How did Earth get its water? New clues discovered in an asteroid sample

The Japanese Hayabusa spacecraft brought an asteroid sample to learn more about Earth’s water. Tiny salt grains discovered in this sample provide strong evidence that liquid water may be more common in the solar system than previously thought.

Table salt, or sodium chloride, while common in our everyday lives, is usually not a substance that intrigues scientists. But at the University of Arizona Lunar and Planetary Laboratory, researchers have found a newfound excitement in minuscule salt crystals that have been found in asteroid samples. These crystals indicate the presence of liquid water.

This asteroid sample, interestingly, is from an S-type asteroid. This category of asteroids is generally known for lacking hydrated, or water-bearing, minerals. Hence, the researchers are now hinting at a possibility that many asteroids speeding through our solar system may not be as dry as we originally thought.

Researchers hope to prove that all of Earth’s water came from asteroids

This discovery, published in the journal Nature Astronomy, breathes new life into the theory that the majority, if not all, of Earth’s water could have been delivered via asteroids during Earth’s early, chaotic stages.

Study senior author Professor Tom Zega, along with lead author Shaofan Che, conducted a thorough analysis. They studied samples gathered from asteroid Itokawa by the Japanese Hayabusa mission in 2005 and returned to Earth in 2010 (see image here).

This is the first study that proves these salt crystals originated on the asteroid’s parent body. It excludes any possibility that the crystals formed due to contamination after the sample reached Earth. This crucial point resolves issues raised by earlier studies that found sodium chloride in similar meteorites but couldn’t rule out the contamination factor.

What the asteroid samples revealed

“The grains look exactly like what you would see if you took table salt at home and placed it under an electron microscope. They’re these nice, square crystals. It was funny, too, because we had many spirited group meeting conversations about them, because it was just so unreal,” explained Zega.

The samples examined are a type of extraterrestrial rock known as ordinary chondrite, which originates from S-type asteroids such as Itokawa. About 87% of meteorites found on Earth fall into this category, but very few of them contain water-bearing minerals.

Zega, also the director of the Lunar and Planetary Laboratory’s Kuiper Materials Imaging & Characterization Facility, commented, “It has long been thought that ordinary chondrites are an unlikely source of water on Earth. Our discovery of sodium chloride tells us this asteroid population could harbor much more water than we thought.”

Most scientists concur that Earth and other rocky planets, including Venus and Mars, formed in the solar nebula’s inner region. This region was a tumultuous cloud of gas and dust surrounding our young Sun, where temperatures were too high for water vapor to condense, according to Che.

“In other words, the water here on Earth had to be delivered from the outer reaches of the solar nebula, where temperatures were much colder and allowed water to exist, most likely in the form of ice. The most likely scenario is that comets or another type of asteroid known as C-type asteroids, which resided farther out in the solar nebula, migrated inward and delivered their watery cargo by impacting the young Earth,” explained Che.

The discovery that water could have originated from ordinary chondrites, which are much closer to the sun than their “wetter” counterparts, is significant for any theory attempting to explain how water reached Earth.

How the study was conducted

The sample studied was a tiny dust particle, about 150 micrometers in diameter (around twice the width of a human hair), from which the team took a 5-micron wide section for analysis.

Che managed to verify that the sodium chloride was not a result of contamination from human sweat, sample preparation, or laboratory moisture. This was achieved through various techniques. 

The researchers also observed that the distribution of sodium chloride grains inside the sample had not changed over a five-year storage period. This observation clearly demonstrated that the grains were not introduced into the sample during that time.

Further assurance came from a control experiment where Che treated terrestrial rock samples identically to the Itokawa sample and studied them under an electron microscope. “The terrestrial samples did not contain any sodium chloride, so that convinced us the salt in our sample is native to the asteroid Itokawa. We ruled out every possible source of contamination,” said Che.

Tom Zega emphasized the everyday influx of extraterrestrial matter to Earth. He pointed out that most of it combusts in the atmosphere and never reaches the surface, but, he added, “You need a large enough rock to survive entry and deliver that water.”

Asteroid water delivery theory first proposed in the 1990s

Michael Drake, the late director of the Lunar and Planetary Lab, proposed in the 1990s a theory that water molecules in the early solar system could become trapped in asteroid minerals and survive an impact on Earth. 

Zega referenced these studies, stating, “Those studies suggest several oceans worth of water could be delivered just by this mechanism. If it now turns out that the most common asteroids may be much ‘wetter’ than we thought, that will make the water delivery hypothesis by asteroids even more plausible.”

The asteroid Itokawa is a peanut-shaped near-Earth asteroid. It’s approximately 2,000 feet long and 750 feet in diameter and is believed to have broken off from a much larger parent body.

Zega and Che speculate that this parent body could have accumulated frozen water and frozen hydrogen chloride. The heat from the natural decay of radioactive elements and constant meteorite bombardments in the early solar system could have sparked hydrothermal processes involving liquid water.

