Study hints that ancient Earth made its own water — geologically

A new study is helping to answer a longstanding question that has recently moved to the forefront of earth science: Did our planet make its own water through geologic processes, or did water come to us via icy comets from the far reaches of the solar system?

The answer is likely “both,” according to researchers at The Ohio State University– and the same amount of water that currently fills the Pacific Ocean could be buried deep inside the planet right now.

At the American Geophysical Union (AGU) meeting on Wednesday, Dec. 17, they report the discovery of a previously unknown geochemical pathway by which the Earth can sequester water in its interior for billions of years and still release small amounts to the surface via plate tectonics, feeding our oceans from within.

In trying to understand the formation of the early Earth, some researchers have suggested that the planet was dry and inhospitable to life until icy comets pelted the earth and deposited water on the surface.

Wendy Panero, associate professor of earth sciences at Ohio State, and doctoral student Jeff Pigott are pursuing a different hypothesis: that Earth was formed with entire oceans of water in its interior, and has been continuously supplying water to the surface via plate tectonics ever since.

Researchers have long accepted that the mantle contains some water, but how much water is a mystery. And, if some geological mechanism has been supplying water to the surface all this time, wouldn’t the mantle have run out of water by now?

Because there’s no way to directly study deep mantle rocks, Panero and Pigott are probing the question with high-pressure physics experiments and computer calculations.

“When we look into the origins of water on Earth, what we’re really asking is, why are we so different than all the other planets?” Panero said. “In this solar system, Earth is unique because we have liquid water on the surface. We’re also the only planet with active plate tectonics. Maybe this water in the mantle is key to plate tectonics, and that’s part of what makes Earth habitable.”

Central to the study is the idea that rocks that appear dry to the human eye can actually contain water–in the form of hydrogen atoms trapped inside natural voids and crystal defects. Oxygen is plentiful in minerals, so when a mineral contains some hydrogen, certain chemical reactions can free the hydrogen to bond with the oxygen and make water.

Stray atoms of hydrogen could make up only a tiny fraction of mantle rock, the researchers explained. Given that the mantle is more than 80 percent of the planet’s total volume, however, those stray atoms add up to a lot of potential water.

In a lab at Ohio State, the researchers compress different minerals that are common to the mantle and subject them to high pressures and temperatures using a diamond anvil cell–a device that squeezes a tiny sample of material between two diamonds and heats it with a laser–to simulate conditions in the deep Earth. They examine how the minerals’ crystal structures change as they are compressed, and use that information to gauge the minerals’ relative capacities for storing hydrogen. Then, they extend their experimental results using computer calculations to uncover the geochemical processes that would enable these minerals to rise through the mantle to the surface–a necessary condition for water to escape into the oceans.

In a paper now submitted to a peer-reviewed academic journal, they reported their recent tests of the mineral bridgmanite, a high-pressure form of olivine. While bridgmanite is the most abundant mineral in the lower mantle, they found that it contains too little hydrogen to play an important role in Earth’s water supply.

Another research group recently found that ringwoodite, another form of olivine, does contain enough hydrogen to make it a good candidate for deep-earth water storage. So Panero and Pigott focused their study on the depth where ringwoodite is found–a place 325-500 miles below the surface that researchers call the “transition zone”–as the most likely region that can hold a planet’s worth of water. From there, the same convection of mantle rock that produces plate tectonics could carry the water to the surface.

One problem: If all the water in ringwoodite is continually drained to the surface via plate tectonics, how could the planet hold any in reserve?

For the research presented at AGU, Panero and Pigott performed new computer calculations of the geochemistry in the lowest portion of the mantle, some 500 miles deep and more. There, another mineral, garnet, emerged as a likely water-carrier–a go-between that could deliver some of the water from ringwoodite down into the otherwise dry lower mantle.

If this scenario is accurate, the Earth may today hold half as much water in its depths as is currently flowing in oceans on the surface, Panero said–an amount that would approximately equal the volume of the Pacific Ocean. This water is continuously cycled through the transition zone as a result of plate tectonics.

“One way to look at this research is that we’re putting constraints on the amount of water that could be down there,” Pigott added.

Panero called the complex relationship between plate tectonics and surface water “one of the great mysteries in the geosciences.” But this new study supports researchers’ growing suspicion that mantle convection somehow regulates the amount of water in the oceans. It also vastly expands the timeline for Earth’s water cycle.

“If all of the Earth’s water is on the surface, that gives us one interpretation of the water cycle, where we can think of water cycling from oceans into the atmosphere and into the groundwater over millions of years,” she said. “But if mantle circulation is also part of the water cycle, the total cycle time for our planet’s water has to be billions of years.”

