We know it from Mars and also from Earth: When iron comes into contact with oxygen, it often forms the iron oxide hematite – it rusts. But now planetary researchers have also discovered hematite on the moon. Spectral data from the Chandrayaan-1 lunar probe confirm the existence of hematite in the lunar polar regions and especially on the side facing us. So far, the formation of this iron oxide on the moon was considered unlikely due to the strongly reducing conditions and the lack of oxygen on the moon’s surface. One possible explanation, however, is the influence of our earth, because it could have supplied the moon with the necessary oxygen.
At first glance, the moon looks cold, barren and dead. Because it has no atmosphere and no magnetic field. Its surface is therefore exposed to the hard radiation of space, the impacts of countless meteorites and also the solar wind. The protons contained in the solar wind also promote reducing chemical reactions in the regolith, which is why it contains more elementary or less oxidizing metals. Oxygen, the gas that triggers oxidation reactions, is missing on Earth’s satellite. That is why there should be no rust on the moon, for example in the form of the iron oxide hematite (Fe2O3). Because this mineral is only formed when iron is in contact with oxygen and water. Tiny traces of hematite were found in some moon rock samples from the Apollo missions. However, it remained a matter of dispute whether it was a matter of earthly contamination or a subsequent reaction.
Hematite in lunar polar regions
But now data from the Moon Mineralogy Mapper on board the Indian lunar probe Chandrayaan-1 provide evidence that there is indeed hematite on the moon. The satellite also rusts accordingly. “When I was evaluating data for the lunar polar regions, I found some spectral signatures that differed from those of the lower latitudes and the Apollo rock samples,” reports first author Shuai Li of the University of Hawaii at Manoa. A closer analysis of the spectra revealed that these signatures must have come from hematite. “At first I couldn’t believe it. Because under the conditions on the moon, this mineral should not exist there, ”says co-author Abigail Fraeman from NASA’s Jet Propulsion Laboratory (JPL). But closer analysis confirmed that there is hematite on the lunar surface. “Hematite-like absorption spectra near 0.85 micrometers are common in the high latitudes of the moon,” the researchers said. According to this, the iron oxide is concentrated in the lunar polar regions. It is also more common on the side of the moon facing us than on the far side.
This makes it clear that, contrary to previous assumptions, there is “rust” on the moon. However, this raises the question of how it could have originated under the rather reducing conditions of the earth’s satellite. “The fact that there is more hematite on the side facing us indicates a connection with the earth,” says Li. As he explains, measurements by the Japanese lunar probe Kaguya have shown some time ago that plasma and oxygen from the upper earth atmosphere can also reach the moon. This transfer occurs mainly during a full moon, when the moon is behind the earth as seen from the sun. Then he dives into the long drawn out magnetic tail of the earth and receives an average of 26,000 oxygen ions per square centimeter and second, as the researchers determined. “That’s enough to form around five to nine percent by weight of hematite,” say Li and his team.
Oxidation and reduction equilibrium
Another factor is the fact that the reducing solar wind does not hit the moon equally hard everywhere. The influx of these high-energy particles is particularly high in the equatorial region and the lower latitudes, while in the higher latitudes it is up to 90 percent lower. “That speaks in favor of a less reducing environment in the high lunar latitudes,” said Li and his colleagues. In addition, at certain times the moon is almost completely shielded from the solar wind: When the moon is full, it passes behind the earth and dips into the foothills of its magnetic tail. This reduces the solar wind influx to less than one percent, as the scientists explain. At these times, another reaction could take place in the lunar regolith that ultimately leads to hematite: the water bound in the lunar regolith could react with the tiny iron granules to form iron hydroxide oxide (FeOOH). This is then converted into hematite by solar radiation and the impact of micrometeorites. This process could especially come to fruition on the far side of the moon. “Interestingly enough, there is also hematite there, albeit less,” says Li.
Overall, all of this suggests that there is a complex equilibrium on the moon consisting of the oxidizing effect of earthly oxygen and lunar water on the one hand and the reducing effect of hydrogen in the solar wind on the other. And especially on the side facing us and in the lunar polar regions, this equilibrium is evidently in favor of oxidation and thus for hematite. “These results suggest that more complex chemical processes take place in our solar system than we previously thought,” says co-author Vivian Sun of JPL.
Source: Shuai Li (University of Hawaii, Manoa), et al., Science Advances, doi: 10.1126 / sciadv.aba1940