Far side of the moon literally cooler than the side we can see

By Stephen Beech

The far side of the moon may be much colder than the side we can see, suggests new research.

The interior of the mysterious far side of the satellite could be chillier than the side constantly facing Earth, according to an analysis of lunar rock samples.

Researchers say their findings, published in the journal Nature Geoscience, indicate that the moon is "two-faced".

The study, co-led by University College London (UCL) and Chinese scientists, looked at fragments of rock and soil scooped up by China’s Chang’e 6 spacecraft last year from a huge crater on the moon's far side.

The researchers confirmed previous findings that the rock sample was about 2.8 billion years old.

The team also analyzed the chemical make-up of its minerals to estimate that it formed from lava deep within the moon’s interior at a temperature of about 1,100 degrees C, about 100 degrees C cooler than existing samples from the near side.

Co-author Professor Yang Li, of UCL, said: “The near side and far side of the moon are very different at the surface and potentially in the interior.

"It is one of the great mysteries of the moon. We call it the two-faced moon.

"A dramatic difference in temperature between the near and far side of the mantle has long been hypothesized, but our study provides the first evidence using real samples.”

Co-author Xuelin Zhu, a PhD student at Peking University, said: “These findings take us a step closer to understanding the two faces of the moon.

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"They show us that the differences between the near and far side are not only at the surface but go deep into the interior.”

The team explained that the far side has a thicker crust, is more mountainous and cratered, and appears to have been less volcanic, with fewer dark patches of basalt formed from ancient lava.

They noted that the far side of the interior may have been cooler due to having fewer heat-producing elements – such as uranium, thorium and potassium - which release heat due to radioactive decay.

Previous studies have suggested that this uneven distribution of heat-producing elements might have occurred after a massive asteroid or planetary body smashed into the far side, shaking up the moon’s interior and pushing denser materials containing more heat-producing elements across to the near side.

Other theories are that the moon might have collided with a second, smaller moon early in its history, with near-side and far-side samples originating from two thermally different moonlets, or that the near side might be hotter due to the tug of Earth’s gravity.

Study first author Sheng He, from the Beijing Research Institute of Uranium Geology, said: “The sample collected by the Chang’e 6 mission is the first ever from the far side of the moon.”

The team mapped selected parts of the sample, made up largely of grains of basalt, with an electron probe, to determine its composition.

The researchers measured tiny variations in lead isotopes using an ion probe to date the rock as 2.8 billion years old.

They then used various techniques to estimate the temperature of the sample while at different stages of its past when it was deep in the moon’s interior.

Although the present temperature of the far and near sides of the moon’s mantle is not known from the study, the research team says that any imbalance in temperature between the two sides will likely persist for a very long time, with the moon cooling down very slowly from the moment it formed from a catastrophic impact.

The research team is now working on getting a definitive answer to that question.