New study finds oceans arrived early to Earth

In this illustration of the early solar system, the dashed white line represents the snow line -- the transition from the hotter inner solar system, where water ice is not stable (brown) to the outer Solar system, where water ice is stable (blue). Two possible ways that the inner solar system received water are: water molecules sticking to dust grains inside the 'snow line' (as shown in the inset) and carbonaceous chondrite material flung into the inner solar system by the effect of gravity from protoJupiter. With either scenario, water must accrete to the inner planets within the first ca. 10 million years of solar system formation. -  Illustration by Jack Cook, Woods Hole Oceanographic Institution
In this illustration of the early solar system, the dashed white line represents the snow line — the transition from the hotter inner solar system, where water ice is not stable (brown) to the outer Solar system, where water ice is stable (blue). Two possible ways that the inner solar system received water are: water molecules sticking to dust grains inside the ‘snow line’ (as shown in the inset) and carbonaceous chondrite material flung into the inner solar system by the effect of gravity from protoJupiter. With either scenario, water must accrete to the inner planets within the first ca. 10 million years of solar system formation. – Illustration by Jack Cook, Woods Hole Oceanographic Institution

Earth is known as the Blue Planet because of its oceans, which cover more than 70 percent of the planet’s surface and are home to the world’s greatest diversity of life. While water is essential for life on the planet, the answers to two key questions have eluded us: where did Earth’s water come from and when?

While some hypothesize that water came late to Earth, well after the planet had formed, findings from a new study led by scientists at the Woods Hole Oceanographic Institution (WHOI) significantly move back the clock for the first evidence of water on Earth and in the inner solar system.

“The answer to one of the basic questions is that our oceans were always here. We didn’t get them from a late process, as was previously thought,” said Adam Sarafian, the lead author of the paper published Oct. 31, 2014, in the journal Science and a MIT/WHOI Joint Program student in the Geology and Geophysics Department.

One school of thought was that planets originally formed dry, due to the high-energy, high-impact process of planet formation, and that the water came later from sources such as comets or “wet” asteroids, which are largely composed of ices and gases.

“With giant asteroids and meteors colliding, there’s a lot of destruction,” said Horst Marschall, a geologist at WHOI and coauthor of the paper. “Some people have argued that any water molecules that were present as the planets were forming would have evaporated or been blown off into space, and that surface water as it exists on our planet today, must have come much, much later—hundreds of millions of years later.”

The study’s authors turned to another potential source of Earth’s water— carbonaceous chondrites. The most primitive known meteorites, carbonaceous chondrites, were formed in the same swirl of dust, grit, ice and gasses that gave rise to the sun some 4.6 billion years ago, well before the planets were formed.

“These primitive meteorites resemble the bulk solar system composition,” said WHOI geologist and coauthor Sune Nielsen. “They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth’s water.

In order to determine the source of water in planetary bodies, scientists measure the ratio between the two stable isotopes of hydrogen: deuterium and hydrogen. Different regions of the solar system are characterized by highly variable ratios of these isotopes. The study’s authors knew the ratio for carbonaceous chondrites and reasoned that if they could compare that to an object that was known to crystallize while Earth was actively accreting then they could gauge when water appeared on Earth.

To test this hypothesis, the research team, which also includes Francis McCubbin from the Institute of Meteoritics at the University of New Mexico and Brian Monteleone of WHOI, utilized meteorite samples provided by NASA from the asteroid 4-Vesta. The asteroid 4-Vesta, which formed in the same region of the solar system as Earth, has a surface of basaltic rock—frozen lava. These basaltic meteorites from 4-Vesta are known as eucrites and carry a unique signature of one of the oldest hydrogen reservoirs in the solar system. Their age—approximately 14 million years after the solar system formed—makes them ideal for determining the source of water in the inner solar system at a time when Earth was in its main building phase. The researchers analyzed five different samples at the Northeast National Ion Microprobe Facility—a state-of-the-art national facility housed at WHOI that utilizes secondary ion mass spectrometers. This is the first time hydrogen isotopes have been measured in eucrite meteorites.

The measurements show that 4-Vesta contains the same hydrogen isotopic composition as carbonaceous chondrites, which is also that of Earth. That, combined with nitrogen isotope data, points to carbonaceous chondrites as the most likely common source of water.

“The study shows that Earth’s water most likely accreted at the same time as the rock. The planet formed as a wet planet with water on the surface,” Marschall said.

