I have a soft spot for the Moon; since I was a young kid, my father would let me see it through his old 30x spotting scope.
I remember waiting patiently for an alien ship to appear from behind the moon. No dad, let me see a bit more… they will come any minute now! Any… minute… now.
The awe I felt back then is still with me today, 30 something years later, and it is renewed every time I aim my telescope at our cosmic companion.
And it is the same awe I see today in my kids and the same spark you can see in people’s eyes when they get to see the Moon up close.
And what they see are not alien ships but countless craters, all different but all equally beautiful.
But why does the Moon have craters in the first place, and why so many?
How do they form? Is the Moon still getting craters today? How big are they and can you photograph them? And where are Earth’s craters? And what about the other moons and planets in the Solar System? Do they have craters too…?
With so many questions, it’s time we answer some of these puzzling moon questions.
What Are Moon Craters
All craters on planets and moons are scars left behind by massive cosmic impacts.
These impacts were frequent in the young Solar Systems.
In time, with the planets “cleaning up” their orbits, such large and catastrophic events became much more rare.
But even today, the Solar System is more than just the Sun, Mercury, Venus, us on Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto: it is littered with cosmic debris.
Countless asteroids, dwarf planets, nano planets, and comets populate the Solar System.
The asteroid belt, for example, is a famous region of space between Mars and Jupiter that is “packed” with asteroids of all sizes and nano planets such as Ceres.
And many more asteroids crisscross the Solar Systems and once in a while, they impact on a larger body, like a Moon or a Planet, sometimes leaving a crater behind.
What Causes The Craters On The Moon?
Impacts from asteroids and comets are extreme and catastrophic events, often releasing an amount of energy equivalent to the explosion of many atomic bombs.
An asteroid as little as 7m in diameter will enter Earth’s atmosphere with a kinetic energy of 16 kilotons, equivalent to the energy released in the explosion of 16000 tons of TNT or 1 Hiroshima bomb.
On Earth, such an asteroid will break up in the atmosphere, causing little to no damage at all.
But on the Moon, with no atmosphere to slow the asteroid down, it would impact on the lunar surface at full speed, leaving a crater behind.
You can use this nice Lunar Impact calculator to calculate the effects a given asteroid will have on the Moon.
Were our 7-meters asteroid to impact on the Moon instead of Earth, it would leave behind a crater almost 140 m in diameter and 40 m deep, melting 3300 tons of rock in the process.
It is estimated a 7-meter asteroid will enter Earth’s atmosphere once every 4.9 years, so such events are fairly common.
Larger asteroids are far more rare: the Chelyabinsk meteor that exploded a few years ago in the skies above Chelyabinsk Oblast (Russia) was estimated to be 20m in diameter.
Such an event should occur once every 60 years.
Should it have struck the Moon instead of Earth, it would have created a crater over 330m in diameter with a depth of 90m, and it would have melted 70700 tons of rocks.
But that would be a puny crater: the typical diameter for lunar craters is 15km, 50 times larger than the one the Chelyabinsk meteor would have created. To create such large craters, you would need impactors of about 1 km in diameter or more.
Today, impacts involving asteroids this large are rare, and we should expect to have only a couple of those in like a million years.
But most lunar craters are hundreds of millions, and even billions, years old and the young Solar System was not a quiet and ordered place to be.
Impact Craters on the moon
If the cause behind a craters’ formation is always the same, the outcome depends on the force of the impact. This depends on:
- The size of the impactor
- The type of the impactor (iron asteroid, rocky asteroid, …)
- The impactor’s Speed at the impact
- The angle at which the impactor strikes the surface
- The type of the impact site
Asteroids up to about 700m in diameter usually form simple craters, which look like a depression in the ground, encompassed by a smooth rim.
Larger asteroids will create complex craters, often involving the formation of one or more central peaks that rise from the crater floor for a few kilometers. The crater rim can show terraces rather than smooth, gentle slopes.
When the Moon was young, the Solar System was a violent place to be, and impacts were so powerful and catastrophic that large parts of the lunar crust were destroyed and the crater was then slowly filled by lava flows from the mantle.
This is what creates the smooth Maria we can see from Earth.
Did You Know?
While impact basins are evenly distributed on the Moon, the Maria are mostly located on the near side of the Moon.
