’s big bully, Jupiter. In the process of barreling into the inner solar system and then getting slung back out, Jupiter may have kicked ice-rich bodies from the outer solar system into new orbits, some headed toward Earth.

But another team argues that Earth didn’t need a special delivery at all. The group, including Michael Drake of the University of Arizona in Tucson, has performed simulations indicating that Earth’s building blocks — cosmic dust grains — managed to grab water molecules so tightly that even the high temperatures of the early inner solar system couldn’t break the bonds.

Understanding whether Earth got its water locally or from the colder outer reaches of the solar system has implications that go far beyond Earth and its planetary neighbors. If water was plentiful in Earth’s building blocks, many extrasolar terrestrial planets could also have been born wet.

The controversy over the origin of Earth’s water may have bearing on the diversity of extrasolar terrestrial planets, including how common an ingredient water may be in these bodies, says Philip Armitage of the University of Colorado at Boulder.

A new model, by Jove

Only four years ago, Morbidelli and his colleagues were satisfied that they had a working model of Earth’s water acquisition. They had proposed that the material came in as a veneer delivered by denizens of the outer part of the asteroid belt, which lies just inside Jupiter’s orbit. The asteroids would have pummeled Earth after the planet had finished forming.

It wouldn’t take much. Although water covers 70 percent of Earth’s surface, that water accounts for only about 0.02 percent of the planet’s mass. (There also may have been, and still may be, 1.3 to 60 times all the oceans’ worth of water present in the planet’s interior.) Meteor­ite measurements indicate that outer-belt asteroids have a water content of about 10 percent, so even if only a relatively few asteroids ferried water to Earth, it might have been enough to account for the planet’s water.

And unlike comets, which orbit the sun farther out than asteroids and must get past Jupiter — no mean feat — to reach Earth, it’s comparatively easy for asteroids to escape their birth region and head inward. Still, something had to jump-start their departure. In Morbidelli’s 2006 model, the researchers envisioned that moon-sized to Mars-sized chunks of material took shape in the youthful asteroid belt. Those embryos scattered some of the rocky material out of the belt, and some of it headed Earth’s way.

But Morbidelli and collaborators soon uncovered problems with their model. Such scattering would have left the asteroids that remained in the belt on tilted orbits, more highly inclined than observed. More significantly, notes Morbidelli, material piling onto the terrestrial planets would have made Mars, the inner solar system planet closest to the asteroid belt, as massive as Earth. In reality, the Red Planet has only about one-tenth Earth’s mass.

Over the last few years “alarm bells started going off,” Morbidelli recalls, prompting the researchers to consider a new model. Their latest proposal provides a sweeping new view about planet formation in the inner solar system as well as a new solution to the puzzle of how Earth got its water.

The stage was set when Brad Hansen of the University of California, Los Angeles began exploring a scenario in which the inner solar system’s planets coalesced from material confined to a width 0.7 to 1.0 times Earth’s distance from the sun, a much narrower disk than normally considered. His simulations with that narrowed disk produced a set of planets similar to the terrestrial orbs, including a properly lightweight Mars, he reported in 2009 in the Astrophysical Journal.

Hansen didn’t know why the planet-forming disk would be so narrow. But Morbidelli and his collaborators, including Kevin Walsh of the Southwest Research Institute in Boulder, Colo., David O’Brien of the Planetary Science Institute in Tucson, and Sean Raymond of the Laboratoire d’Astrophysique de Bordeaux in France, thought Jupiter’s movement could be the culprit.

The team’s new model begins after the gaseous Jupiter was fully formed, a scant few million years after the birth of the solar system. The inner planets — Mercury, Venus, Earth and Mars — had just started to coalesce from bits of cosmic dust. The asteroid belt, which now lies between the orbits of Mars and Jupiter and marks the transition to the outer solar system, was a much denser jumble of rocks and dust than it is today, and Jupiter’s outlying sister planet, Saturn, had yet to reach full size.

As the gas from the planet-forming disk spiraled into the sun, the researchers propose, Jupiter was dragged along, plowing through and emptying the asteroid belt and entering the inner solar system. The combination of Jupiter’s heft and motion opened up a gap in the inner part of the planet-forming disk, effectively narrowing it just as Hansen required to make a svelte Mars, Walsh reported in October in Pasadena, Calif., at a meeting of the American Astronomical Society’s Division for Planetary Sciences (SN Online: 10/4/10).

Jupiter’s proposed motion may also have played a crucial role in delivering water to Earth, O’Brien noted at the meeting. In the new scheme, gravitational interactions with Saturn, which trailed its big brother in migrating inward, caused Jupiter to stop in its tracks about where Mars now resides and then reverse direction.

As Jupiter and Saturn traveled outward, they moved farther than ever into the frigid outer region of the solar system (though not as far out as the reservoirs of bodies that would become comets). There Jupiter and Saturn encountered an icier, water-rich population of bodies. The duo would have scattered some of this icy material into the outer part of the asteroid belt. More important for Earth, Jupiter and Saturn’s foray would have placed icy outer solar system material on elongated orbits aimed into the inner solar system. Some of the material headed inward would have bashed into Earth, baptizing the planet with water.

So as Morbidelli and colleagues now envision it, Earth’s water didn’t come from the asteroid belt. Instead, water-rich objects that smacked into Earth and the objects now in the outer part of the belt came from denizens of the outer solar system flung inward by gravitational encounters with Jupiter and Saturn.

Two recent findings may dovetail with this new model. Until about a year ago, no one had found evidence of ice on an asteroid’s surface.  At the Pasadena meeting, Humberto Campins of the University of Central Florida in Orlando announced the discovery of frozen water on the surface of outer asteroid 65 Cybele. His team and others had previously reported the detection of ice on another outer-belt rock called 24 Themis, the first discovery of ice on one of these space rocks (SN: 11/7/09, p. 9).

