Geology of the Solar System 3: Cratering of planets



I think craters are pretty cool, and would love to visit one some day (luckily, there is one fairly close to where I live. Unluckily, it is largely off limits, and is covered in vegetation anyways).

They are created by collisions of bolides, which is a general term for things that come from outside of the earth system and crash into it. Going with the solar nebula and cold accretion theories/assumptions, there is exponentially less loose, floating debris in the solar system over time, as it gradually collects together into single larger bodies. Therefore, one can tell the age of the surface of a planet (not the age of the entire planet itself), by looking at how much cratering there is.  For example, because its plate tectonic system is constantly recycling Earth’s surface, it has relatively few craters because all of the rock from the older, more heavily impacted surfaces, has been recycled already, so we can see that Earth’s surface is relatively young.  Mercury, on the other hand, most likely has a much older surface, judging by how heavily cratered it is.

Mechanism of Crater Formation

There are three stages in the creation of a crater:

1. Compression stage – a shock wave expands out from the point of impact

2. Excavation stage – the pressure of the compression stage is released upward and outward, and the target rocks expand explosively, traveling out in an expanding conical sheet that breaks apart to form rays, all in a matter of a few seconds

3. Modification stage – after the crater has been hanging out for a while, the center of it rebounds upwards, the sides of the crater slump inwards, and weathering or erosion might occur

Features of Impacts

There are many features that can differentiate a crater from something else of a similar size and shape, like a caldera for instance.  There will be impact melt on the crater floor, created when the heat caused by impact melted some of the surrounding material to become molten, which can be found by drilling a core in the floor.  That core may also contain breccia, materials below the crater that were shattered by the force of impact.  There will also be tektites in the area, which are small, relatively spherical beads of glassy material created when molten materials splashed out of the crater and cooled instantaneously in the air. You can also look for an ejecta blanket, the broken debris spread out from the crater during the excavation stage, which gradually thins with distance from the crater, eventually appearing as thin rays. And finally, there will be the crater itself, which comes in three types:

Simple craters are bowl-shaped and relatively small, complex – central peak craters are a bit larger and are caused by gravitational adjustment, with a peak of material rising in the center to compensate for the missing mass, and complex multirin craters, which are the largest, and are caused by concentric slump blocks falling into the crater to create a terraced effect around it.  Sometimes, complex multiring craters will also have a central peak.

Yeah, craters!


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