Erosion
Geologic processes shape planets. Remember, there are only 4 important ones: volcanism, tectonic, impact cratering and erosion . The topic for this chapter is the one most taken for granted, erosion.
Erosion can be caused by wind, rain, or just gravity pulling stuff down. You might think this doesn't amount to much but over time erosion has worn down mountains chains, carved canyons and filled in craters. Some planets don't have air, so they don't have wind or rain. Without, wind and rain, there isn't much erosion and geological features, like a mountain, survive for billions of years.
You can check for signs of erosion on a planet just by carefully looking at geological features. Are they crisp and new? Most every feature you see is millions of years old. If there is erosion, over time they get more worn down and look old and battered. It isn't any more sophisticated then that. Features that look new either are new or they haven't changed since they were new.
There are only a couple of ways to erode something, so we might as well review them all. On Earth, water is the biggest eroding force. Rain falling from the sky beats up whatever it hits. Also, water can, over time, dissolve rocks. It isn't hard to imagine an atmosphere where the rain isn't nice clean water. Instead it could be a very nasty acid or caustic chemical that wears rock more quickly.
The force of individual rain drops might be limited, but rain water can form rivers that wash away anything in its path. Earth's Grand Canyon demonstrates a river's force quite nicely. Flowing water can transport really, really huge amounts material downstream.
Small quantities of water can fill a small crack. If the weather turns cold, the water freezes. When water freezes it expands and can split a rock into smaller pieces. This makes more surface area available to weathering as well as making pieces small enough to be carried away by wind or water.
Wind, espically when it is blowing sand or other small particles, will erode rock. This sandblasting, over time, can batter features that stick up (like volcanoes or mountains) and fill in features that are holes (like impact craters). Wind flowing around features forms eddies. Where the wind has blown, you can often find evidence of windstreaks. They are often shapped like teardrops.
If tiny meteorites can reach a surface, they will erode it. They can pulverize rock into dust. They can also melt small rock fragments and fuse them together. Fast micrometeorites change a surface a little like the rain we're familiar with. If a planet has an atmosphere, tiny meteorites burn up and never reach the surface. The atmosphere acts as a protective blanket. On a place like the Moon where there is no atmosphere, tiny meteorites pummel the surface turning rock into dust and fusing the dust into rock. On the Moon this has created a surface that looks like soil.
Just repeatedly heating and cooling rocks makes them crack and break. As the rocks weaken, geological features crumble. How important this is depends on the planet's weather. On Venus, the total change in temperature (from seasonal change or day/night change) is probably under 20 degrees. Other places, like the Earth, have temperature swings of over 100 degrees.
Gravity can erode geological features that aren't extremely stable. Over time, a hollow volcano might not be able to withstand the constant tug of gravity and it can collapse.
Sometimes, just chemical reactions can cause erosions. If an atmosphere has strong chemcials, like a powerful acid, it will eat way features and cause them to crumble.
On Earth, biological activity contributes to erosion. Living things work their way into so many niches allowing water to enter and freeze and create soil to be blown around or washed away. As you look at other planets, remember that Earth erodes much faster than some other worlds.
To recap, there probably isn't much you learned about erosion in this chapter. You've heard about it and seen it at work. I hope the review of all of it's different forms was useful and the comments about how it is different on other worlds helps you think about it little more clearly.