Impact Craters
The easiest geological process to understand is impact cratering. Perhaps you went to a Hollywood movie about this, or at least saw the trailer for it. An impact crater is created when a big astoroid or comet runs into a planet like the Earth. It leaves a big hole in the ground and throws up a lot of dust.
The object that runs into a planet is called the "impactor". That saves us from having to say "astoroid or comet" over and over again.
In geology, we use the word "planet" to mean terrestrial planet or moon. That saves us from having to say "terrestrial planet or moon" over and over again.
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When a big impactor hits a planet, it blows up. The impactor is moving so fast and has so much enegry it can't just smash into the ground and stop. When it blows up it creates a big, round hole and throws up an enourmous amount of debris and dust. (The above picture isn't of an impactor hitting Earth, but just of a big explosion.)
Impactors come in all sizes. There are more small ones and fewer bigger ones.
The really small impactors are the size of grains of sand. Have you ever seen a shooting star? They are caused by tiny impactors buring up in Earth's atmosphere.
An atomsphere acts like a defensive shield that protects the surface from smaller impactors. But not the big impactors. Big impactors don't burn up in the atmosphere. Instead they crash into the surface and explode. The explosion creates a big hole and throws up a lot of dust.
Some planets don't have an atmosphere. So, small impactors don't burn up. Instead all the little impactors pummel the surface. Over time, they erode a surface just like rain. These micrometerorites can break apart rocks. Micrometerorites can also melt small rock fragments and fuse them together. Over time, this creates a surface a little like soil and that's what our Moon has.
A planet can have much more atmosphere then Earth. For example, Venus has almost 100 times more air then we do. More air means more air resistance as the impactors plunge towards the surface. When you have more air, you burn up bigger impactors. Having more air is like having a better defensive shield.
Big impactors don't come by very often these days. A planet could go tens of millions of years before crashing into something really big. But over a couple of billions years, a planet gets hit a lot and that makes a lot of craters. Since some planets are pretty geologcally active places (with volcanoes, tectonic forces, and weather), craters can be covered up or worn away. When you look at a planet if you see lots of impact craters remember that it takes a long time to create a lot of craters. When you see craters, remember what you aren't seeing - volcanoes, lava flood plains mountains, and erosion. Lots of craters imply a planet or a region without anything new happening; a very old surface.
The above two pictures show similarly sized regions of different planets. On the top one, every round feature is an impact crater. As you can see, there are many craters, both big and small. The bottom picture doesn't have a single crater. That planet must have some active geological process that destroys or covers up craters while the planet from the top image doesn't. One planet is geologically active, continually making the surface anew while the other planet is geologically dead.
Big impactors don't come along very often these days. Perhaps the last big one to hit Earth wiped out the dinasours a couple dozen million years ago. But when the solar system was young, there were many more impactors of all sizes. It was much more dangerous way back then.
When a big impactor hits the surface of a planet, it explodes. That throws up a lot of debris and dust. If the planet has an atmosphere, all the stuff settles back down very near explosion and forms a ring. If there isn't any atmosphere, the stuff isn't slowed down and it travels much further.
If a you've ever stood close to a moving subway or a train you know it is surrounded by a breeze. When the train goes by, you feel the wind. Of course, that only happens if there is an atmosphere. As a big asteroid streaks through an atmosphere, it will also create a wind, a big wind. After the asteroid hits the surface, this wind blows the debris around. If the asteroid hits the surface at a sharp angle, most of the debris is pushed to the far side of the crater.