Surface features of the Moon

What are those big circles with streaks coming out of them? Why are there light and dark patches? And why do some bits look smooth and others jagged? Surprisingly, this varied landscape mainly comes from the same source: space debris smashing into the lunar surface over billions of years.

Moon fact: The 8-kilometre-deep South Pole-Aitken Basin stretches across nearly a quarter of the lunar surface. It is the largest and oldest impact basin on the Moon. The violent asteroid crash that made this crater was around a thousand times more explosive than the one that ravaged the dinosaurs on Earth 65 million years ago.

Why does the Moon look like that?

Most ancient religions worshipped a god or goddess of the Moon. In fact, we still refer to all things Moon as ‘lunar’, which comes from the Roman Goddess Luna. When Christianity came to the fore, the Moon was demoted to being just one of thousands of ‘heavenly bodies’. These heavenly bodies were perfect spheres circling the Earth. The Moon’s patchy appearance was simply explained as being a trick of the light.

But then, over 400 years ago, Galileo Galilei pointed a telescope at the Moon for the first time. And what he saw was startling. The Moon’s surface had what looked like valleys, plains and mountains much like the distinctly unheavenly surface of the Earth. Galileo’s conclusion led him to search for imperfections in other objects in the night sky ­– and ultimately to prove that the Earth was not the centre of the Universe.

Impact craters

Despite Galileo finding similarities between Earth and the Moon, it was and is still a very alien world. For example, the huge circles that dot the surface were a mystery. A few scientists over the following centuries correctly predicted they were impact craters from meteorites smashing into the lunar surface. But most thought they came from ancient volcanic eruptions right up to the mid-20th century.

The reason they clung to this idea for so long was simple. If these circles were impact craters from bombardment, then the Earth should be similarly littered with them, which was clearly not the case. These scientists hadn’t accounted for two important features of our planet. Asteroids heading for Earth first have to get through our thick atmosphere, which destroys most before reaching the ground. For the ones that do manage to break through and smash to Earth, their imprint is usually buried or destroyed over millions of years by our continuously changing weather, climate and geology.

We now know that without our shield-like atmosphere and active climate and geology, the Earth would be the Moon’s doppelganger. And knowing how the lunar craters formed, we can also see that the streaks, known as ‘rays’, radiating from where the meteorites crash-landed are fresher material tossed out during impact. These rays are most visible in the crater named Copernicus. Located slightly west of the Moon's Earth-facing centre, it can be seen with the naked eye.

Mountains and seas

As it turns out, impacts not only formed the Moon’s craters, but also had a large part to play in shaping the lunar ‘mountains’, ‘seas’ and ‘soil’. (Why these features are in inverted commas will become clear below.)

Near the Engel'gardt crater on the lunar far side, the Moon boasts a summit that is almost 2 kilometres higher than Mount Everest. Climbers should not get excited though. Unlike the steep approach to Everest, the ascent to the summit is easy: a pleasant stroll sloping up just 3°. The reason this and all of the Moon’s mountains are so gentle compared to the Earth’s is that they were created by a completely different process.

Most of the Earth’s mountains were formed from tectonic plates smashing or grinding together, or from volcanic activity. In contrast, the Moon’s mountains are much more ancient. They are what is left of gigantic impacts in the Moon’s young life. These impacts left huge inner craters surrounded by colossal rims that would later become mountain ranges.

As the Moon was still volcanically active at that time, lava flowed across the surface. This lava swamped some craters and ridges and then cooled to form what we call ‘maria’ (meaning ‘seas’ because Galileo thought they resembled large smooth areas of water). Maria are darker than the rest of the surface because the volcanic rock they are made from is full of iron, which is less reflective than other Moon rock materials. Further impacts over the course of billions of years added more craters, shaping the Moon we see today.

Alien soil

Apollo 11 landed in one of these maria – Mare Tranquillitatis (or the ‘Sea of Tranquillity’), which is a large dark patch east of centre as you look at the Moon. While there, the astronauts described and collected the first ever samples of soil from the Moon. This was an important job, as before Apollo 11 no one knew whether the soil might affect the crew's ability to move about or work on the lunar surface. There were even worries that the soil might be so soft the lander would sink, or cause an explosion if exposed to air. Luckily, most of these fears were soothed by the crew’s safe landing and detailed inspection. Although, annoyingly, the lunar soil got into everything, jamming seals, damaging spacesuit fabric, and scratching camera lenses and spacesuit helmets.

The 382 kilograms of lunar soil brought back to Earth by the Apollo and Soviet Luna missions show it’s nothing like what you would find in your garden. Covering almost the entire surface, it was created from the constant bombardment of micrometeorites on the Moon for the last 4.5 billion years. This violent birth, when combined with the lack of wind and rain, has made it sand-like, made up of tiny sharp particles that are abrasive to the touch. Rich in oxygen, silicon, iron, calcium and magnesium, the only soil remotely similar on Earth is basalt rock found in volcanic regions such as Fuji Mountain in Japan or Lake Bolsena in Italy.

Today, scientists take rock from these earthly locations, and then crush, grind, sieve and mix them with other minerals. Using this fake lunar soil, they are growing plants and building bricks with it. Why? So that one day, astronauts returning to the Moon can eat lunar turnips and shelter themselves from the harsh environment.