How did Earth get its water?

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Earth — a planet of oceans, rivers and rainforests — grew up in an interplanetary desert.

When the solar system formed about 4.6 billion years ago, shards of calcium- and aluminum-rich minerals stuck together, building ever-larger pebbles and boulders that smashed together and assembled the rocky planets, including Earth.

But Earth’s signature ingredient was nowhere to be found. Heat from the young sun vaporized any ice that dared to come near the inner planets. Earth’s relatively feeble gravity couldn’t grab on to the water vapor, or any other gas for that matter. And yet, today, Earth is a planet that runs on H₂O. Water regulates the climate, shapes and reshapes the landscape and is essential to life. At birth, humans are about 78 percent water — basically a sack of the wet stuff.

To get water, Earth had to have help from somewhere else.

Researchers recently found traces of Earth’s aquatic starter kit locked away inside several meteorites, chunks of rock that fell to the planet’s surface. Those meteorites were a gift from Vesta, the second largest body in the asteroid belt between Mars and Jupiter. Vesta is thought to have formed earlier than Earth, roughly 8 million to 20 million years after the start of the solar system. (Earth needed 30 million to 100 million years to pull itself together.)

Well before the rocky planets formed, recent research suggests, ice-infused asteroids were forged beyond Jupiter and subsequently swarmed the inner solar system. These space rocks delivered water to Vesta and to Earth after being hurled at our planet by the gravity of Jupiter and Saturn. Whether the giant planets were a help or a hindrance is anybody’s guess. But if what happened here can happen anywhere, then water might be prevalent on other worlds, giving life a good chance of thriving throughout the galaxy.

Comets vs. asteroids

For decades, researchers have debated whether comets or asteroids delivered Earth’s water. At first glance, comets seemed a likely source. Originating beyond the orbit of Neptune, comets are the deep-freeze storage units of the solar system. They hold a lot of ice that has been locked away within their interiors since the formation of the solar system. Some comets are occasionally thrown inward after a close brush with a planet or passing star. It makes sense that, during the chaos of the early solar system, Earth would have been pummeled with comets, bringing plenty of water to fill the oceans.

In recent years, however, the comet hypothesis has lost favor. “It looks like comets are pretty much out,” says cosmochemist Conel Alexander of the Carnegie Institution for Science in Washington, D.C. Most of the comet water tested so far doesn’t match that of Earth’s oceans. Plus, it’s incredibly difficult to bring a comet toward Earth, much less a whole slew of them. “It just shouldn’t be part of the discussion anymore,” he says.

Part of the problem lies in a subtle chemical difference between water on Earth and water in most comets. Water is a simple molecule resembling a pair of Mickey Mouse ears: two hydrogen atoms grab a single oxygen atom. But sometimes deuterium, a slightly heavier version of hydrogen, weasels its way into the mix. The nucleus of a deuterium atom contains one proton and one neutron; in hydrogen, the proton stands alone. On Earth, only about 156 out of every 1 million water molecules contain deuterium.

Researchers have long used the relative amount of deuterium compared with hydrogen — known as the D/H ratio — to trace water back to where it originated. At colder temperatures, deuterium starts to show up in ice more frequently. So bodies that formed in the frigid backwaters of the solar system, such as comets, should be enriched in deuterium, whereas the water vapor that swirled around the infant Earth should have little to none.

Most comets appear to follow that logic; their D/H ratio is typically about twice what has been measured on Earth.

Two comets, however, threw a curveball at scientists who had counted out comets as the source of Earth’s water. In 2010, researchers used the Herschel space telescope to measure the D/H ratio of comet 103P/Hartley 2. They reported that 103P’s water nearly matched that found on Earth. Observations of comet 45P/Honda-Mrkos-Pajdušáková three years later also found abnormally low D/H ratios. Suddenly one, possibly two, comets were carrying Earthlike water.

Jupiter’s pull

Both of these comets are part of a community known as Jupiter family comets. They originated in the Kuiper belt, the ring of icy debris beyond Neptune where Pluto lives. The gravity of first Neptune and then Jupiter gradually nudged these comets into relatively short orbits that bring them closer to the sun. All previous D/H measurements were of comets that hail from the far more distant Oort cloud, a shell of ice fragments that envelops the solar system. Comets 103P and 45P suggested that researchers may have been hasty in dismissing all comets as Earth’s water source. Perhaps just the Jupiter family comets were responsible.

