Where do the crystals of salt in space come from? Japan's Hayabusa spacecraft has found tiny grains of salt in a sample of asteroids, suggesting that the presence of liquid water may be more common in the solar system's largest asteroid population than previously thought. Sodium chloride, better known as table salt, is not the kind of mineral that captures the imagination of scientists. However, researchers at the University of Arizona's Lunar and Planetary Laboratory were intrigued by some tiny salt crystals found in asteroid samples. This is because such crystals can only form in the presence of liquid water.

Asteroid Itokawa as seen by the Hayabusa spacecraft. The peanut-shaped S-shaped asteroid is about 1,100 feet in diameter and rotates once every 12 hours. Source: Japan Aerospace Exploration Agency

What's even more interesting, according to the team, is that the sample came from an S-type asteroid, which is known to mostly lack hydrated or hydrous minerals. The discovery strongly suggests that the large number of asteroids hurtling through the solar system may not be as dry as previously thought. The findings, published in Nature Astronomy, once again give impetus to the hypothesis that most, if not all, of Earth's water may have reached Earth via asteroids during the planet's turbulent youth.

The study's senior author, Tom Zega, a professor of planetary science at the University of Arizona's Lunar and Planetary Laboratory, and the study's lead author, Shaofan Cheh, a postdoctoral fellow at the Lunar and Planetary Laboratory Che conducted a detailed analysis of samples collected by the Japanese Hayabusa mission from the asteroid Itokawa in 2005 and brought to Earth in 2010.

Artistic impression of Japan's Hayabusa spacecraft landing on asteroid Itokawa in 2005. University of Arizona researchers Shaofan Cha and Tom Zega analyzed a particle brought to Earth by the Hayabusa mission in 2010. Source: JAXA/ Akihiro Ikeshita

The study is the first to prove that the salt crystals originated from the asteroid's parent body, ruling out the possibility that the samples formed from contamination after reaching Earth, a problem that has plagued previous studies finding sodium chloride in meteorites of similar origin.

Zega said: "These particles look like what you would see if you put table salt in your home under an electron microscope. They're beautiful square crystals. It was also funny because we had a lot of heated discussions about them in group meetings because it was so unreal."

The samples represent a type of extraterrestrial rock known as common nephrite. Such meteorites come from so-called S-type asteroids such as Itokawa, which account for 87 percent of meteorites collected on Earth. Very few of them have been found to contain water-bearing minerals.

"Ordinary meteorites have long been considered an unlikely source of water on Earth," said Zega, director of the Lunar and Planetary Laboratory's Kuiper Materials Imaging and Characterization Facility. "Our discovery of sodium chloride tells us that this asteroid cluster may contain a lot more water than we thought."

In the lab, Che and Zega embedded dust particles from the asteroid Itokawa into epoxy resin and prepared to thinly slice them. The scale says 200 microns, about the width of two or three human hairs placed side by side. Photo Credit: Shaofan Cha and Tom Zega/University of Arizona

Today, Che said, scientists largely agree that Earth, and other rocky planets like Venus and Mars, formed around the young sun, in the inner regions of swirling clouds of gas and dust known as solar nebulae, where temperatures were very high - too high for water vapor to condense out of the gas.

In other words, Earth's water would have to be transported from the outside of the solar nebula, where temperatures are much cooler, allowing for water to exist, and most likely in the form of ice. The most likely scenario is that comets or another type of asteroid known as a C-type asteroid migrated inwards farther into the solar nebula and delivered their water cargo by hitting the young Earth.

The discovery that water may be present in common nephrite, and therefore its source closer to the sun than its "wetter" counterpart, has implications for any attempt to explain the delivery of water to the early Earth.

The sample used in the study was a tiny dust particle with a diameter of about 150 microns, about twice the diameter of a human hair, from which the team cut a small section about 5 microns wide for analysis, just big enough to cover a yeast cell.

Using a variety of techniques, the researchers were able to rule out that sodium chloride was caused by sources such as human sweat, sample preparation processes, or exposure to laboratory moisture.

Since the samples had been stored for five years, the team took before and after photos of the samples and compared them. The photos showed no change in the distribution of sodium chloride particles within the sample, ruling out the possibility that any particles were deposited into the sample during this period. In addition, they conducted a controlled experiment in which they processed a set of land rock samples identical to the Itokawa samples and examined them with an electron microscope.

The researchers cut through the epoxy resin with a diamond knife to reveal slices inside the dust particles, which are seen under an electron microscope. Photo Credit: Shaofan Che and Tom Zega/University of Arizona

"The land samples did not contain any sodium chloride, which gives us confidence that the salt in our sample is the primary salt of asteroid Itokawa," he said. We have ruled out all possible sources of contamination."

Zega said tons of extraterrestrial material land on Earth every day, but most of it burns up in the atmosphere and never reaches the surface. You need a rock that's big enough to survive entering the Earth and deliver that water."

Research led in the 1990s by the late Michael Drake, former director of the Lunar and Planetary Laboratory, proposed a mechanism by which water molecules in the early solar system could have become trapped in asteroid minerals and even survived impact with Earth.

"These studies show that several oceans' worth of water can be provided through this mechanism alone," Zegar said. "If it turns out now that the most common asteroids may be much 'wetter' than we thought, that would make the water-carrying asteroid hypothesis more plausible."

Itokawa is a peanut-shaped near-Earth asteroid about 2,000 feet long and 750 feet in diameter that is believed to have split off from a much larger parent body. It is conceivable that frozen water and frozen hydrogen chloride could have accumulated there, and that the natural decay of radioactive elements and frequent bombardment by meteorites in the early solar System could have provided enough heat to sustain hydrothermal processes involving liquid water, Che and Zega argue. Eventually, the parent body succumbs to the impact and breaks into smaller pieces, leading to the formation of Itokawa.

Zega said: "Once these ingredients come together to form an asteroid, it is possible to form liquid water. And once liquids form, we can assume that they occupy the asteroid's cavity and potentially engage in water chemistry."

However, the evidence that the salt crystals in the Itokawa sample have been present since the dawn of the solar system does not end there. In the sample, the researchers found a vein of plagioclase rich in sodium chloride, a sodium-rich silicate mineral.

Che Shaofan said: "When we see these altered veins in land samples, we know they were formed by water alteration, which means there must have been water involved. "The fact that we see this texture associated with sodium and chlorine is another strong piece of evidence that this is happening on asteroids because water flows in this sodium-containing silicate."