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Lab #13 - Impact Craters
Craters are seen on virtually every world out there. They are pretty simple things and you’ll learn about how they are made and how they can help us learn about other worlds. First some basic concepts.
Part I - Crater Types and Features
Go to the website and check out the images there. Here are some general crater descriptions.
Very large craters - these tend to be so large that the centers will smooth over due to the rebounding of the surface. While a crater rim may be visible, there is also the tendency for concentric rings to exist around the crater. The term impact basin is often used to refer to the largest craters. Due to the size of the impact that produces the largest craters, there are often secondary craters around them.
Medium sized craters - these will on occasion have central peaks where the surface has rebounded. They can also have rather smooth crater floors, apart from the central peak.
Small craters - basic crater shape, usually don’t fill in the central bowls, so have a rough, sloped interior.
Because the material on the surfaces of the various planets and satellites is different, the guidelines for crater sizes can be a bit fuzzy. What might look like a large impact crater on one world, could be only a medium sized crater on another.
There are also some general features of craters that can be distinguished. Take a look at the crater Danilova on Venus along with the corresponding identification map that goes with the crater. Make sure you can distinguish the various crater features from the identification map, because you’re going to have to do that with the next few images. I should mention something about the images of Venus that you are seeing. Since these are radar images, they are not really pictures but show the roughness of the surface. Dark regions are very smooth, while lighter colored regions are rough. Even though you may think you are seeing shadows and elevations, you really aren’t - but knowing how rough or smooth the various parts of the crater are can help you distinguish them. Here are the main crater features -
CentralPeak - as the name says, this is usually a peak in the center of the crater caused by the rebounding of the surface. Generally only medium sized craters have these.
Crater Floor - another fairly obvious name, usually a fairly smooth part, since it may have filled in with lava.
Crater Wall - what you might also think of as the rim of the crater.
Crater Ejecta - the stuff blasted out by the impact, pretty much the splatter from the impact.
Crater Outflow - some impacts cause further damage, such as lava flows or landslides that may happen well after the impact.
On the image of Crater Yablochkina identify the various crater features that are labeled.
a.
b.
c.
d.
e.
Now for a little more difficulty, this time you’ll just have the crater image to look at, no nice neat diagrams that mark off the different types of crater features. Use the image of Crater Danilova (and Yablochkina) as a guide to help you. Not all of the letters are in the location of the feature they indicate - some have lines indicating the features that they refer to.
Crater AddamsCrater Golubkina
f.j.
g.k.
h.l.
i.m.
Part II - Craters on Different Worlds
Take a peak at the images of craters on various worlds. First, there are some Earth craters, Barringer and Wolfe Creek. One feature about the craters on the Earth is that they are subjected to some fierce erosion effects, so much so that there are very few visible craters on the Earth. They are easily wiped out or covered over by erosion. It is possible to estimate the age of craters using several methods. One is how well defined the crater features are.
n. Of these craters, which one looks the “freshest” (this would also indicate which is the younger of the two)? What crater features give you this impression?
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Lunar Craters
There are several pictures of lunar craters. It is important to note that there is no atmosphere or liquid water on the Moon, so there are not as many mechanisms for erosion occurring on the Moon as there are on the Earth.
o. How are the lunar craters different from the Earth craters?
p. Is there any evidence of erosion on the Moon, and if so, what form does it take?
Martian Craters
q. Take a look at the Martian craters. Is there any erosion going on there? What do you base your answer on?
Part III - Making Craters
Now you’ll experiment with a little simulation that shows what happens when objects hit planets. The program will show the size (red dot) and energy of an impact on the surface of the Earth. The impacts start off on Latham hall, and then extend outwards to various scales until you reach the point where all life on Earth will be wiped out. You can change a few things, like the diameter of the object hitting the Earth, its velocity and the angle at which it hits (90 degrees is straight down, 10 degrees is a glancing blow). You can also change the composition of the object hitting the Earth (Ice, Rock or Metal) and you can even change the planet that gets hit - but let’s stick with the Earth. Let’s do a few tests first to see what happens. To avoid confusion, the thing that hits the planet, the impactor, is always described in terms of its diameter, while the crater that is produced by the hit will be described in terms of its radius.
r. How does the size of the crater change with the different compositions? Provide an explanation as to why it changesin that manner.
s. Change the angle (values range from 10 to 90). How does that change the size of the crater? Provide an explanation as to why it changes in that manner.
Now we’ll do a more quantitative experiment. Set the composition to “Rock” and the angle to 45. The velocity should still be 20 km/s and you should still be on the Earth. You’ll change the value of the diameter according to the table below and determine various features for the crater. For the energy value we’ll use the Log (Energy) value, since that is easier to write. Fill in the other values as well.
Impactor Diameter Crater Radius Ejecta Spread Log (Energy)
1
10
100
1000
10000
100000
Now that you have a range of crater sizes and information about the requirements needed to make those craters, you can use that to figure out how some Earth craters were formed. Use the radii of these craters to estimate the size of the object that created them, the size of the ejecta spread, and the Log (Energy) of the impact. You’ll want to compare these craters’ radii to those that you got in the table. This will give you starting point from which to begin experimenting with values for the impactor diameter. You should just keep on experimenting with the program until you get craters of about the appropriate size (try to get within 10% of the crater size). We’ll use the same velocity and angle values as before, so the only thing you’ll change is the impactor size.
Manson Iowa Crater (crater radius = 17,000 m)
t. Impactor Diameter =
u. Ejecta spread =
v. Log (Energy) =
SudburyCanada (crater radius = 125,000 m)
w. Impactor Diameter =
x. Ejecta spread =
y. Log (Energy) =
Barringer Arizona(radius = 600 m)
z. Impactor Diameter =
aa. Ejecta spread =
bb. Log (Energy) =
cc. Now let’s speculate on these impacts. Do you think you’d be influenced by any of these impacts if they were to happen today, and if so, which ones? Assume that you are in Iowa when the impact occurs and don’t change this answer later – just use the information given above, such as the energy of the impact and the crater size and ejecta spread to come to a conclusion about each impact.
Let’s see how bad things actually would be. Using the value of the energy for each impact, determine the frequency of impacts - how often do these critters hit. There is a program on the web page to do this. When you click on the line on this graph, the value for the energy of the impact and the frequency (how often it occurs) will be displayed. There is also a utility to determine the number of fatalities that would occur in the event of an impact (when you click on the line). This also uses the energy of the impact. Fill those values in the appropriate place below for each of these impacts.
Manson Iowa Crater (crater radius = 17,000 m)
dd. Frequency =
ee. Impact Deaths =
SudburyCanada (crater radius = 125,000 m)
ff. Frequency =
gg. Impact Deaths =
Barringer Arizona (radius = 600 m)
hh. Frequency =
ii. Impact Deaths =
jj. Now what do you think of your chances for surviving any of these impacts?
kk. Are you going to lose any sleep over this?
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