Scientists think the Earth's oxygen may have been rusting the Moon for billions of years.
The oxidised iron mineral haematite has been discovered at high latitudes on the Moon, to the surprise of planetary scientists.
Iron is highly reactive with oxygen, and forms the reddish rust commonly seen on Earth.
But the surface and interior of the Moon are virtually devoid of oxygen and highly oxidised iron has not been confirmed in samples returned from the Apollo missions.
Additionally, hydrogen in solar wind blasts the lunar surface, which acts in opposition to oxidation.
The presence of highly oxidised iron-bearing minerals, such as haematite, on the Moon is therefore an unexpected discovery.
Lead author Shuai Li is an assistant researcher at the Hawai'i Institute of Geophysics and Planetology (HIGP) in the UH Manoa School of Ocean and Earth Science and Technology (SOEST).
He said: "Our hypothesis is that lunar haematite is formed through oxidation of lunar surface iron by the oxygen from the Earth's upper atmosphere.
"It 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."
Researchers - including co-authors from Nasa's Jet Propulsion Laboratory (JPL) and elsewhere - analysed data acquired by the Moon Mineralogy Mapper (M3) designed by Nasa JPL onboard India's Chandrayaan-1 mission.
Prof Li said: "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.
"I was curious whether it is possible that there are water-rock reactions on the Moon. After months of investigation, I figured out I was seeing the signature of haematite."
According to the researchers, the locations where haematite is present are strongly correlated with water content at high latitude.
Prof Li added: "More haematite on the lunar nearside suggested that it may be related to Earth.
"This reminded me of 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 haematite. Water and interplanetary dust impact may also have played critical roles."
The study, published in Science Advances, also found that haematite is not completely absent from the far side of the Moon where Earth's oxygen may have never reached.