But then in 2014, the European Space Agency’s Rosetta probe arrived at Comet 67P/Churyumov–Gerasimenko, another Jupiter family comet. As the spacecraft sidled up to the comet, it sampled the water streaming from the comet body and found 67P’s D/H ratio to be staggeringly high — more than three times that of Earth’s oceans (SN: 1/10/15, p. 8).

“Each new comet measurement is giving us a different picture,” says Karen Meech, a planetary scientist at the University of Hawaii in Honolulu. The Rosetta results show that even among a single family of comets, there is incredible diversity in water composition. “Comets formed over a huge range of distances, so it’s no surprise that there’s a huge range in D/H,” she says.

But even if some comets have an Earth-like D/H ratio, it’s still really hard to get comets to hit our planet in the first place. “Any comet that’s going to bash into Earth has to get past this really big linebacker of Jupiter,” says planetary scientist Sean Raymond of the Laboratoire d’Astrophysique de Bordeaux in France. Jupiter has a tendency to take comets that come too close and fling them out of the solar system. The few that do end up on Earth-crossing orbits don’t stay there for long.

“The comet only has a certain number of tries to get in close and either hit Earth or get scattered on to another orbit,” Raymond says.

So Jupiter’s gravity may be too big a hurdle for comets to overcome. But it may be just the ticket for flinging asteroids at the inner planets.

A more ‘tack’-ful approach

In 2011, a team of researchers including Raymond were tackling a different problem: Why is Mars so small? There should have been plenty of raw material available 4.6 billion years ago to turn Mars into a planet closer in size to Venus or Earth. But Mars is just about half Earth’s diameter and about one-tenth its mass. One possible explanation is that something prematurely robbed the nascent Red Planet of its building blocks.

One solution, known as the Grand Tack model, describes a solar system far less sedate than the one we inhabit today (SN Online: 3/23/15). In the Grand Tack scenario, Jupiter and Saturn stride back and forth across the solar system like schoolyard bullies, hurling rocks at and stealing food from the other planets. The gas that encircled the sun dragged Jupiter and then Saturn inward. Once Jupiter arrived at about the current orbit of Mars, a gravitational tug from Saturn flung both back out from where they came (the “tack” in “Grand Tack”).  Jupiter’s encroachment on the inner solar system carved a gap in the debris field from which the rocky planets were forming, depriving Mars of raw ingredients.

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WATER HERE AND THERE 

Along with Earth, a couple of dwarf planets and several moons have shown evidence of water, in one form or another. Their potential to support life varies. 

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The same planetary tango that robbed Mars of resources might also explain how icy asteroids pummeled Earth. As Jupiter and Saturn wandered back out, their gravity latched on to asteroids that formed beyond the snow line — the boundary beyond which temperatures are low enough for ice to form — and flung them inward. About 1 percent of these ice-infused boulders, known as C-type asteroids, were dropped into the outer regions of the asteroid belt. But for every C-type asteroid relocated to the belt, at least 10 were sent careening into the region where the rocky planets were materializing.

This bombardment of asteroids a few million years after the start of the solar system could have easily delivered enough ice — locked inside the rocks, safe from the sun’s heat — to account for Earth’s oceans, computer simulations indicate. Water makes up to about 20 percent of the mass of some of these asteroids. On Earth, despite having more than 70 percent of its surface blanketed in blue, water accounts for only 0.023 percent of the planet’s mass. Compared with some asteroids, Earth is positively parched.

The Grand Tack nicely explains the formation of Mars, the layout of the asteroid belt and the delivery of water to Earth via icy asteroids. But Raymond stresses that it’s just one way to match all the data. “It’s an evolution of thinking,” he says. “It’s not meant to be a final solution.”

The same D/H ratio that exonerated comets is now pointing a finger at these asteroids. In 2012, Alexander and colleagues concluded in the journal Science that the bulk of Earth’s water arrived via bodies similar to a class of meteorites known as CI carbonaceous chondrites. Researchers think that these meteorites, which were knocked off asteroids that formed beyond Jupiter, are among the oldest objects in the solar system.

