Salt crystals found in asteroid sample suggest liquid water may be more common in the solar system

Scientists at the University of Arizona Lunar and Planetary Laboratory have made an exciting discovery involving sodium chloride, commonly known as table salt. They found tiny salt crystals in a sample from an asteroid, indicating the presence of liquid water during their formation. What makes this finding even more intriguing is that the sample comes from an S-type asteroid, which is typically devoid of hydrated minerals. This suggests that many asteroids in our solar system may contain more water than previously believed. The research, published in Nature Astronomy, supports the hypothesis that asteroids played a significant role in delivering water to Earth during its early history.

The study focused on samples collected from asteroid Itokawa in 2005 by the Japanese Hayabusa mission and brought back to Earth in 2010. By conducting a detailed analysis, the researchers confirmed that the salt crystals originated on the asteroid itself and were not a result of contamination after the sample's return to Earth. Previous studies had found sodium chloride in meteorites of similar origin but faced doubts about possible contamination.

Tom Zega, a professor of planetary sciences at the UArizona Lunar and Planetary Laboratory and the senior author of the study, described the salt grains as resembling those found in a household salt shaker when observed under an electron microscope. The samples belonged to an ordinary chondrite, a type of extraterrestrial rock derived from S-type asteroids like Itokawa. Although ordinary chondrites make up approximately 87% of collected meteorites, only a small number have been found to contain water-bearing minerals.

In summary, this research highlights the presence of salt crystals and liquid water on an S-type asteroid, challenging previous assumptions about the water content of such asteroids. The findings support the idea that asteroids may have contributed to the delivery of water to Earth during its early stages of development.

Artist's impression of the Japanese spacecraft Hayabusa touching down on the asteroid Itokawa in 2005. UArizona researchers Shaofan Che and Tom Zega analyzed a particle that the Hayabusa mission brought to Earth in 2010. Credit: JAXA

Tom Zega, the director of the Kuiper Materials Imaging & Characterization Facility at the Lunar and Planetary Laboratory, emphasized the significance of their discovery, stating, “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.”

Scientists now widely agree that Earth, along with neighboring planets like Venus and Mars, formed in the inner region of the solar nebula—a turbulent cloud of gas and dust surrounding the young Sun. In this region, temperatures were too high for water vapor to condense. Shaofan Che, the lead author of the study, explained, “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.”

The discovery that water could have existed in ordinary chondrites challenges previous assumptions and has implications for theories explaining the delivery of water to Earth during its early stages.

The sample analyzed in the study was a minute dust particle measuring approximately 150 micrometers, equivalent to twice the diameter of a human hair. The researchers carefully extracted a thin section about 5 microns wide, just enough to cover a single yeast cell, for analysis.

Shaofan Che employed various techniques to confirm that the sodium chloride present was not a result of contamination from sources like human sweat, the sample preparation process, or exposure to moisture in the laboratory.

In the lab, Che and Zega embedded the dust particle from asteroid Itokawa in epoxy resin to prepare it for thin sectioning. The scale indicates 200 micrometers, about the width of two or three human hairs placed side by side. Credit: Shaofan Che and Tom Zega / University of Arizona

To ensure the integrity of their findings, the research team conducted several measures. Since the sample had been stored for five years, they took before and after photos and compared them. The comparison revealed that the distribution of sodium chloride grains remained unchanged, indicating that no new grains were deposited during that period. Moreover, a control experiment was performed on terrestrial rock samples, treating them in the same manner as the Itokawa sample and examining them under an electron microscope.

Shaofan Che explained, “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.”

Tom Zega pointed out that while a significant amount of extraterrestrial matter enters Earth's atmosphere daily, most of it burns up before reaching the surface. He highlighted the necessity of a sufficiently large rock to survive the atmospheric entry and deliver water.

In the 1990s, former Lunar and Planetary Lab director Michael Drake led previous research proposing a mechanism by which water in the early solar system could become trapped in asteroid minerals and survive impact on Earth.

Zega mentioned, “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.”

Researchers used a diamond knife to slice through the epoxy and expose a section through the inside of the dust particle, seen here under an electron microscope. Credit: Shaofan Che and Tom Zega / University of Arizona

Itokawa, the asteroid from which the sample was collected, is an elongated peanut-shaped near-Earth asteroid. It measures around 2,000 feet in length and 750 feet in diameter, and scientists believe it originated from a much larger parent body. According to Che and Zega, it is possible that frozen water and frozen hydrogen chloride accumulated on Itokawa. The decay of radioactive elements and frequent meteorite impacts during the early stages of the solar system could have generated enough heat to sustain hydrothermal processes involving liquid water. Eventually, the parent body would have suffered significant impacts and fragmented, giving rise to Itokawa.

Zega explained, “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.”

The evidence supporting the presence of salt crystals on Itokawa since the early stages of the solar system does not end there. The researchers discovered a vein of plagioclase, a sodium-rich silicate mineral, running through the sample. This vein exhibited enrichment in sodium chloride.

Che elaborated, “When we see such alteration veins in terrestrial samples, we know they formed by aqueous alteration, which means it must involve water. 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.”

Source: University of Arizona

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