While the findings don’t preclude a late addition of water on Earth, it shows that it wasn’t necessary since the right amount and composition of water was present at a very early stage.

“An implication of that is that life on our planet could have started to begin very early,” added Nielsen. “Knowing that water came early to the inner solar system also means that the other inner planets could have been wet early and evolved life before they became the harsh environments they are today.

Vitamin B3 might have been made in space, delivered to Earth by meteorites

Karen Smith crushes meteorites with a mortar and pestle in Goddard's Astrobiology Analytical Laboratory to prepare them for analysis. Vitamin B3 was found in all eight meteorites analyzed in the study. -  Karen Smith
Karen Smith crushes meteorites with a mortar and pestle in Goddard’s Astrobiology Analytical Laboratory to prepare them for analysis. Vitamin B3 was found in all eight meteorites analyzed in the study. – Karen Smith

Ancient Earth might have had an extraterrestrial supply of vitamin B3 delivered by carbon-rich meteorites, according to a new analysis by NASA-funded researchers. The result supports a theory that the origin of life may have been assisted by a supply of key molecules created in space and brought to Earth by comet and meteor impacts.

“It is always difficult to put a value on the connection between meteorites and the origin of life; for example, earlier work has shown that vitamin B3 could have been produced non-biologically on ancient Earth, but it’s possible that an added source of vitamin B3 could have been helpful,” said Karen Smith of Pennsylvania State University in University Park, Pa. “Vitamin B3, also called nicotinic acid or niacin, is a precursor to NAD (nicotinamide adenine dinucleotide), which is essential to metabolism and likely very ancient in origin.” Smith is lead author of a paper on this research, along with co-authors from NASA’s Goddard Space Flight Center in Greenbelt, Md., now available online in the journal Geochimica et Cosmochimica Acta.

This is not the first time vitamin B3 has been found in meteorites. In 2001 a team led by Sandra Pizzarello of Arizona State University, in Tempe discovered it along with related molecules called pyridine carboxylic acids in the Tagish Lake meteorite.

In the new work at Goddard’s Astrobiology Analytical Laboratory, Smith and her team analyzed samples from eight different carbon-rich meteorites, called “CM-2 type carbonaceous chondrites” and found vitamin B3 at levels ranging from about 30 to 600 parts-per-billion. They also found other pyridine carboxylic acids at similar concentrations and, for the first time, found pyridine dicarboxylic acids.

“We discovered a pattern – less vitamin B3 (and other pyridine carboxylic acids) was found in meteorites that came from asteroids that were more altered by liquid water. One possibility may be that these molecules were destroyed during the prolonged contact with liquid water,” said Smith. “We also performed preliminary laboratory experiments simulating conditions in interstellar space and showed that the synthesis of vitamin B3 and other pyridine carboxylic acids might be possible on ice grains.”

Scientists think the solar system formed when a dense cloud of gas, dust, and ice grains collapsed under its own gravity. Clumps of dust and ice aggregated into comets and asteroids, some of which collided together to form moon-sized objects or planetesimals, and some of those eventually merged to become planets.

Space is filled with radiation from nearby stars as well as from violent events in deep space like exploding stars and black holes devouring matter. This radiation could have powered chemical reactions in the cloud (nebula) that formed the solar system, and some of those reactions may have produced biologically important molecules like vitamin B3.

Asteroids and comets are considered more or less pristine remnants from our solar system’s formation, and many meteorites are prized samples from asteroids that happen to be conveniently delivered to Earth. However, some asteroids are less pristine than others. Asteroids can be altered shortly after they form by chemical reactions in liquid water. As they grow, asteroids incorporate radioactive material present in the solar system nebula. If enough radioactive material accumulates in an asteroid, the heat produced as it decays will be sufficient to melt ice inside the asteroid. Researchers can determine how much an asteroid was altered by water by examining chemical and mineralogical signatures of water alteration in meteorites from those asteroids.

When asteroids collide with meteoroids or other asteroids, pieces break off and some of them eventually make their way to Earth as meteorites. Although meteorites are valued samples from asteroids, they are rarely recovered immediately after they fall to Earth. This leaves them vulnerable to contamination from terrestrial chemistry and life.

The team doubts the vitamin B3 and other molecules found in their meteorites came from terrestrial life for two reasons. First, the vitamin B3 was found along with its structural isomers – related molecules that have the same chemical formula but whose atoms are attached in a different order. These other molecules aren’t used by life. Non-biological chemistry tends to produce a wide variety of molecules — basically everything permitted by the materials and conditions present — but life makes only the molecules it needs. If contamination from terrestrial life was the source of the vitamin B3 in the meteorites, then only the vitamin should have been found, not the other, related molecules.