Alexander’s research, along with that of many others, builds a strong case for a chemical match between Earth’s water and chondrites’ water. But it doesn’t address when the water arrived. Brown University geologist Alberto Saal argues that part of the answer lies on the moon.

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The bounty of lunar samples brought to Earth by Apollo astronauts included volcanic glass hauled in during the Apollo 15 and 17 missions. The glass formed from rapidly cooling magma that was spat out from the moon’s interior long ago. In 2013, Saal and colleagues reported in Science that the D/H ratio of water trapped within the glass matched that measured in both Earth’s oceans and Alexander’s carbonaceous chondrites (SN: 6/29/13, p. 8). Saal’s findings suggest two things: Earth and the moon have a common source of water and the water was already here when the moon formed.

The moon started with a literal bang. A planet the size of Mars is thought to have smashed into Earth toward the end of our planet’s formation. The collision blasted part of Earth, as well as the unfortunate interloper, into a ring of vaporized rock that encircled Earth before sticking together to build the moon (SN: 7/12/14, p. 14). Water must have been present at the time of impact for it to be sealed into the moon, Saal notes, or it at least arrived before the moon’s surface had time to cool and solidify. This puts water near Earth about 150 million years after the start of the solar system. But based on the moon data alone, we can’t say how much earlier, says Sune Nielsen, a geologist at the Woods Hole Oceanographic Institution in Massachusetts.

To narrow in on a more precise time for water’s arrival, researchers have turned to the asteroid Vesta. Or, more specifically, meteorites nicked off Vesta after the asteroid got whacked by another space rock. Woods Hole geologist Adam Sarafian, Nielsen and colleagues analyzed small amounts of water trapped within minerals of apatite locked inside a sample of Vesta meteorites. The team reported last fall in Science that the D/H ratio of the meteorites’ water matched Earth’s. That discovery implies that whatever delivered Vesta’s water brought along Earth’s as well and that this water had to have arrived before Vesta finished forming (SN Online: 11/1/14).

That finding pushes the influx of water back, possibly as early as 8 million years after the start of the solar system. This is the oldest stockpile of water ever dated in the solar system, Nielsen says. These observations place water in the inner solar system well after Jupiter and Saturn were on the prowl, lobbing asteroids around the solar system.

Nailing down how and when water arrived at Earth is about more than just understanding how our planet was built. “If you have to have some sort of external delivery mechanism for getting water to terrestrial planets,” says Alexander, “it becomes harder to make a habitable planet.” Rocky planets forming around other stars will face the same problem that Earth faced. These planets in the habitable zones of their stars, while able to support liquid water on their surfaces, develop in dry environments and need to have ice sent in from farther out. Did Earth get lucky by having Jupiter and Saturn as neighbors, or are there other ways to move water around?

Just because Earth formed one way doesn’t mean all habitable planets must follow the same path. “I would be cautious,” Nielsen says, about saying that gas giants are the only way to bring water to rocky planets.

In fact, gas giants may even be a hindrance. “Jupiter and Saturn just screw things up,” says Raymond. Their gravity is strong enough that they tend to kick asteroids and comets right out of the solar system. If Jupiter and Saturn didn’t exist, he notes, Earth’s gravity could have stolen 10 times as much water from the outer edge of the asteroid belt. In the absence of giant planets, water delivery could happen naturally as planets pull in debris from different parts of the solar system. Recent observations from the Kepler space telescope suggest that planets the size of Jupiter are relatively uncommon around other stars. Perhaps most habitable planets do just fine on their own.

If that’s the case, then maybe the galaxy is teeming with ocean worlds waiting to be discovered. “From my point of view,” Raymond says, “having water on a planet like Earth is an everyday occurrence.” 

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This article appears in the May 16, 2015, issue with the headline, “Water, water everywhere: Every bit of Earth’s H₂O was delivered by space rocks, but which ones?”

Editor’s note: This story was corrected on May 18, 2015. A caption incorrectly referred to hydrogen molecules, instead of hydrogen atoms. The Water Here and There slideshow was corrected and updated on May 20.