There’s water on the Moon. It is not flowing like on Earth, but it is still there, trapped in ice deep inside craters constantly in darkness, or in hydrated rocks. This has been observed from space and samples, and now Chang’E-5 has shown that not only water is really there, but also its origin. It mostly comes from inside the Moon.

The findings, reported in Nature Communications, are the first laboratory evidence taken on Earth and on the Moon of the presence of native lunar water. Samples returned to Earth in late 2020 have revealed the presence of the hydroxyl group (an oxygen and hydrogen atom stuck together), which is the most common result of a chemical reaction involving water.  

The team did not find droplets of water within the rock, but it's the chemical equivalent of smoke to a fire. Chang’E-5 was in the Oceanus Procellarum - the Ocean of Storms - but the hydroxyl quantity in this vast basaltic flood was just 30 parts per million.

"For the first time in the world, the results of laboratory analysis of lunar return samples and spectral data from in-situ lunar surface surveys were used jointly to examine the presence, form and amount of 'water' in lunar samples," co-corresponding author LI Chunlai from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), said in a statement. 

"The results accurately answer the question of the distribution characteristics and source of water in the Chang’E-5 landing zone and provide a ground truth for the interpretation and estimation of water signals in remote sensing survey data."

The team also stressed that the conditions during the sample collection and the direct analysis conducted by Chang’E-5 on the Moon were extremely dry. The ground temperature was over 90°C (around 200 °F) as it was almost lunar noon over the region.

It also coincides with a time of lower solar wind. The constant stream of particles from the Sun can create hydroxyl on the Moon, something that was determined not just by Chang’E-5 but also by samples collected by NASA astronauts during the Apollo missions. But the Chinese mission was able to establish that the water in these rocks came from inside the Moon, back when the volcanic eruptions were still a common feature of our satellite.

"This excess hydroxyl is indigenous, demonstrating the presence of lunar-originated internal water in the Chang’E-5 lunar samples, and that water played an important role in the formation and crystallization of the late lunar basaltic magma," LI said, referring to the composition of Chang’E-5 landing site in the mare basalt of Oceanus Procellarum. 

"By investigating lunar water and its source, we are learning more about the formation and evolution of not just the Moon itself, but also the solar system. In addition, lunar water is expected to provide support for future human lunar in-situ resources."

Future missions such as Chang’E-6 and Chang’E-7 will continue this line of investigation to best establish the chemistry of the lunar surface.