When you were a kid, how often did you look up at the full moon and wonder how that face got up there? Well, we humans have a knack for looking for (and finding) faces in the weirdest places.
If a certain period during the formation of the Solar System called the Late Heavy Bombardment (LHB) had not happened, those big, dark splotches called “maria” (Latin for “seas”) that make up the face of the “Man in the Moon” probably wouldn’t have had as much of an effect on that face-finding tendency of ours.
These maria were formed relatively late in the moon’s formative years, after the molten surface had cooled into a crust, somewhat like the Earth’s. After that LHB period had for the most part petered out, the still molten magma underlying the crust would occasionally erupt onto the surface, often obliterating any surface features into a relatively smooth, flat plain.
But wait! I seem to have skipped over the fun part! What happened during that totally cool-sounding Late Heavy Bombardment period?
I’m so glad you asked…
The Solar System Was (And Still Is) A Cosmic Shooting Gallery
Just as some carnival shooting galleries build their rifles and pistols to miss their targets, in the shooting gallery of today’s solar system, most shots also miss. At least that’s the case with the larger asteroids and comets. But that wasn’t the case during the Late Heavy Bombardment period.
As the planets started to cool from the earlier accretion phase, in which the larger planetary bodies, through gravitational attraction, pulled in and absorbed smaller bodies, they began to develop outer crusts. But there were still plenty of potential impactors whizzing around. And quite a few of them had the Moon’s name written on them.
Since scientists are known to pick apart each other’s work, there are several hypotheses concerning where the material that made up the LHB came from, as well as exactly when the LHB occurred.
The most popular theory, called the Nice model, named after an observatory in Nice, France, is that the giant planets (Jupiter, Saturn, Uranus and Neptune) migrated outward from more compact orbits to more distant ones over a period of several hundred million years. This caused resonances to pass through the asteroid and Kuiper belts.
As these resonances swept through the belts, many of the asteroids and comets were scattered into more elliptical, inner planet-crossing orbits. As a result, the LRB pummeled the inner terrestrial planets (Mercury, Venus, Earth and Mars) for a period between 4.1 and 3.8 billion years ago.
No Brakes! No Brakes!
Now that you know where the impactors (most likely) came from, have you ever wondered why the Moon is so thoroughly pockmarked with craters, but the Earth isn’t? Well, our planet has something the Moon lacks.
Oh, and a minor little thing called an atmosphere.
During the Late Heavy Bombardment period, Earth was just as much of a target as every other terrestrial body in the inner solar system.
But while Venus, Earth, the Moon and Mars all have had tectonic activity since the formation of their outer crusts, only the Earth’s crust is fractured into tectonic plates. As a result, many of the craters created by impacting asteroids were subducted under the crust and melted back into the mantle as one plate slid under the other.
Smaller asteroids simply burned up in the Earth’s atmosphere, never to leave any mark at all on an uncaring surface.
Craters created by the larger rocks that managed to make it through Earth’s atmosphere became the victims of erosion caused by hundreds of millions of years worth of water, wind and weather (if they managed to avoid being subducted).
The Moon, however is a different story. There is no atmosphere to speak of on the Moon to slow down or burn up the incoming projectiles. And tectonic activity, limited to volcanic eruptions, has been dormant since the Moon’s early history.
Moon craters, also known as impact craters come in three basic sizes with correspondingly different shapes. Of course, the size of the crater is determined by the size of the impactor that created it.
- Simple craters – Small and bowl-like with no central peak. Most simple craters are less than 15 kilometers (km) in diameter. This is also the most common type of crater on the moon.
- Complex craters – These craters have a well-defined rim, a central peak and are usually from 20 to 175 km in diameter.
- Crater basins – The largest crater type, crater basins usually have a diameter of at least 300 km. These are also the only craters where the rim at least partially survived later volcanic eruptions. The erupting magma would fill the basin and cool into what are now called lunar maria. Mare Orientalis is a great example of this.
Being unusual always gets you noticed
There are some lunar craters that have unusual properties. One such crater is Tycho, close to the Moon’s southern highlands, which displays a prominent ray structure, emanating like the spokes of a wheel out from the rim of the crater.
For the most part, Crater Tycho is a typical complex moon crater, about 85 km in diameter with a central peak, but the prominent rays make this crater spectacular to look at. There are other craters which also have ray structures associated with them, but none stand out quite as much as Tycho.
At 93 km in diameter, Crater Copernicus is another ray crater which falls into the complex crater category. It’s located up and to the left of Tycho, just to the north and west of the center of the Moon’s near side. Copernicus’s rays are highlighted by the darker background of the largest of the lunar maria, called Oceanus Procellarum.
Rays Coming Off The Craters? What’s THAT About?
Ray craters are thought to be the result of fairly young craters formed by relatively recent impacts. The ejecta sprayed out during their impact event has not had time to be obscured by more recent nearby impacts, usually of the simple crater category, and their ejecta.
Such A Small Piece Of The Night Sky, So Much To See!
Though the Moon’s craters are fascinating features of the moon’s topography, there are lots of other features to marvel at. From the dark plains of the maria to the mountains of the lunar highlands, you won’t get bored any time soon scanning and identifying the features of our nearest celestial neighbor!
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