In 2011, NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) probe confirmed previous Earth-based detections of water ice from Mercury’s North Pole. A pretty unexpected discovery for a planet with a daytime temperature of around 400°C (750°F).
Until now, the scientific consensus suggested that most of Mercury’s water was delivered by asteroids, similar to water’s arrival on Earth. However, a new mechanism, proposed by scientists at the Georgia Institute of Technology, suggests Mercury itself is a “gigantic ice-making chemistry lab,” aided by its stellar neighbor, the Sun. This process could easily account for up to 10 percent of Mercury’s total ice, the researchers say.
"This is not some strange, out of left field idea. The basic chemical mechanism has been observed dozens of times in studies since the late 1960s," Brant Jones, a researcher in Georgia Tech's School of Chemistry and Biochemistry and the paper's first author, said in a statement. "But that was on well-defined surfaces. Applying that chemistry to complicated surfaces like those on a planet is groundbreaking research."
The process, detailed in Astrophysical Journal Letters, begins with the delivery of protons to Mercury’s surface via strong solar winds.
"These are like big magnetic tornados, and they cause huge proton migrations across most of the surface of Mercury over time," Thomas Orlando, a professor in Georgia Tech's School of Chemistry and Biochemistry and the study's principal investigator, explained.
Having embedded themselves about 10 nanometers deep into the planet’s surface, the protons produce chemically stable mineral-bound hydroxyl groups (OH). Due to the extreme heat on the planet, these entities are freed up and smashed together, resulting in the production of water molecules and hydrogen.
In the model, these molecules drift around Mercury, where they are either broken down by sunlight, rise far above the surface, or land in the planet’s polar regions. Lying in the permanent shadows of craters, the molecules are shielded from the Sun. In the -200°C (-328°F) temperatures, the water molecules become part of the polar glacial ice.
"It's a little like the song Hotel California. The water molecules can check in to the shadows but they can never leave," Orlando joked.
All this talk of craters may have you wondering whether this same mechanism is applicable to another impacted celestial body, our Moon. Although they’re roughly similar in size, have no significant atmosphere, and have ice deposits in their polar craters where the Sun never shines, the researchers say that their model would not be compatible with the Moon.
"The process in our model would not be anywhere near as productive on the Moon. For one, there's not enough heat to significantly activate the chemistry," Jones said. However, on Mercury the researchers postulate that the process would yield around 1013 kilograms (10,000,000,000,000 kg or 11,023,110,000 tons) of ice, over a period of about 3 million years.
"I would concede that plenty of the water on Mercury was delivered by impacting asteroids," Jones remarked, "but there's also the question of where asteroids laden with water got that water. Processes like these could have helped make it."
Orlando’s lab is now looking to exploit this chemical process to help generate water in future human missions to the Moon.