By Sabine Stanley, Ph.D., Johns Hopkins University
There are many interesting features present on the surface of the Moon. There are light highlands, dark lowlands, anorthosites, and maria, but the most intriguing feature of the topography of the Moon are the lunar swirls.
The Light Highlands on the Moon
The lighter regions on the Moon are known as the lunar highlands. They are higher in altitude than the darker regions—on average, about 1 to 2 kilometers higher. The highlands are also the oldest regions on the Moon’s surface. This was where Apollo 15 found a so-called Genesis rock, though that was only 4.1 billion years old.
The highlands are made of a coarse, igneous rock known as anorthosite. The key mineral for this rock is plagioclase feldspar, which consists predominantly of silicon, followed by calcium, aluminum, sodium, and oxygen. The Earth has less anorthosite on its surface than the Moon, but there are some big formations in Labrador, Canada, and the Bushveld of South Africa.
The earliest Moon was hot enough to be molten and had a giant magma ocean. But this magma had a complex composition, like a soup of all sorts of different minerals that had melted together.
As the magma ocean cooled, certain mineral compositions crystallized first out of the liquid. The first minerals to solidify—things like olivine and pyroxene—sank to the bottom of the magma ocean because they were denser. When about 75% of the magma ocean had crystallized, plagioclase feldspar began to solidify out of the magma ocean.
The important thing about this mineral is that it’s less dense than the remaining magma ocean, and so it floated to the upper layers of the ocean rather than sinking to the bottom. Eventually, enough floated to the top to form a solid crust surrounding the entire Moon. And that crust produced the lunar highlands, dating back over 4.5 billion years.
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The Dark Lowlands on the Moon
Dark regions, conversely, are very flat, low-lying regions on the Moon. Some of these regions are fairly circular, as well. Early astronomers thought that they were seas and so named these features maria, the Latin word for ‘seas’. However, the maria aren’t really seas or even liquid. Instead, they are solidified ancient lava flows.
Imagine a circular impact basin. Then imagine lava from volcanic eruptions that flowed into the low-lying impact basins. That’s why a lot of the maria are circular. The maria are made up of basalt, a volcanic rock that we see in the Earth’s volcanic regions as well. The basalts in the maria are darker than the surrounding regions because they have more metallic iron. The lunar maria are generally younger than the rest of the surface.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
Lunar Swirls on the Moon
An interesting feature of the lunar surface are the lunar swirls, like Reiner Gamma, which you can even see from Earth with a small telescope. It’s located in Oceanus Procellarum near a crater named Reiner. You notice it as an unusually bright feature on the surface.
The Lunar Reconnaissance Orbiter mission got a much closer view, revealing some of its strange features. It looks a bit like a tadpole, with a ‘body’ region that’s around 50 kilometers in diameter and a ‘tail’ that extends another 100 kilometers. The bright parts are also kind of swirly, hence the name lunar swirl.
What is particularly intriguing is that the ‘body’ region isn’t just a uniform bright spot. Instead, the feature seems to have a sharp, dark oval ring inside it. And even though the feature is really bright and noticeable, it doesn’t seem to be related to craters, or mountains, or any other tectonic process that might produce a noticeable topographic feature on the Moon.
What could create such a structure? Scientists like to joke that when you don’t understand some feature or process in astrophysics, just say, “It’s due to magnetic fields.” As it turns out in this case, magnetic fields are definitely involved. Lunar swirls are located in places that have strong magnetic fields in the rocks. And these magnetic fields may explain the bright and dark features in the swirls because of how they protect the lunar surface from high-energy particles in the solar wind.
Here’s the leading theory for how that works. Magnetized rocks in the near-surface are rather like bar magnets. They have magnetic field lines that stick up out of the ground on one side and travel back into the ground on the other side. High-energy particles from the solar wind get trapped around magnetic field lines, and so the region under the dome created by the magnetic field doesn’t get hit with so many particles as the regions where the magnetic fields stick in and out of the surface.
That means that parts of the lunar surface are sheltered from the space weathering process that makes the lunar surface darker over time. The bright regions in the lunar swirls are places where there is very little weathering. Scientists found that ancient buried magma channels under the surface of the lunar swirls could produce a pattern of magnetization to explain the shape of the lunar swirls on the surface.
However they are caused, lunar swirls are some of the most otherworldly looking features seen in the solar system. And the only place we’ve ever seen them is on the Moon.
Learn more about robotic and crewed missions to the Moon.
Common Questions about the Topography of the Moon
The surface of the Moon has many features, such as highlands and lowlands, craters, and maria.
During the exposure to the Sun, the temperature on the Moon’s surface can reach up to 260 degrees Fahrenheit. But when the Sun goes down, the temperature can dip to minus 280 Fahrenheit.
The Moon’s atmosphere consists of some unusual gases such as sodium and potassium, which are generally not found in the atmosphere on Earth.