Second, the amount of vitamin B3 found was related to how much the parent asteroids had been altered by water. This correlation with conditions on the asteroids would be unlikely if the vitamin came from contamination on Earth.

The team plans to conduct additional interstellar chemistry experiments under more realistic conditions to better understand how vitamin B3 can form on ice grains in space. “We used pyridine-carbon dioxide ice in the initial experiment,” said Smith. “We want to add water ice (the dominant component of interstellar ices) and start from simpler organic precursors (building-block molecules) of vitamin B3 to help verify our result.”

Kansas scientists probe mysterious possible comet strikes on Earth

It’s the stuff of a Hollywood disaster epic: A comet plunges from outer space into the Earth’s atmosphere, splitting the sky with a devastating shock wave that flattens forests and shakes the countryside.

But this isn’t a disaster movie plotline.

“Comet impacts might be much more frequent than we expect,” said Adrian Melott, professor of physics and astronomy at the University of Kansas. “There’s a lot of interest in the rate of impact events upon the Earth. We really don’t know the rate very well because most craters end up being destroyed by erosion or the comets go into the ocean and we don’t know that they’re there. We really don’t have a good handle on the rate of impacts on the Earth.”

An investigation by Melott and colleagues reveals a promising new method of detecting past comet strikes upon Earth and gauging their frequency. The results will be unveiled at the American Geophysical Union’s Fall Meeting, to be held Dec. 14-18 in San Francisco.

The research shows a potential signature of nitrate and ammonia that can be found in ice cores corresponding to suspected impacts. Although high nitrate levels previously have been tied to space impacts, scientists have never before seen atmospheric ammonia spikes as indicators of space impacts with our planet.

“Now we have a possible new marker for extraterrestrial events in ice,” Melott said. “You don’t just look for nitrates, you also look for ammonia.”

Melott studied two possible cometary airbursts with Brian Thomas, assistant professor of physics and astronomy at Washburn University, Gisela Dreschhoff, KU adjunct associate professor of physics and astronomy, and Carey Johnson, KU professor of chemistry.

In June 1908, a puzzling explosion rocked central Siberia in Russia; it came to be known as the “Tunguska event.” A later expedition found that 20 miles of trees had been knocked down and set alight by the blast. Today, scientists have coalesced around the idea that Tunguska’s devastation was caused by a 100-foot asteroid that had entered Earth’s atmosphere, causing an airburst.

Some 13,000 years earlier, an occurrence thought by some researchers to be an extraterrestrial impact set off cooler weather and large-scale extinctions in North America. The “Younger Dryas event,” as it is known, coincided with the end of the prehistoric Clovis culture.

Melott and fellow researchers examined data from ice cores extracted in Greenland to compare atmospheric chemistry during the Tunguska and Younger Dryas events. In both instances, Melott’s group found evidence that the Haber process – whereby a nitrogen fixation reaction produces ammonia – may have occurred on a large scale.

“A comet entering the atmosphere makes a big shock wave with high pressure – 6,000 times the pressure of air,” said Melott. “It can be shown that under those conditions you can make ammonia. Plus the Tunguska comet, or some fragments of it, landed in a swamp. And any Younger Dryas comet presumably hit an ice sheet, or at least part of it did. So there should have been lots of water around for this Haber process to work. We think the simplest way to explain the signal in both objects is the Haber process. Comets hit the atmosphere in the presence of a lot of water and you get both nitrate and ammonia, which is what both ice cores show.”

Melott cautions that the results are inconclusive because the ice cores are sampled at five-year intervals only, not sufficient resolution to pinpoint peaks of atmospheric nitrates and ammonia, which rapidly would have been dissipated by rains following a comet strike.

But the KU researcher contends that ammonia enhancement resulting from the Haber process could serve as a useful marker for detecting possible comet impacts. He encourages more sampling and analysis of ice cores to see where the nitrate-ammonia signal might line up with suspected cometary collisions with the Earth.

Such information could help humankind more accurately gauge the danger of a comet hitting the Earth in the future.

“There’s a whole program to watch for near-Earth asteroids as they go around the sun repeatedly, and some of them have close brushes with the Earth,” said Melott. “But comets are a whole different ball game. They don’t do that circular thing. They come straight in from far, far out – and you don’t see them coming until they push out a tail only a few years before they would enter the inner solar system. So we could be hit by a comet and only have a few years’ warning – possibly not enough time to do anything about it.”