Moon cannot rust without oxygen, Earth supplies it

Moon is rusting, particularly more on the side facing the Earth. While it cannot do so without oxygen, Earth has supplied it.

In a paper published in Science Advances, researchers have found the answer why the Moon has traces of rust, or hematite, which usually requires water and oxygen.

Study lead author Shuai Li said a rusting Moon is “very puzzling.”

“The Moon is a terrible environment for hematite to form in,” he said.

Mystery

Iron rusts with the aid of an oxidizing agent to remove the electrons, which is very unlikely to happen on the Moon.

As a starter, it has an enormous amount of hydrogen, coming from the solar wind bombarding the Moon and the Earth with hydrogen.

Earth’s magnetic field shields it from solar wind. Its satellite does not have this kind of shield.

Abundant with hydrogen, adding electrons to the materials it interacts with, Moon’s surface can’t rust.

Presence of hematite

Li turned to Abigail Fraeman and Vivian Sun, Jet Propulsion Laboratory (JPL) scientists, to review the data from the Moon Mineralogy Mapper instrument of Chandrayaan-1.  JPL built the instrument which discovered water ice on the Moon.

Fraeman and Sun studied the data to confirm Li’s discovery of hematite.

“At first, I totally didn’t believe it. It shouldn’t exist based on the conditions present on the Moon,” Fraeman said.

“But since we discovered water on the Moon, people have been speculating that there could be a greater variety of minerals than we realize if that water had reacted with rocks.”

After reviewing the data, both scientists confirmed the existence of hematite at the lunar poles.

The data, said Sun, showed that hematite is present.

How rust form

While the Moon has no atmosphere, it has traces of oxygen, which comes from Earth.

The paper explained that Earth’s magnetic field trails behind the planet like a windsock. Kaguya orbiter had discovered in 2007 that the oxygen of the Earth’s upper atmosphere can hitch on the trailing magnetotail to the Moon at a speed of 385,000 kilometers (239,000 miles).

This fits with the data from Chandrayaan-1, which showed there are more hematite on the Moon’s surface facing the Earth.

“This suggested that Earth’s oxygen could be driving the formation of hematite,” said Li.

The researchers believed that more oxygen had crossed the chasm between the Earth and its satellite billion of years ago when the two were closer than today.

Why facing Earth

The enormous amount of hydrogen prevent oxidation of the Moon’s surface. But the magneto tail of the Earth has changed the equation.

At the time the magneto tail ferries the oxygen to the Moon, it also blocks about 90 percent of the solar wind. This happens specifically during the phase when the Moon is on full view from the Earth.

During this phase of the lunar cycle, the rust on its surface, facing the Earth, can form. (Antonio Manaytay/ Mindanao Sun)

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