The dark spots on the Moon are called maria. The largest of the maria, Oceanus Procellarum, may or may not be an ancient impact crater, but the Moon is littered with different types of impact craters. Let’s explore these craters and try to differentiate them.
Most of the maria, or crystallized basalt plains on the low-lying impact basins, are dated between 3 billion and 3.5 billion years ago, although some of the youngest maria may only be about 1.2 billion years old.
The largest of the maria is so large that it is called an ‘ocean’ rather than a ‘sea’—Oceanus Procellarum, which translates as ‘the ocean of storms’. It has about the same area as the six largest US states combined: that’s Alaska, Texas, California, Montana, New Mexico, and Arizona.
Oceanus Procellarum isn’t circular, and there isn’t any sign of a surrounding crater rim. So one hypothesis is that a giant impact crater occurred very early in lunar history, and any signs of the impact have been erased by later volcanism and impacts. That would make Oceanus Procellarum the largest impact basin on the Moon.
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The Evidence Behind Oceanus Procellarum Being an Impact Basin
It seems strange to suggest that an impact must have occurred if there is no sign of it, but there are some compositional features of Oceanus Procellarum that would be explained by such an impact, even if the circular rim is now missing. For example, this basin is relatively high in a set of trace elements called KREEP.
KREEP is an acronym that stands for potassium (potassium’s atomic symbol is K), rare earth elements (REE), and phosphorus (P). KREEP behaved in a very particular way when the magma was cooling. The elements in KREEP concentrated into the liquid phase of the magma, so they didn’t become part of the minerals that crystallized early on. It was only the last of the magma to solidify that ended up having the KREEP elements.
The first minerals to solidify from the magma ocean sank to the bottom, then eventually, plagioclase feldspar floated to the top, forming the crust. So the last magma to solidify would have been below the surface, at about 100 kilometers depth, and this would have been relatively high in KREEP elements.
The fact that Oceanus Procellarum has more of the KREEP elements could
be explained by a giant impact because that impact would have created a hole deep enough to reach down to the KREEP layer. The heat from the impact would have caused melting, so the impact basin could have been filled with lava coming from the deep, KREEP-enriched region.
But a giant impact isn’t the only possibility for how Oceanus Procellarum could have formed. Another possibility is contraction as the Moon’s early surface cooled. This theory is based on data from the GRAIL mission, which mapped out the Moon’s gravity field at very high resolution.
The mission found that Oceanus Procellarum is surrounded by a rectangular pattern of ancient rift valleys. On the early Moon, rift valleys were formed when a surface made of one material was cooling, contracting, and separating from the surrounding material. This contraction would have caused deep cracks that would act as conduits for lava to flow up as the deep rock depressurized. The lava would have filled the depressed surface.
In this way, the rift valleys would have created a signature in the gravity field that the GRAIL mission could measure. Now, the important thing here is that the contraction cracks tend to have a rectangular pattern rather than a circular boundary. So, Oceanus Procellarum might be caused by how the crust contracted rather than by a giant impact.
Both are reasonable explanations. We’ll need more data before we can decide whether an impact crater or contraction cooling was the cause of this enormous basin.
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The Differences between Simple and Complex Craters
Although Oceanus Procellarum may or may not be an ancient impact crater, the Moon is covered in impact craters. Craters can have different shapes and features based on how big they are. These shapes and features also help us determine the size of the meteor that impacted the surface to form the crater.
On the smallest end are simple craters. These are the bowl-shaped craters with sharp rims. On the southern edge of Mare Tranquillitatis, 50 kilometers from the Apollo 11 landing site, there is a good example called Moltke crater. It has a diameter of 6.5 kilometers and a depth of 1.3 kilometers.
Notice that the depth of the crater is about 20% of its diameter. It turns out that all simple craters—whether on the Moon, or Earth, Mercury, or Mars—have depth to diameter ratios in the range of 14−20%. Bigger craters end up having a different shape than simple craters.
For instance, let’s look at Tycho crater near the south pole of the Moon. You can actually see Tycho from Earth because it is so bright. It has long spokes, some up to 1,500 kilometers, shooting out from its center.
It is so noticeable because it is a fairly young crater, only about 100 million years old. The impact ejected much of the older, darker material from the surface, revealing the fresher layer underneath.
Tycho is about 85 kilometers in diameter, so more than 10 times bigger than the simple crater Moltke. Tycho has some typical features that occur in craters of this larger size. First, the floor is quite flat, but then there is a peak, or mountain, at its center. The walls of the crater are also terraced due to the slumping of the material at the rim of the crater after it formed. Craters like this are known as complex craters.
Interestingly, the transition point, where simple craters transition to complex craters, is different for different planets, based on their gravity. Smaller bodies, like the Moon, where the gravity is only 17% of that on Earth, have simple craters even at larger diameters. On Earth, more intense gravity causes complex craters to form at smaller sizes than we find on the Moon.
As craters get bigger, the features become even more complex. The bigger a crater is, the more complex its morphology. The impacts that create the largest craters have a lot of energy that results in lots of melting and deformation, even far away from the crater itself.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
SPA: The Largest Crater on the Moon
The largest confirmed crater on the Moon, and the second largest in the entire solar system, is the South Pole-Aitken basin or SPA. It is about 2500 kilometers in diameter. It would span the distance from New York City to Denver. SPA’s crater bottom is about 13 kilometers below the highest point on its mountainous rim.
The Chinese mission Chang’e 4 began exploring SPA in January 2019. The mission was defined as focusing inside the enormous SPA crater, on a smaller crater, named Von Karman, exploring a crater’s crater. The Chang’e 4 lander was the first spacecraft to soft-land on the far side of the Moon.
The term ‘soft land’ distinguishes Chang’e from the NASA Ranger 4 spacecraft, which impacted on the far side of the Moon but didn’t return any data. And Chang’e 4 soft-landed autonomously, without humans to guide the landing.
The SPA basin is important for understanding the Moon. Firstly, the SPA is very old, so it offers a window into some of the processes going on in the earliest times of the solar system.
Secondly, because the basin is so big and deep, it is likely to be made of excavated material from much deeper in the Moon, bringing that deeper material to the surface. SPA can, therefore, act as a window to the Moon’s interior that we don’t get by exploring smaller craters.
Thirdly, we just don’t have that many big craters in the solar system, so we don’t understand all the physics going on when they form. For instance, how much heat is imparted to the Moon during such an impact? What happens to the Moon’s interior when shock waves travel through it from such a large impact? Studying the rocks in the SPA will help us answer all of these questions.
Common Questions about the Craters on the Moon
The craters on the Moon are caused by asteroids and meteorites colliding with the surface of the planet.
South Pole-Aitken basin is the largest crater on the Moon. It’s about 2500 kilometers in diameter.
Scientists believe that water ice could possibly exist in cold, permanently shadowed craters present at the Moon’s poles.
The dark spots on the Moon are called maria, from the Latin word for ‘sea’. They cover nearly 17 percent of the surface of the Moon.