Those discoveries can be consistent with the new model by Morbidelli and his collaborators, Campins says. Water would be delivered to such asteroids just as it was delivered to Earth.

“Work like this is overdue, and I’m excited to see where these ideas lead,” says Fred Ciesla, a theorist at the University of Chicago. The migrating Jupiter model “makes our lives more difficult,” he says, because it may force scientists to rethink their ideas on the possible sources of water for the inner solar system and the conditions that the fledgling planets were exposed to. “However, as the authors state, our old ideas weren’t working, so maybe we do have to start over to some degree,” Ciesla notes.

A watery birth

But for other researchers, starting over means that Earth got its water from the very start. “For 30-odd years, I taught my students that Earth got its water from comets or wet asteroids,” says the University of Arizona’s Drake. He stopped being a true believer a decade ago.

That’s when studies began revealing that comets contain too much deuterium, a heavy form of hydrogen, relative to the amount of ordinary hydrogen. Comets have about twice the ratio of deuterium to hydrogen found in terrestrial oceans. And asteroids, Drake and some others argue, don’t have the right isotopic abundances of certain noble gases, such as xenon, to match Earth’s waters.

So Drake began considering a model in which, despite the hot early temperatures, Earth could have acquired its water as it formed. In recent simulations Drake, Nora de Leeuw of University College London and their collaborators examined the interactions of individual water molecules and grains of olivine, a dry mineral common on Earth and in planet-forming disks around other stars. The team found that magnesium on the surface of olivine grains grabs hold of the oxygen in water molecules so tightly that the water remains bound at temperatures over 600° C. The researchers describe their study in the Dec. 21 Chemical Communications.

It’s likely that at least some and perhaps most of Earth’s water came from the cosmic dust from which the planet arose, Drake says.

And for any rocky, terrestrial planet whose building blocks do contain water, it should be relatively easy to transform that interior fluid into a surface ocean, says Linda Elkins-Tanton of MIT. Her new simulations show that as Earth, which formed molten, began to cool, minerals within its magma skin would solidify and crystallize. Those crystallized minerals leave most of the water behind in the magma fluid. Eventually, the water in the magma becomes so concentrated that it bubbles out of the molten material, creating a steam-rich atmosphere that ultimately condenses on the surface as oceans.

Even super-Earths — planets two to 10 times as massive as Earth — would commonly produce an ocean within tens to hundreds of millions of years after formation. That could bode well for supporting the onset of life, says Elkins-Tanton, who describes her work in a paper posted online November 16 in Astrophysics and Space Science.

But there’s one catch in Drake’s scenario, notes Raymond. Studies suggest that the young sun and the dust around it had a deuterium-to-hydrogen ratio that’s only one-sixth the amount in Earth’s oceans.

Drake acknowledges the mismatch and says his team has begun computer simulations to determine if the deuterium-to-hydrogen ratio might somehow have increased as the dust coalesced to make Earth.

Drake and his colleagues give further support to the idea that water could be adsorbed onto the olivine grains that later aggregated to form the building blocks of the Earth, Ciesla says. But it remains unclear how much of that water would have been retained during planet formation, when dust particles bashed into each other, he adds.

Beyond the solar system

If Drake’s model holds up, it could paint a rosier picture for the prevalence of water in extrasolar terrestrial planets, Armitage says. Observations with infrared telescopes over the last decade have revealed there’s no dearth of water in the planet-forming regions around other stars. And water molecules would probably stick to dust particles in those systems, too.

Alternatively, says Armitage, if the rocks that coalesced to form terrestrial planets are truly dry, “then we have to rely on water delivery from the extrasolar analogs of the solar system’s asteroid belt.”

To acquire fluid, the planets would have to depend on a host of other factors, including the architecture of those systems. For instance, systems that have an analog to Earth’s asteroid belt or a reservoir of comets might be more likely to deliver water to inner, Earthlike planets, some researchers speculate. Or if the new model by Morbidelli and collaborators is correct, water delivery might require some highly specialized choreography from a Jupiter-like planet journeying in and out of the terrestrial zone to help ferry the fluid.

While a migrating Jupiter may bode well for the presence of water-rich rocky planets, a hot Jupiter could be the kiss of death. These planets orbit in the hot zone, much closer to their parent star.

Only two years ago, hot Jupiters were assumed to migrate gently inward from the outskirts of their planetary system and quickly settle into sedate, circular orbits that would keep them well away from the zone in which terrestrial planets could form. But beginning in 2009, studies revealed that several hot Jupiters either orbit backward or at a tilt relative to the direction in which their parent star is spinning (SN: 5/8/10, p. 11; SN: 9/12/09, p. 12). Such unruly orbits indicate that hot Jupiters don’t settle down for millions of years. And until they do, they rampage through the inner solar system, destroying or expelling any water and the other building blocks of Earthlike planets, Raymond says.

Clues from Earth’s neighborhood and beyond may prove to be invaluable in looking for habitable planets in the flood of new data from missions like NASA’s Kepler spacecraft. In February, Kepler researchers are expected to announce a trove of new planets, possibly dozens, including several super-Earths. Kepler finds planets by detecting the tiny amount of light that they block each time they pass in front of their parent stars. Follow-up studies with other telescopes can reveal the mass of a planet and whether its parent star harbors other planets as well.

Recent observations suggest water-rich planets may indeed await discovery beyond the solar system. In the Dec. 2 Nature, researchers reported that a recently found super-Earth called GJ 1214b has an atmosphere containing either steam or large clouds that mask its true composition. Additional observations at several more wavelengths are likely to distinguish between the two possibilities.

Before too long, new observations of extrasolar planetary systems may even help settle the debate about water on Earth.

From the Nature Index

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