The Hamilton Spectator

Scientists Create Ice From the Far Beyond

By KENNETH CHANG

Scientists have discovered two new forms of salty ice that probably do not exist naturally on Earth but might be found on icy moons farther out in the solar system.

“These structures are nothing like anything that has been described before,” said Baptiste Journaux, an acting assistant professor of earth and space sciences at the University of Washington in Seattle.

In a recent issue of the Proceedings of the National Academy of Sciences, Dr. Journaux and his colleagues describe two new solid, icy combinations of two of the most common substances on Earth: water and sodium chloride, better known as table salt. The new crystals formed, unexpectedly, when salty water was chilled and squeezed to high pressures.

Sodium chloride — each molecule consists of one sodium atom and one chlorine atom — is often thought of first as an antifreeze. When salty water does freeze, the ice crystals that form are made of pure water with the sodium and chloride ions pushed out into the remaining liquid.

At cold enough temperatures, the residual supersalty

A salty experiment provides clues to galactic mysteries.

water begins to solidify, forming hydrohalite, a rigid, water-containing crystal or hydrate. Hydrohalite consists of two water molecules for each sodium chloride.

In recent decades, scientists have discovered several worlds in the outer solar system that possess liquid water oceans under their icy crusts. Those include Europa and Ganymede, two moons of Jupiter, and Titan and Enceladus, two moons of Saturn. Dr. Journaux wanted to study the role that salt might play in keeping those oceans from freezing.

To reproduce those conditions, a small amount of salty water was chilled to as low as minus 123 degrees Celsius and squeezed between two pieces of diamond to pressures up to 25,000 times the usual 1,013.53 millibars that air presses against us at the Earth’s surface.

“We were expecting to see something somewhat similar to what we see on Earth, which is the salts would be rejected from the ice as it grows,” Dr. Journaux said.

Instead, the salts froze: “We had a new crystal that came out of nowhere that we were not at all expecting.”

The crystals were at most about one-one-hundredth of a centimeter wide, roughly the width of a human hair.

Bouncing X-rays off the crystals showed that two new hydrates had been created. One had a crystal structure of two sodium chloride molecules for every 17 water molecules. That one formed at about minus 73 degrees and a pressure of 5,000 times the usual atmospheric pressure. At higher pressures, a less salty hydrate formed, with 13 water molecules for every sodium chloride molecule.

The scientists also saw signs of a third form, but the needlelike crystals were too thin to study the crystal structure. “It’s very pretty,” Dr. Journaux said, “but it’s so thin, it’s hard to get the data.”

The new hydrates could help explain a mystery on Europa. Observations in 2019 using the Hubble Space Telescope unambiguously identified sodium chloride in yellowish streaks on the moon’s surface. It is highly unlikely to be in the form of grains of pure salt, but other observations — colors of infrared light absorbed by the surface, which serve as identifying fingerprints of specific compounds — offered no convincing signs of hydrohalite, the known salt hydrate.

The new hydrate that formed at 5,000 times atmospheric pressure remained stable after the pressure was removed and at perhaps temperatures as warm as minus 40 degrees, suggesting this hydrate could have formed in Europa’s subsurface and would remain in that form if it were pushed to the surface.

The other hydrate, with 13 water molecules for every sodium chloride, might be found at the bottom of the oceans of these icy worlds, Dr. Journaux said.

Several robotic spacecraft will be headed to the outer solar system in the coming years to study these intriguing ice worlds, which many scientists say are the most promising places in the solar system to search for extraterrestrial life. The European Space Agency’s Jupiter Icy Moons Explorer mission is scheduled to launch this month. NASA is planning to launch its Europa Clipper spacecraft in October 2024 to study Europa and the Dragonfly in 2026 to head toward Titan.

Possibly, the hydrates might even turn out to be a way to store energy generated by solar panels and wind turbines.

“So there could be some real-life implication to this as well,” Dr. Journaux said.

CULTURE

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2023-04-01T07:00:00.0000000Z

2023-04-01T07:00:00.0000000Z

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