Pass the salt: This space rock holds clues as to how Earth got its water


Date:
June 13, 2023
Source:
University of Arizona
Summary:
The discovery of tiny salt grains in a sample from an asteroid
provides strong evidence that liquid water may be more common in
the solar system than previously thought.


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FULL STORY ========================================================================== Sodium chloride, better known as table salt, isn't exactly the type
of mineral that captures the imagination of scientists. However,
a smattering of tiny salt crystals discovered in a sample from an
asteroid has researchers at the University of Arizona Lunar and Planetary Laboratory excited, because these crystals can only have formed in the
presence of liquid water.

Even more intriguing, according to the research team, is the fact that
the sample comes from an S-type asteroid, a category known to mostly lack hydrated, or water-bearing, minerals. The discovery strongly suggests
that a large population of asteroids hurtling through the solar system
may not be as dry as previously thought. The finding, published inNature Astronomy, gives renewed push to the hypothesis that most, if not all,
water on Earth may have arrived by way of asteroids during the planet's tumultuous infancy.

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

The study is the first to demonstrate that the salt crystals originated
on the asteroid's parent body, ruling out any possibility they might have formed as a consequence of contamination after the sample reached Earth,
a question that had plagued previous studies that found sodium chloride
in meteorites of a similar origin.

"The grains look exactly like what you would see if you took table salt
at home and placed it under an electron microscope," Zega said. "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." Zega said the samples represent a type of extraterrestrial rock
known as an ordinary chondrite. Derived from so-called S-type asteroids
such as Itokawa, this type makes up about 87% of meteorites collected on
Earth. Very few of them have been found to contain water-bearing minerals.

"It has long been thought that ordinary chondrites are an unlikely
source of water on Earth," said Zega who is the director of the Lunar
and Planetary Laboratory's Kuiper Materials Imaging & Characterization Facility. "Our discovery of sodium chloride tells us this asteroid
population could harbor much more water than we thought." Today,
scientists largely agree that Earth, along with other rocky planets such
as Venus and Mars, formed in the inner region of the roiling, swirling
cloud of gas and dust around the young sun, known as the solar nebula,
where temperatures were very high -- too high for water vapor to condense
from the gas, 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," Che said. "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 been present in ordinary chondrites,
and therefore been sourced from much closer to the sun than their "wetter"
kin, has implications for any scenario attempting to explain the delivery
of water to the early Earth.

The sample used in the study is a tiny dust particle spanning about 150 micrometers, or roughly twice the diameter of a human hair, from which
the team cut a small section about 5 microns wide -- just large enough
to cover a single yeast cell -- for the analysis.

Using a variety of techniques, Che was able to rule out that the sodium chloride was the result of contamination from sources such as human sweat,
the sample preparation process or exposure to laboratory moisture.

Because the sample had been stored for five years, the team took
before and after photos and compared them. The photos showed that the distribution of sodium chloride grains inside the sample had not changed, ruling out the possibility that any of the grains were deposited into the sample during that time. In addition, Che performed a control experiment
by treating a set of terrestrial rock samples the same as the Itokawa
sample and examining them with 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,"
he said. "We ruled out every possible source of contamination." Zega said
tons of extraterrestrial matter is raining down on Earth every day, but
most of it burns up in the atmosphere and never makes it to the surface.

"You need a large enough rock to survive entry and deliver that water,"
he said.

Previous work led by the late Michael Drake, a former director of the
Lunar and Planetary Lab, in the 1990s proposed a mechanism by which water molecules in the early solar system could become trapped in asteroid
minerals and even survive an impact on Earth.

"Those studies suggest several oceans worth of water could be delivered
just by this mechanism," Zega said. "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."
Itokawa is a peanut-shaped near-Earth asteroid about 2,000 feet long
and 750 feet in diameter and is believed to have broken off from a much
larger parent body. According to Che and Zega, it is conceivable that
frozen water and frozen hydrogen chloride could have accumulated there,
and that naturally occurring decay of radioactive elements and frequent bombardment by meteorites during the solar system's early days could have provided enough heat to sustain hydrothermal processes involving liquid
water. Ultimately, the parent body would have succumbed to the pummeling
and broken up into smaller fragments, leading to the formation of Itokawa.

"Once these ingredients come together to form asteroids, there is
a potential for liquid water to form," Zega said. "And once you have
liquids form, you can think of them as occupying cavities in the asteroid,
and potentially do water chemistry." The evidence pointing at the salt crystals in the Itokawa sample as being there since the beginning of the
solar system does not end here, however. The researchers found a vein of plagioclase, a sodium-rich silicate mineral, running through 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,"
Che said. "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."
* RELATED_TOPICS
o Space_&_Time
# Asteroids,_Comets_and_Meteors # Solar_System #
Space_Missions # Space_Exploration # Nebulae # NASA #
Sun # Extrasolar_Planets
* RELATED_TERMS
o Definition_of_planet o Asteroid_belt o Asteroid
o Ceres_(dwarf_planet) o Gas_giant o Solar_system o
Extrasolar_planet o Comet

========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Daniel Stolte.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Shaofan Che, Thomas J. Zega. Hydrothermal fluid activity on asteroid
Itokawa. Nature Astronomy, 2023; DOI: 10.1038/s41550-023-02012-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/06/230613190805.htm

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