The dark side of the Moon is surprisingly dry

There might not be as much water on the Moon as we thought.

While lunar ‘seas’ might not be awash with water, the Moon is far from bone dry. Water has been found in many different forms on the Moon, hidden in its craters, soils, and locked within its rocks.

New research, however, suggests that this water isn’t spread equally across the Moon. Analysis from rock samples returned by the Chang’e 6 lunar lander suggest that there is much less water in the soils on the far side of the Moon than those on the side closest to Earth.

Professor Sara Russell, a scientist who investigates the origin of water in the Solar System, says this finding was completely unexpected.

“For a long time, it was believed that there wasn’t any water on the Moon at all,” Sara explains. “This has only started to change relatively recently, when measurements taken from lunar meteorites and Apollo samples showed that there is water trapped in the Moon’s minerals.”

“Even then, I don’t believe anyone had suggested that water wasn’t evenly spread across the Moon. We’ve been biased by our use of samples from the near side of the Moon, and this discovery, if confirmed, has a lot of implications for understanding how much water there is on our celestial neighbour.”

The findings of the study were published in the journal Nature.

What does the dark side of the Moon look like?

The dark side of the Moon is more lumpy than the near side, and covered in large craters. The biggest is called the South Pole-Aitken Basin (SPA Basin), which is 2,500 kilometres in diameter. This is about the same length as five Grand Canyons all lined up in a row.

One of the reasons for the sheer number of craters on the lunar far side is because it has a much thicker crust. Asteroids striking this side of the Moon were unable to break through the surface, so left their mark behind in the rock.

In contrast, the crust on the near side is much thinner. This means that when asteroids hit the surface during the Moon’s early years, it split it open and released molten magma from beneath the surface. This formed the smooth lunar seas that we can see today.

This new research has now identified another difference between the Moon’s two hemispheres. While one cubic metre of lunar soil from the near side of the Moon contains around 350 millilitres of water, the same amount from the far side appears to contain ten to 100 times less.

Finding out exactly why this is the case will need more research. It’s thought that it might be related to how the Moon cooled down in its early years, with its near side kept warm by the still-molten Earth as the far side solidified faster.

It’s possible, of course, that the area of the SPA Basin sampled by Chang’e 6 is an unusually dry part of the Moon’s far side. More samples will need to be taken by future missions to confirm this difference once and for all.

How did the Moon get its water?

This new discovery feeds into wider questions about how the Moon has water in the first place. When the Moon formed, it would have been a lump of molten rock – more than hot enough to boil away any water that was originally there.

As a result, the water must have been arrived later in the Moon’s development. There are a couple of ideas about how this happened.

“The main idea is that most of the Moon’s water came from meteorite impacts,” Sara explains. “Clay-rich meteorites such as carbonaceous chondrites have large amounts of water bound up in their structure that they could have delivered to the Moon. This probably provided most of the Moon’s water, as the chemical signature is more similar to these meteorites.”

“The solar wind could also have helped to form water. Hydrogen particles streaming away from the Sun could have reacted with oxygen in the Moon’s minerals to make water.”

While much of this water is bound up in the Moon’s rocks and soil, some also exists as ice on the surface. This survives in the depths of craters which are permanently in shade.

This means the Moon could provide all the water astronauts might need for living in space. As well as using it for drinking, the water can also be split into hydrogen and oxygen to provide the raw materials for rocket fuel and air.

“We’re hoping to get astronauts back on the Moon in the near future, and potentially in lunar bases, so it’s important to understand what resources are there,” Sara says. “The far side of the Moon would be an especially important place to have a base, as it would allow us to carry out astronomy and other research that’s completely shielded from the Earth.”

“If this is to become a reality, then we need to send more probes to the Moon so we can find out as much as we can about it.”