On Earth, we are blessed to have a planet with very active layers directly under our feet. These layers play an essential role in our existence and are vital for sustaining life.
Stepping away from Earth, the Moon appears to be cold and quiet. Perhaps there’s something more than meets the eye happening deep within the layers of the Moon. Let’s spend a bit of time exploring our closest neighbor.
What Are the Layers of the Moon?
The Moon, like the other spherical rocky objects in our Solar System, consists of three distinct layers. Let’s drill into the different layers of the Moon and see what we can discover about each one of them.
The crust represents the outermost layer of the Moon and averages 38 miles (60 kilometers) thick on the Moon’s Earth-facing side. The crust on the far side of the Moon sits a bit wider at approximately 63 miles (100 kilometers). This crust reaches up to the Moon’s surface, including the very soil that Apollo astronauts walked on some 50 years ago.
Today we know this layer is covered with impact craters, dead volcanoes, and old lava flows. Even from Earth, some of these can even be seen with the naked eye. While researchers have long agreed that the Moon is made of rock, there has been some confusion about the Moon’s darker areas.
Early scientists believed these areas were actually large bodies of water and went so far as to use the Latin word mare, meaning sea, to identify them. It turns out these areas are oceans of hardened lava, but we still refer to them as seas to this day.
With a minimal atmosphere to break up stellar debris, the Moon’s surface was subject to heavy bombardments for much of its formative years. Although this has slowed significantly, the Moon is still at risk of the occasional wandering object.
Most of the Moon’s surface is covered in regolith, which is a fancy term for dust and broken rock formed as those impacts threw Lunar material out from the crash site.
The Moon’s crust is composed mainly of oxygen, silicon, iron, calcium, aluminum, and magnesium. The fact that these elements are also common on Earth lends itself to the theory that the Moon was once a part of our planet. The Moon’s seas are composed of volcanic material that’s mainly basalt.
Heading deeper, we reach the Moon’s middle layer, the mantle. Starting just under the crust, the mantle stretches some 800 miles (1300 kilometers) inward. Representing 80% of its total volume, this is by far the largest layer inside the Moon.
Interestingly, the Moon’s mantle is split into two layers of its own.
Mantle’s Upper Layer – Lithosphere
The rigid upper layer of the Moon is known as the lithosphere and is estimated to be 600 miles (1000 kilometers) thick. Scientists are able to calculate this approximate distance by measuring the depth of moonquakes, which only affect solid rock.
Today, this layer is too cool to flow and has hardened into a massive solid chunk. At one point in the Moon’s history, this layer must have been hot enough to flow. This is evidenced by the now-dead volcanoes and cooled lava flows that would have come from this layer.
Mantle’s Lower Layer – Asthenosphere
The second layer of the mantle is called the asthenosphere. This layer is still semi-solid but is finally warm enough to be at least partially molten. This layer is likely only 200 miles (300 kilometers) thick and is way too far from the surface to cause any sort of volcanic activity.
Both sections of the mantle are believed to be rich in iron and composed mainly of the minerals olivine, orthopyroxene, and clinopyroxene.
The core is the densest layer of any planetary object. Since the Moon’s density is a lot less than that of Earth, scientists have surmised that the Moon’s core must make up a lot less of its overall composition than Earth’s.
At just 20% of the Moon’s total thickness, the core is surprisingly small compared to other rocky objects. In comparison, the Earth’s core makes up some 50% of its radius. The core also only attributes to about 3% of the Moon’s total mass.
Although unproven, scientists believe the Moon has a layer of molten core before reaching a solid one. Layer of liquid or no, the core has a radius of only approximately 200 miles (300 kilometers).
Just like on Earth, the Moon’s inner core is hot enough to be a liquid. However, internal pressure is so high that it keeps the Moon’s center compressed into a solid form.
This core is believed to consist of mainly metallic iron with a small amount of sulfur and nickel mixed in.
Tools of the Trade
Scientists have worked to discover the Moon’s exact composition for some time, but this is no easy feat.
On Earth, the furthest humans have ever traveled towards the core was just 7.6 miles (12 kilometers). That feat took an incredible 20 years to accomplish and means the crew averaged just over a third of a mile (half of a kilometer) per year!
On the Moon, astronauts from the combined Apollo missions spent little more than three days out on the lunar surface. With deep drilling not an option, the astronauts used that time to set up seismometers that sent data back to Earth for eight years.
Scientists on Earth were able to use this information to approximate the thickness of each of the Moon’s layers.
If there was ever a chance for life on the Moon, it ceased to be as its layers cooled and solidified. The lack of volcanic and seismic activity on the crust and mantle has left the Moon an unchanging, dead rock out in space.
Nevertheless, if the plan is to some day have a colony on its surface, we need to continue learning all we can about the layers of the Moon. For now, it’s enough to wonder and snap a few pictures of our closest neighbor while we’re at it.