China’s Chang’e-5 lunar lander has found evidence of hydroxyl (OH) on the Moon. Hydroxyl is a close chemical cousin of water, H2O. While several other orbital missions have detected OH on the Moon previously, Chang’e-5 marks the first time it has been detected by a spacecraft sitting on the lunar surface.
Chang’e-5 is most famous for its sample return mission, which brought a sample of lunar regolith back to Earth in December of 2020. However, for the detection of hydroxyl, the Chang’e-5 lander used instruments on board to look at the area around the landing site. Scientists were able to use data from a panoramic camera, lunar mineralogical spectrometer (LMS) and lunar penetrating radar to look for evidence of water molecules.
Curiously, a rock near the landing site contained the most hydroxyl, almost twice as much as the surrounding regolith. The regolith has less than 120 parts per million (ppm) of water molecules, while the rock has about 180 ppm.
Context images and water content at the Chang’E-5 landing site. Image credit: LIN Honglei.
“The spectrum of CE5-Rock exhibits a strong absorption 2.85 ?m [water’s spectral signature] because of the presence of OH/H2O,” the team wrote in their paper. “By contrast, most of the lunar regolith at the landing site exhibit no/weak absorptions … similar to the Moon Mineralogy Mapper (M3) spectra over the region. The 2.85-?m absorption band in the rock spectrum is about twice stronger than that in the regolith spectra.”
An international team of scientists led by researchers from the Chinese Academy of Sciences in Beijing published their paper in the journal Science Advances.
Water molecules were first detected on the Moon in 2009. In September of that year, scientists from three different space missions announced they had detected widespread water across the surface of the Moon. Then in November of 2009, the LCROSS science team held a press briefing to announce they had detected “buckets” of water inside the crater, where the LCROSS impactor blasted material from inside Cabeus Crater, with a spacecraft making observations of the debris.
It is largely accepted, based on numerous studies, that the water molecules on the Moon’s surface arrive through a process called solar wind implantation. Charged particles from the sun drive hydrogen atoms to the lunar surface where they later bonded with oxygen to form water and hydroxyl. While that process creates water molecules that exist diffusely in low concentrations across the Moon as hydroxyl or water molecules, the LCROSS discovery could mean underground reservoirs of water ice.
The scientists say the lunar rock with a higher water content could have been blasted from under the Moon’s surface from an meteor/asteroid impact.
“The results of compositional and orbital remote sensing analyses show that the rock may have been excavated from an older basaltic unit and ejected to the landing site of Chang’e-5,” said the team, in a press release from the Chinese Academy of Sciences. “Therefore, the lower water content of the soil, as compared to the higher water content of the rock fragment, suggests that degassing of the mantle reservoir beneath the Chang’E-5 landing site took place.”
The team also said this finding will help in their studies of the Chang’e-5 returned samples, which you can read about here.