Basins on the far side of the Moon didn’t get flooded by lava because of a fundamental asymmetry in the Moon: the lunar crust on the far side has about twice the thickness of the crust on the near side.
Are All Craters On The Moon The Same Depth?
No. The depth of a crater depends on the type, size, and speed of the impactor and on the type of surface it impacted on.
Why Are Moon Craters So Shallow?
Wait, what? A crater a few kilometers deep does not look shallow to me at all!
Here, the term “shallow” is used in a relative way: it refers to the ratio between the depth and the diameter of a crater.
The most famous craters have diameters of 80-100 km or more, with depths ranging from about 4 to over 9 kilometers: their depth is only about 1/15 to 1/25 of their diameter.
In more familiar terms, those large craters are “shallower” than your dinner plate.
In contrast, smaller craters often have a greater depth-to-diameter ratio, looking more like soup bowls.
How Many Craters Are On The Moon?
We don’t know. It is easy to spot and count the larger craters, but for the small ones, those that are a few tens of cm across or less, that is another story.
We only know there are more than 5800 craters that are wider than 20 km, and there could be half a billion that are wider than 10m.
What Is The Biggest Crater On The Moon?
The largest impact crater on the Moon is the South Pole-Aitken Basin. It stretches for about 2500km and it reaches depths of 13km.
In time, many craters have formed inside the South Pole-Aitken basin, as you can see in the image above.
Famous Moon Crater Names
This is why you should not dismiss the Moon if you are into astrophotography: almost all the famous craters can be easily photographed with some details with amatorial equipment, along with mountain ranges and peaks, maria, and valleys.
Here are the top three most famous craters on the Moon.
- Diameter: 101 km
- Depth: 1468 m
- Shallowness: 0.015
Plato is a large crater located between Mare Imbrium (at South) and Mare Frigoris (at North). Because of its size and location, it is one of the easiest craters to identify.
The crater floor appears smooth with no central peak.
- Diameter: 93 km
- Depth: 3800 m
- Shallowness: 0.040
Copernicus is another classic. It is a large complex crater with three central peaks, climbing up to 1.2 km from the crater’s floor. The rim shows terraces.
Copernicus crater is located at the Southern edge of Mare Imbrium, at the end of the Montes Apenninus mountain range and it has been photographed from lunar orbit by Apollo 12.
- Diameter: 86 km
- Depth: 4800 m
- Shallowness: 0.055
Being relatively young (about 108 million years old), Tycho is one of the brightest craters on the Moon, and it is surrounded by a wide ray system of ejected material.
Because of its high albedo, it is a challenging area for Moon photography, particularly if you want to photograph the whole lunar disc, as it is easy to overexpose it.
The single central peak is about 1600m tall and is very easy to spot and photograph.
Why Does The Moon Have More Craters Than Earth?
In one word: erosion.
Earth has an atmosphere, weather, running water, ice caps, oceans, tectonic plates in constant movement, active volcanoes, and … life. Old Earth crust is constantly consumed and replaced by new ones.
Craters will not stay around for long.
The Meteor Crater in Arizona (USA) is large, 1.2km and 170m deep, and is only 50,000 years old: being so young, it is one of the best-preserved impact craters on Earth.
Most of the older craters, even those created by the asteroid that wiped out the dinosaurs 60-something million years ago, are only detectable by studying satellite images.
On the Moon, the primordial fire at its core is probably completely gone, so no volcanoes to flood and erase the more ancient craters and no tectonic plates or formation of new crust.
Cosmic weather can, to some extent give rise to erosion, but this is a very small and slow effect.
Did You Know?
Craters are found on all rocky planets and moons, but Earth is not the only place where craters get “quickly erased”.
Moons such as Europa (Jupiter) and Enceladus (Saturn) are almost without craters. This is due to their icy surface and to the presence of an ocean of liquid water underneath the surface so that the ice is constantly renewed and impact craters are erased.
One of the things that make the Moon so interesting is the zoo of craters that scars its surface.
Small, large, shallow, deep, with smooth floors or majestic central peaks, with smooth or complex rims, you can find pretty much everything on the Moon.
And the beauty is you can see and photograph most of the larger crater with a budget astronomy setup. And if you can’t see them all, apps such as Moon Globe will allow you to explore the Moon comfortably from your couch.