The Moon appears to be 'rusting' due to a wind flowing from Earth
Science is a step closer to discovering the source of iron oxide found on the Moon.
Hematite, a form of iron oxide similar to rust, has been found on the Moon, and until now, the source was a confounding mystery.
When scientists examined the data returned by India's first lunar orbiter, Chandrayaan-1, they found something very unexpected. There was hematite on the Moon's surface.
A sample of hematite from Brazil is shown here next to the chemical formula and structure of the iron oxide molecules it is composed of. (Houston Museum of Natural Science/Scott Sutherland)
Hematite is a mineral form of iron oxide, the same substance as rust. Both are produced when iron is exposed to water and oxygen. The only difference between them, really, is that the iron oxide molecules in hematite are arranged in an orderly crystalline lattice, whereas the same iron oxide found in the reddish flakes peeling off of an exposed iron fence post, for example, are far more disorderly.
There's a problem with finding hematite on the Moon, though.
We've known about water on the Moon for some time now. The permanently shadowed craters at the lunar south pole potentially have vast stores of it, locked away as ice. This water could be released into the environment, temporarily, due to dust particles and micrometeorites striking the surface.
These highlighted craters at the Moon's south pole are in permanent darkness, which has allowed water to collect in them over millions to billions of years. (NASA)
READ MORE: NASA has its first detailed map of water on the Moon
However, the Moon's thin atmosphere contains no oxygen. So, how did the hematite form?
Based on a study from 2020, researchers believe it may be due to what's known as the 'Earth-wind'.
Just as the Sun produces the solar wind — a flow of charged particles originating from the solar corona — the Earth-wind is a flow of molecules that escapes from the top of Earth's atmosphere. It includes ionized oxygen, nitrogen, and even water molecules, which are drawn into space by the planet's magnetosphere. The Earth-wind then carries them well beyond the orbit of the Moon.
This diagram shows the Earth-wind flowing into the planet's magnetosphere and influencing the Moon. (H.Z. Wang et al 2021/The Astrophysical Journal Letters 907 L32)
READ MORE: NASA finds mysterious force that lofts Earth's atmosphere into space
For up to five days out of every lunar orbit, the Moon passes straight through this flow from Earth's atmosphere. So, hypothetically, that could provide the needed oxidation for iron deposits on the lunar surface to become hematite.
In a new study published earlier this month, another set of researchers took this idea into the laboratory to test the hypothesis. They simulated the Earth-wind in action by bombarding the kind of iron minerals found on the Moon with high-energy ionized oxygen. Their results show that some of the iron samples were converted to hematite.
Since the Moon spends more time bathed in the solar wind than the Earth-wind, the research team also exposed the samples to both high-energy and low-energy streams of hydrogen ions. This simulated the solar wind's impacts at different points of the Sun's 11-year solar cycle. While the high-energy hydrogen did convert some of the hematite back to iron, the low-energy hydrogen ions did not.
Thus, depending on the flow of oxygen from Earth, and high-energy vs low-energy hydrogen from the Sun, hematite could build up in sufficient amounts to be detected from orbit.
Shuai Li, the planetary scientist at the University of Hawaii at Mānoa who led the 2020 study, told Nature that if a future mission brought back samples of lunar hematite, researchers could analyse the oxygen contained within the minerals to confirm if it traces back to the Earth-wind.
(While the iron oxide on the Moon does not turn it red, to go along with the concept of 'rusting', the thumbnail image — provided by NASA's Scientific Visualization Studio — depicts the Moon with a rust-red filter added using image processing software.)