Rust on the Moon. How is that possible without oxygen and liquid water?

Humanity’s fleet of robotic explorers peppered throughout the solar system continue to reveal new and exciting pieces of information regarding other members of the Sun’s stellar family.  

New research from India’s Chandrayaan-1 lunar mission shows the Moon is rusting.  The baffling discovery led scientists on an intriguing search for the answer to how this seemingly impossible formation was happening on the oxygen-less Moon away from the satellite’s polar water deposits.

On Earth, when water and oxygen interact with iron, rust forms.  On Mars, the planet’s past abundance of water and concentration of atmospheric oxygen long lost to space by the solar wind combined with iron in its surface to give the planet its iconic red color.

But how can rust form far from water ice deposits on a barren oasis devoid of oxygen?

Scientists pouring over data obtained by the Indian Space Research Organisation’s (ISRO’s) Chandrayaan-1 spacecraft discovered telltale, if not surprising, signatures that clearly showed the presence of hematite, a common iron oxide (or rust), on the lunar surface.

Chandrayaan-1 — India’s first lunar mission which ceased operating on 22 October 2009 after a nearly three year mission — was a highly successful undertaking that discovered water ice in the permanently shadowed craters at the lunar poles.

The discovery proved a long-held theory about water ice and its relationship to the Moon.  But new research using Chandrayaan-1 findings returned a surprise: rust far from those polar water deposits.

Digging through the spectral data returned by Chandrayaan-1 a decade ago, Shuai Li, Researcher at the Hawaii Institute of Geophysics and Planetology at the University of Hawaii saw a signature that closely matched hematite.  

But how could rust have possibly formed when the needed ingredients are not present?

Turns out, Earth is to blame — with a little help from the Sun.

The solar wind routinely bombards the Moon with hydrogen, which actually makes it harder for hematite to form because hydrogen adds electrons to the material it interacts with.  Hematite needs the opposite: an oxidizer to remove electrons.

“It’s very puzzling,” Li said.  “The Moon is a terrible environment for hematite to form in.”

“At first, I totally didn’t believe it.  It shouldn’t exist based on the conditions present on the Moon,” said Dr. Abigail Fraeman, co-author of the study and an Earth and Planetary Scientist at NASA’s Jet Propulsion Lab (JPL).  “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.”

Dr. Vivian Sun, Planetary Scientist and Systems Engineer in the Science Planning Group at JPL, who with Dr. Fraeman was asked to corroborate the findings, added, “In the end, the spectra were convincingly hematite-bearing, and there needed to be an explanation for why it’s on the Moon.”

That brought the team back to the overall question of how hematite could form when hydrogen is being delivered by the solar wind to areas devoid of oxygen and water.

The largest part of the answer stems from Earth’s protective magnetic field.  In regard to the lunar rust conundrum, Earth’s magnetic field bends the solar wind around the planet, shielding us from the constant stream of energetic particles — and the hydrogen so detrimental to rust formation.

Earth’s magnetic field and magnetotail. (Credit: NASA/Goddard/Aaron Kaase)

For most of its 29.5-day synodic orbit (the period of the lunar phases), the Moon moves outside of Earth’s magnetic field, taking the full brunt of all of the hydrogen delivered by the solar wind

But our planet’s magnetic field is compacted and streamlined by the solar wind and forms a tail (called the magnetotail) that extends backward (directly away from the side of Earth facing the Sun) more than 385,000 kilometers. 

When the Moon is on the opposite side of Earth from the Sun (full moon period), it is possible for it to be inside the magnetotail where nearly all of the hydrogen delivered by the solar wind is blocked.

This opens a small window every orbit during which hematite (rust) could form if all conditions are correct and providing the Moon is close enough to be inside the magnetotail as it approaches “full” status.

Of note, the Moon’s orbit is slightly eccentric, with a perigee of 363,228 km and an apogee of 405,440 km — so not every full moon results in the satellite’s passage through our planet’s magnetotail.

Nevertheless, this passage is the critical first — and second — piece of the lunar rust puzzle.

As it turns out, the magnetotail is also responsible for the delivery of trace amounts of upper atmospheric oxygen to the Moon. 

According to a 2007 publication using data from Japan’s SELENE (Selenological and Engineering Explorer) — better known as Kaguya — lunar orbiter, Earth’s magnetic field ferries small amounts of oxygen in the upper atmosphere along the magnetotail to the surface of the Moon.

As the Moon passes through the magnetotail, not only is nearly all of the hydrogen carried by the solar wind blocked, but that same blocking force also carries trace amounts of oxygen from Earth to the lunar surface.

Artist’s conception of the lunar dust exosphere surrounding the Moon. (Credit: University of Colorado Boulder/Daniel Morgan/Jamey Szalay

When these first two elements are combined, one would expect to find more hematite on the Earth-facing side of the Moon than the far side.  And that is exactly what data from the Chandrayaan-1 mission shows.

With two important elements of the conundrum solved, the remaining question became a matter of determining where the water needed to form rust was coming from.

While there are significant quantities of water ice trapped in permanently shadowed craters near the lunar poles, the hematite detected by Chandrayaan-1 is not located at this water source.

As it turns out, a seemingly unrelated element of the Moon’s environment likely provides the answer. 

The Moon is actually engulfed in a permanent dust cloud — a discovery made using data from NASA’s LADEE (Lunar Atmosphere and Dust Environment Explorer) mission which launched in September 2013 on a now-Northrop Grumman Minotaur V rocket from the Mid-Atlantic Regional Spaceport on Wallops Island, Virginia.

Li et al. propose that water molecules found naturally and in small quantities in the lunar regolith (a fine, powdery top soil) are kicked up and distributed by this dust cloud just enough for them to mix with the iron in lunar rocks and the oxygen already delivered by the magnetotail.  

If this combination happens while the Moon is passing through Earth’s magnetotail when almost all hydrogen is blocked, the stage is set for the formation of rust.

Speaking of the discovery, Dr. Sun said, “I think these results indicate that there are more complex chemical processes happening in our solar system than have been previously recognized.  We can understand them better by sending future missions to the Moon to test these hypotheses.”

Unlocking the process for how rust formed on Earth’s airless Moon could also provide insight to how rust forms on other airless bodies throughout the solar system, such as asteroids.  According to Dr. Fraeman, “It could be that little bits of water and the impact of dust particles are allowing iron in these bodies to rust.”

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