Eventually, the parent body could have broken up due to the relentless battering, leading to the formation of Itokawa. 

“Once these ingredients come together to form asteroids, there is a potential for liquid water to form. And once you have liquids form, you can think of them as occupying cavities in the asteroid, and potentially do water chemistry,” said Zega.

Another discovery by the research team

The team also discovered a vein of plagioclase, a sodium-rich silicate mineral, within the sample, enriched with sodium chloride. 

“When we see such alteration veins in terrestrial samples, we know they formed by aqueous alteration, which means it must involve water,” said Che. “The fact that we see that texture associated with sodium and chlorine is another strong piece of evidence that this happened on the asteroid as water was coursing through this sodium-bearing silicate.”

This finding and the unchanging salt crystals imply that the process was already occurring when the solar system was still in its infancy. 

In conclusion, the research suggests that ordinary chondrites, once considered an unlikely source of water, may have played a more significant role in the arrival of water on Earth than previously believed.

More about the Hayabusa mission 

The Hayabusa mission was a remarkable endeavor by the Japan Aerospace Exploration Agency (JAXA). The mission targeted asteroid 25143 Itokawa, named after Hideo Itokawa, a Japanese rocket scientist.

Hayabusa, which translates to “Peregrine Falcon” in English, was launched in May 2003. The spacecraft was tasked with an ambitious goal: to travel to Itokawa, land on the asteroid, collect samples, and then return to Earth. This kind of mission had never been attempted before.

Exploring near-Earth asteroid Itokawa 

Asteroid Itokawa is a near-Earth object, classified as an Apollo asteroid, the most common group of near-Earth asteroids. It’s a relatively small asteroid, with a length of about 540 meters (1770 feet) and a width of approximately 270 meters (890 feet). Itokawa’s surface varies greatly, with a smoother area named the “Muses Sea” and a rough, rocky region called the “Sagamihara.”

After a journey of two and a half years, Hayabusa reached Itokawa in September 2005. Over the next few months, the spacecraft studied the asteroid from a close distance, captured hundreds of images, and made several attempts to touch down on the asteroid’s surface to collect samples.

Although the mission encountered numerous challenges, including a malfunction that led to the loss of the spacecraft’s Minerva mini-robot, Hayabusa managed to touch down on Itokawa twice in November 2005. While it’s uncertain if the spacecraft was successful in firing a small metal ball into the asteroid’s surface to collect samples, as planned, it did likely stir up and capture some dust during touchdown.

Returning the sample to Earth

In 2007, Hayabusa began its journey back to Earth, enduring further malfunctions and difficulties. Despite the odds, the spacecraft returned to Earth in June 2010, becoming the first mission to return asteroid samples to our planet. The re-entry capsule, containing the collected samples, landed in the Woomera Prohibited Area, Australia.

Scientists confirmed that the capsule held over a thousand grains of asteroid material. This discovery opened a new window into understanding the early solar system, as these grains could carry valuable information about the formation and history of asteroids like Itokawa.

The Hayabusa mission was considered a success despite its difficulties, and it paved the way for subsequent asteroid sample return missions like Hayabusa2 and NASA’s OSIRIS-REx.

More theories about how Earth got its water 

There are several theories about how Earth got its water. Here are some of the primary ones:

Cometary delivery

This theory suggests that comets, which are essentially ‘dirty snowballs’ made of ice and rock, delivered water to Earth through impacts. During the early stages of the solar system, it’s thought there was a period known as the “Late Heavy Bombardment” when the inner planets were frequently hit by comets and asteroids. However, the isotopic composition of water in comets doesn’t always match that of Earth’s oceans, which casts some doubt on this theory.

Asteroid delivery 

Asteroids could have also contributed significantly to Earth’s water. Unlike comets, which originate from the outer regions of the solar system, asteroids come from the asteroid belt located between Mars and Jupiter. Some types of asteroids, particularly carbonaceous chondrites, are known to contain water and could potentially match the isotopic composition of Earth’s water more closely than comets.

Volcanic outgassing 

This theory posits that water was always present in the Earth, locked within its rocks. Over time, volcanic activity brought this water to the surface in the form of water vapor, which then condensed to form oceans. The outgassing of water from the interior of the Earth continues today, albeit at a slower rate.

Local synthesis 

Some studies suggest that water could have formed right here on Earth through a chemical reaction between the oxygen in the atmosphere and hydrogen in the solar wind. However, this theory is still a topic of ongoing research and is considered less likely than the others.

Hydrated dust grains

Another theory suggests that during the Earth’s formation, it collected lots of tiny, hydrated dust grains from the protoplanetary disk surrounding the young Sun. These grains could have then released their water content as the planet heated up, contributing to Earth’s oceans.

These theories are not mutually exclusive and it’s likely that Earth’s water came from a combination of these sources. As our technology and understanding of the universe continues to grow, so too will our understanding of where Earth’s water originated.

image Credit: JAXA

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