Moons
Moons

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Moons

2.2.7 Making an impact

In the decades since Gene Shoemaker’s discoveries in Arizona, the impact model of crater formation has gained wide acceptance.

Using data from computer-based modelling and real lab-based experiments, planetary scientists now know that when an impactor hits the ground at hypervelocity, most of the impactor material is vaporised, and the crater itself is excavated by intense shock waves, radiating out from the point of impact.

This video includes some classic experiments from the mid 1980s.

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NARRATOR
The origin of the craters on the moon was a matter of controversy which lasted from the middle of the 19th century until sometime after the first Moon landings in 1969. So how can we be sure that most craters are caused by impacts? One way is to study impact sites, such as this - the Barringer Crater in Arizona, which clearly is not the result of volcanic activity. Instead, this crater and the ejecta deposits around it resemble very closely the results of a major explosion in which an expanding shockwave excavates the cavity and disperses a layer of ejected material across the surrounding area. Another way of investigating crater formation is by simulating impact on a small scale in the laboratory - as was done here at the NASA Ames Research Centre in a classic series of experiments during the 1980s.
PETE SCHULTZ
We can produce a hole in the ground. We can produce it with a very small object. A quarter-inch ball might produce a 20-inch diameter crater. It'll look conical in shape, but that won't be necessarily what a lunar impact crater will look like. Here's an example of a lunar creator. And to give you an idea of scale, a Meteor Crater - the Barringer Crater in Arizona-- would probably be about the size of this crater right here. This gives you an idea of the amount of energy that's involved when one of these events actually happens. It's a catastrophic process that is very difficult to imagine. And these experiments provide us only sort of a small glimpse of really what's going on.
NARRATOR
In this experiment, a thin layer of red dust - called 'tempera' - has been spread over the surface of the target so that the trajectories of the particles in the surface layer can be followed in detail. The chamber must then be evacuated slowly so air escapes gently from between the dust grains without disturbing the surface. An electronic control system will fire the gun having first switched on three high-speed cameras and their lights to record the impact in slow motion.
PETE SCHULTZ
Well, I hope it works.
SPEAKER 3
I hope so.
PETE SCHULTZ
Any bets?
SPEAKER 3
No.
[LAUGHING]
[WHIRRING]
PETE SCHULTZ
Wow! Beautiful! Beautiful! Wow!
SPEAKER 4
Beautiful!
PETE SCHULTZ
Good job! Good job!
SPEAKER 4
Just great!
PETE SCHULTZ
Beautiful!
SPEAKER 4
Very, very good. Yay! Very good!
PETE SCHULTZ
Every time you do one of these things, it's a little bit different. I don't know. It's sort of like opening a present. You never know what exactly is going to happen, especially when you take perhaps a day preparing a target. And then, suddenly to have it work. I feel like I'm -
This is what's the beauty of doing experiments. You never know what's going to happen next. Ah! Great! That's great. OK. That did just what we expected.
Here we can see where the red tempera that we had on the surface of the target has been covered by the lighter material that was below it. And you can see the decay of the thickness of that ejecta. And eventually, it gives out about here where it predominately becomes red. And as you go farther out, you can see just primarily red clumps that have impacted. And these would have produced secondary craters had they been formed on the Moon at a much larger scale.
NARRATOR
And this is what the cameras recorded - an impact slowed down to 1/16th its normal speed. What does that picture tell us?
PETE SCHULTZ
One of the things that we can tell out of these experiments is that the process of cratering is a very orderly process. It's not this chaotic jumble that you might envision but looking at an old war movie or looking at the nuclear explosion. It looks like a kind of jumble, a cloud. Instead, in a vacuum condition - which we have for the moment-- it forms a nice series of conical-shape, inverted cones that marches outward.
And as it marches outward, it doesn't change its form very much. And this marching outward is a result of the locus of ejecta material coming from the inside first and at the highest velocities and contributing at the top of the ejecta plume. The material that is down at the bottom of the plume is coming from deeper in the crater. And even after the crater has formed, this ejecta curtain simply keeps on marching outward in a beautiful, regimented way.
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