For about half of Earth’s 4.6 billion-year existence, the atmosphere contained only carbon dioxide and nitrogen, with no oxygen. But this changed when cyanobacteria, also known as blue-green algae, started producing the first oxygen using nitrogenase, which led to the Great Oxidation Event, fueling the evolution of life on Earth.
Rust Discovered at Moon’s High Latitudes
To the surprise of many planetary scientists, the oxidized reddish-brown iron mineral hematite has been discovered at high latitudes on the Moon, reshaping our knowledge about the Moon’s polar regions. “Earth may have played an important role on the evolution of the Moon’s surface,” says Shuai Li, assistant researcher at the Hawai’i Institute of Geophysics and Planetology (HIGP) in the University of H Hawai’i Mānoa School of Ocean and Earth Science and Technology (SOEST).
Thought to be a Scientific Impossibility
Scientists led by Li now speculate in the journal Science Advances that Earth’s life-giving oxygen highly reactive with iron was carried to the poles of the Moon by wind creating what was thought to be a scientific impossibility, rust—a reddish-brown oxide left behind when iron atoms react with oxygen and water in what is known as an oxidizing, or electron-losing, reaction–where solar winds constantly blast its cratered surface with charged hydrogen, causing it to have highly reducing, or electron-gaining, condition that prohibit oxidation.
The lunar surface and interior, however, are virtually devoid of oxygen, so pristine metallic iron is prevalent on the Moon and highly oxidized iron has not been confirmed in samples returned from the Apollo missions. In addition, hydrogen in solar wind blasts the lunar surface, which acts in opposition to oxidation. So, the presence of highly oxidized iron-bearing minerals, such as hematite, on the Moon is an unexpected discovery.
When the Moon is in Earth’s Magnetotail
“Our hypothesis is that lunar hematite is formed through oxidation of lunar surface iron by the oxygen from the Earth’s upper atmosphere that has been continuously blown to the lunar surface by solar wind when the Moon is in Earth’s magnetotail during the past several billion years,” said Li, who’s research was inspired by his previous discovery of water in the Moon’s polar regions in 2018.
“I don’t think anyone expected this on the Moon’s surface,” said Li, the first author of the paper. “This is basic chemistry—we all know that the lunar surface is highly reducing, so there is no reason you would be able to see a high-valence iron like hematite.”
To make this discovery, Li, HIGP professor Paul Lucey and co-authors from NASA’s Jet Propulsion Laboratory (JPL) and elsewhere analyzed the hyperspectral reflectance data acquired by the Moon Mineralogy Mapper (M3) designed by NASA JPL onboard India’s Chandrayaan-1 mission.
“When I examined the M3 data at the polar regions, I found some spectral features and patterns are different from those we see at the lower latitudes or the Apollo samples,” said Li. “I was curious whether it is possible that there are water-rock reactions on the Moon. After months investigation, I figured out I was seeing the signature of hematite.”
Concentrated on the Moon’s Nearside
The team found the locations where hematite is present are strongly correlated with water content at high latitude Li and others found previously and are more concentrated on the nearside, which always faces the Earth.
“More hematite on the lunar nearside suggested that it may be related to Earth,” said Li. “This reminded me a discovery by the Japanese Kaguya mission that oxygen from the Earth’s upper atmosphere can be blown to the lunar surface by solar wind when the Moon is in the Earth’s magnetotail. So, Earth’s atmospheric oxygen could be the major oxidant to produce hematite. Water and interplanetary dust impact may also have played critical roles”
“Interestingly, hematite is not absolutely absent from the far-side of the Moon where Earth’s oxygen may have never reached, although much fewer exposures were seen,” said Li. “The tiny amount of water (< ~0.1 wt.%) observed at lunar high latitudes may have been substantially involved in the hematite formation process on the lunar far-side, which has important implications for interpreting the observed hematite on some water poor S-type asteroids.”
The research team hopes the NASA’s ARTEMIS missions can return hematite samples from the polar regions. The chemical signatures of those samples can confirm their hypothesis whether the lunar hematite is oxidized by Earth’s oxygen and may help reveal the evolution of the Earth’s atmosphere in the past billions of years.
The Daily Galaxy, Jake Burba, via UH Mānoa School of Ocean and Earth Science and Technology (SOEST)
Image Credit: NASA