The Two-faced Moon

The far side of the Moon, mostly never seen from Earth, looks very different from the face shining through the night sky. Now, a new study found that it might be colder, too.

“The near side and far side of the moon are very different at the surface and potentially in the interior,” study author Yang Li said in a statement. “It is one of the great mysteries of the moon. We call it the two-faced moon.”

The far side has a thicker crust, more mountains and craters, and fewer dark basalt areas formed by volcanic activity.

“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,” said Li.

Researchers examined about 10.6 ounces of rock and soil collected last year from a vast crater on the far side of the moon by China’s Chang’e 6 spacecraft – the first ever sample to be collected on that side. They confirmed previous findings that it was about 2.8 billion years old.

The team studied the sample’s mineral composition and compared it with computer simulations to estimate how hot it was when it crystallized from molten lava to basalt billions of years ago. They estimated it formed at roughly 2,012 degrees Fahrenheit, which is about 180 degrees cooler than estimated for samples collected from the near side of the Moon.

“These findings take us a step closer to understanding the two faces of the moon,” study author Xuelin Zhu said in the statement. “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 also determined that the “parent rock” – the original material that melted into magma to then solidify into the sample collected by Chang’ – also did so at a temperature about 180 degrees Fahrenheit lower compared with near-side rocks.

Researchers said this difference in temperature between the Moon’s faces might be due to an uneven distribution of heat-producing elements – such as uranium, thorium, and potassium – which release heat due to radioactive decay in the Moon’s mantle.

While scientists do not know the exact temperature of the far and near sides of the Moon’s mantle, any imbalance between the two will likely persist for a long time, as the Moon has been cooling down very slowly since it formed from a catastrophic impact. Meanwhile, the team is working on finding the precise temperatures.